zong-liang yang guo-yue niu robert e. dickinson the university of texas at austin
DESCRIPTION
Modeling Surface and Subsurface Runoff in CLM. Zong-Liang Yang Guo-Yue Niu Robert E. Dickinson The University of Texas at Austin. Prepared for Land Model Working Group Meeting, March 14, 2005 Funded under NASA grant NAG5-12577. Outline. Introduction - PowerPoint PPT PresentationTRANSCRIPT
Zong-Liang Yang Guo-Yue Niu
Robert E. Dickinson
The University of Texas at Austin
Modeling Surface and Subsurface Runoff in Modeling Surface and Subsurface Runoff in CLMCLM
Prepared for Land Model Working Group Meeting, March 14, 2005
Funded under NASA grant NAG5-12577
OutlineOutline Introduction
Current treatment of runoff in CLM and problems Saturation area
Surface runoff Ksat, macropores and anisotropic factor
Subsurface runoff Constant versus exponential Ksat
Continental-scale simulations Water table
Regional-scale simulations Comparison with observations
Sensitivity to parameters f Rsub,max
OutlineOutline Introduction
Current treatment of runoff in CLM and problems Saturation area
Surface runoff Ksat, macropores and anisotropic factor
Subsurface runoff Constant versus exponential Ksat
Continental-scale simulations Water table
Regional-scale simulations Comparison with observations
Sensitivity to parameters f Rsub,max
Performance of Baseline CLM (1)Performance of Baseline CLM (1)
Soil moisture (Sleepers River Catchment):Too lowOdd profile (9th layer driest)
Daily runoff (Sleepers River Catchment, Vermont, USA):Negative modeling efficiency because of large spikesSurface runoff (fast component) too high
Performance of Baseline CLM (2)Performance of Baseline CLM (2)
Monthly runoff (GSWP2 Project):Overestimated Surface runoff (fast component) too high Surface runoff is 80% of total runoff.
Parameterization of Runoff in Baseline CLMParameterization of Runoff in Baseline CLM
Guided by four considerations:
1) TOPMODEL:
topographic control on the growth and decay of saturated area and groundwater flow
2) 1-D 10-layer soil structure:
3) Topographic data availability:
a simple determination of the saturated area, allowing room for improvement when the topographic parameters are available globally.
4) BATS:
success in PILPS experiments, esp. PILPS 1c (The Red-Arkansas River Basin)
Parameterization of Runoff in Baseline CLMParameterization of Runoff in Baseline CLMRunoff = Surface runoff + Subsurface runoff
Surface runoff Rs = Fsat Qwat + (1 – Fsat) ws4 Qwat
TOPMODEL BATS
Qwat = Input of water at the soil surfaceFsat = Fractional saturated area = Fmax exp(–Dw)ws = averaged soil wetness in the top three soil layers
Subsurface Runoff Rsb = Fsat lb exp(–Dw) + (1 – Fsat) Kb wb2B+3
lb = maximum baseflow rate = 10-5 mm s-1 Kb = maximum drainage rate = 0.04 mm s-1
wb = averaged soil wetness in the bottom three soil layers
Ksat (z) = Ksat(0) exp(–f z )
Ksat(0) = saturated hydraulic conductivity at the soil surface, determined by soil texture following Cosby et al. (1984); f = 2 (tunable parameter)
Problems in the Baseline CLMProblems in the Baseline CLM1) The second term in surface runoff is redundant and too large.
Rs = Fsat Qwat + (1 – Fsat) ws4 Qwat
TOPMODEL BATS
2) The second term in subsurface runoff is redundant and too large.
Rsb = Fsat lb exp(-Dw) + (1 – Fsat) Kb wb2B+3
3) How to determine Ksat (0) and Ksat(z)? Following Cosby et al. (1984)? Allowing macorpores? How to account for vertical and horizontal Ksat?
4) How to compute Fsat?Constrained by a global constant? By topography?
5) How to determine the water table?By the total head equilibrium? The moving boundary? An explicit groundwater model?
Proposed Runoff Scheme in CLMProposed Runoff Scheme in CLM1) Surface runoff
Rs = Fsat Qwat + (1 – Fsat) max(0, Qwat – Imax)2) Subsurface runoff
Rsb = Rsb,max exp (-f zw) simplified from
Rsb = [ α Ksat (0) / f ] exp(- λm) exp(- f zw)
α= anisotropic factor for different Ksat in vertical and horizontal directionsλm= grid-cell averaged topographic indexzw= grid-cell mean water table depth
3) Ksat (0) = ksat exp (f Dc) Ksat (z) = Ksat(0) exp(–f z )ksat is determined by following Cosby et al. (1984).Allowing macropores.
4) Fsat = ∫λ ≥ (λm + f*zw) pdf(λ) dλ
5) The water table is diagnosed from an equilibrium relationshipψ(z) – z = ψsat – zw (i.e., the total head is equal across the soil column layers)
Topography-based Runoff SchemeRunoff production mechanism
Surface runoffSaturation excessInfiltration excess
Subsurface runoff Topographic control Bottom drainage “Over-saturated” water
recharged into upper unsaturated layers
Infiltration Excess
Wat
er T
able
Dep
th
Saturation Excess
Super-saturationTopography Bottomm
w
ef
KR
eRR
satsb
fzsbsb
)0(max,
max,
OutlineOutline Introduction
Current treatment of runoff in CLM and problems Saturation area
Surface runoff Ksat, macropores and anisotropic factor
Subsurface runoff Constant versus exponential Ksat
Continental-scale simulations Water table
Regional-scale simulations Comparison with observations
Sensitivity to parameters f Rsub,max
Maximum Fractional Saturated Area (FMaximum Fractional Saturated Area (Fsat,maxsat,max))
Using 1 km × 1 km topographic index (λ)
Using Γ-distribution fit to the 1 km data
Differences of (Middle – Top)
Fsat = ∫λ ≥ (λm + f*zw) pdf (λ) dλ when the water table is at the surface (zw = 0)
Defining the Maximum Fractional Saturated AreaDefining the Maximum Fractional Saturated Area
Fsat = ∫λ ≥ (λm + f*zw) pdf (λ) dλ
Fsat,max results when the water table is at or above the surface (zw ≤ 0)
Topographic Index λ
Simulations over the Sleepers River BasinSimulations over the Sleepers River Basin
TOPMODEL: Fsat = ∫λ ≥ (λm + f*zw) pdf (λ) dλ
SIMTOP: Fsat = Fsat,max exp (–0.5 f zw) Fsat,max = 0.42
OutlineOutline Introduction
Current treatment of runoff in CLM and problems Saturation area
Surface runoff Ksat, macropores and anisotropic factor
Subsurface runoff Constant versus exponential Ksat
Continental-scale simulations Water table
Regional-scale simulations Comparison with observations
Sensitivity to parameters f Rsub,max
Ksat, macropores and anisotropic factor
ksat depends on soil type (Cosby et al., 1984)
Stiglietz et al. (1997) :Ksat(0) = 1000 × ksat α=1, f=3.26
Chen and Kumar (2001):Ksat(0) = exp(f Dc) × ksat
= 6 × ksat α=2000, f=1.8
This study: Ksat(0) = exp(f Dc) × ksat
= 6 × ksat α=20, f=2 (global); =3.26 (Sleepers River)
or Rsb,max = 1.45×10–7m/s
fzsatsat eKzK )0()(
10–7 m/s 10–3 m/s0 m
1 m
2 m
3 m
10–10 m/s
Baseline CLM
Stiglietz et al.
mef
KR satsb
)0(
max,
Chen & Kumar
Ksat, macropores and anisotropic factor
fzsatsat eKzK )0()(
mef
KR satsb
)0(
max,10–7 m/s 10–3 m/s
0 m
1 m
2 m
3 m
10–10 m/s
Baseline CLM
Stiglietz et al.
Chen & Kumarα = 1
α = 20
OutlineOutline Introduction
Current treatment of runoff in CLM and problems Saturation area
Surface runoff Ksat, macropores and anisotropic factor
Subsurface runoff Constant versus exponential Ksat
Continental-scale simulations Water table
Regional-scale simulations Comparison with observations
Sensitivity to parameters f Rsub,max
Simulations over Various Regional BasinsSimulations over Various Regional Basins
West SiberiaEast Siberia
NW Canada
CongoAmazon India
E USAW USAC Europe
S AfricaSahara Australia
N America
EurasiaS Hemisphere
Simulations over the Sleepers River BasinSimulations over the Sleepers River Basin
TOPMODEL: Fsat = ∫λ ≥ (λm + f*zw) pdf (λ) dλ
Rsb,max = 1.45 ×10–7 m/s
Chen & Kumar
10–7 m/s 10–3 m/s0 m
1 m
2 m
3 m
10–10 m/s
Baseline CLM
Stiglietz et al.
Bottom sealed
Bottom NOT sealed
Simulations over the Sleepers River BasinSimulations over the Sleepers River Basin
10–7 m/s 10–3 m/s0 m
1 m
2 m
3 m
10–10 m/s
Baseline CLM
Stiglietz et al.
Bottom NOT sealed
Bottom sealed
Chen & Kumar
TOPMODEL: Fsat = ∫λ ≥ (λm + f*zw) pdf (λ) dλ
Rsb,max = 1.45 ×10–7 m/s
OutlineOutline Introduction
Current treatment of runoff in CLM and problems Saturation area
Surface runoff Ksat, macropores and anisotropic factor
Subsurface runoff Constant versus exponential Ksat
Continental-scale simulations Water table
Regional-scale simulations Comparison with observations
Sensitivity to parameters f Rsub,max
Comparison of Comparison of Simulated Water TableSimulated Water Table with with MeasurementsMeasurements in Illinois in Illinois
OutlineOutline Introduction
Current treatment of runoff in CLM and problems Saturation area
Surface runoff Ksat, macropores and anisotropic factor
Subsurface runoff Constant versus exponential Ksat
Continental-scale simulations Water table
Regional-scale simulations Comparison with observations
Sensitivity to parameters f Rsub,max
Sensitivity to f:Sensitivity to f:Simulations over the Sleepers River Simulations over the Sleepers River
Sensitivity to RSensitivity to Rsb,maxsb,max
Simulations over the Sleepers River Simulations over the Sleepers River
Simulations over the Sleepers RiverSimulations over the Sleepers River
Simulations over the Amazon BasinSimulations over the Amazon Basin
Coupled CAM2-CLM2 Results in AmazonSu
rfac
e ru
noff
(m
m/d
)
Soil
Moi
stur
e (m
m/d
)
ET
(mm
/d)
Prec
ipita
tion
(mm
/d)
Simplified TOPMODEL produced less surface runoff, allowing more water to infiltrate into deeper soil and to increase soil moisture. Transpiration increases significantly, more than compensating the decrease in the interception loss. As a result, both ET and precipitation show favorable increases.
1-2mm/d
Conclusions1) Based on offline tests for a small catchment or global continents, the
proposed runoff scheme is shown to be robust for a wide range of assumptions including
a) Different methods of Fsat,• Based on 1-km topographic parameters• Assuming a global constant
b) Constant versus exponential Ksat• In the constant profile case, results depend on whether the bottom
is sealed or not c) Different methods of water table.
2) The simulations of soil moisture and runoff are all improved over the baseline version.
3) In the Amazon region, canopy evaporation and surface runoff are reduced, soil is wetter, and both ET and precipitation are increased.
Future Work1) Increase the total soil thickness to ~10 m and make it
a geographic variablea) Need bedrock data,b) Adjust root depth and distribution,c) Collect the water table data,d) Compare with the GRACE data.
2) Global optimization of two calibration parameters (f and Rsub,max).
3) Include (unconfined) aquifer into CLM to study groundwater recharge, discharge, and climate-groundwater interactions.
Land Surface, Surface Water and
Groundwater
Can be detected by GRACE