Hydrologic effects and implications of vegetation in semiarid mountain regions

Huade GuanAdvisor: Dr. John Wilson

SAHRA 4th Annual meetingOct. 15, 2004

South Baldy, Magdalena Mountains, New Mexico, 2001

N

• SAHRA vegetation question: What are the impacts of vegetation change on the basin-scale water balance?

• Today I’ll focus on two issues related to understanding and modeling vegetation hydrologic effects in mountain areas.– Effects of vegetation on hydrologic processes– Mountain Block Recharge (MBR) at hillslope

scale under various conditions, including vegetation types, and vegetation change

SAHRA Scientific Question and My Study

Precip

itation

Bedrock

Soil Soil water

How does water partition on the mountain hillslopes? In particular, what is the percolation across the soil-bedrock interface?

FS

DS

FR

DR

MASTER FAULT

FAULT

OBLIQUEFAULT

Surface Fault Trace

FAULT

Evapotranspiration

?

Precipitation

BedrockInterflow

Percolation

What is the contribution of distributed recharge to mountain-front recharge?

From this percolation, what is the distributed recharge to the mountain block?

Effects of vegetation on hydrologic processes

• Modifies surface albedo

• Intercepts precipitation

• Transpires soil water, actively responds to the atmospheric condition and soil moisture

• Modifies soil

structure

and hydraulic

conductivity

Precip

itation

Bedrock

Soil Soil water

Effects of vegetation on hydrologic processes

We separate these effects into two categories,

– one contributes to the boundary of the model: PE and PT (including Fr, albedo, interception, stomatal resistance, vegetation structure, etc);

– the other contributes to the model parameters: K, root water uptake model.

Hydrologic effects of vegetationNew surface energy partitioning model that we use to separately generate PE and PT on mountain hillslope: SEP4HillET

Hydrologic modeling of the surface and vadose zone: currently HYDRUS

PT PE

Root macropore

Modified from Shuttleworth and Wallace (1985)

SEP4HillET model: Considers effects of the vegetation coverage and slope (aspect and steepness) on surface energy partitioning for ET (E and T separately) modeling

Potential E and T on a shrub-covered surfaceSevilleta NWR, NM (2002), Fr=0.3, LAI=1

0

2

4

6

8

10

1 31 61 91 121 151 181 211 241 271 301 331 361

Julian day

PT

or

PE

(m

m/d

ay)

PT

PEPE=83%, PT=17%

PE=83%, PT=17%

Potential E and T on a grass-covered surfaceSevilleta NWR, NM (2002), Fr=0.55, LAI=1.5

0

2

4

6

8

1 31 61 91 121 151 181 211 241 271 301 331 361

Julian day

PT

or

PE

(m

m/d

ay)

PT

PEPE=60%, PT=40%

PE=60%, PT=40%

The model was tested for estimating both potential evaporation (PE) and potential transpiration (PT) on two surfaces of Sevilleta LTER by comparing to stable isotopic measurements (Boulanger,2003)

Measured: E =79~84%, T = 16~21%Modeled: PE = 83%, PT = 17%

Measured: E = 52~70%, T = 30~48%Modeled: PE = 60%, PT = 40%

Modeled PE and PT

shrub grass

N

Test the hypothesis for distinct vegetation

Different atmospheric demands for ET lead to differentsoil moisture regimes, and support different vegetation.

100 cm soil

30 cm soil

Duration of dry root zone soil

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Creosote:P/PET ~ 0.18

Boundary P/PET ~ 0.2

Picture from Bruce Harrison

Juniper:P/PET ~ 0.24

Model results with 8-year micrometeological Data at Red Tank station, Sevilleta LTER

Long-term vegetation changes with climate

Mountain Block Recharge (MBR) at hillslope scale

– under various conditions, • including vegetation types, and vegetation change

Precip

itation

Bedrock

Soil Soil water

Granite

Tuff

Granite

Tuff

Annual P=565mmFr=50%

S N S N

Annual P=565mmFr=5%

Percolation: in % of Precip

Aspect effect

4%

4%

31%

17%

Aspect effect

Vegetation control

Soi

l and

bed

rock

ef

fect

s

Generic hillslope hydrologic modeling MBR sensitivity to: bedrock permeability, soil thickness, vegetation

coverage, and slope aspect. (climate variability, rainfall intensity, soil structure change and erosion due to vegetation change not considered)

Soil

Soil

6%

7%

43%

22%

3%

1%

23%

6%

2%

0.3%

16%

1.8%

Los Alamos hillslope experiments(data from Newman, 2003)

Simulations of Los Alamos hillslope experiments

Site description:• ponderosa pine, • 6% slope, • ~ 500 mm annual precip,• permeable tuff

• However, little recharge

Tuff

Simulations of Los Alamos hillslope experiments

Lab measured K

Soil is layered, with lowest measured hydraulic conductivity at 40 cm. However the soil moisture ponds at 60-70 cm. Why?

Tuff

Root

zoneIm

peding layer for percolation

Because of root macropore !

Simulations of Los Alamos hillslope experiments

root zone

tuff

barrier

0

3

6

9

12

Po

ten

tia

l re

ch

arg

e (

cm

/yr)

P=52cm

root zone

tuff

root zone

tuff

0

3

6

9

12 P=52cm Ponderosa

root zone

tuff

0

3

6

9

12 P=38cm

Juniper

Q1: Does ponderosa pine site naturally leads to soil impeding layer?

Q2:Percolation

<?

Los Alamos hillslope experiments(data from Newman, 2003)

Ro

ot

zon

e

Permeable Tuff

Imp

edin

g layer fo

r p

ercolatio

n

Valles Caldera field sites

Hillslope modeling

What controls soil thickness at these sites?

Impeding soil layer?

What is the distributed MBR?

Of the Los Alamos experiments suggests an impeding soil layer below the root zone of ponderosa pine forest. What about Valles Caldera?

Thank you！