Evapotranspiration (ET) in the Lower Walker River

Basin, West-Central Nevada

ByKip K. Allander, J. LaRue Smith,

Michael J. Johnson, U.S. Geological Survey, Nevada Water Science Center

and Robert PattisonU.S. Department of Agriculture, Agricultural Research

Service

Nevada Water Resources Nevada Water Resources AssociationAssociation

2006 Annual Conference; Mesquite, 2006 Annual Conference; Mesquite, NVNV

February 23, 2006February 23, 2006

Overview

• Objective• Study Design

– ET Network– Scaling up of ET

• Results– ET at Walker

Lake– ET at Saltcedar– ET summary

Main area of interest for ET study

Objective

The primary objective of the ET component of the Walker River Basin project is to estimate ET losses from . . .

Natural Vegetation,

Agricultural Vegetation,

and from open-water surfaces.

Photo by Jim CromptonPhoto by Jim Crompton

Study Design

• 8 ET Stations• 5 are Bowen-ratio

method• 3 are Eddy-

covariancemethod (3D)

RAWS Dead Camel Site

Quantifying ET across the Study Area

• Create map of ET units– Airborne imagery– Lidar

This image is an example of an ET unit map obtained from Laczniak and others, 2001, Ground-water discharge determined from estimates of ET, Death Valley Regional Flow System, NV and CA: USGS WRIR 01-4195, p. 23.

Imagery (infrared)

• Color infrared imagery will be used to classify vegetation based on its relative vigor and soil conditions.

Imagery (natural color)• Natural color imagery will be used to define the

extent of riparian and phreatophytic vegetation.

Lidar• Lidar imagery will be used to estimate

vegetation density and canopy height.

Results – Evaporation (E) from Walker Lake

Measured E on Walker Lake

Water Year 2005

Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct

Eva

po

rati

on

(m

m)

0

2

4

6

8

10

12

14

Daily EData gaps

E compared with Net Radiation on Walker Lake

Water Year 2005

Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct

Evap

ora

tio

n (

mm

)

0

2

4

6

8

10

12

14

Mean

Da

ily

Ne

t R

ad

iati

on

(W

/m2 )

-50

0

50

100

150

200

250

300Daily ENet RadiationData gaps

RAWS – Dead Camel Site

Photo obtained from Photo obtained from http://www.wrcc.dri.edu/wraws/pictures/ndea/200509/ndea.ST3.jpg

E at Walker Lake compared with RAWS solar radiation

Water Year 2005

Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct

Eva

po

rati

on

(m

m)

0

2

4

6

8

10

12

14

Dai

ly S

ola

r R

adia

tio

n (

kW-h

r/m

2)

0

2

4

6

8

10

12

Daily E RAWS Solar Radiation

Relation between E at Walker Lake and RAWS solar radiation

y = 0.0548x2 + 0.5356x

R2 = 0.6533

0

2

4

6

8

10

12

14

0 2 4 6 8 10 12

Daily Solar Radiation (x) kW-hr/m^2

Dai

ly E

vapo

ratio

n (y

) in

mm

Total E for Water Year (WY) 2005

Water Year 2005

Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct

Ev

ap

ora

tio

n (

mm

)

0

2

4

6

8

10

12

14

Daily EEstimated daily E

Total E, WY 2005 = 1,814 mm or 6.0 ft*

* Provisional data has not received final approval and is subject to change with further review

Results – ET from Walker Lake

• Preliminary estimate of ET for WY 2005 was 6.0 ft*, up from previous estimates of 4.1 ft.

• Surface area of Walker Lake in June 2005 was 32,000 acres.

• Estimated water volume evaporated from Walker Lake in WY2005 was 191,000 acre-ft*.

• ~ 50% increase over previous estimates.• If relation between lake ET and RAWS solar

radiation data holds, may be able to estimate annual ET back to 1999.

* Provisional data has not received final approval and is subject to change with further review

Results – Saltcedar site

Saltcedar site• Site underwent substantial defoliation due

to introduction of a biologic control agent: Saltcedar Leaf Beetle (Diorhabda elongata)

• Changed characteristics and ET rate for large stand of Saltcedar

Photo by Robert Pattison Photo by Robert Pattison

Date 2005

Mar Apr May Jun Jul Aug Sep Oct Nov

ET

(m

m)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Leaf

Cov

er (

%)

0

20

40

60

80

100

SaltcedarSaltcedar Leaf Cover

Measured ET at SaltcedarRed line represents a “Conceptual ET Curve” for a vegetation with shallow depth to ground water, such as is the case with this site. This conceptual curve is intended to demonstrate seasonal variability more than seasonal magnitude.

Error bars represent standard error

Comparison with Saltgrass ET

Date 2005

Mar Apr May Jun Jul Aug Sep Oct Nov

ET

(m

m)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Leaf

Cov

er (

%)

0

20

40

60

80

100

SaltcedarSaltgrassSaltcedar Leaf Cover

Period used in regression

Relation between ET at Saltcedar and Saltgrass

y = 0.7985x + 0.7609

R2 = 0.5962

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0.0 0.5 1.0 1.5 2.0 2.5

Saltgrass ET (mm)

Sal

tced

ar E

T (

mm

)

Saltgrass ET normalized to Saltcedar ET

Date 2005

Mar Apr May Jun Jul Aug Sep Oct Nov

ET

(m

m)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Leaf

Cov

er (

%)

0

20

40

60

80

100

SaltcedarNormalized SaltgrassSaltcedar Leaf Cover

Reduction of ET using Saltgrass comparison

Date 2005

Mar May Jul Sep Nov

ET

(m

m)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Lea

f C

ove

r (%

)

0

20

40

60

80

100

SaltcedarNormalized SaltgrassSaltcedar Leaf Cover

203 mm*

57 mm*

This is a 22% reduction in ET*

* Provisional data has not received final approval and is subject to change with further review

Comparison with Willow ET

Date 2005

Mar Apr May Jun Jul Aug Sep Oct Nov

ET

(m

m)

0

2

4

6

8

10

12

Leaf

Cov

er (

%)

0

20

40

60

80

100

SaltcedarWillowSaltcedar Leaf Cover

Period used in regression

Relation between ET at Saltcedar and Willow

y = -0.0015x + 1.8512(mean difference is 1.85 mm/day)

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

0.00 2.00 4.00 6.00 8.00 10.00

Willow ET (mm)

Sal

tced

ar E

T (

mm

)

Reduction of ET using Willow comparison

Date 2005

Mar Apr May Jun Jul Aug Sep Oct Nov

ET

(m

m)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Le

af C

ove

r (%

)

0

20

40

60

80

100

SaltcedarNormalized WillowSaltcedar Leaf Cover

203 mm*

58 mm*

This is a 22% reduction in ET*

* Provisional data has not received final approval and is subject to change with further review

Results – Saltcedar site

• Saltcedar underwent substantial defoliation and associated reduction of ET due to introduction of biologic agent.

• ET rate may have been reduced by more than 20%, but a reduction even greater than this is likely.

• Saltcedar ET rates of 4.0 ft/yr have been observed in Southern Nevada.

• Actual pre-Saltcedar Beetle ET rates from Saltcedar in study area are undetermined at this time.

ET Station

Wa

lke

r L

ak

e

Wil

low

Alf

alf

a B

11

Alf

alf

a B

01

Sa

ltg

ras

s

Sa

ltc

ed

ar

Ra

bb

itb

rus

h

Gre

as

ew

oo

d

ET

(in

ch

es

)

0

20

40

60

80

2005 Growing Season2005 Water Year

Results – ET Summary6.0 ft*

3.5 ft* 3.3 ft*2.8 ft*

0.7 ft* 0.7 ft*0.4 ft* 0.4 ft*

* Provisional data has not received final approval and is subject to change with further review

Summary

• Open water has the greatest ET rates, followed by Willows and then Alfalfa.

• E from Walker Lake for 2005 water year was nearly 6 ft*, almost 2 ft greater than previous estimates.

• Estimated volume evaporated from Walker Lake is approximately 50%* greater than previous estimates.

• ET rates in Saltcedar were reduced by more than 20%* with newly introduced biologic control, but a reduction even greater than this is likely.

* Provisional data has not received final approval and is subject to change with further review

http://nevada.usgs.gov/walker/