7 eric small drought presentation - sahraweb.sahra.arizona.edu/.../7_eric_small.pdf · microsoft...
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SAHRASAHRA
Drought, vegetation, and water cyclingin the semiarid southwest
Eric SmallUniversity of Colorado [email protected]
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