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Scientific Reports Manuscript Page 1 of 18

Relative sea level rise at Galveston Pier 21, Texas, USA: Contributions from 1

land subsidence 2

Yi Liu1,*, Jiang Li1, John Fasullo2, Devin L. Galloway3 3

1Morgan State University, Department of Civil Engineering, Baltimore, MD 21251; 4 2National Center for Atmospheric Research, Climate and Global Dynamics Lab, Boulder, CO 5

80305; 6 3U.S. Geological Survey, Water Mission Area, Earth System Processes Division, Indianapolis, IN 7

46278 8

*Correspondence to: [email protected] 9

Abstract 10

Relative sea level rise at tide gauge Galveston Pier 21, Texas, is the combination of absolute sea 11

level rise and land subsidence. We estimate subsidence rates of 3.54 mm/a during 1909–37, 6.08 12

mm/a during 1937–83, and 3.50 mm/a since 1983. Subsidence attributed to aquifer-system 13

compaction accompanying groundwater extraction contributed as much as 85% of the 0.7 m 14

relative sea level rise since 1909, and an additional 1.9 m is projected by 2100, with contributions 15

from land subsidence declining from 30 to 10% over the projection interval. We estimate a uniform 16

absolute sea level rise rate of 1.10±0.19 mm/a in the Gulf of Mexico during 1909–92 and its 17 acceleration of 0.270 mm/a2 at Galveston Pier 21 since 1992. This acceleration is 87% of the value 18

for the highest scenario of global mean sea level rise. Results indicate that evaluating this extreme 19

scenario would be valid for resource-management and flood-hazard-mitigation strategies for 20

coastal communities in the Gulf of Mexico, especially those affected by subsidence. 21

Introduction 22

Many severe hurricane-induced urban floods have occurred in U.S. coastal communities along the 23

Gulf of Mexico in recent decades (including Harvey (2017), Isaac (2012), Ike (2008), Gustav 24

(2008), Katrina (2005), Rita (2005), and Ivan (2004)). The Great Galveston hurricane on Sept. 8, 25

1900, killed more than 6,000 people and destroyed approximately 3,000 homes at Galveston City 26

with a 4.6 m storm surge that swept through the city1. The latest severe flood caused by Hurricane 27

Harvey (2017) in the Houston-Galveston region was regarded as one of the costliest disasters in 28

U.S. history, with damages exceeding $100 billion. Flood risk in this region is elevated in part 29

because relative sea level rise (RSLR2) in the Galveston Bay is about four times greater than global 30

mean sea level rise (GMSLR2). RSLR, measured at any tide gauge, is the combination of absolute 31

sea level rise (ASLR) due to global warming and land subsidence (LS) due to tectonic downward 32

movement, subsurface fluid withdrawal and creep of soil and rock. Lying within one of the globe’s 33

key hot spots of sea level rise3, tide gauge Galveston Pier 21 is one of 22 tide gauges along the 34

U.S. coast of the Gulf of Mexico in the U.S. (these gauges and five others along the Atlantic coast 35

of Florida are shown in Fig. 1). This gauge has the longest tide record (110 years, 1909−2018 are 36

analyzed in this study) since 1904 among the 27 gauges. Linear trends of relative sea level over 37

time for the entire period of record for each gauge vary from 2.13 mm/a at Cedar Key, Florida to 38

9.65 mm/a at Eugene Island, Louisiana4, where ‘a’ in the denominator denotes annum throughout 39

the paper. Sea level in Galveston Bay has risen about 71 cm with a linear trend of 6.51 mm/a at 40

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tide Gauge Galveston Pier 21 since 19044. This RSLR rate is 3.8 times larger than the GMSLR 41

rate of 1.7 mm/a5. An additional 0.3–1.2 m of GMSLR is projected to occur by 21005. While 42

current and future GMSLR is associated with global warming5, the primary cause of local RSLR 43

in the Houston-Galveston region during the past 50 years has been LS associated with groundwater 44

extraction. In the next several decades, storm surges and high tides are likely to combine with 45

GMSLR and LS to further increase flooding in many regions6,7. GMSLR will continue in response 46

to the current state of global warming beyond 2100 because the oceans take a very long time to 47

equilibrate with warmer conditions at the Earth’s surface6. Ocean waters will therefore continue 48

to warm and sea levels will keep rising for many centuries8. Recent research indicates that present 49

day carbon dioxide levels are sufficient to cause Greenland to melt completely over the next 50

several thousand years9. Therefore, the Houston-Galveston region will likely continue to be one 51

of the world’s large coastal communities that is most susceptible to coastal and inland flooding 52

from hurricanes and other extreme storms. Improved understanding of RSLR, particularly 53

contributions from LS, is fundamental in adapting resource-management and flood-mitigation 54

policies. 55

56 Fig. 1. Geological map with tide gauges and GPS stations along the Gulf of Mexico in the United 57

States of America (Based on U.S. Geological Survey digital geological map10 with the GIS data 58

from NOAA4, JPL (Jet Propulsion Laboratory)11 and GLOSS12). Geological cross-section A-A′ is 59

shown in Fig. 2. (mapped by using ArcGIS 10.6.1; https://desktop.arcgis.com/en/) 60

The ASLR7,13,14 is equivalent to eustatic sea level rise (SLR)15,16 or global-mean geocentric SLR2 61

and attributed to global warming. In this paper it is assumed that LS7,13,14,17 includes two 62

components LSBR and LSnBR, where LSBR is subsidence contributed from bedrock systems or non-63

compacting strata owing to tectonic subsidence (TS)18–20 and creep of bedrock systems (SCBR)21; 64

and LSnBR is subsidence contributed from the compaction of susceptible (compressible, non-65

bedrock) aquifer systems owing to primary compaction (SPC) caused by subsurface fluid 66

withdrawal22–27 and creep of these aquifer systems (SCnBR)28–30. Thus, RSLR = ASLR + LS, where 67

LS = LSBR + LSnBR; LSBR = TS + SCBR; and LSnBR = SPC + SCnBR (see Table 1). By application, 68

due to geological similarity (see Fig. 2), LS = LSBR + LSnBR = LSBR can be found at the two adjacent 69

locations of tide gauge Cedar Key and GPS station XCTY at Cross City, Florida as LSnBR = 0. 70

Therefore, RSLR = ASLR + LSBR at tide gauge Cedar Key and similarly, RSLR = ASLR + LSBR 71



+ (SPC + SCnBR) at tide gauge Galveston Pier 21 where LSBR can be measured at GPS station 72

SG32 at College Station, TX. 73

This paper quantifies RSLR at tide gauge Galveston Pier 21 as the combination of ASLR, LSBR 74

and LSnBR based on an estimate of ASLR for the Gulf of Mexico and an analysis of the historical 75

subsidence in the Houston-Galveston region, and forecasts the RSLR in 2100 at tide gauge 76

Galveston Pier 21. Firstly, an estimate of ASLR for the Gulf of Mexico is computed based on the 77

estimated LS at tide gauge Cedar Key, Florida, where it is assumed that only LSBR contributes to 78

LS because LSnBR is considered negligible owing to a lack of groundwater-level declines and an 79

overconsolidated stress condition31. Next, the estimated ASLR is used along with estimates of each 80

of the two components of LS (LSBR and LSnBR) to evaluate RSLR in the historical record at tide 81

gauge Galveston Pier 21 and project RSLR to 2100. SPC and SCnBC resulting from compressible 82

aquifer systems at tide gauge Galveston Pier 21 are estimated through analysis of 1) historical 83

LSnBR measurements and coupled groundwater-flow and LS simulation results; 2) annual mean 84

RSLR data from long-term tide gauge records (Supplementary Fig. S1A); 3) the uniform ASLR in 85

the Gulf of Mexico; and 4) LSBR in the Houston-Galveston region estimated from the 86

measurements at GPS station SG32 (Fig. 1). 87

88 Fig. 2. Geological cross-section showing locations of GPS receivers, and tide gauges from College 89

Station to Galveston Pier 21, TX to Eugene Island and Grand Isle, LA to Cedar Key, Cross City 90

and Daytona Beach, FL (A-A′ in Fig. 1). GPS receivers SG32 and XCTY measure LSBR19; tide 91

gauges Galveston Pier 21, Eugene Island, Grand Isle, Cedar Key and Daytona Beach measure 92

RSLR. The dashed lines are based on previous studies32–34. Land-surface elevation is from Google 93

Earth Pro. RSLR values are from NOAA4. GPS LS values are from GLOSS12 and JPL11. Upward 94



pointing red arrows indicate RSLR at tide gauge stations and purple downward pointing arrows 95

indicate LS at GPS stations. See Table 1 for geological layer symbols and Supplementary Fig. S2 96

for aquifer systems in the Houston-Galveston region. 97

98

Geological materials

Symbol of Station type

Geological time Stress history

Co

nso

lidat

ion

de

gre

e**

** Subsidence Type

Period Epoch Start, MYBP

Exp

eri

en

ced

eff

ect

ive

st

ress

(σ

c′)

Cu

rre

nt

eff

ect

ive

st

ress

(σ

o′)

Geo

logi

cal s

yste

m

Aquifer Systems

Qh Quaternary Holocene 0.00117 Very low

σo′ ≈ σc

′

1 SPC, SCnBR

LSnBR

Qp Pleistocene 2.58 Low 2

T Tertiary Pliocene to Paleocene

66 High 3

C Cretaceous 145 Very high 4

TOC Tertiary Pliocene to Paleocene

66 High σo

′ < σc′ 5

SCnBR* COC Cretaceous 145 Very high

Bedrock System

pre-C Jurassic to Precambrian

4600 Highest σo

′ < σc′ **

σo′ ≈ σc

′ *** 6

TS, SCBR

LSBR

*SCnBR from the TOC and COC strata in the human observation period is assumed to be insignificant after a long-term accumulative creep such as the length CD in Supplementary Fig. S3 after an overburden removal event. **In the College Station, Texas area and the region from De Soto Canyon to Daytona Beach, Florida in Fig. 2. ***In the region from the west of Galveston Pier 21, Texas to De Soto Canyon, Florida in Fig 2. ****The consolidation degree of geological strata is based on the strata’s stress history in column stress history of this Table 1; 1 – Very unconsolidated; 2 – Unconsolidated; 3 – Semi-consolidated 35; 4 – Highly semi-consolidated; 5 – Over semi-consolidated; and 6 – Consolidated (Bedrock). LSnBR – Land subsidence due to compaction of non-bedrock systems (aquifer systems); LSBR – Land subsidence due to TS and SCBR of the bedrock system. SCnBR – Subsidence due to creep of non-bedrock systems (aquifer systems); SPC – Subsidence due to primary compaction; TS – Tectonic subsidence; and SCBR – Subsidence due to creep of the bedrock system.

Table 1. Geological type and symbol of tide gauge and GPS receiver site in the study area 99

Results 100

Absolute sea level rise (ASLR) in Gulf of Mexico. Of the 22 tide gauges shown in Fig. 1 along 101

the Gulf coast of the U.S., 21 are situated on more compressible Quaternary strata. Only tide gauge 102

Cedar Key (Figs. 1 and 2) and its nearby reference benchmarks are situated directly over 103

outcropped over semi-consolidated (its geohistorical overburden pressure is larger than the current 104

overburden pressure; see Table 1) Tertiary limestone (TOC), for which SPC and LSnBR are 105

negligible due to no significant decline of groundwater level and the removal of geohistorical 106

overburden layers, i.e., the absence of Quaternary strata Qh and Qp in Fig. 2. Moreover, the creep 107

magnitude during the period of human observation is negligibly small (e.g., the length C-D in 108

Supplementary Fig. S3). Only one GPS station XCTY is established on the same limestone, 55 km 109

distant from the Cedar Key gauge. LS (≈LSBR) measured at XCTY (Supplementary Fig. S4A) and 110

the minimum RSLR gauged at Cedar Key (Supplementary Fig. S1B) are used to estimate a uniform 111

ASLR in the Gulf of Mexico before 1992. 112

Assuming tide gauge Cedar Key measures RSLR comprising ASLR and LS (where LS=LSBR 113

owing to negligible LSnBR), and that the LS measured at the GPS station XCTY at Cross City, FL 114

(where LS≈LSBR) can be applied at tide gauge Cedar Key, then ASLR can be evaluated by 115



subtracting the LS measurement at XCTY from RSLR measured at Cedar Key. Height time series 116

at GPS station XCTY during 2004–13 is shown in Supplementary Fig. S4A. A long-term LS rate 117

of 0.88 mm/a was derived by SONEL12 (Système d'Observation du Niveau des Eaux Littorales) at 118

GPS station XCTY using Ellipsoid GRS8036. A piecewise trend equation set (1) applied to 119

simulate annual mean sea level at tide gauge Cedar Key with a regression coefficient (R) of 0.864 120

(Fig. 3) follows through PEST37 mimic linking a Fortran code of equation set (1) for identifying 121

best parameter values: 122

AMSL=1.98t+3081.85, t∈[1939 to 1992] (1-1) 123

AMSL=0.09276(t-1992)2+1.98t+3081.85, t∈(1992 to 2018] (1-2) 124

where AMSL is annual mean sea level (PSMSL38) in mm and t is time in years. The constant 125

(linear) RSLR rate at tide gauge Cedar Key is 1.98 mm/a from equation set (1). A constant (linear) 126

ASLR rate of 1.10 mm/a at tide gauge Cedar Key before 1992, which is selected based on the 127

linear trend of GMSLR before 1992 in the 20th century and the quadratic trend since 199239, is 128

derived from the difference between the RSLR rate of 1.98 mm/a and the LS rate of 0.88 mm/a. 129

This value is used as the regional constant (linear) ASLR rate for the Gulf of Mexico and is 130

represented by ar in supplementary equation set (S1) for tide gauge Galveston Pier 21. An ASLR 131

acceleration at tide gauge Cedar Key after 1992 of 0.1856 mm/a2 (2 × 0.09276) is estimated from 132

equation (1-2). 133

134 Fig. 3. Annual mean sea level (AMSL) and simulated trends (equations (1-1) and (1-2)) at tide 135

gauge Cedar Key, Florida (Data source: PSMSL38 based on NOAA tide records). Note: Datum is 136

RLR (revised local reference) defined to be approximately 7000mm below mean sea level by 137

PSMSL at each tide gauge). 138

Land subsidence due to tectonics (LSBR) in the Houston-Galveston region. The following 139

strata underlie tide gauge Galveston Pier 21 (Fig. 2, Supplementary Fig. S6; Table 1) and its nearby 140

reference benchmarks (GPS station TXGA and extensometer site Texas City-Moses Lake; see 141

Supplementary Fig. S6): 1. Quaternary unconsolidated layer which includes surficial thin 142

Holocene sediments and the more than 400-m thick Pleistocene deposits constituting the Chicot 143

aquifer; 2. Tertiary semi-consolidated deposits which includes the Evangeline aquifer, Burkeville 144



confining unit, Jasper aquifer and the Catahoula confining system with a total thickness of as much 145

as 350 m; 3. Cretaceous highly-semi-consolidated limestone10; and 4. pre-Cretaceous rocks 146

referred to here as basement rock. The Tertiary strata are outcropped and (or) uplifted in the 147

western area such as at College Station, Texas (Fig. 2 and Supplementary Fig. S2). The outcropped 148

and (or) uplifted Tertiary strata are over semi-consolidated (Figs. 1 and 2 and Supplementary Fig. 149

S2). Therefore, LS at Galveston Pier 21 should include LSBR from pre-Cretaceous and its 150

underlying strata (bedrock systems) and LSnBR from both SPC of the compressible Chicot and 151

Evangeline aquifer systems and the Burkeville confining unit due to groundwater withdrawal, and 152

SCnBR of all the compressible Quaternary, Tertiary and Cretaceous strata. 153

GPS station SG32, established on the outcropped over semi-consolidated Tertiary Yegua 154

formation (Ey) comprising sandstone, clay and lignite deposits with a thickness of 229–305 m, 155

measured land elevation changes from 2003 to 2014. Supplementary Fig. S8 shows the silty 156

sandstone outcrops in College Station, Texas. The uplifted Cretaceous layer and pre-Cretaceous 157

basement rocks underlie the Tertiary strata. JPL’s height time series at GPS station SG32 from 158

2003 to 2014 is shown in Supplementary Fig. S4C from which a long-term constant (linear) LS 159

rate of 2.67 mm/a11 is derived. GPS station LDBT (Fig. 1), 107 km southwest of GPS station SG32, 160

is anchored on the outcropped, over semi-consolidated Tertiary Calvert Bluff formation (Ecb) 161

composed of mudstone. SPC is negligible in the Ecb because no fluids are available for 162

development from the formation. A long-term constant (linear) LS rate of 2.68 mm/a was derived11 163

at this station using GPS station LDBT elevation data from 2003 to 2009 (Supplementary Fig. 164

S4D). LDBT is 257 km from Galveston Pier 21. A regional LSBR value can be evaluated from the 165

measured LS (height changes) on over semi-consolidated strata underlying GPS station SG32 in 166

Fig. 2 and Supplementary Figs. S3 and S4 due to negligible primary compaction (SPC≈0) and 167

creep (SCnBR≈0) in these materials and the absence of geohistorical overburden layers – 168

Quaternary strata (Qh and Qp in Fig. 2). Moreover, the creep magnitude of these strata (SCnBR) 169

during the human observation period under the oversonsolidation stress condition at GPS station 170

SG32 is negligible (see the path C-D in Supplementary Fig. S3). The similar LS values measured 171

at the two GPS stations support the estimate of LSBR in the Houston-Galveston region. LSBR at 172

tide gauge Galveston Pier 21 has a value of 2.67 mm/a used as coefficient sBR in supplementary 173

equation set (S1) to compute the RSLR at this tide gauge underlain by compressible aquifer 174

systems. 175

Subsidence due to primary compaction (SPC) at tide gauge Galveston Pier 21. During 1918 176

and subsequent years, millions of barrels of oil were removed from the Goose Creek Oilfield (see 177

location in Supplementary Fig. S6), 46 km northwest of Galveston Pier 2125. Between 1918 and 178

1926 a maximum LS of 115 mm/a (92 cm for 8 years) was measured in the oilfield. No subsidence 179

attributed to production in the oil field was observed within 40 km of Galveston Pier 21. This 180

implies that SPC attributed to oil and gas production from the oil field was negligible at Galveston 181

Pier 21. By 1937, groundwater levels were falling in a growing set of gradually coalescing cones 182

of depression centered on the areas of intensive groundwater withdrawal from the Chicot and 183

Evangeline aquifers25. One of these areas was in Texas City about 16 km from Galveston Pier 21. 184

About 1.6 mm/a LS (6 cm for 37 years) was estimated at Galveston Pier 21 from regional leveling 185

measurements during 1906 to 1943 (Supplementary Fig. S5A). A average LS of 0.85 mm/a 186

(cumulative LS of 4 cm for 47 years) occurring at Texas City from 1890 to 1937 was simulated 187



using a coupled groundwater-flow and subsidence model HAGM.2013 (Supplementary Fig. 188

S5D)32. This indicates that prior to about 1937 SPC at Galveston Pier 21 was small (probably much 189

less than 0.85 mm/a) because this location is on the periphery of the subsidence depression in 190

Texas City. About 7.0 mm/a LS rate (21 cm of SPC for 30 years) at Galveston Pier 21 was 191

estimated during 1943–73 (Supplementary Fig. S5B). Less than 6.8 mm/a SPC (15 cm of SPC for 192

22 years) was estimated during 1973–95 (Supplementary Fig. S5C). Though no significant 193

subsidence at Texas City was simulated by HAGM.201332 during 1974–2009 (Supplementary Fig. 194

S5D), 7.5 mm/a LS (6 cm of subsidence (compaction) for 8 years) was observed during 1973–81 195

from the Texas City–Moses Lake borehole extensometer (see location in Supplementary Fig. S6, 196

Supplementary Fig. S5D). No demonstrable subsidence was observed after 1983 at Texas City. 197

Therefore, it is reasonable to assume that SPC occurred principally during 1937–83 at Galveston 198

Pier 21. Combining with the analysis of annual mean RSLR at tide gauge Galveston Pier 21 (Fig. 199

5), it is determined that 1937 and 1983 represent the initial year and the ending year, respectively, 200

of SPC at this tide gauge. 201

Subsidence due to creep of non-bedrock aquifer system (SCnBR) estimated based on borehole-202

extensometer data in the Houston-Galveston region. A previously established instrumentation 203

system monitoring LSnBR of compressible aquifer systems in the Houston-Galveston region 204

includes 11 borehole extensometer stations comprising 13 borehole extensometers32 205

(Supplementary Fig. S6). Two of the stations Baytown and Clear Lake have shallow and deep 206

borehole extensometers and each of other 9 stations have only one borehole extensometer. 207

Supplementary Fig. S7 shows measured LS in terms of compaction measured at each extensometer 208

from the 1970s or 1980s to 2017. Supplementary Fig. S7 shows the negligibly variable SCnBR of 209

Quaternary and Tertiary strata (Qp + T) after inelastic SPC ended around 2000, for various periods 210

(Supplementary Table S1) from the mid-to late-2000s onward during which groundwater levels in 211

the Chicot and Evangeline aquifers were stable (Supplementary Table S1) owing to effective 212

groundwater resource management practices30. SCnBR rates range from 0.08 to 8.49 mm/a 213

(Supplementary Table S1) (corresponding to the slopes of the SCnBR trendlines which range from 214

2.22×10-4 to 2.327×10-2 mm/d in Supplementary Fig. S7)30, where ‘d’ in the denominator denotes 215

day throughout the paper. Determination of 3.87 mm/a SCnBR at extensometer Southwest is shown 216

in Fig. 4. These results indicate that the SCnBR from the Quaternary unconsolidated and Tertiary 217

semi-consolidated strata also occurs at the location of tide gauge Galveston Pier 21. Note, the 218

SCnBR of Holocene strata (Qh) in the Mississippi Delta was found to be as much as 5 mm/a based 219

on analysis of a series of radiocarbon-dated sediment cores29. 220



221 Fig. 4. Inelastic SPC ended in about 2000 and SCnBR became apparent at extensometer Southwest 222

in Houston when groundwater-level (GL) trends were stable. I: Inelastic SPC dominated LS; II: 223

elastic rebound dominated LS; III: SCnBR and SPC offset rebound; IV: elastic SPC and SCnBR > 0 224

while inelastic SPC approached 0; V: elastic rebound offset SCnBR; and VI: SCnBR apparent in trend 225

line (red line) while SPC absent. Slope of SCnBR trend (red line) is 0.0106 mm/d or 3.87 mm/a. 226

(modified from30) 227

Subsidence due to primary compaction (SPC) and absolute sea level rise (ASLR) acceleration 228

estimated from tide gauge records at Galveston Pier 21. ASLR of 1.10 mm/a (ar in 229

supplementary equation set (S1)) and LSBR of 2.67 mm/a (sBR in equation set (S1)) contributed to 230

RSLR at Galveston Pier 21 for the period of record analyzed as noted above. Also noted above, 231

SPC occurred during 1937–83 at the location of tide gauge Galveston Pier 21, and its value was 232

determined by differencing the linear trend of RSLR during 1937–83 from the linear trend during 233

1909–37 and 1983–92 when SPC was considered negligible at the location of the tide gauge. A 234

piecewise trend of the AMSL, expressed by equation set (2) was obtained using PEST40 simulation 235

to estimate all other coefficients (i.e., -1994.78 mm, 7.16 mm/a and -6953.15 mm, -1877.03 mm, 236

and 0.1349 mm/a2 applied in equation set (2)) with a regression coefficient of 0.98 (blue dashed 237

line, Fig. 5) as below: 238

AMSL=4.60t-1994.78, t∈[1909 to 1937] (2-1) 239

AMSL=7.16t-6953.15, t∈(1937 to 1983] (2-2) 240

AMSL=4.60t-1877.03, t∈(1983 to 1992] (2-3) 241

AMSL=4.60t-1877.03+0.1349(t-1992)2, t∈(1992 to 2018] (2-4) 242

Equation (2-2) shows a constant (linear) RSLR rate of 7.16 mm/a during 1937–1983, which is 243

increased by groundwater withdrawal, from 4.60 mm/a before 1937 and after 1983. The difference 244



of 2.56 mm/a is the estimated SPC rate at Galveston Pier 21 during 1937–1983 with a cumulative 245

SPC of 118 mm for the subperiod. Equation (2-4) is quadratic with an ASLR acceleration of 0.270 246

mm/a2 (2 × 0.1349) in addition to a constant (linear) rate of 4.60 mm/a, which is the combination 247

of ASLR, LSBR and the SCnBR rates determined for the period 1992–2018. Note: The negligibly-248

variable SCnBR is assumed in equation set (2) for identification of SPC (see details in section of 249

SCnBR in Supplementary Materials). This acceleration is considered to be driven by climate 250

change.41 251

252 Fig. 5. Annual mean sea level (AMSL) and simulated trend lines at tide gauge Galveston Pier 21 253

(Data source: PSMSL38 based on NOAA tide gauge records). 254

Variable subsidence due to creep of non-bedrock aquifer systems (SCnBR) estimated from 255

tide gauge records at Galveston Pier 21. For a more accurate projection of long-term RSLR, 256

variable SCnBR is considered in the piecewise supplementary equation set (S1) of RSLR with 257

parameter CH based on the creep theory of compressible sedimentary materials31,42. Two unknown 258

parameters CH and C in the supplementary equation set (S1) can be evaluated as 3825.51 mm and 259

6780.61 mm, respectively, using PEST37 simulation. The other parameters are given as the 260

following: ar = 1.10 mm/a, sBR = 2.67 mm/a, pl= 2.56 mm/a , ac = 0.270 mm/a2, t0 = 1908, t1 = 261

1937, t2 = 1983, and t3 = 1992, where: ar denotes a reginal uniform ASLR rate; sBR and pl, the 262

annual rates of LSBR and SPC, respectively; ac, the regional ASLR acceleration rate; and t0, t1, t2 263 and t3, the start times of each of the four subperiods. The red solid line in Fig. 5 shows the trend 264

line of RSLR found with supplementary equation set (S1). From 1983 to 1992 estimated SCnBR 265

rates range narrowly from 0.84 to 0.83 mm/a, which is comparable to 0.83 mm/a derived from 266

equation (2-3) for example, by removing the ASLR of 1.10 mm/a and LSBR of 2.67 mm/a from the 267

computed RSLR rate of 4.60 mm/a. The estimated variable SCnBR rates computed using the 268

formula 0.4343CH/t were 0.87 mm/a in 1909, 0.82 mm/a in 2018 and 0.79 mm/a in 2100. 269

Projection of mean relative sea level rise (RSLR) at tide gauge Galveston Pier 21. Figure 5 270

shows that sea level has risen by about 0.7 m since 1909 at Galveston Pier 21. Supplementary 271

equation set (S1) with all parameter values identified above was used to project RSLR at tide gauge 272



Galveston pier 21 from 2018 to 2100. The projected RSLR of 1.9 m is about 90% and 146% of 273

the highest (2.1 m) and intermediate-high (1.3 m) scenarios of GMSLR39, respectively. The 274

projected ASLR acceleration of 0.270 mm/a2 is about 87% and 155% of the highest (0.312 mm/a2) 275

and intermediate-high (0.1742 mm/a2) GMSLR scenarios, respectively. The projected ASLR 276

acceleration of 0.1856 mm/a2 computed previously at tide gauge Cedar Key for the same period is 277

about 60% and 107% of the highest and intermediate-high scenarios of GMSLR39, respectively. 278

Therefore, the results in this paper indicate that it may be prudent to consider the highest scenario 279

of GMSLR in resource-management and flood-hazard-mitigation strategies for coastal 280

communities in the Gulf of Mexico, especially those affected by LS. 281

Contributions to relative sea level rise (RSLR) at tide gauge Galveston Pier 21. RSLR was 282

computed for tide gauge Galveston Pier 21 for four subperiods during the period 1909–2100 using 283

supplementary equation set (S1) (note: the first three subperiods are the same as used in equation 284

set 2; the fourth subperiod is different from, but inclusive of, the fourth subperiod [1992-2018] 285

used in equation set 2). Contributions from ASLR, LSBR, SPC and SCnBR to RSLR vary in different 286

subperiods and are estimated to be 24, 58, 0 and 18% of the 4.63 mm/a RSLR during 1909–37; 287

15, 37, 36 and 12% of the 7.16 mm/a RSLR during 1937–83; and 24, 58, 0 and 18% of the 4.60 288

mm/a RSLR during 1983–92, respectively (see Table 2). Thus, ASLR contributed an estimated 289

15–24% to RSLR at Galveston Pier 21 from 1909 to 1992, while LS contributed 76–85%. The 290

estimated LS contribution (Fig. 6) to RSLR during 1992–2100 decreased from 76% in 1992 to 291

30% in 2018 and is projected to decrease to 10% by 2100. The estimated LS contribution to RSLR 292

at Galveston Pier 21 in 2000 was 52%. The estimates indicate that LS dominated RSLR in the 20th 293

century but since 2001, ASLR driven by global warming has dominated RSLR at tide gauge 294

Galveston Pier 21. 295

296 Fig. 6. Projected annual mean sea level (AMSL) and estimated land subsidence (LS) contribution 297

to relative sea level rise (RSLR; RLR: See note in Fig. 3 caption) at tide gauge Galveston Pier 21 298



shown with projections for the highest and intermediate-high scenarios of global mean sea level 299

rise (GMSLR)39. 300

301

Item

Contribution

1909-37 1937-83 1983-92

Rate (mm/a) % Rate (mm/a) % Rate (mm/a) %

ASLR 1.10 24 1.10 15 1.10 24 LSBR 2.67 58 2.67 37 2.67 58 SPC 0.00 0 2.56 36 0.00 0 SCnBR 0.86 18 0.83 12 0.83 18

RSLR 4.63 100 7.16 100 4.60 100

Table 2. Contributions of ASLR, LSBR, SPC and SCnBR to RSLR from 1909 to 1992. 302

Discussion 303

Tide gauge Cedar Key is the sole tide gauge station anchored on the over semi-consolidated 304

Tertiary strata with minimal local groundwater development along the U.S. coast of the Gulf of 305

Mexico. Assuming the constant (linear) ASLR rate of 1.10 mm/a with uncertainty of ±0.19 mm/a 306 (comparable to ±0.18 mm/a in Supplementary Fig. S1B from NOAA), obtained from tide gauge 307 Cedar Key and the GPS station XCTY, is regionally representative, then the ASLR rate estimated 308

from any other tide gauge along the Gulf coast should be very close to 1.10±0.19 mm/a. Tebaldi 309 et al.7 assumed ASLR along the U.S. coasts is approximately equal to GMSLR of 1.70 mm/a for 310

estimating LS values at each tide gauge location. To evaluate the representativeness of the ASLR 311

rate estimated for tide gauge Cedar Key, ASLR rate was estimated from tide gauge Galveston Pier 312

21 and its reference GPS station TXGA, 3 km distant (Supplementary Fig. S6). The LS rate at 313

TXGA of 3.44 mm/a was estimated by SONEL12 (see Supplementary Fig. S4B) for the period 314

2005–14. This LS is the sum of LSBR and SCnBR at this station because LSnBR represents only 315

SCnBR as there was no SPC. A constant (linear) ASLR rate of 1.16 mm/a was derived by subtracting 316

the LS rate of 3.44 mm/a from the constant (linear) RSLR rate of 4.60 mm/a in equation (2-4), 317

assuming 3.44 mm/a approximately represents the constant (linear) LS rate at Galveston Pier 21 318

and recognizing that the acceleration of 0.270 mm/a2 is only related to global warming. The 319

derived ASLR rate of 1.16 mm/a leads to a difference of 5% relative to 1.10 mm/a estimated at 320

tide gauge Cedar Key. The ASLR rate difference of 0.06 mm/a may be due to differences between 321

the Quaternary and Tertiary strata underlying GPS station TXGA and those underlying tide gauge 322

Galveston Pier 21. However, the similar rates estimated at the two tide gauges support the use of 323

the estimated 1.10 mm/a as a regionally representative rate of ASLR (ar in supplementary equation 324

set (S1)) in the Gulf of Mexico, which is comparable to an ASLR rate of 1.11 mm/a estimated 325

from the tide gauge in Baltimore43. (Note: Wang et al.44 estimated ASLR for the Gulf of Mexico 326

using a new reference frame but the results presented for ASLR for the same tide gauges analyzed 327

in this article are not comparable to our estimate that is based on tide gauge Cedar Key for the 328

period before 1992, because the measured LS (designated as VLM in their table 2) in Wang et al.44 329



was relative and not based on an absolute reference frame such as GRS80 from SONEL used in 330

this study.) 331

If only a single linear equation is used to simulate the RSLR trend for the entire period of record 332

(1908–2018), the effects of LS (particularly SPC) and global warming acceleration may not be 333

accounted for in the analysis. Supplementary Figs. S1A and S1B show linear RSLR trends of 6.51 334

mm/a at tide gauge Galveston Pier 21 and 2.13 mm/a at tide gauge Cedar Key. Supplementary 335

Table S2 shows that the resulting ASLR rates are 3.07 mm/a at tide gauge Galveston Pier 21 and 336

1.25 mm/a at tide gauge Cedar Key, respectively, computed using LSBR measured at GPS station 337

TXGA and LS (LSBR + SCnBR) at GPS station XCTY. The 1.25 mm/a value at tide gauge Cedar 338

Key is closer to the previously estimated, pre-1992 regionally representative rate of ASLR in the 339

Gulf of Mexico (1.10 mm/a). The resulting ASLR rate of 3.07 mm/a at tide gauge Galveston Pier 340

21 differs by 146% relative to the 1.25 mm/a value at tide gauge Cedar Key. This large difference 341

underscores the importance of accounting not only for the historical (pre-1992) SPC but also the 342

ASLR acceleration since 1992 when estimating the pre-1992 linear trend of ASLR. 343

Nearly identical LSBR rates of 2.67 and 2.68 mm/a were measured at GPS stations SG32 344

(Supplementary Fig. S4C) and LDBT (Supplementary Fig. S4D), respectively, due to negligible 345

SPC and SCnBR at these station locations. The two stations are 107 km apart (Fig. 1). From 2005 346

to 2014 the LS rate at GPS station TXGA is 3.44 mm/a (Supplementary Fig. S4B) where SPC is 347

absent, the SCnBR rate at GPS station TXGA (see location in Supplementary Fig. S6) of 0.77 or 348

0.76 mm/a was evaluated by subtracting the LSBR of 2.67 mm/a (Supplementary Fig. S4C) or 2.68 349

mm/a (Supplementary Fig. S4D) at GPS stations SG32 or LDBT from that at GPS station TXGA, 350

assuming GPS station TXGA is located in the same tectonic zone as GPS stations SG32 and 351

LDBT (see locations in Supplementary Fig. S6). Compared to the SCnBR rate of 0.83 mm/a 352

estimated at tide gauge Galveston Pier 21 from 2005 to 2014, a difference of about 0.06 or 0.07 353

mm/a in the SCnBR rate at GPS station TXGA is reasonable due to geological material variation 354

between tide gauge Galveston Pier 21 and GPS station TXGA, located 3 km apart. The above 355

analysis demonstrates spatial stability of the estimated LSBR in the Houston-Galveston region and 356

the associated insights that can be gained regarding contributions to RSLR in the vulnerable region. 357

The LS rates at the four GPS stations (i.e., XCTY in Florida, and SG32, LDBT and TXGA in 358

Texas) are derived from a short period 2003–14 but work well systematically with the RSLR trend 359

for the longer period 1909–2018. This indicates that the LS rates in different tectonic zones, from 360

Florida to the Houston-Galveston region, Texas (Fig. 2), may also be temporally stable within 361

short time scales of the observations compared to the geological time scale of tectonics. 362

The analyses here for the Galveston Pier 21 tide gauge show that for the period record RLSR is 363

dominated by SPC attributed to groundwater-level declines accompanying groundwater 364

extraction. However, the magnitude of historical aquifer-system compaction and land subsidence 365

in the Houston-Galveston region in inland and other coastal locations (Supplementary Figs. S5 366

and S7) is far greater than that experienced at the location of the Galveston Pier 21 tide gauge and 367

GPS station TXGA. This indicates that potential impacts of subsidence and RSLR in terms of 368

coastal and inland flooding are likely greater in other areas of this region. Further, the variable 369

spatial and temporal distributions of historical subsidence that arise from the variable distributions 370



of compressible sediments, hydraulic properties in the aquifer systems, and groundwater 371

extractions from the aquifer systems, result in variable potential impacts of subsidence in the 372

region. Another related point is that the RSLR projections for tide gauge Galveston Pier 21 (Fig. 373

6) assume no changes in future management of groundwater resources in the region. Unlike ASLR, 374

RSLR with substantial contributions from land subsidence can vary locally and can change quickly 375

in response local changes in groundwater extraction. These variabilities indicate that in coastal 376

regions where SPC is an important contributor to RSLR, a more complete vulnerability assessment 377

is needed, one that accounts for the historical and future subsidence and potential future 378

groundwater management practices. 379

Materials and Methods 380

Identification of geological and hydrogeological conditions at tide gauges and GPS stations. 381

A tide gauge measures ASLR and LS and a GPS receiver at the tide gauge’s paired reference 382

station measures LS. Where both the tide gauge and paired reference station are seated on basement 383

rocks or on over semi-consolidated sediments without significant SPC and LSnBR, the LS at both 384

sites has only the component of LSBR. In contrast, where both the tide gauge and its paired 385

reference station are seated on unconsolidated and/or semi-consolidated sediments, LS constitutes 386

LSnBR (SPC and SCnBR) and LSBR. Geological and hydrogeological data were used to determine 387

the LS components measured at tide gauges and their paired GPS stations (Table 1). 388

Identification of regional absolute sea level rise (ASLR) before 1992. In general, ASLR can 389

be determined by RSLR minus LS that are measured with tide gauges and their nearby GPS 390

stations, respectively. Due to complexity of geological and geohydrological conditions and stress 391

history, LS varies at different locations. ASLR was estimated at tide gauges Cedar Key and 392

Galveston Pier 21 using ASLR=RSLR−LS. LS at tide gauges was estimated using 393

LS=LSnBR+LSBR, where LSnBR=SPC+SCnBR and LSBR=TS+SCBR. TS was estimated from 394

measurements at GPS stations anchored on bedrock (XCTY for gauge Cedar Key; and SG32 and 395

and LDBT for gauge Galveston Pier 21), assuming SCBR was negligible over the human time-scale 396

of observations, thus LS at the GPS stations could be represented by LS=LSBR=TS. These values 397

for LS=LSBR were translated to the tide gaging stations and used to compute ASLR at those gaging 398

stations. At tide gauge Cedar Key where LSnBR was assumed to be negligible, ASLR was estimated 399

using RSLR measured at the gaging station minus the translated estimate of LS=LSBR. For tide 400

gauge Galveston Pier 21 anchored in non-bedrock material, it was necessary to also estimate 401

LSnBR=SPC+SCnBR at the tide gaging station. 402

Identification of subsidence due to primary compaction (SPC) and absolute sea level rise 403

(ASLR) acceleration. SPC in the Houston-Galveston region accrued during a time period when 404

subsurface fluid was developed. The starting and ending years for a period when SPC occurred at 405

a location was determined by analyzing regional LS leveling data and simulation results as well as 406

annual mean RSLR data. RSLR trends during periods when SPC was either active or inactive were 407

simulated using long-term tide gauge records. ASLR acceleration and RSLR trends in active and 408



inactive SPC periods were further estimated with PEST40. Then the SPC rate was estimated from 409

the difference of RSLR trends between SPC active and inactive periods. 410

Identification of subsidence due to creep of non-bedrock aquifer system (SCnBR). The 411

existence of SCnBR at a tide gauge station was demonstrated by analyzing aquifer-system 412

compaction measurements and groundwater-level observations in the study area. Negligibly 413

variable SCnBR was used to analyze and estimate SPC before simulation of its variation through 414

compaction due to creep in supplementary equation set (S1)31,42 after uniform ASLR rate before 415

1992, ASLR acceleration, LSBR and SPC were determined at tide gauge Galveston Pier 21. 416

References 417

1. USACE. Galveston’s Bulwark Against the Sea: History of the Galveston Seawall. (1981). 418

2. Gregory, J. M. et al. Concepts and Terminology for Sea Level: Mean, Variability and 419

Change, Both Local and Global. Surveys in Geophysics 40, (2019). doi:10.1007/s10712-420

019-09525-z 421

3. Sallenger, A. H., Doran, K. S. & Howd, P. A. Hotspot of accelerated sea-level rise on the 422

Atlantic coast of North America. Nat. Clim. Chang. (2012). doi:coast of North America. 423

Nature Climate Change doi:10.1038/NCLIMATE1597 424

4. NOAA. Sea Level Trends. Available at: 425

https://tidesandcurrents.noaa.gov/sltrends/sltrends.html. (Accessed: 21st June 2019) 426

5. Walsh, J. et al. Ch. 2: Our Changing Climate. in Climate Change Impacts in the United 427

States: The Third National Climate Assessment 19–67 (U.S. Global Change Research 428

Program, 2014). doi:doi:10.7930/J0KW5CXT 429

6. Moser, S. C. et al. Coastal Zone Development and Ecosystems. in Climate Change 430

Impacts in the United States: The Third National Climate Assessment (ed. J. M. Melillo, 431

Terese (T.C.) Richmond, and G. W. Y.) 579–618 (2014). doi:doi:10.7930/J0MS3QNW. 432

http://nca2014.globalchange.gov/report/regions/coasts 433

7. Tebaldi, C., Strauss, B. H. & Zervas, C. E. Modelling sea level rise impacts on storm 434

surges along US coasts. Environ. Res. Lett. 7, (2012). doi:10.1088/1748-9326/7/1/014032 435

8. Gillett, N. P., Arora, V. K., Zickfeld, K., Marshall, S. J. & Merryfield, W. J. Ongoing 436

climate change following a complete cessation of carbon dioxide emissions. Nat. Geosci. 437

4, 83–87 (2011). doi:10.1038/ngeo1047 438

9. Robinson, A., Calov, R. & Ganopolski, A. Multistability and critical thresholds of the 439

Greenland ice sheet. Nat. Clim. Chang. 2, 429–432 (2012). doi:10.1038/nclimate1449 440

10. U.S. Geological Survey [USGS]. Geologic maps of US states. (2019). Available at: 441



https://mrdata.usgs.gov/geology/state/. (Accessed: 21st June 2019) 442

11. JPL. GNSS Time Series. Available at: https://sideshow.jpl.nasa.gov/post/series.html. 443

(Accessed: 21st June 2019) 444

12. SONEL. SONEL. Available at: https://www.sonel.org/?lang=en. (Accessed: 21st June 445

2019) 446

13. Zervas, C., Gill, S. & Sweet, W. Estimating vertical land motion from long-term tide 447

gauge records. NOAA Tech. Rep. NOS CO-OPS 065 (2013). 448

14. Eggleston, J. & Pope, J. Land Subsidence and Relative Sea-Level Rise in the Southern 449

Chesapeake Bay Region. U.S. Geological Survey Circular 1392 (2013). 450

doi:10.3133/cir1392 451

15. Letetrel, C. et al. Estimation of vertical land movement rates along the coasts of the Gulf 452

of Mexico over the past decades. Cont. Shelf Res. 111, 42–51 (2015). 453

doi:10.1016/j.csr.2015.10.018 454

16. Erkens, G., Bucx, T., Dam, R., De Lange, G. & Lambert, J. Sinking coastal cities. Proc. 455

Int. Assoc. Hydrol. Sci. 372, 189–198 (2015). doi:10.5194/piahs-372-189-2015 456

17. Kolker, A. S., Allison, M. A. & Hameed, S. An evaluation of subsidence rates and sea-457

level variability in the northern Gulf of Mexico. Geophys. Res. Lett. 38, (2011). 458

doi:10.1029/2011GL049458 459

18. Benford, B., Demets, C. & Calais, E. GPS estimates of microplate motions, northern 460

Caribbean: Evidence for a Hispaniola microplate and implications for earthquake hazard. 461

Geophys. J. Int. 191, 481–490 (2012). doi:10.1111/j.1365-246X.2012.05662.x 462

19. Goudarzi, M. A., Cocard, M. & Santerre, R. Present-Day 3D Velocity Field of Eastern 463

North America Based on Continuous GPS Observations. Pure Appl. Geophys. 173, 2387–464

2412 (2016). doi:10.1007/s00024-016-1270-7 465

20. Mitrovica, J. X., Milne, G. A. & Davis, J. L. Glacial isostatic adjustment on a rotating 466

earth. Geophys. J. Int. 147, 562–578 (2001). 467

21. Chigira, M. Long-term gravitational deformation of rocks by mass rock creep. Eng. Geol. 468

32, 157–184 (1992). 469

22. Terzaghi, K. Settlement and consolidation of clay. in Principles of Soil Mechanics, vol. IV 470

874–878 (McGraw-Hill, 1925). 471

23. Poland, J. F. Land subsidence in the San Joaquin Valley and its effect on estimates of 472

ground-water resoruces. in lASH Publication 52 325–335 (International Association of 473

Scientific Hydrology, 1960). 474



24. Helm, D. C. One-dimensional simulation of aquifer system compaction near Pixley, 475

Calif., part 1. Constant parameters. Water Resour. Res. 11, 465–478 (1975). 476

25. Galloway, D. L., Jones, D. R. & Ingebritsen, S. E. Land subsidence in the United States. 477

(U.S. Geological Survey, Circular 1182, 1999). 478

26. Pope, J. P. & Burbey, T. J. Multiple-aquifer characterization from single borehole 479

extensometer records. Ground Water 42, 45–58 (2004). 480

27. Liu, Y. & Helm, D. C. Inverse procedure for calibrating parameters that control land 481

subsidence caused by subsurface fluid withdrawal: 2. Field application. Water Resour. 482

Res. 44, (2008). doi:10.1029/2007WR006605 483

28. Gabrysch, R. K. & Bonnett, C. W. Land-surface subsidence in the Houston-Galveston 484

Region, Texas. (1975). 485

29. Törnqvist, T. E. et al. Mississippi Delta subsidence primarily caused by compaction of 486

Holocene strata. Nat. Geosci. 1, 173–176 (2008). doi:10.1038/ngeo129 487

30. Liu, Y., Li, J. & Fang, Z. N. Groundwater Level Change Management on Control of Land 488

Subsidence Supported by Borehole Extensometer Compaction Measurements in the 489

Houston-Galveston Region, Texas. Geosciences 9, 19 (2019). 490

doi:10.3390/geosciences9050223 491

31. Taylor, D. W. & Merchant, W. A. A theory of clay consolidation accounting for 492

secondary compression. J. Math. Phys. 19, 167–185 (1940). 493

32. Kasmarek, M. C. Hydrogeology and Simulation of Groundwater Flow and Land-Surface 494

Subsidence in the Northern Part of the Gulf Coast Aquifer System, Texas, 1891–2009. 495

(U.S. Geological Survey, Scientific Investigation Report 2012-5154, 2013). 496

33. Heinrich, P., Paulsell, R., Milner, R., Snead, J. & Peele, H. Investigation and GIS 497

development of the buried Holocene-Pleistocene surface in the Louisiana coastal plain. 498

140 (2015). 499

34. Scott, T. M. Text To Accompany the Geology Map of Florida. (2001). 500

35. U.S. Geological Survey [USGS]. GROUND WATER ATLAS of the UNITED STATES - 501

Introduction and National Summary. Available at: https://pubs.usgs.gov/ha/ha730/ch_a/A-502

text3.html. (Accessed: 2nd March 2020) 503

36. Santamaría-Gómez, A. et al. Uncertainty of the 20th century sea-level rise due to vertical 504

land motion errors. Earth Planet. Sci. Lett. 473, 24–32 (2017). 505

doi:10.1016/j.epsl.2017.05.038 506

37. Doherty, J. PEST Model-Independent Parameter Estimation. (2002). 507



38. PSMSL. Tide Gauge Data. Available at: http://www.psmsl.org/data/obtaining/. (Accessed: 508

30th July 2019) 509

39. Parris, A. et al. Global Sea Level Rise Scenarios for the United States National Climate 510

Assessment. (2012). 511

40. Doherty, J. PEST, Model-independent parameter estimation-User manual (5th ed.). 512

(2004). 513

41. Nerem, R. S. et al. Climate-change–driven accelerated sea-level rise detected in the 514

altimeter era. Proc. Natl. Acad. Sci. 0, 201717312 (2018). doi:10.1073/pnas.1717312115 515

42. Taylor, D. W. Research on consolidation of clays, Department of Engineering, 516

Massachusetts Institute of Technology, Cambridge, Mass. Serial 82, 147 (1942). 517

43. Liu, Y., Rashvand, M. & Li, J. Preliminary Investigation of Land Subsidence Impacts on 518

Sea Level Change in Baltimore Inner Harbor, Maryland. in World Environmental and 519

Water Resources Congress 2020 236–243 (2020). 520

44. Wang, G. et al. GOM20 : A Stable Geodetic Reference Frame for Subsidence , Faulting , 521

and Sea-Level Rise Studies along the Coast of the Gulf of Mexico. 1–29 (2020). 522

doi:10.3390/rs12030350 523

Acknowledgements 524

We are grateful to Zheng N. Fang for funding acquisition, Donald C. Helm for partial review and 525

comments on an early version of the manuscript, Chris Zervas, Susanne Moser, Jim Neumann, 526

John Ellis, Ramage Jason, Michael Heflin, Frank Tsai, Zhong Lu, Etienne Poirrier, Elizabeth 527

Prouteau, Guy Woppelmann and Mojtaba Rashvand for data access and/or fruitful discussions, 528

and Tranell Griffin for GIS mapping and Ermei Liu for mapping support. The project described in 529

this publication is supported by the U.S. National Science Foundation (NSF) grant 1832065 on 530

“Identification of urban flood impacts caused by land subsidence and sea level rise in the Houston-531

Galveston region”. The authors appreciate three anonymous journal peer reviewers and U.S. 532

Geological Survey colleague reviewer Jason Pope for their insightful and constructive comments 533

that led to an improved manuscript. Any use of trade, firm, or product names is for descriptive 534

purposes only and does not imply endorsement by the U.S. Government. 535

Author contributions 536

Y.L. conceived this study, developed the methodology, conducted data curation, analysis and 537

simulation, as well as wrote the manuscript and the Supplemental Information. J.L., J.F. and 538

D.L.G. contributed to the methodology development. D.L.G., J.L. and J.F. contributed to review 539



and revision of the manuscript and the Supplemental Information. All authors reviewed the final 540

manuscript and the Supplemental Information. 541

Competing interests: The authors declare no competing interests. 542

Data and materials availability: All data needed to evaluate the conclusions in the paper are 543

present in the paper and/or the Supplementary Information. Additional data related to this paper 544

may be requested from the corresponding author. Groundwater level and borehole extensometer 545

raw data used in this paper are available from the U.S. Geological Survey (USGS) 546

(https://txpub.usgs.gov/houston_subsidence/home/ etc.) etc. Sea level data used in this paper are 547

available from the National Oceanic and Atmospheric Administration (NOAA) 548

(https://tidesandcurrents.noaa.gov/sltrends/sltrends.html) and/or the Permanent Service for Mean 549

Sea Level (PSMSL) (https://www.psmsl.org/). GPS height data used in this paper are available 550

from the Jet Propulsion Laboratory (JPL) of NASA 551

(https://sideshow.jpl.nasa.gov/post/series.html) or SONEL (https://www.sonel.org/?lang=en). 552

Additional information 553

Supplementary information 554

555

https://urldefense.proofpoint.com/v2/url?u=https-3A__txpub.usgs.gov_houston-5Fsubsidence_home_&d=DwMFAg&c=0CCt47_3RbNABITTvFzZbA&r=QnAmYETmHst5ST6_b5ylXqYzMEkMsaJDrLJtLdvA0ls&m=P3iZkJpxcdhn1Xr-FnX4mVrJUhyd_yBggnN3VQm33ls&s=_o064-ju97m7jaFY5cbPLFKAY11_fC5wIkAWVWYe0qU&e=https://tidesandcurrents.noaa.gov/sltrends/sltrends.htmlhttps://www.psmsl.org/https://sideshow.jpl.nasa.gov/post/series.html

1

Supplementary Information

Land subsidence contributions to relative sea level rise at tide gauge Galveston

Pier 21, Texas

Yi Liu1,*, Jiang Li1, John Fasullo2, Devin L. Galloway3

1Morgan State University, Department of Civil Engineering, Baltimore, MD 21251; 2National Center for Atmospheric Research, Climate and Global Dynamics Lab, Boulder, CO

80305; 3U.S. Geological Survey, Water Mission Area, Earth System Processes Division, Indianapolis,

IN 46278

*Correspondence to: [email protected]

mailto:[email protected]

2

Supplementary Materials and Methods

Geological materials

Compressible aquifer systems at tide gauge Galveston Pier 21: From northwest to southeast the

Houston-Galveston region includes Grimes County with the region’s highest elevation of about

122 m, and Montgomery, Waller, Harris and Galveston counties with the lowest elevations of

about 0 to 15 m near the coast of the Gulf of Mexico (Fig. 2)1. The three primary Quaternary and

Tertiary aquifers in the Gulf Coast aquifer system in the region are the Chicot, Evangeline, and

Jasper (Fig. 2 and Supplementary Fig. S1)1–4, which comprise laterally discontinuous deposits of

gravel, sand, silt, and clay. The youngest and uppermost Quaternary aquifer, the Chicot aquifer,

consists of Holocene- and Pleistocene-age sediments; the underlying Tertiary Evangeline aquifer

consists of Pliocene- and Miocene-age sediments; and the oldest and most deeply buried Tertiary

aquifer, the Jasper aquifer, consists of Miocene-age sediments (Fig. 2 and Supplementary Fig.

S1)5,6. The lowermost unit of the Gulf Coast Tertiary aquifer system is the Miocene-age Catahoula

confining system, which includes the Catahoula Sandstone. The Catahoula confining system

comprises sands in the upper section and clay and tuff interbedded with sand in the lower section.

Detailed stratigraphic information regarding the underlying Cretaceous and pre-Cretaceous strata

is unavailable in this area. The Cretaceous strata are generally considered semi-consolidated. The

pre-Cretaceous strata are bedrocks or basement rocks here (Table 1). A maximum subsidence due

to primary compaction (SPC) of 3 m was measured in the Houston-Galveston region and attributed

to oil, gas and groundwater development. Subsidence due to creep of aquifer systems or non-

bedrock systems (SCnBR) measured from 13 borehole extensometers in this region is shown in

Supplementary Table S1 and Supplementary Fig. S7. Thus, the compaction subsidence (LSnBR) in

the compressible aquifer systems includes SPC and SCnBR. Land subsidence contributed from the

bedrock system (LSBR) (Table 1) includes components of tectonic subsidence (TS) and subsidence

due to creep (SCBR) in this region. Therefore, the LS records at tide gauge Galveston Pier 21 and

its reference GPS station TXGA includes vertical motion attributed to both LSnBR (SPC + SCnBR)

and LSBR (TS + SCBR)

Ocala limestone at tide gauge Cedar Key and Cross City: The limestones exposed near Ocala,

Marion County, in central peninsular Florida are referred to as the Ocala Limestone7–10. The Ocala

Limestone consists of nearly pure limestones and occasional dolostones. Numerous disappearing

streams and springs occur within these areas. The permeable, highly transmissive carbonates of

the Ocala Limestone are one of the most permeable rock units of the Floridan Aquifer System

(FAS)11. Though details of the underlying Cretaceous and pre-Cretaceous strata are not well

known11, it is believed that the Tertiary limestone and Cretaceous strata were uplifted and are

currently over semi-consolidated because their geohistorical overburden materials, Quaternary

sediments Qh and Qp, were removed. Because minimal groundwater development has occurred in

the Cedar Key and Cross City areas, which are 55 km apart within the same tectonic zone, the

Cedar Key and Cross City areas should experience identical LSBR (TS + SCBR) and negligible

LSnBR (SPC + SCnBR). Both tide gauge Cedar Key and its reference GPS station XCTY are

important to identify ASLR from the long-term tide-gauge and GPS station records along the coast

of the Gulf of Mexico in the U.S. The other tide gauges and their reference GPS stations along the

Gulf of Mexico in the U.S. (Fig. 2) are affected to various degrees by LSnBR and LSBR.

Outcropped over semi-consolidated Tertiary strata at GPS station SG32 in College Station, Texas

(Yegua Formation): The Yegua Formation (Ey) consists of sandstone, clay, and lignite with a

3

thickness of 229 to 305 m12. The sandstone near SG32 is shown in Supplementary Fig. S8. In East

Texas and along the Gulf Coast to the Rio Grande River, the Yegua Formation overlies the Cook

Mt. Formation, and is overlain by the Caddell Formation. SCnBR is negligible in the over semi-

consolidated strata (TOC + COC). Minimal groundwater has been developed from this minor aquifer

in Texas and no significant groundwater-level decline has been measured by the Texas Water

Development Board (TWDB)13. Thus, SPC in the strata is negligible. Therefore, land subsidence

(LS) measured from GPS station SG32 is attributed solely to LSBR.

Outcropped over semi-consolidated Tertiary strata (Calvert Bluff Formation) at GPS station

LDBT: The Calvert Bluff Formation underlying GPS station LDBT 107 km southwest of GPS

station SG32 is mostly14. The sandstone is medium to fine grained, well sorted, crossbedded, and

lenticular. This formation is about 305 m thick. Owing to the over semi-consolidated strata (TOC +

COC) SCnBR is negligible. Because no groundwater has been developed from the strata in the region,

SPC is negligible. Thus, LS measured from the paired reference GPS station LDBT is attributed

to LSBR.

Piecewise equation of sea level rise (SLR) at tide gauge Galveston Pier 21

Future GMSLR has been projected using the quadratic equation Eglobal(t)=0.0017t+bgt2, where

0.0017 m/a refers to the trend for the GMSLR determined from observations from 1900 to 1992,

with an acceleration-related constant bg [m/a2] determined to be 0.0, 2.71×10-5, 8.71×10-5, and

1.56×10-4 m/a2 for the lowest, intermediate-low, intermediate-high, and highest GMSLR

scenarios, respectively15,16. The highest scenario of GMSLR by 2100 is derived from a

combination of estimated ocean warming from the IPCC Fourth Assessment Report (AR4),

GMSLR projections and a calculation of the maximum possible glacier and ice sheet loss by the

end of the century15. The intermediate-high scenario is based on an average of the high end of

semi-empirical, GMSLR projections15. Semi-empirical projections utilize statistical relations

between observed global sea level change, including recent ice sheet loss and air temperature. The

intermediate-low scenario is based on the upper end of IPCC AR4 GMSLR projections resulting

from climate models using the B1 emissions scenario15. The lowest scenario is based on a linear

extrapolation of the historical SLR rate derived from tide gauge records beginning in 1900 (1.7

mm/a) 15. Note, bg = 0.5ag where ag denotes GMSLR acceleration [m/a2] with values of 0.0,

5.42×10-5, 1,74×10-4, and 3.12×10-4 m/a2 for the lowest, intermediate-low, intermediate-high, and

highest scenarios of GMSLR, respectively. By comparison, SLR acceleration in the Chesapeake

Bay, U.S. was estimated to be about 0.05−0.10 mm/a2 (5.0×10-5 − 1×10-4 m/a2) from the 1930s to

201117. The historical and estimated future magnitudes of RSLR are two additional factors for

scientists, engineers and planners to consider when devising sustainable solutions involving

adaptive engineering, such as seawalls18–20, levee systems21–23, and pile-elevated building

foundations24 for future coastal settlements in the U.S. A set of piecewise equations representing

a time-dependent model of RSLR at a tide gauge was developed as follows:

RSLR(t)=(ar+sBR)(t-t0)+0.4343CH ln(t/t0) +C, t∈(t0,t1] (S1-1)

RSLR(t)=(ar+sBR)(t-t0)+0.4343CH ln(t t0⁄ ) +pl(t-t1)+C, t∈(t1,t2] (S1-2)

RSLR(t)=(ar+sBR)(t-t0)+0.4343CH ln(t t0⁄ ) +pl(t2-t1)+C, t∈(t2,t3] (S1-3)

RSLR(t)=(ar+sBR)(t-t0)+0.4343CH ln(t t0⁄ ) +pl(t2-t1)+0.5ag(t-t3)

2+C, t∈( t3, 2100] (S1-4)

where ar and sBR denote a regional uniform ASLR rate [L/T] and LSBR rate [L/T], respectively; t

represents any year [T] such as 1909 in the period 1909-2100; t0 denotes year 1908, the previous

4

year of the starting year 1909 for the period of record and the first subperiod 1909–37, and t1, t2

and t3 denote the starting years 1937, 1983 and 1992, respectively for the specified subperiods

(1937–83, 1983–92 and 1992–2100) (note: the last subperiod differs from the last subperiod used

in equation set (2) in the main text); RSLR(t) signifies the mean RSLR in year, in meters [L]; pl

represent rates of SPC; ag denotes regional RSLR acceleration [L2/T]; CH = CαH [L] where Cα and

H represent the compression coefficient of creep [dimensionless] 25 and total thickness [L] of

compressible aquifer systems for variable SCnBR, respectively; and C [L] is a constant the offset

of the fit to measured or observed sea level. Model parameters CH and C were estimated using

PEST26 to optimize the fit to observed RSLR. Values of t1 and t2 were determined by analyzing RSLR data with information of regional and local land subsidence, subsurface fluid withdrawal,

groundwater level and subsurface fluid-flow simulation. The value of 1992 for t3 is from Parris

et al.15 and represents the start of the period of ASLR acceleration which extends to 2100 in this

analysis. Note, after 1983, the term pl(t1-t1) in equations (S1-3–4) is constant and represents the

SPC contribution for subperiods beyond 1983. The supplementary equation set (S1) for a particular

tide gauge reflects the fact that RSLR varies along the U.S. coastline27. The U.S. Army Corps of

Engineers (USACE)16 has incorporated sea level change into civil works programs using a single

equation that is similar to supplementary equation (S1-4) in Regulation No. 1100-2-8162.

Land subsidence from bedrock systems (LSBR)

In this study, LSBR is defined as the portion of land subsidence (LS) attributed to TS and SCBR in

bedrock systems. LSBR can be measured at a tide gauge’s paired reference GPS station that is

anchored on bedrock (pre-Cretaceous) or over semi-consolidated Tertiary and Cretaceous strata

for which SPC and SCnBR are negligible. The TS is assumed to be caused by comprehensive

intraplate or interplate tectonic activities28–30, which include regional and local faulting, and glacial

isostatic adjustment (GIA)30. SCBR represents subsidence due to creep of bedrock systems31,32. For

purposes of this study, the LSBR trend in terms of an annual rate is assumed to be constant for a

specific station during the relatively short time period (human time scale) represented by these

analyses. Global GPS height data from 2,567 GPS stations on land are available from the Jet

Propulsion Laboratory (JPL)33. Of those, 1,961 GPS stations are distributed on the North American

Plate and JPL provides trends (rates) computed for each GPS station. The GPS data are also

available from SONEL34. For example, GPS station SG32 at College Station, Texas, about 206

km northwest of tide gauge Galveston Pier 21 (Figs. 1 and 2), is a reference GPS station used in

this study, where LS is attributed solely to LSBR. The trend of height changes over time at SG32

during 2003–14 is -2.67±0.67 mm/a (Supplementary Fig. S4C) representing a rate of LS equal to

LSBR of 2.67 mm/a. This value was used to represent LSBR at reference GPS station TXGA for

tide gauge Galveston Pier 21 as described in the main article.

Regional ASLR estimation

For purposes of this study, the tectonic conditions in a coastal region were assumed to be identical

in a small zone with similar strata. For tide gauges and their paired reference GPS stations where

SPC and SCnBR are negligible, LSBR was used to estimate ASLR in a coastal region because those

tide gauges were assumed to measure height changes attributed solely to ASLR and LSBR. For

example, tide gauge Cedar Key and its reference GPS station XCTY (Figs. 1 & 2) 55 km distant

are established over the same outcropped over semi-consolidated (see Table 1) Tertiary limestone

(TOC)7, where superscript OC denotes an overconsolidated stress status (σ0'

5

measured trend in relative sea level referred to as RSLR in this paper is 2.13 mm/a (Fig.

Supplementary S1B)35, while the NOAA measured LS in trend (LSBR) is 0.88 mm/a (± 0.43 mm/a

standard deviation) (Supplementary Fig. S4A)34. At this gauge, RSLR comprises only ASLR and

LSBR, as LSnBR (SPC and SCnBR) in the TOC and the Cretaceous strata COC (Fig. 2 and Table 1) are

negligible. Groundwater development from the unconfined and semiconfined Floridan aquifer

along the Gulf coast of peninsular Florida north of Tampa Bay is minimal at Cedar Key in Levy

County and Cross City in Dixie County36,37. The water table at Cedar Key is hydraulically

connected with sea water36. Water table measured in USGS Rosewood Tower Well with a depth

of 134.7 m, completed in the Tertiary Floridan limestone aquifer 14 km northeast of Cedar Key,

ranged from 2.7 to 3.7 m above NGVD29 without significant decline during the available period

of record (1976–2011). Six USGS groundwater wells are completed in the Tertiary Floridan

aquifer with well depths of 3.8 to 121.3 m in Dixie County where GPS station XCTY is located.

No significant trends in groundwater-level decline were evident in the available water-level

records (1961–94). Therefore, SPC in the Tertiary over semi-consolidated Floridan limestone

aquifer system at Cedar Key and Cross City is considered to be negligible, and as discussed in the

previous section, SCnBR is also considered to be negligible in these strata.

Subsidence due to primary compaction (SPC)

SPC is the compaction of compressible aquifer systems caused by subsurface fluid withdrawal.

SPC in this paper includes the elastic and inelastic (virgin) deformation of aquitards (fine-grained

deposits [clays and silts] with low permeability) and the elastic deformation of aquifers (coarse-

grained deposits with moderate to high permeability) in aquifer systems. An analytical solution

was developed38,39 to simulate SPC, a coupled compaction and fluid-flow process, in a water

saturated, doubly draining clay layer. The clay layer has a uniform initial pore-fluid pressure

wherein only vertical water flow is permitted, and identical, instantaneous step changes in

hydraulic head (or equivalent fluid pressure) occur at the bottom and top of the clay layer38,39. This

process, based on changes in effective stress resulting from changes in pore-fluid pressure,

describes the equilibration of fluid-pressure and resulting compaction, which was subsequently

extended to the analysis40 and simulation41,42 of aquitard/confining unit drainage. This concept,

known as “the aquitard drainage model”43 has formed the theoretical basis of many successful

subsidence investigations39–41,44,45. The time constant for the drainage and compaction of an

aquitard is τ0' =Ssk

' (b0'

2⁄ )2/K', where Ssk

', b0

' and K' are expressed in hydrogeologic terms denoting

vertical skeletal specific storage [dimensionless], thickness [L], and vertical hydraulic conductivity

[L/T] of the aquitard, respectively40,41. The degree of compaction in the aquitard reaches 93.1%,

99.4% and 100% of the ultimate compaction for a normalized time factor Tv = ∆ t τ0'⁄ , where ∆t is

the change in time since the initial step change in hydraulic head at the upper and lower boundary

of the aquitard, equals 1, 2 and ∞, respectively. SPC is deemed fully completed when Tv reaches 2 with 99.4% of the ultimate compaction realized46. Thus, a SPC period (∆t) can be theoretically

estimated from the above expression for Tv with specified values for τ0' and Tv. In practice,

generally SPC is dominated by inelastic deformation which occurs when the pore-fluid pressure

declines result in increased effective stresses in aquitard(s) and confining unit(s) that are greater

than the historical maximum effective stress (i.e., σo' >σc

' ), typically defined by the previous

minimum pore-fluid pressure in those units. The deformation of the coarse-grained deposits

constituting the aquifers generally proceeds only elastically for both decreases and increases in

aquifer pore-fluid pressure. The elastic skeletal specific storage of the aquifers governs their

deformation and is usually much smaller than Ssk' 47. Elastic deformation of both aquifers and

6

aquitard/confining units occurs during periods of pore-fluid pressure increase (or groundwater

level recovery; for example, see period II in Fig. 4) 47. For the aquitard drainage model, inelastic

compaction is governed by Ssk'

which is typically about 1-3 magnitude orders larger than the elastic

skeletal specific storage of the aquifers and the aquitards/confining units48–51. Notably, SPC owing

to inelastic compaction of aquitards and confining units can result in appreciable land subsidence

and thus, increased RSLR. In the Houston-Galveston region it was demonstrated that SPC began

in about 1937 and proceeded for at least the next 63 years (∆t)47. SPC stopped at many borehole extensometer sites between about 2000 and 2004 (Fig. 4).

Subsidence due to creep of non-bedrock aquifer systems (SCnBR)

SCnBR represents the deformation caused by creep behavior of sedimentary materials under a

constant load. Due to the weight of the overburden (geostatic stress) and the inelastic compaction

characteristics of the aquitards/confining units, about 90 percent of the deformation is permanent52.

With regard to the degree of self-weight compression, three main sedimentation stages are

defined: the clarification regime, zone-settling regime, and compression regime53. Quaternary,

Tertiary and Cretaceous aquifer systems with a stress condition of σo' >σc

' , remain in the

compression regime and experience compaction under self-weight, which has been referred to as

creep25,54. The path A-B in Fig. S3 shows SCnBR at a constant historical maximum effective stress

σc' due to the overburden (geostatic) stress. For an unconsolidated/semi-consolidated sediment

layer with an initial thickness of H [L], SCnBR can be approximated using Sc(t)=CαHlog(t t0⁄ ), which is employed in supplementary equations (S1-1–4) for the variable SCnBR rate, where t0

denotes an initial reference time for compression of creep, and t signifies time larger than or equal

to t025. Taking the derivative with respect to time t of the expression above for Sc gives

Ṡc= CαH (t ln 10)⁄ , the subsidence rate. The decreased percentage (DS) of Ṡc from t to t+∆t was

derived using [Ṡc(t)-Ṡc(t+∆t)]/Ṡc(t) as follows

47:

DS(t)= (1-t

t+∆t) ×100. (S2)

For t≫∆t, where ∆t can be a short observing period such as 10–20 years, DS approaches zero which implies that Ṡc is approximately a constant. In other words, the changing value of Ṡc over the ∆t period can be ignored. For example, if a current observation period (∆t) is considered to be 10

years, 990, 1990, and 9990 years of SCnBR are needed to achieve decreased percentages of 1.0, 0.5

and 0.1% of the specified subsidence rate, respectively47. This negligibly-variable SCnBR rate47 is

used to estimate SPC.

Negligible SCnBR in over semi-consolidated Tertiary and Cretaceous strata

SCnBR is assumed to be negligible in Tertiary (TOC) and Cretaceous (C

OC) strata shown in Figs. 1,

2 and Table 1. This is because the current effective stress σ𝑜′ in TOC or C

OC is less than its historical

maximum effective stress σc' (Supplementary Fig. S3) owing to the outcropping or uplifting of the

strata and the geological removal of the overlying Quaternary sediments Qh and Qp, which is

referred as an overconsolidated condition25. Experimental investigations show that sediment creep

rate with an overconsolidated stress status is significantly slower than that without55. In addition,

the primary factor is that when compared to the age of geological strata our human observation is

in such a short time period that the change in SCnBR is insignificant, which has also been discussed

after supplementary equation S2. For instance, the length C-D in Supplementary Fig. S3 represents

the negligible subsidence of creep during a period of 100 years under effective stress σo' .

7

Supplementary Figure S1. Monthly mean sea level with average seasonal cycle removed, and

linear trend showing relative RSLR at tide gauges: A. Galveston Pier 21; and B. Cedar Key

(Image source: NOAA35).

A

B

8

Supplementary Figure S2. Geologic and hydrogeologic units of the Gulf Coast aquifer system

in the Houston-Galveston region study area, Texas1–4. (Map source: USGS1)

9

Supplementary Figure S3. Subsidence due to creep and historical changes in effective stress

presented with a conceptual isotache model25,55–57. Path A-B shows subsidence due to 10n year

creep under a given overburden pressure σc' . Path B-C displays rebound due to an instant

overburden removal at the age of 10n years and reduction of effective stress to σo' from σc

' . Path A-

C represents combination of both the rebound and creep processes that may occur simultaneously.

Path C-D at σo' shows insignificant subsidence of creep during 100 years (e.g., 1900 – 2000) from

10n years to 10n + 100 years.

log σ' σc' σo

'

Isotaches:

Lan

d s

ub

sid

ence

A

B

C C

D

10

Supplementary Figure S4. Land subsidence (LS) derived from GPS stations: A. Subsidence of

bedrock systems (LSBR) of 0.88 mm/a from GPS station XCTY at Cross City, Florida34; B. Total

LS of 3.44 mm/a, which is the sum of LSBR and SCnBR, from GPS station TXGA, Texas (see

location in Supplementary Fig. S6)34; C. LSBR of 2.67 mm/a from GPS station SG32 at College

Station, Texas33; D. LSBR of 2.68 mm/a from GPS station LDBT near Lake Bastrop, Texas.

(Observations in B and D: Black points with error bars; Fit: Red points; and breaks (or

discontinuities) in position: Green bars33). (Image source: SONEL34 for A & B and Courtesy NASA/JPL-Caltech58 for C & D)

Velocity (mm/a): -0.88 +/- 0.43

Velocity (mm/a): -3.44 +/- 0.79

A

B

C

D

11

Supplementary Figure S5. Subsidence due to primary compaction (SPC) measured and

simulated in groundwater withdrawal areas close to Galveston Pier 21 (modified from45: A. B.

and C . measured subsidence contours; D. Simulated subsidence at Texas City, Galveston

County, Texas using HAGM.20135 and observed subsidence at the Texas City–Moses Lake

borehole extensometer (see Supplementary Fig. S6 for location). (Image source for A, B & C:

USGS45)

00.10.20.30.40.50.60.70.80.9

1

1890 1910 1930 1950 1970 1990 2010

Sub

sid

en

ce, m

Time, year

Observed with

extensometer

A B C

Extensometer

Simulated with HAGM.2013

1937 to 1983

D

12

Supplementary Figure S6. Location of borehole extensometer sites and selected groundwater

level monitoring well sites, Houston Galveston region, Texas (modified from59). (Note: The

location of wells LJ-65-21-229 and LJ-65-21-227 is at the same location of extensometer

Southwest) (Base map soure: USGS59)

13

Supplementary Figure S7. Time series of observed cumulative compaction (CC; Data source:

USGS) and linear trendlines (dashed red lines) of CC for SCnBR (or creep) at 13 borehole

extensometer sites in the Houston-Galveston region located: A. Very near or on the coast; B.

Near the coast and generally near bayous; and C. Inland, farther from the coast. The compaction

period for SCnBR for each site is given in Supplementary Table S1. The slopes in the trendline

equations are in mm/d. (modified from47)

CC = 3.693E-03t + 2.126E+01

CC = 5.956E-03t + 4.908E+01

CC = 8.317E-03t + 1.280E+02

CC = 2.222E-04t + 2.061E+01

0

100

200

300

400

500

600

3/8/1971 5/25/1979 8/11/1987 10/28/1995 1/14/2004 4/1/2012 6/18/2020

Cu

mu

lati

ve c

om

pac

tio

n, m

m

Texas City BaytownShallow BaytownDeep Seabrook

CC = 4.926E-03t + 2.061E+02

CC = 3.033E-03t + 9.128E+01

CC = 6.032E-03t - 1.659E+02

CC = 5.062E-03t + 1.324E+02

CC = 1.211E-03t + 1.590E+02

0

50

100

150

200

250

300

350

400

450

500

3/8/1971 5/25/1979 8/11/1987 10/28/1995 1/14/2004 4/1/2012 6/18/2020

Cu

mu

lati

ve c

om

pac

tio

n, m

m

PasadenaClear Lake ShallowClear Lake DeepJohnson Space CenterEast End

CC = 1.042E-02t + 6.716E+01

CC = 1.150E-02t – 2.031E+02

CC = 2.327E-02t + 1.438E+02

CC = 3.760E-03t + 2.563E+01

0

200

400

600

800

1000

1200

3/8/1971 5/25/1979 8/11/1987 10/28/1995 1/14/2004 4/1/2012 6/18/2020

Cu

mu

lati

ve c

om

pac

tio

n, m

m

Lake Houston

Addicks

Northeast

Southwest

B

C

A

14

Supplementary Figure S8. Tertiary silty sandstone outcrops (Yegua Formation) at College

Station, Texas.

15

Extensometer Well # Aquifer

SCnBR appearance period Groundwater level

Trend

SCnBR rate

Depth, m starting date ending date m/d m/a mm/d mm/a

Texas City 244 KH-64-33-901 Chicot 1/24/2008 1/14/2017 -8.73E-05 -0.03 2.222E-04 0.08

Seabrook 421 LJ-65-32-519 Chicot

1/25/2008 1/14/2017 -2.77E-05 -0.01

8.317E-03 3.04 LJ-65-32-630 Evangeline -7.67E-05 -0.03

Space Center Clear

Lake

239 LJ-65-42-422 Chicot 1/25/2007 12/20/2017

9.09E-05 0.03 5.062E-03 1.85

937* LJ-65-42-424 Evangeline 6.79E-05 0.02 3.033E-03 1.11

Baytown Shallow 131 LJ-65-16-933 Chicot 5/26/2005 5/28/2009

3.82E-04 0.14 3.630E-03 1.33

Baytown Deep 450 LJ-65-16-931 Evangeline 1/11/2007 9.21E-05 0.03 5.956E-03 2.17

Addicks 549 LJ-65-12-729 Chicot

10/1/2007 5/15/2014 2.73E-04 0.10

2.327E-02 8.49 LJ-65-12-726 Evangeline 0.00E+00 0.00

EastEnd 304 LJ-65-22-623 Chicot

7/27/2007 1/13/2015 -2.72E-04 -0.10

4.926E-03 1.80 LJ-65-22-622 Evangeline 5.64E-04 0.21**

Northeast 663 LJ-65-14-745 Chicot

1/4/2008 3/1/2011 5.27E-04 0.19

1.150E-02 4.20 LJ-65-14-746 Evangeline 1.07E-03 0.39***

Pasadena 864 LJ-65-23-321 Chicot 1/5/2007 1/5/2011 3.76E-06 0.00

6.032E-03 2.20 LJ-65-23-326 Evangeline 2/6/2007 3/30/2010 8.39E-04 0.31****

Lake Houston 591 LJ-65-07-902 Chicot

1/7/2004 4/4/2007 8.38E-05 0.03

3.760E-03 1.37 LJ-65-07-908 Evangeline 1.24E-04 0.05

Southwest 719 LJ-65-21-229 Chicot

9/18/2003 10/18/2018 -1.13E-04 -0.04

1.042E-02 3.80

LJ-65-21-227 Evangeline 4.59E-04 0.17

*: Clear Lake Deep is 937 m deep and Clear Lake Shallow is 530 m deep. **: The difference of groundwater level between -42.75 m on 5/7/2008 and -42.77 m on 11/3/2014 is 0.02 m. ***: The difference of groundwater level between -54.14 m on 1/4/2008 and -54.18 m on 3/1/2011 is 0.04 m. ****: The difference of groundwater level between -40.03 m on 2/6/2007 and -40.17 m on 3/30/2010 is 0.14 m.

Supplementary Table S1. SCnBR (creep) appearance periods based on groundwater level trend at or near extensometer sites in the

Houston-Galveston region (modified from47)

16

Tide gauge

Galveston Pier 21 Cedar Key

Linear trend of

RSLR from

NOAA

LSBR + SCnBR

from GPS

TXGA

Linear trend of

RSLR from

NOAA

LS (LSBR +

SCnBR) from

GPS XCTY

Constant (linear)

rate, mm/a 6.51 3.44 2.13 0.88

ASLR, mm/a 3.07 1.25

Difference,

mm/a 1.82*

*: 3.07 mm/a has 146% difference relative to 1.25 mm/a.

Supplementary Table S2. ASLR values obtained with linear RSLR trend method

17

Supplementary References

1. Kasmarek, M. C., Ramage, J. K. & Johnson, M. R. Water-level altitudes 2016 and water-

level changes in the Chicot, Evangeline, and Jasper aquifers and compaction 1973–2015

in the Chicot and Evangeline aquifers, Houston–Galveston region, Texas. (USGS, 2015).

doi:10.3133/SIM3308

2. Sellards, E. H., Adkins, W. S. & Plummer, F. B. The Geology of Texa

scientific reports...32 sea level rise (aslr) due to global warming and land subsidence (ls) due to...

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