SAHRASAHRA

Drought, vegetation, and water cyclingin the semiarid southwest

Eric SmallUniversity of Colorado Bouldereric.small@colorado.edu

s

Soil

Sediment/Rock

Precipitation~250 mm/yr

Runoff? mm/yr

Evapotranspiration(ET) ~250 mm/yr

Recharge? mm/yr

Vegetation plays a key role in water cycling

July-August1999

July-August1995

Dry WetVariability of rainfall

August 1999 August 1995

Vegetation responds to rainfall variability

1950’s drought

Annual Precipitation: Central New Mexico

100

200

300

1900 1920 1940 1960 1980 2000

Year

Pre

cip

itat

ion

(mm

)

How does vegetation response to drought influence water cycling?

Shrub invasion in the southwest:Was drought important?

Early 1800’s: Black Grama Today: Creosotebush

1 m

Vegetation response to drought:Depends on plant type

Invasion of woody species into grassland

1964

1903 Southern Arizona

•Soil erosion•Nutrient availability in soil•hydrology

Environmental impacts of shrub invasion

Runoff in shrubland and grassland

Time (min)

5

10

15

00 20 40 60

Shrubland

Grassland

Dis

char

ge(c

m3

s-1m

-2)

Abrahams et al. 1995

Annual Precipitation: Central New Mexico

100

200

300

1900 1920 1940 1960 1980 2000

Year

Pre

cip

itat

ion

(mm

)

“1950’s drought”

Shrubs more drought resistant

•Deeper roots

•Able to grow in dryer soil

•Grow throughout the year

Untangle multiple factorsdrought & land use

Shrubs favored under grazing

•Livestock prefers grass

•Grazing reduces fire

•Nutrient and soils modified

1800’s: 5 million sheep1900’s: > 2 million cattle

SAHRASAHRA

Large scale field experimentsto study drought impacts

Shrubland: 28% Canopy Grassland: 54% canopy3 meters

Shrubland Grassland3 meters

Annual Precipitation: Central New Mexico

100

200

300

1900 1920 1940 1960 1980 2000

Year

Pre

cip

itat

ion

(mm

)

1950’s drought

How does climatic change or variability influence water cycling and ecosystems?

-3

-2

-1

0

1

2

3

0 500 1000 1500 2000

Year

Rec

on

stru

cted

Pre

cip

itat

ion

(sta

nd

ard

dev

iati

on

s)

Grissino-Mayer, 1995

-3

-2

-1

0

1

2

3

0 500 1000 1500 2000

Year

Rec

on

stru

cted

Pre

cip

itat

ion

(sta

nd

ard

dev

iati

on

s)

Grissino-Mayer, 1995

How does precipitation amount influence critical zoneprocesses in desert grassland and shrubland?

•Does ecosystem response influence water cycling at basin scale?

•Are water-plant-soil interactions important?

•Does drought lead to shrub invasion?

Sevilleta LTER

Model of drought response in grassland

Grassland

Decreased productivityHigh mortality

More evaporationLess transpiration

Less infiltrationMore heterogenous

Less CanopyMore bare soil

Connected drainage networkMore runoff

Soil erosion andNutrient redistribution