Prepared in cooperation with the Natural Resources Conservation Service, Texas State 

Soil and Water Conservation Board, Victoria County Groundwater Conservation District, 

Victoria Soil and Water Conservation District, and the San Antonio River Authority 

Effects of Huisache Removal on Rangeland 

Evapotranspiration in Victoria County, 

South-Central Texas, 2015–18

Scientific Investigations Report 2020–5008

U.S. Department of the Interior

U.S. Geological Survey
Cover.

Top, from left to right

1. Photograph showing the huisache site, U.S. Geological Survey (USGS) site 283526096594800 

evapotranspiration (ET) gage (the treatment site), near McFaddin, Texas, prior to huisache removal at the 

site (photograph by Richard Slattery, USGS, August 31, 2016).

2. Photograph showing the huisache site ET gage prior to huisache removal at the site (photograph by 

George Umphres, USGS, October 6, 2015).

3. Photograph showing the sonic anemometer and gas analyzer at the grassland site, USGS site 

283534097002100 ET gage (the reference site), near McFaddin, Texas (photograph by Richard Slattery, 

USGS, March 23, 2016).

4. Photograph showing the huisache site ET gage prior to huisache removal at the site (photograph by 

George Umphres, USGS, July 30, 2015).

Center

5. Photograph showing the huisache site after huisache removal (photograph by George Umphres, 

USGS, May 23, 2018).

Bottom, from left to right

6. Photograph showing the huisache site ET gage after huisache removal at the site (photograph by 

George Umphres, USGS, September 7, 2017).

7. Photograph showing the huisache site prior to huisache removal (photograph by George Umphres, 

USGS, June 24, 2015).

8. Photograph showing the grassland site ET gage (photograph by Richard Slattery, USGS, May 28, 2015).
Effects of Huisache Removal on Rangeland 

Evapotranspiration in Victoria County, 

South-Central Texas, 2015–18

By Richard N. Slattery, Darwin J. Ockerman, Matt Bromley, Justin Huntington, 

and John R. Banta

Prepared in cooperation with the Natural Resources Conservation Service, 

Texas State Soil and Water Conservation Board, Victoria County Groundwater 

Conservation District, Victoria Soil and Water Conservation District, and the 

San Antonio River Authority 

Scientific Investigations Report 2020–5008

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

DAVID BERNHARDT, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2020

For more information on the USGS—the Federal source for science about the Earth, its natural and living 

resources, natural hazards, and the environment—visit https://www.usgs.gov or call 1–888–ASK–USGS.

For an overview of USGS information products, including maps, imagery, and publications, 

visit https://store.usgs.gov.

Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Slattery, R.N., Ockerman, D.J., Bromley, M., Huntington, J., and Banta, J.R., 2020, Effects of huisache removal on 

rangeland evapotranspiration in Victoria County, south-central Texas, 2015–18: U.S. Geological Survey Scientific 

Investigations Report 2020–5008, 27 p., https://doi.org/10.3133/sir20205008.

The associated data release is

Slattery, R.N., 2020, Evapotranspiration measured at two rangeland sites in Victoria County, south-central Texas, 

2015–2018: U.S. Geological Survey data release, https://doi.org/10.5066/P950QSRE.

ISSN 2328-0328 (online)
iii

Acknowledgments

The authors thank McFaddin Enterprises Ltd. for allowing installation of the evapotranspiration 

stations on the McFaddin ranch, for providing ongoing access to the sites for operation and 

maintenance of the equipment, and for their contribution of equipment and resources needed for 

the huisache removal surrounding the huisache site evapotranspiration station.

v

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Purpose and Scope ..............................................................................................................................2

Description of Study Area ..................................................................................................................3

Data-Collection Methods..............................................................................................................................6

Rainfall....................................................................................................................................................6

Eddy-Covariance Evapotranspiration................................................................................................7

Instrumentation..........................................................................................................................11

Data Processing.........................................................................................................................12

Quality Assurance of Energy Flux and Meteorological Data ......................................................12

Source Area of Measurements........................................................................................................13

Evapotranspiration Data ...................................................................................................................13

Energy-Balance Closure....................................................................................................................15

Remote Sensing of Evapotranspiration...........................................................................................16

Effects of Huisache Removal on Evapotranspiration ............................................................................17

Mapping Evapotranspiration by Remote Sensing..................................................................................21

Summary........................................................................................................................................................23

References Cited..........................................................................................................................................24

Figures

1. Map showing locations of evapotranspiration monitoring sites and other

data-collection stations providing data that were used in the study, Victoria

County, Texas.................................................................................................................................4

2. Photograph showing landscape surrounding the instrument tower at the

grassland site evapotranspiration station, Victoria County, Texas, March 2015 ...............5

3. Photograph showing landscape beginning to leaf out at the huisache site

evapotranspiration station, Victoria County, Texas, March 2015 .........................................5

4. Photograph showing landscape in full leaf out surrounding the huisache site

evapotranspiration station, Victoria County, Texas, August 2016.........................................6

5. Photograph showing landscape surrounding the huisache site evapotranspiration

station, Victoria County, Texas, on February 2, 2017, after huisache removal ...................7

6. Image showing satellite view of evapotranspiration measurement sites,

March 25, 2017, Victoria County, Texas.....................................................................................8

7. Photograph showing landscape surrounding the huisache site evapotranspiration

station after huisache removal, April 2017, Victoria County, Texas .....................................9

8. Graph showing source-area contributions of average daytime cumulative turbulent

flux for the grassland and huisache evapotranspiration sites, Victoria County,

Texas, before and after huisache removal.............................................................................13

9. Graph showing daily eddy-covariance evapotranspiration values at the grassland

and huisache sites, Victoria County, Texas, March 2015–August 2018.............................14

10. Graph showing three-month moving average of the monthly energy-balance

ratio, grassland site and huisache site, Victoria County, Texas, March 2015–

August 2018..................................................................................................................................16
vi

11. Graphs showing comparison of corrected eddy-covariance monthly 

evapotranspiration and remotely sensed monthly evapotranspiration at the 

grassland site and the huisache site, Victoria County, Texas, March 2015–

August 2018..................................................................................................................................18

12. Graph showing monthly cumulative difference in evapotranspiration between

huisache and grassland sites, by eddy-covariance and remotely sensed

evapotranspiration methods, Victoria County, Texas, March 2015–August 2018 ............19

13. Map showing remotely sensed 2015 annual evapotranspiration rates for the 

study sites and surrounding area, Victoria County, Texas ..................................................22

Tables

1. Annual (2015–18) and long-term average annual (1962–2018) rainfall at Victoria

Regional Airport, Texas................................................................................................................3

2. Stations providing data for the rangeland evapotranspiration study, Victoria

County, Texas, March 2015–August 2018..................................................................................3

3. Instruments used to measure evapotranspiration, energy balance, and rainfall 

at eddy-covariance evapotranspiration sites, Victoria County, Texas ..............................11

4. Number of daily evapotranspiration values collected at the grassland and 

huisache sites, Victoria County, Texas, March 2015–August 2018.....................................14

5. Summary of average energy-balance data and calculated energy-balance ratio 

at the grassland and huisache evapotranspiration sites, March 2015–August 2018......15

6. Summary of rainfall and evapotranspiration totals for the grassland and huisache

study sites, March 2015–August 2018 .....................................................................................17

7. Selected statistics for the monthly difference in evapotranspiration between the

huisache site and grassland site (ET huisache – ET grassland) before and after

huisache removal .......................................................................................................................20

8. Average monthly difference between huisache site and grassland site

evapotranspiration before huisache removal (pre-removal period) and after

huisache removal (post-removal period) for eddy-covariance and 

remote-sensing estimates.........................................................................................................21
vii

Conversion Factors

International System of Units to U.S. customary units

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Area

square meter (m2) 0.0002471 acre 

hectare (ha) 2.471 acre

square kilometer (km2) 247.1 acre

square meter (m2) 10.76 square foot (ft2) 

hectare (ha) 0.003861 square mile (mi2) 

square kilometer (km2) 0.3861 square mile (mi2)

Volume

cubic centimeter (cm3) 0.06102 cubic inch (in3) 

liter (L) 61.02 cubic inch (in3) 

cubic meter (m3) 35.31 cubic foot (ft3)

cubic kilometer (km3) 0.2399 cubic mile (mi3) 

cubic meter (m3) 0.0008107 acre-foot (acre-ft) 

Mass

gram (g) 0.03527 ounce, avoirdupois (oz)

kilogram (kg) 2.205 pound, avoirdupois (lb)

Pressure

kilopascal (kPa) 0.009869 atmosphere, standard (atm)

kilopascal (kPa) 20.88 pound per square foot (lb/ft2) 

kilopascal (kPa) 0.1450 pound per square inch (lb/ft2) 

Density

kilogram per cubic meter (kg/m3) 0.06242 pound per cubic foot (lb/ft3) 

gram per cubic centimeter (g/cm3) 62.4220 pound per cubic foot (lb/ft3) 

Energy

joule (J) 0.2390 calorie (cal)

watt per square meter (W/m2) 0.0222 calorie per second per square foot 

[(cal/s)/ft2]

 Frequency

hertz (Hz) 1.0 cycles per second (cps)
U.S. customary units to International System of Units

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 4,047 square meter (m2)

acre 0.004047 square kilometer (km2)

square foot (ft2) 0.09290 square meter (m2)

square inch (in2) 6.452 square centimeter (cm2)

square mile (mi2) 259.0 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2) 

Volume

cubic inch (in3) 16.39 cubic centimeter (cm3) 

cubic inch (in3) 0.01639 liter (L)

cubic foot (ft3) 0.02832 cubic meter (m3) 

cubic mile (mi3) 4.168 cubic kilometer (km3) 

acre-foot (acre-ft) 1,233 cubic meter (m3)

Mass

ounce, avoirdupois (oz) 28.35 gram (g) 

pound, avoirdupois (lb) 0.4536 kilogram (kg) 

Pressure

atmosphere, standard (atm) 101.3 kilopascal (kPa)

pound per square foot (lb/ft2) 0.04788 kilopascal (kPa) 

pound per square inch (lb/in2) 6.895 kilopascal (kPa) 

Density

pound per cubic foot (lb/ft3) 16,020 gram per cubic meter (kg/m3)

pound per cubic foot (lb/ft3

) 0.01602 gram per cubic centimeter (g/cm3)

Energy

calorie (cal) 4.184 joule (J)

calorie per second per square foot [(cal/s)/ft2] 45.045 watt per square meter (W/m2)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows: 

°F = (1.8 × °C) + 32.

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows: 

°C = (°F – 32) / 1.8.

viii
ix

Datum

Vertical coordinate information is referenced to either the National Geodetic Vertical Datum of 

1929 (NGVD 29) or the North American Vertical Datum of 1988 (NAVD 88).

Horizontal coordinate information is referenced to either the North American Datum of 1927 or 

the North American Datum of 1983 (NAD 83).

Altitude, as used in this report, refers to distance above the vertical datum.

Abbreviations

CIMEC Calibration Using Inverse Modeling at Extreme Conditions

DRI Desert Research Institute

EBR energy-balance ratio

EC eddy covariance

ET evapotranspiration

ETC corrected eddy-covariance evapotranspiration

ETEC eddy-covariance evapotranspiration

ETr daily reference evapotranspiration

ETrF fraction of daily reference evapotranspiration

ETRS remotely sensed evapotranspiration

G soil heat flux

H sensible-heat flux

LE latent-heat flux

METRIC Mapping Evapotranspiration at High Resolution With Internalized Calibration

NWS National Weather Service

Rn net radiation

SSEBop Operational Simplified Surface Energy Balance

USGS U.S. Geological Survey

Effects of Huisache Removal on Rangeland 

Evapotranspiration in Victoria County, South-Central 

Texas, 2015–18

By Richard N. Slattery,1 Darwin J. Ockerman,1 Matt Bromley,2 Justin Huntington,2 and John R. Banta1

Abstract

The U.S. Geological Survey and Desert Research 

Institute, in cooperation with the Natural Resources 

Conservation Service, Texas State Soil and Water 

Conservation Board, Victoria County Groundwater 

Conservation District, Victoria Soil and Water Conservation 

District, and the San Antonio River Authority, evaluated the 

hydrologic effects of Vachellia farnesiana var. farnesiana 

(huisache) removal on rangeland evapotranspiration in 

Victoria County, Texas. Measurements of evapotranspiration, 

rainfall, and related properties were made at two sites 

during March 2015 through August 2018. One site was 

predominantly grassland. The other site was dominated by 

dense huisache vegetation that was removed about halfway 

through the study period. The resulting evapotranspiration 

data were examined for differences between the locations 

and differences between the pre-removal (2015–16) and 

post-removal (2017–18) periods to assess the effects of 

huisache removal on evapotranspiration. Evapotranspiration 

measurements were made using the eddy-covariance technique 

and were supplemented by remote-sensing estimates of 

evapotranspiration derived from thermal and optical satellite 

images. A map of remotely sensed evapotranspiration was 

generated for the area surrounding the study sites for 2015 

and demonstrates the capability of remote sensing to evaluate 

land-management effects on evapotranspiration for larger scale 

areas, such as a county or stream-basin area. 

During the pre-removal period (March 2015–

December 2016), evapotranspiration was greater at the 

huisache site than at the grassland site. Evapotranspiration 

at the grassland site (average of the eddy-covariance 

evapotranspiration and average remotely sensed 

evapotranspiration) was 87.6 millimeters per month 

(mm/mo) and at the huisache site was 100.8 mm/mo, with 

the differences in evapotranspiration rates being attributed to 

the difference in site vegetation. After huisache was removed 

in January 2017, evapotranspiration at the huisache site was 

substantially lower than at the grassland site, the changes in 

evapotranspiration rates being attributed not only to removal 

of huisache vegetation but also to possible disruption of soil 

runoff and infiltration characteristics. During the post-removal 

period (February 2017–August 2018), evapotranspiration 

was 88.5 mm/mo at the grassland site and 72.9 mm/mo at the 

huisache site (average of the eddy-covariance and average 

remotely sensed evapotranspiration).

The monthly differences in evapotranspiration 

between the grassland and huisache sites, determined 

by eddy-covariance and remote-sensing methods, were 

statistically significant between the pre-removal and 

post-removal periods. Also, the pre-removal period provided 

the best conditions to evaluate the differences between 

huisache site and grassland site evapotranspiration. During the 

pre-removal period, evapotranspiration from the huisache site 

as measured by the eddy-covariance method was, on average, 

10.7 mm/mo greater than evapotranspiration measured at the 

grassland site. As determined by the average of the remotely 

sensed methods, huisache site evapotranspiration was 

15.8 mm/mo greater than grassland site evapotranspiration. 

These average differences in evapotranspiration rates by 

the two methods indicate that evapotranspiration at the 

grassland site was, on average, 13.2 mm/mo less than that at 

the huisache site during the pre-removal period. This average 

difference in evapotranspiration rates also indicates potential 

increased groundwater recharge and (or) surface-water runoff 

at the grassland site.

Introduction

Brushy or woody plants in grassland prairies of the 

Southwestern United States have increased during the 

past 150 years (Van Auken, 2000). Historically, grassland 

prairies in Texas were often interspersed with stands of 

oak trees and used as rangelands for grazing domestic 

animals. Much of the historical oak-grassland land cover

_________________

1

U.S. Geological Survey.

2

Desert Research Institute.
2 Effects of Huisache Removal on Rangeland Evapotranspiration in Victoria County, South-Central Texas, 2015–18

across Texas has undergone woody-plant encroachment 

attributed to fire suppression and overgrazing (Bray, 1904; 

Van Auken, 2000; Ansley and Hart, 2012). Some scientific 

studies have reported evidence that land use and altered 

vegetation composition may affect streamflow, downstream 

spring discharge, and groundwater recharge (Thurow and 

Hester, 1997; Tennesen, 2008; Saleh and others, 2009; Banta 

and Slattery, 2011). 

Vachellia farnesiana var. farnesiana (huisache) is one 

type of woody plant that can overtake a grassland prairie in 

Texas, often in a generation, resulting in areas with diminished 

grazing benefits (Mutz and others, 1978; Bontrager and 

others, 1979). Huisache can grow 5–8 meters (m) tall, and its 

extensive root system is able to access both shallow soil water 

and deeper groundwater. Huisache is a prolific seed producer, 

and once established, it can outcompete native grasses for 

space (Teveni, 2017).

By removing woody vegetation such as huisache and 

allowing native grasses to reestablish, the hydrologic budget 

of the watershed might change to increase groundwater 

recharge and surface-water supply (Teveni, 2017). One 

method of documenting changes to the hydrologic budget 

resulting from differences in vegetative cover is by measuring 

evapotranspiration rates (Dugas and others, 1998). 

Evapotranspiration is the combined processes of 

evaporation and transpiration wherein water is converted from 

a liquid to a vapor and is transferred from Earth’s surface to the 

atmosphere. Evaporation occurs from open bodies of water, soil 

water, and water condensate on land and vegetation surfaces. 

Transpiration processes occur in plants, wherein liquid water 

is changed to a vapor as it passes through the stomata of the 

plant. Evapotranspiration rates are affected by available energy 

and water, as well as by air temperature, humidity, wind and 

air movement, soil moisture, depth to groundwater, and plant 

phenology (U.S. Geological Survey, 2019a). By quantifying 

evapotranspiration rates for different vegetative species, a 

better understanding of the hydrologic budget in a watershed is 

possible (Stannard and others, 2013). 

Previous studies have shown that different vegetative 

land cover can affect evapotranspiration and might influence 

the hydrologic cycle (Tennesen, 2008; Saleh and others, 2009; 

Banta and Slattery, 2011). For example, at the Honey Creek 

State Natural Area, near Spring Branch, Tex., Juniperus ashei

(Ashe juniper) was removed, and the post-brush-management 

hydrologic data exhibited a reduction in evapotranspiration 

compared to the pre-brush-removal period (Banta and 

Slattery, 2011). Similarly, in the North Concho River Basin 

near San Angelo, Tex., a reduction in evapotranspiration 

was observed after removal of genus Prosopis (mesquite) 

vegetation (Saleh and others, 2009). 

The U.S. Geological Survey (USGS) and the Desert 

Research Institute (DRI), in cooperation with various 

Federal, State, and local agencies, studied the effects 

of huisache removal on evapotranspiration and the 

hydrologic budget on rangeland in Victoria County in 

south-central Texas during 2015–18. Federal, State, and 

local agencies that cooperated on the study were the Natural 

Resources Conservation Service, Texas State Soil and 

Water Conservation Board, Victoria County Groundwater 

Conservation District, Victoria Soil and Water Conservation 

District, and the San Antonio River Authority.

Quantifying evapotranspiration rates is essential for 

understanding the effects of woody-vegetation removal at a 

site (Dugas and others, 1998; Saleh and others, 2009; Banta 

and Slattery, 2011). Typically, the scales of evapotranspiration 

studies range from plot size (a few hectares) to small watershed 

size (less than 20 square kilometers [km2]). One concern of sitebased measurement studies of evapotranspiration is how to best 

incorporate measurements from smaller scale (plot or small 

watershed size) studies into larger scale studies that can then be 

used for basin-scale hydrologic models (Liu and others, 2016). 

One technique to “scale up” from smaller scale studies is 

to use remote-sensing techniques based on satellite imagery 

(Liu and others, 2016). Satellite images have much larger 

footprints than do ground-based monitoring systems (for 

example, a Landsat satellite image covers approximately 

33,000 km2 of Earth’s surface) while maintaining relatively 

high spatial resolution (as detailed as 30 x 30 m) (National 

Aeronautics and Space Administration, 2019a). By using 

satellite images of surface temperature and surface reflectance, 

evapotranspiration rates can be estimated with the application 

of surface energy-balance models. Remote-sensing estimates 

are commonly used to complement and scale up ground-based 

measurements of evapotranspiration in space and time 

(Kustas and Norman, 2009). Spatial and temporal estimates 

of evapotranspiration derived from remote-sensing models 

can be incorporated into hydrologic models to support the 

model calibration process and ultimately improve estimates 

of the water budget and prediction of individual water budget 

variables, such as streamflow and groundwater recharge 

(Zhang and others, 2009; Szilagyi and Jozsa, 2013; Carroll 

and others, 2015). For example, two remote-sensing-based 

surface energy-balance models that have been extensively 

applied and assessed across the Western United States 

include the Mapping Evapotranspiration at High Resolution 

With Internalized Calibration (METRIC) model (Allen and 

others, 2007) and the Operational Simplified Surface Energy 

Balance (SSEBop) model (Senay, 2018). 

The present study combined ground-based 

evapotranspiration measurements at the study site with 

remotely sensed evapotranspiration. This approach provides 

detailed estimates of evapotranspiration combined with 

onsite measurements for assessing the remote-sensing 

evapotranspiration data. 

Purpose and Scope

The purpose of this report is to document the effects of 

huisache removal on evapotranspiration rates at two sites 

within a rangeland area in Victoria County, Tex., during 

March 2015–August 2018. The eddy-covariance (EC) and 
Introduction 3

remote-sensing methods used to measure evapotranspiration 

are described. Differences in evapotranspiration 

measurements made at the two sites (one predominantly 

grassland and the other dominated by dense huisache that 

was removed in January 2017, approximately halfway 

through the study period) are presented, and the spatial and 

temporal effects of huisache removal on evapotranspiration 

are evaluated. 

Description of Study Area 

The study area is on private ranchland in Victoria 

County, Tex., about 24 kilometers south of Victoria, Tex. 

(fig. 1). Long-term average annual rainfall for the area, as 

calculated from the 1962–2018 annual rainfall record at the 

National Weather Service (NWS) station at Victoria Regional 

Airport, Tex. (fig. 1), is 997 millimeters per year (mm/yr) 

(table 1). Annual rainfall is highly variable, ranging from 

332 millimeters (mm) (2011) to 1,870 mm (2004) (National 

Oceanic and Atmospheric Administration, 2019). Rainfall 

during each year of the study was approximately the same 

as or greater than the long-term average (table 1). The 

average monthly temperature in the study area (measured 

at Victoria Regional Airport, 1962–2018) ranges from 

53.7 degrees Fahrenheit (oF) in January to 84.6 oF in August 

(National Oceanic and Atmospheric Administration, 2019). 

Two evapotranspiration measurement sites, USGS station 

number 283534097002100 and USGS station number 

283526096594800, hereinafter referred to as the “grassland 

site” and the “huisache site,” respectively, were established on 

the private ranch in March 2015 (fig. 1, table 2).

Table 1. Annual (2015–18) and long-term average annual 

(1962–2018) rainfall at Victoria Regional Airport, Texas.

2015 2016 2017 2018 1962–

2018

Annual and period 

rainfall (millimeters 

per year)

1,360 997 1,180 1,100 997

The study area is relatively flat (less than 1 percent 

slopes), and soils are sandy loam and clay loam (Natural 

Resources Conservation Service, 2019). The grassland site 

includes somewhat poorly drained Edna fine sandy loam 

soils and moderately well drained Fadden sandy loam soils 

(Natural Resources Conservation Service, 2019). The huisache 

site includes Edna fine sandy loam soils and moderately well 

drained Dacosta-Contee clay loam soils (Natural Resources 

Conservation Service, 2019). Because of low slope and some 

areas with somewhat poor drainage, soils in the study area 

can remain wet for parts of the year after periods of excessive 

rainfall. The depth to the water table in the study area is 

greater than 80 inches (Natural Resources Conservation 

Service, 2019).

The grassland site (USGS station number 

283534097002100) was installed in an area dominated by 

homogenous grass vegetation. The huisache site (USGS 

station number 283526096594800) was installed in an area 

dominated by huisache vegetation. The grassland site had a 

variety of species including Schizachyrium scoparium (little 

bluestem), Sorghastrum nutans (yellow Indiangrass), and 

Sporobolus compositus (tall dropseed), which were once 

dominant across the area (Griffith and others, 2007). The 

huisache site was dominated by huisache woody vegetation. 

However, because of fire suppression, overgrazing, and 

other disturbances, huisache and other woody or thorn-shrub 

species, including Prosopis glandulosa (honey mesquite), 

Vachellia rigidula (blackbrush acacia), and Celtis pallida 

(granjeno), are of concern because they can outcompete 

grassland species in the region (Griffith and others, 2007).

Figure 2 shows a photograph of the grassland site in 

March 2015. Figure 3 shows a photograph of the huisache 

site in March 2015, when the trees were just beginning to 

leaf out. Figure 4 shows a photograph of the huisache site in 

August 2016, when the huisache trees were in full leaf out.

Huisache is one of the most formidable woody species for 

removal in the Texas coastal prairie (Mutz and others, 1978). 

Its seeds, like those of honey mesquite, have high germination 

potential immediately upon dissemination, especially if 

scarified and exposed to temperatures of 26 to 30 degrees 

Table 2. Stations providing data for the rangeland evapotranspiration study, Victoria County, Texas, March 2015–August 2018.

[USGS, U.S. Geological Survey; ET, evapotranspiration; NWS, National Weather Service]

Site identifier 

(fig. 1)

Station 

number

Station 

name

Latitude Longitude Type of data 

collected

USGS grassland 

evapotranspiration station

283534097002100 ET station (reference site) 

near McFaddin, Texas

28° 35′ 37.1′′ 97° 00′ 20.9′′ Rainfall, eddy-covariance 

evapotranspiration

USGS huisache 

evapotranspiration station

283526096594800 ET station (treatment site) 

near McFaddin, Texas

28° 35′ 26.5′′ 96° 59′ 48.1′′ Rainfall, eddy-covariance 

evapotranspiration

USGS streamflow-gaging 

station San Antonio River 

near McFaddin, Tex.

08188570 San Antonio River near 

McFaddin, Texas

28° 31′ 52.5′′ 97° 02′ 33.7′′ Rainfall

NWS station at Victoria 

Regional Airport

12912 Victoria Regional Airport 28° 51′ 45′′ 96° 55′ 47′′ Rainfall
4 Effects of Huisache Removal on Rangeland Evapotranspiration in Victoria County, South-Central Texas, 2015–18

Lavaca Bay

San Antonio Bay

Guadalupe River

San Antonio River

Port

Lavaca

Victoria

Goliad

DE WITT

COUNTY

JACKSON

COUNTY

VICTORIA

COUNTY

GOLIAD COUNTY

CALHOUN COUNTY

REFUGIO COUNTY

59

87

77

35

59

183

National Weather

Service—Victoria

Regional Airport

San Antonio

River near

McFadden McFaddin

Huisache

site

Grassland

site

National Weather

Service—Victoria

Regional Airport

28°50'

28°40'

28°30'

28°20'

97°20' 97°10' 97°00' 96°50' 96°40' 96°30'

EXPLANATION

Evapotranspiration site and identifier—See

table 2

U.S. Geological Survey streamflow-gaging

station and name—See table 2

National Weather Service rainfall station

and name—See table 2

Huisache

site

San Antonio

River near

McFaddin

0 5 10

0 5 10 15 KILOMETERS

MILES

20 Base modified from U.S. Geological Survey 1:100,000 scale digital data

Texas South-Central State Plane coordinate system

North American Datum of 1983

Area enlarged

TEXAS

Area shown on fig. 6

Figure 1. Locations of evapotranspiration monitoring sites and other data-collection stations providing data that were used in 

the study, Victoria County, Texas.
Introduction 5

Figure 2. Landscape surrounding the instrument tower at the grassland site evapotranspiration 

station, Victoria County, Texas, March 2015.

Figure 3. Landscape beginning to leaf out at the huisache site evapotranspiration station, Victoria 

County, Texas, March 2015 (photograph taken from 

near the top of the instrument tower).
6 Effects of Huisache Removal on Rangeland Evapotranspiration in Victoria County, South-Central Texas, 2015–18

Figure 4. Landscape in full leaf out surrounding the huisache site evapotranspiration 

station, Victoria County, Texas, August 2016 (photograph taken from near the top of 

the instrument tower).

Celsius (Scifres, 1974). Once established, the seedlings grow 

rapidly; they can also resprout readily following damage or top 

removal. In addition, huisache sprouts can grow to almost half 

their original total plant height within 5 months after shredding 

(Powell and others, 1972). Thus, mechanical top removal 

results in only short-term suppression of huisache (Mutz and 

others, 1978) and gives the species competitive advantage over 

associated, slower growing woody plants. Almost pure, dense 

stands of huisache may develop within two to three growing 

seasons following brush control methods that disturb the soil 

(Bontrager and others, 1979).

In January 2017, huisache was removed at the huisache 

site. The huisache removal was accomplished by uprooting 

the entire trees and roots with a bulldozer-mounted device that 

pulled the huisache tree and roots from the ground. The brush 

was then hauled away from the site. This huisache removal 

method ensured that the woody plant would not resprout during 

the study. Figure 5 shows a photograph of the area surrounding 

the instrument tower at the huisache site in February 2017, 

after huisache removal. In figure 6 (satellite imagery of study 

area on March 25, 2017), the extent of huisache removal 

can be seen as the gray area, extending about 150 m radially 

from the instrument tower. After huisache removal, native 

grasses quickly became established, as seen in the April 2017 

photograph taken about 3 months after huisache removal 

(fig. 7). No seeding of the site was required.

Data-Collection Methods

Evapotranspiration and rainfall data were collected for the 

grassland and huisache sites during March 2015–August 2018 

(table 2). To measure evapotranspiration by the EC method, 

the grassland and huisache sites were equipped with the 

following instrumentation: an open path gas analyzer to 

measure atmospheric water concentrations, a three-dimensional 

sonic anemometer to measure wind speed and direction, a net 

radiometer to measure longwave and shortwave radiation, air 

temperature and humidity sensors, soil heat flux plates, and 

soil temperature and soil moisture sensors (table 3). Rainfall 

data were collected at each site with tipping-bucket rain gages 

and augmented with rainfall data from two nearby stations as 

needed during periods of missing rainfall data.

Rainfall

Rainfall was measured in inches at each site 

(Slattery, 2020) with a NovaLynx 12-inch tipping-bucket 

rain gage (table 3) mounted 2.4 m above the land surface; 

for analysis purposes, rainfall data were converted from 

inches to millimeters. To maintain the accuracy of the rain 

gages, the instruments were periodically inspected and 

cleaned, and calibration checks were performed as described 
Data-Collection Methods 7

Figure 5. Landscape surrounding the huisache site evapotranspiration station, Victoria County, 

Texas, on February 2, 2017, after huisache removal.

by the manufacturer and USGS protocols (U.S. Geological 

Survey, 2005; NovaLynx Corporation, 2019). Instruments 

not meeting calibration standards (calibration values differed 

from expected values by more than 8 percent) were replaced. 

Rainfall data were also reviewed by comparison of data 

between the two sites to better identify periods when a rain 

gage might have been clogged and therefore under-recorded 

rainfall. Affected data were removed from the USGS National 

Water Information System database (U.S. Geological 

Survey, 2019b). 

Rainfall totals were recorded every 30 minutes, with 

daily rainfall totals calculated from the sum of the 30-minute 

values for each day. Daily rainfall totals were not reported for 

days missing more than 30 percent of the 30-minute values 

for a day. Other than removing anomalous values caused by 

instrumentation noise (anomalous values not corroborated by 

preceding and subsequent values), no further corrections were 

made to the rainfall data. 

Based on field-calibration checks, the rainfall data are 

considered accurate to within 8 percent of actual rainfall. The 

daily rainfall totals at the two sites were highly correlated 

during 2015–18, with an average site-to-site R2

 value 

(coefficient of determination) (Helsel and Hirsch, 2002) of 

0.97. Given the proximity of the sites and similar physical 

site conditions (for example, similar slopes and aspects), a 

representative daily rainfall amount for the study area was 

developed by averaging the available daily rainfall totals from 

the two rainfall sites. 

For days when measured rainfall data were not available 

from one of the sites, the measured rainfall at the other site 

was used as the daily rainfall amount for the study area. For 

days when rainfall data were not available from either site, 

daily rainfall was obtained from one of two sources (table 2). 

The first source was the NWS station at the Victoria Regional 

Airport. The second source was USGS streamflow-gaging 

station 08188570 San Antonio River near McFaddin, Tex. 

(hereinafter referred to as the “McFaddin station”). Compared 

to the NWS station at the Victoria Regional Airport, the 

McFaddin station is closer to and likely more representative 

of rainfall conditions at the evapotranspiration study sites. 

However, a complete record of daily rainfall for the study 

period was not available from the McFaddin station. Rainfall 

data from the McFaddin station were only used to substitute 

for missing data during extensive rainfall caused by Hurricane 

Harvey in August–September 2017. A total of 64 days (about 

5 percent of the study period) were missing rainfall data from 

the evapotranspiration sites; those daily values were filled by 

using a combination of data from the NWS Victoria Regional 

Airport station and the McFaddin station.

Eddy-Covariance Evapotranspiration

The process of evapotranspiration consumes energy 

from the environment, and measuring this transfer of energy 

is the basis for estimating evapotranspiration (Laczniak 

and others, 1999). The instrumentation for estimating 
8 Effects of Huisache Removal on Rangeland Evapotranspiration in Victoria County, South-Central Texas, 2015–18

Huisache

site

Grassland

site

97°00'30" 97°00' 96°59'30"

28°36'

28°35'30"

28°35'

EXPLANATION

Huisache Evapotranspiration site and identifier

site

Base map data from Google, Maxar Technologies 2019

Texas South-Central State Plane coordinate system

North American Datum of 1983

0 0.25 0.5 0.75 1 KILOMETER

0 0.25 0.5 MILE

Area enlarged

TEXAS

Figure 6. Satellite view of evapotranspiration measurement sites, March 25, 2017, Victoria County, Texas.
Data-Collection Methods 9

Figure 7. Landscape surrounding the huisache site evapotranspiration station after huisache 

removal, April 2017, Victoria County, Texas.

evapotranspiration by the EC method measures vertical 

turbulent fluxes (associated with energy transfer) from the 

land surface to the atmosphere. These turbulent fluxes are 

commonly compared to measured radiative fluxes to assess the 

energy budget.

The energy at Earth’s surface can be described by a 

surface energy budget. Assuming that energy fluxes from other 

sources and sinks are negligible, the simplified form of the 

energy budget can be expressed as follows (Brutsaert, 1982):

R G n  W L   E H (1)

where

Rn is net radiation, 

G is soil heat flux, 

W represents changes in heat energy stored in the 

vegetation, 

LE is latent-heat flux, and

H is sensible-heat flux.

All terms in equation 1 are in watts per square meter, and 

each term is positive during typical daytime conditions. Rn

 is 

positive when incoming longwave and shortwave radiation 

exceeds outgoing longwave and shortwave radiation, G is 

positive when heat moves from the surface into the subsurface, 

and W is positive when the huisache vegetation gains energy 

from the sun. LE and H are positive when moving upward 

from the surface to the atmosphere. The left side of equation 1 

(Rn – G – W) represents the available energy for the right side 

of equation 1; the right side (LE + H) represents the turbulent 

flux of mass and energy into the atmosphere. For this study, W

was considered negligible and was not measured or considered 

in any calculations.

Turbulent fluxes occur in the lowest layer of the 

atmosphere, the atmospheric boundary layer, through 

eddy diffusion. Above a flat, level surface, the average air 

movement is commonly recognized as being horizontal and 

is quantified as wind speed. However, superimposed on the 

average wind is the random turbulent upward and downward 

movement of wind (eddies), resulting in instantaneous 

wind vectors with nonzero vertical components. Within 

an eddy, wind movements are highly correlated and can 

be quite distinct from those in nearby eddies. Similarly, 

concentrations of admixtures (for example, the heat, water 

vapor, or carbon dioxide content) may vary from eddy to eddy. 

Vertical transport of eddy admixtures results from the vertical 

components of eddy motions, a process similar to molecular 

diffusion but on a much larger scale (Brutsaert, 1982; Stannard 

and others, 2013).

As an eddy moves upward or downward, it carries the 

admixtures along with it, giving rise to vertical turbulent 

flux. Above an extensive uniform surface, the net vertical 

transport of an admixture is equal to the algebraic sum of 

the transport contributions from all eddies (or a statistically 

significant sample thereof) passing over the surface (Stannard 

and others, 2013). Thus, over an evaporating surface, 

upward-moving eddies contain, on average, more water vapor 

than do downward-moving eddies. If certain criteria are met, 

the flux of an admixture can be measured with high-speed 

measurements (measured at 10 times per second and averaged 
10 Effects of Huisache Removal on Rangeland Evapotranspiration in Victoria County, South-Central Texas, 2015–18

over 30-minute intervals for this study) of vertical eddy 

motion and the admixture concentration (Stannard and 

others, 2013). The EC technique provides the most direct 

measure of turbulent energy flux available (Baldocchi, 2003; 

Foken and others, 2004; Stannard and others, 2013). 

Therefore, fluxes of water vapor and heat can be estimated 

directly without the application of empirical constants (Foken 

and others, 2004). 

In the case of water vapor, the EC equation takes the 

following form (Dyer, 1961): 

LE    w v ' ' (2)

where

LE is the measured latent-heat flux, in watts per 

square meter per day;

λ is the latent heat of vaporization, in joules per 

gram;

w ́ is the vertical component of wind speed 

and direction, in meters per second; the 

prime symbol denotes deviations from the 

average value over the averaging period 

(upward is positive); and

ρ ́v is water vapor density, in grams per cubic 

meter; the prime symbol denotes 

deviations from the average value over the 

averaging period. 

The quantity w v ' ' ρ is the covariance of w’ and ρ ́v; the 

overbar denotes average value.

The sensible-heat flux is similarly computed, but variations 

in temperature are considered rather than water vapor:

H c  a p w T a ' ' (3)

where

H is the measured sensible-heat flux, in watts 

per square meter;

ρa is air density, in kilograms per cubic meter;

cp is specific heat of air at constant pressure, in 

joules per degree Celsius; 

w ́ is the vertical component of wind speed 

and direction, in meters per second; the 

prime symbol denotes deviations from the 

average value over the averaging period 

(upward is positive); and

T ́a is air temperature, in degrees Celsius; the 

prime symbol denotes deviations from the 

average value over the averaging period.

The quantity w T' ' a is the covariance of w ́ and T ́a; the 

overbar denotes average value.

In addition to turbulent flux of heat and mass between 

the surface and atmosphere, a turbulent flux of horizontal 

momentum also occurs from the atmosphere to the land 

surface (Stannard and others, 2013). Horizontal wind speed 

is zero at the land surface and increases with height above 

the land surface—a wind-speed profile that indicates greater 

momentum at height, decreasing to zero momentum at 

the land surface. This gradient implies a flux of horizontal 

momentum from the upper atmosphere downward to the land 

surface, also recognized as a drag force exerted on the surface 

by the air. The transfer of momentum downward occurs 

through eddy diffusion and can be expressed as 

  m a   w u' ' (4)

where

τm is the calculated momentum flux or drag per 

unit area of land surface, in kilograms per 

meter per second squared;

ρa is air density, in kilograms per cubic meter;

w ́ is the vertical component of wind speed, 

in meters per second; the prime symbol 

denotes deviation from the average value 

(upward is positive); and

u ́ is the horizontal component of wind speed, 

in meters per second; the prime symbol 

denotes deviations from the average value.

The quantity w u' ' is the covariance of w and u; the overbar 

denotes average value. τm is calculated concurrently with LE

and H (eq. 2 and eq. 3) and is used to calculate corrections to 

those fluxes. 

The relation between a given evapotranspiration rate and 

the associated latent-heat flux is

ET   LE w / ( ) (5)

where

ET is evapotranspiration, in millimeters per day;

LE is the latent-heat flux, in watts per square 

meter;

ρw is density of water, in kilograms per cubic 

meter; and 

λ is the latent heat of vaporization, in joules per 

kilogram.

Both ρw and λ are very weak functions of temperature, 

and they are roughly constant over the range of temperatures 

recorded in this study. Therefore, to express ET as a 

function of LE, equation 5 can be simplified and rewritten 

approximately as

ET   0.035 LE (6)

where

ET is evapotranspiration, in millimeters per day;

LE is the latent-heat flux, in watts per square 

meter; and

0.035 is a factor that converts the energy 

transfer of the latent-heat flux into an 

evapotranspiration rate of water in 

millimeters per day. 
Data-Collection Methods 11

Instrumentation

To obtain the meteorological and surface energy flux 

data needed for the calculation of evapotranspiration, the 

two sites were instrumented with the LI-COR Biosciences 

EC system (table 3). Each EC system included an LI-7500A 

open path gas analyzer with an LI-7550 interface unit 

to measure atmospheric water concentrations (LI-COR 

Biosciences, 2019a) and a Campbell Scientific CSAT3 or 

Gill WindMaster Pro three-dimensional sonic anemometer 

to measure wind speed and direction and sonic temperature 

(Campbell Scientific, Inc., 2019; Gill Instruments, 2019). 

These measurements were made and recorded by the 

LI-COR LI-7550 at a rate of 10 measurements per second 

(10 hertz [Hz]). Each EC system also included a LI-COR 

Biomet system (LI-COR Biosciences, 2016), composed of 

a Sutron 9210 data logger (Sutron Corporation, 2019); a 

Kipp & Zonen CNR4 net radiometer (Kipp & Zonen, 2019) 

to measure incoming and reflected solar radiation in 

both longwave and shortwave bands; a Vaisala HMP155 

humidity and temperature probe (Vaisala, 2019); three 

Hukseflux HFP01 soil heat flux plates (Hukseflux Thermal 

Sensors, 2016); three LI-COR 7900-180 soil temperature 

sensors (LI-COR Biosciences, 2019b); three Decagon ECH20 

EC-5 soil moisture sensors (Decagon Devices, 2019); and 

a NovaLynx 12-inch tipping bucket rain gage (NovaLynx 

Corporation, 2019). The LI-COR Biomet instruments 

measured data every 5 seconds; these data were averaged or 

summed (rainfall only) and recorded by the Sutron data logger 

every minute. The EC system also included a SMARTFlux 

system module to process raw EC data (.ghg format files 

which are LICOR Bioscience custom raw file type), 

computing fully corrected atmospheric fluxes of water 

vapor and energy by using the EddyPro software version 6.2 

(LICOR Biosciences, 2016).

The gas analyzer, sonic anemometer, radiometer, and 

humidity and temperature sensor at the grassland site were 

mounted on a 3-m tower at 2.6 m above the land surface and 

over a vegetation height of approximately 0.6 m.

The gas analyzer, sonic anemometer, radiometer, and 

humidity and temperature sensor at the huisache site were 

mounted on a 12-m tower at 8.4 m above the land surface and 

over a huisache canopy approximately 4.5 m in height. After 

the removal of huisache in January 2017, these instruments at 

the huisache site were lowered to a height of 2.8 m above the 

land surface and over existing vegetation with a canopy height 

of about 0.6 m. 

At the grassland site, the three sets of soil heat flux plates, 

soil moisture sensors, and soil temperature sensors were 

placed at locations within 10 m of the tower. At the huisache 

site, the three sets of soil heat flux plates, soil moisture 

sensors, and soil temperature sensors were placed at different 

locations with varied amount of sun and shade. At both sites, 

the ground sensors were buried at depths approximately 

5 centimeters (cm) below the land surface. The calibration of 

the soil moisture sensors was verified by using soil moisture 

data collected from near the location of the ground sensors. 

Soil cores with a diameter of approximately 2 cm and a 

length of about 8–10 cm were collected using a straight 

barrel sampler and then sealed in a glass container. The soil 

cores were then transported to the USGS Oklahoma-Texas 

Table 3. Instruments used to measure evapotranspiration, energy balance, and rainfall at eddy-covariance evapotranspiration sites, 

Victoria County, Texas.

Type of measurement Manufacturer Model number and 

instrument type Source

Evapotranspiration (water vapor 

concentration)

LI-COR Biosciences LI-7500A open path gas analyzer LI-COR Biosciences, 2019a

Evapotranspiration (turbulent 

fluctuations of horizontal and 

vertical wind)

Campbell Scientific, Inc., or 

Gill Instruments

CSAT3 three-dimensional sonic 

anemometer or WindMaster Pro 

sonic anemometer

Gill Instruments, 2019; Campbell 

Scientific, Inc., 2019

Air humidity/temperature Vaisala Corporation HMP155 humidity and temperature probeVaisala, 2019

Net radiation Kipp & Zonen CNR4 net radiometer Kipp & Zonen, 2019

Soil temperature LI-COR Biosciences Three 7900-180 soil temperature 

sensors

LI-COR Biosciences, 2019b

Soil moisture Decagon Devices Three ECH20 EC-5 soil moisture 

sensors

Decagon Devices, 2019

Soil heat flux Hukseflux Thermal Sensors Three HFP01 soil heat flux plates Hukseflux Thermal Sensors, 

2016

Rainfall NovaLynx Corporation 260-2500-12, 12-inch tippingbucket rain gageNovaLynx Corporation, 2019

Data processer and software LI-COR Biosciences SmartFlux system and EddyPro 

software, version 6.2

LI-COR Biosciences, 2016

Data logger Sutron Corporation 9210 data logger Sutron Corporation, 2019
12 Effects of Huisache Removal on Rangeland Evapotranspiration in Victoria County, South-Central Texas, 2015–18

Water Science Center office in San Antonio, Tex., where they 

were weighed, oven dried for 24 hours, reweighed, and soil 

moisture calculated and compared with the measured sensor 

data (Burt, 2014). 

The sites were visited periodically for maintenance and 

repairs. During the visits, the lenses of the radiometers and gas 

analyzers were inspected, cleaned, and rinsed with deionized 

water. At least annually, the gas analyzers were returned to 

LI-COR Biosciences for recalibration. Other sensors such 

as the humidity and temperature sensors and the soil sensors 

were checked to ensure operation and replaced if needed.

Data Processing

Evapotranspiration-related data collected by the various 

sensors were merged into self-contained, compressed .ghg 

files every half hour and were processed with the LI-COR 

SmartFlux module and LI-COR EddyPro software, version 

6.2 (LI-COR Biosciences, 2016). To operate correctly and to 

produce valid EC datasets, site-specific variables were entered 

into the LI-7500 SmartFlux module by using the LI-COR 

Greenhouse Gas (GHG) software interface. These variables 

included the height of the gas analyzer and sonic anemometer 

above the land surface, height of the vegetation canopy around 

the site, displacement distance of the gas analyzer relative to 

the sonic anemometer, and the sonic anemometer orientation 

from magnetic north. On the half hour, the EddyPro software 

would process the half hour of 10-Hz .ghg files into a 

30-minute average. At the end of the day, the files were 

merged into a single file containing the 30-minute averages 

for the previous 24 hours. The 10-Hz .ghg files, 30-minute 

EddyPro outputs, and the daily summary files were recorded 

to a universal serial bus (USB) drive for storage. At the end of 

each day, an office computer running the LI-COR file transfer 

program would connect with the LI-7500 system, through 

a cellular modem, and download the recorded file. EddyPro 

software was then used to reprocess the raw 10-Hz EC data 

(.ghg files) and to compute fully corrected water vapor and 

energy fluxes. 

To improve flux measurements, the EddyPro software 

applies several corrections and statistical tests to correct or 

filter the raw 10-Hz EC flux data. These corrections and tests 

include tilt correction (Wilczak and others, 2001), extracting 

turbulent fluctuations from time-series data (Gash and 

Culf, 1996), and compensation for possible time lags between 

anemometric variables and variables measured by the gas 

analyzer (Fan and others, 1990).

Soil heat flux (G) is calculated by using measurements 

obtained from the soil heat flux plates, soil temperature 

sensors, and soil moisture sensors buried near the tower. Three 

soil heat flux plates were buried at a depth of about 5 cm 

where the overlying soil is exposed to varying amounts of 

shade and sun. Soil moisture sensors measure the volumetric 

water content of the soil. The change in heat stored in the soil 

solids and soil water above the soil heat flux plates was added 

to the heat flux measured by the soil heat flux plates to obtain 

G (Campbell Scientific, Inc., 2016). A single value of G was 

estimated by averaging the three Hukseflux HFP01 soil heat 

flux estimates that included heat storage above the plates. Heat 

storage above each plate was calculated from measurements 

of soil bulk density, organic content, and soil temperature and 

volumetric soil moisture by using the calorimetric method 

(Liebethal and others, 2005). Soil temperature was measured 

by averaging the data from the three LI-COR 7900-180 

soil temperature sensors. Soil moisture was measured by 

averaging the data from the three Decagon ECH20 EC-5 soil 

moisture sensors. 

Quality Assurance of Energy Flux and 

Meteorological Data

Review of the measured energy flux data included net 

radiation (Rn), latent-heat flux (LE), sensible-heat flux (H), and 

G. Some loss of data occurred as a result of station power loss. 

Edits were also made to the data during site visits, primarily 

removal from the data file of erroneous sensor readings that 

could occur during instrument service and cleaning.

The 30-minute measurements of LE and H processed by 

the EddyPro software included a data quality “flag” based on 

Mauder and Foken (2006). Data-quality flag values of 0, 1, or 

2 were assigned, with a value of 0 indicating a measurement 

of high quality, a value of 1 indicating a measurement suitable 

for energy-budget analysis, and a value of 2 indicating a 

measurement of possible poor quality that should be reviewed 

and possibly not used. If a data-quality flag value was 2, 

the 30-minute LE and H flux data were generally deleted. 

LE and H data were also reviewed for outliers by following 

recommendations of the AmeriFlux network for data archiving 

(Law and others, 2005) and by comparing data from the 

two sites. Acceptable thresholds for both LE and H were 

set to between 675 and −50 watts per square meter (W/m2) 

for daytime and between 75 and −75 W/m2 for nighttime. 

Values above or below these thresholds were discarded. LE

and H were also inspected, and values considered spurious 

were rejected. Data that were rejected by using this approach 

were typically values measured during periods of rainfall 

or condensation. 

Meteorological data, including air temperature, air 

pressure, water vapor density, water vapor partial pressure, 

specific humidity, relative humidity, wind speed, and wind 

direction, also were reviewed to identify missing data or 

potentially spurious values (identified as individual, or a 

few values that were greater than 50 percent different from 

adjacent values when values at the other site did not exhibit 

the same trend). For missing, or potentially spurious, data 

intervals of less than 3 hours, the missing 30-minute values 

were replaced with linearly interpolated values, interpolated 

between the value immediately preceding and immediately 

following the problematic data interval. Missing data intervals 

of greater than 3 hours were not filled. 
Data-Collection Methods 13

Source Area of Measurements

The source area for EC turbulent-flux measurements 

is the dynamic upwind land-surface area contributing to 

measured water vapor and heat fluxes, whereas source areas 

for available-energy measurements are constant and depend 

on instrument placement. Thus, turbulent-flux source areas are 

larger and more variable than available-energy source areas 

(Garcia and others, 2015).

The area surrounding the EC stations that influences 

the measurements is referred to as the “fetch footprint” of 

the study sites. The fetch footprint is the area “seen” by 

the instruments on the tower, such that the water fluxes 

generated in this area are registered, or measured, by the tower 

instruments. Fetch is often reported as a distance from the 

tower when describing the footprint. The fetch also describes 

the land features such as ground cover, vegetation types, water 

bodies, and structures that influence airflow patterns and thus 

affect measured fluxes (Burba, 2013). 

The area of the flux footprint of each of the sites was 

estimated by using the EddyPro software with one of two 

flux models built into this software, either the Kljun model 

(Kljun and others, 2004) or the Kormann and Meixner model 

(Kormann and Meixner, 2001). The EddyPro software uses 

the Kljun model as the default. When values are outside of 

predefined ranges of atmospheric conditions of stability and 

turbulence, the EddyPro software automatically switches to 

the Kormann and Meixner model. Both models depend on 

the height of the instruments, height of the vegetation canopy, 

surface roughness, and stability of the atmosphere. 

The EddyPro software output provided an estimate of the 

measured turbulent-flux footprint as a set of peak distances 

from the instrument tower, expressed as a percentage (10, 30, 

50, 70, and 90 percent) of contributions to the total fluxes, 

and from the corresponding wind direction. A plot of the 

average daytime cumulative turbulent-flux contributions by 

distance from the instrument tower at each site for the pre- and 

post-removal periods is shown in figure 8.

By using the two flux models built into the EddyPro 

software, it was estimated that 90 percent of the measured 

atmospheric fluxes at the grassland site during daytime hours 

originated from within about 200 m of the instrument tower 

during the pre-removal period and from within about 150 m of 

the tower during the post-removal period (fig. 8); the EddyPro 

software provides a confidence interval of 95 percent for each 

of these estimates. For the huisache site, it was estimated that 

90 percent of the measured atmospheric fluxes originated from 

within 250 m of the instrument tower during the pre-removal 

period and within 200 m during the post-removal period, 

also at a confidence interval of 95 percent (fig. 8). During the 

pre-removal period, the vegetation within 250 m of the tower 

at each site was uniform grasses or huisache, respectively, 

and thus the fetch was considered to appropriately represent 

the grassland and huisache-dominated sites. During the 

post-removal period, the fetch at the huisache site appropriately 

represented an area where huisache had been removed. 

Evapotranspiration Data 

The 30-minute eddy-covariance evapotranspiration 

(ETEC) data were reviewed in several steps. If the LE value 

was discarded, the ETEC value also was discarded. For data 

gaps of less than 3 hours, the missing 30-minute ETEC values 

were replaced with values interpolated between the ETEC

0 50 100 150 200 250

Distance from instrument tower, in meters

0

10

20

30

40

50

60

70

80

90

100

Average daytime cumulative turbulent flux, in percent

EXPLANATION

Grassland site 

Pre-removal

Post-removal

Huisache site 

Pre-removal

Post-removal

Figure 8. Source-area contributions of average daytime cumulative turbulent flux (expressed as percentages 

of 10, 30, 50, 70, or 90) for the grassland and huisache evapotranspiration sites, Victoria County, Texas, before 

(pre-removal, March 2015–December 2016) and after (post-removal, February 2017–August 2018) huisache 

removal. 
14 Effects of Huisache Removal on Rangeland Evapotranspiration in Victoria County, South-Central Texas, 2015–18

value immediately preceding and the ETEC value immediately 

following the data gap. For data gaps of 3 hours or more 

occurring at night, missing values were set to zero. For data gaps 

of 3 hours or more occurring in daytime and during periods of 

rainfall, a time interval was selected at the peak of the rain event 

or in the middle of the rain event and set to zero; remaining 

missing values were then replaced with interpolated values. 

Daily ETEC values were totaled from the 30-minute ETEC

values for a 24-hour period. The daily total included ETEC

values that were estimated as described earlier. However, 

daily totals were not reported if more than 30 percent of 

the 30-minute ETEC values measured during the daytime 

period were missing or estimated. Daily ETEC totals were 

available for the grassland site from March 14, 2015, to 

August 31, 2018. Daily ETEC totals were available for the 

huisache site from March 13, 2015, to August 22, 2018. 

Table 4 summarizes the number of daily ETEC values 

collected at each site and also the number of days for which 

concurrent daily values were available (number of days where 

direct comparison of ETEC rates between the sites can be 

made). For the overall data-collection period, the daily-value 

Table 4. Number of daily evapotranspiration values collected at 

the grassland and huisache sites, Victoria County, Texas, March 

2015–August 2018.

[ET, evapotranspiration]

2015 2016 2017 2018 Total

Total daily ET values—

grassland site

274 287 304 232 1,097

Total daily ET values— 

huisache site

242 284 313 215 1,054

Days for which daily ET 

values were available for 

both sites

233 253 280 189 955

record is about 86 percent complete for the grassland site and 

82 percent complete for the huisache site. 

The effects of huisache removal were also evaluated 

by using estimates of evapotranspiration derived from 

remote-sensing-based surface energy-balance models (Allen 

and others, 2007; Senay, 2018). Remote-sensing-based 

estimates of evapotranspiration complemented station-based 

estimates of evapotranspiration made by the EC method and 

provided additional information to evaluate the spatial and 

temporal effects of huisache removal on evapotranspiration. 

Missing values were estimated to compare ETEC rates between 

the two sites for the pre-removal and post-removal periods 

and ETEC rates with evapotranspiration determined by remote 

sensing. The method used to estimate missing daily values 

is based on a relation between measured evapotranspiration 

and daily reference evapotranspiration (ETr), where ETr was 

computed using the American Society of Civil Engineers 

standardized reference evapotranspiration equation for a 

tall reference surface (Allen and others, 2005) and weather 

data acquired from gridMET (University of Idaho, 2019; 

Abatzoglou, 2013). 

Measured daily ETEC values were divided by ETr to 

calculate the fraction of reference evapotranspiration (ETrF). 

This calculated ETrF value was then used in creating a 7-day 

running average of ETrF. Days with missing ETEC data were 

filled by multiplying the daily total ETr by the gap-filled 

daily ETrF value derived from the 7-day moving average 

(centered on day 4) of ETrF. For gaps in station data greater 

than or equal to 7 days, the moving average of ETrF was used 

until the 7-day averaging window no longer contained two 

valid calculated ETrF values, at which point the remainder 

of the gap was bridged by linear interpolation. The resulting 

gap-filled dataset of daily ETEC values includes the period 

March 2015 through August 2018 for both sites. A time series 

of measured daily ETEC values is shown in figure 9.

Daily evapotranspiration, in millimeters

0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

2015 2016 2017 2018

Mar. 1

June 1

Sept. 1

Dec. 1

Mar. 1

June 1

Sept. 1

Dec. 1

Mar. 1

June 1

Sept. 1

Dec. 1

Mar. 1

June 1

Sept. 1

EXPLANATION

Grassland site Huisache site

Figure 9. Daily eddy-covariance evapotranspiration values at the grassland and huisache sites, Victoria County, Texas, 

March 2015–August 2018.
Data-Collection Methods 15

Daily totals of ETEC and daily average values of energy 

fluxes from the grassland and huisache sites are provided 

in a USGS data release (Slattery, 2020). The data release 

also includes 30-minute values of micrometeorological data 

collected at each station.

Energy-Balance Closure

Based on the principle of energy conservation, available 

energy (Rn – G in eq. 1; that is, the net radiation occurring 

on the landscape minus the soil heat flux and ignoring small 

changes in the heat stored by the vegetation) is equal to the 

turbulent flux (LE + H in eq. 1; that is, the latent-heat flux plus 

the sensible-heat flux). The conservation of energy requires 

that both sides of equation 1 balance. The simplified left 

side of equation 1 (Rn – G) is estimated from measurements 

of net radiation, soil heat flux, and soil heat storage via 

measurements of soil temperature and water content, whereas 

the right side of equation 1 (LE + H) is estimated by the EC 

method. The fundamental criterion of the conservation of 

energy is that the energy balance is satisfied, and therefore the 

available energy is equal to the turbulent flux. This concept 

commonly is referred to as “energy-balance closure” (Foken 

and others, 2012). The energy-balance closure is often 

evaluated using the energy-balance ratio (EBR), or ratio of the 

turbulent flux to available energy: 

EBR   ( ) LE H R / ( n  G) (7)

The EBR at the grassland and huisache sites were 

computed to assess energy-balance closure of the 

measurements of available energy (Rn – G) and turbulent fluxes 

(LE + H). Table 5 summarizes the average energy-balance data 

and the calculated EBR for the study period. 

Ideally, if all energy fluxes are accounted for, measured 

accurately (within the limits of measurement accuracy), and 

representative of energy source areas, then the EBR will 

equal 1, but studies have shown that EC-derived turbulent fluxes 

(LE + H) are consistently less than the available energy (Rn − G). 

Leuning and others (2012) and Foken (2008) studied the results 

of other investigators and reported EBR values ranging from 

0.39 to 1.69 at 22 EC sites. Typical EBR values range from 

0.6 to 1.0, but most frequently range from 0.7 to 0.8 (Moreo and 

others, 2017). Various theories have been advanced to explain 

the discrepancy between EC flux and available energy, including 

(1) lack of coincidence of the source areas among various flux 

components; (2) flux divergence arising from transport that is 

not one dimensional such as insufficient fetch; (3) measurement 

errors related to sensor separation, frequency response, alignment 

problems, and interference from tower or instrument-mounting 

structures (Twine and others, 2000); and (4) inaccurate 

accounting of heat storage (Leuning and others, 2012).

Because knowledge regarding sources of energy-balance 

discrepancies is limited, the decision of whether to force 

closure is often subjective (Garcia and others, 2015). For 

closure adjustments to be accurate, several assumptions are 

necessary, including (1) available-energy estimates are error 

free; (2) EC measurements and the application of accepted 

corrections and filters are accurate; (3) discrepancy between 

available energy and the turbulent flux can be attributed to 

turbulent-flux measurements, possibly caused by advective 

flux divergence with similar scalar ratios to those measured 

by the EC system; and (4) available-energy estimates are 

representative of the same source area as EC measurements 

(Garcia and others, 2015). 

For the grassland site, the average EBR was 0.87 during 

the overall study period (table 5). For the huisache site, the 

average EBR was 0.93 overall. However, there was some 

variation in the EBRs during the study, as shown in figure 10. 

During various periods of the study, the difference 

in EBRs between the sites changes. The reasons for these 

changes in EBRs are not known. Because of these differences 

in EBRs over time and between sites, measured ETEC values 

at both sites were adjusted with the application of a monthly 

closure factor to attempt to account for sensor or measurement 

biases previously described. Although the overall EBRs are 

similar at each site, the EBR at the grassland site is noticeably 

higher during 2017 than at the huisache site (fig. 10). 

For this study, similar to the approach by Garcia and 

others (2015), the measured turbulent fluxes were considered 

a probable minimum. A probable maximum was computed by 

dividing the monthly ETEC by the 3-month moving average 

EBR (fig. 10). The most probable estimate of the latent-heat 

(evaporative) flux for this study was considered to be the 

average of the probable minimum and probable maximum 

estimates. The most probable estimate is hereinafter referred 

to as “corrected,” and the resulting corrected monthly 

values of ETEC are referred to as “corrected eddy-covariance 

evapotranspiration” (ETC). Monthly values of ETC are 

available in Slattery (2020).

Table 5. Summary of average energy-balance data and calculated energy-balance ratio 

at the grassland and huisache evapotranspiration sites, March 2015–August 2018.

[Rn, net radiation; W/m2, watt per square meter; G, soil-heat flux; LE, latent-heat flux; H, sensible-heat 

flux; EBR, energy-balance ratio]

Rn

(W/m2)

G

(W/m2)

Available 

energy 

(W/m2)

LE

(W/m2)

H

(W/m2)

Turbulent 

flux 

(W/m2)

EBR

(unitless)

Grassland site 164 4.8 159 96.4 42.7 139 0.87

Huisache site 154 1.7 152 88.6 53.2 142 0.93
16 Effects of Huisache Removal on Rangeland Evapotranspiration in Victoria County, South-Central Texas, 2015–18

0.60

0.70

0.80

0.90

1.00

1.10

1.20

Energy-balance ratio

2015 2016 2017 2018

Mar. 1

June 1

Sept. 1

Dec. 1

Mar. 1

June 1

Sept. 1

Dec. 1

Mar. 1

June 1

Sept. 1

Dec. 1

Mar. 1

June 1

Sept. 1

EXPLANATION

Grassland site Huisache site

Figure 10. Three-month moving average of the monthly energy-balance ratio, grassland site and huisache site, Victoria 

County, Texas, March 2015–August 2018.

Remote Sensing of Evapotranspiration

Evapotranspiration estimates were derived from 

Landsat optical and thermal imagery by using the METRIC 

and SSEBop surface energy-balance models (Allen and 

others, 2007; Senay, 2018). These estimates were developed 

to both complement the EC data and provide additional 

information about how evapotranspiration rates differ between 

grassland and huisache sites at larger spatial scales. New 

Landsat imagery is available every 16 days from the Landsat 8 

satellite; two types of imagery are collected by Landsat 8, 

Optical Land Imager (OLI) imagery and Thermal Infrared 

Sensor (TIRS) imagery (National Aeronautics and Space 

Administration, 2019a). Landsat imagery acquired during the 

study period has a native spatial resolution of 30 m for optical 

channels and 60–100 m for thermal channels. Landsat imagery 

is also collected by the Landsat 7 satellite Enhanced Thematic 

Mapper Plus (ETM+) that, when combined with imagery from 

Landsat 8, provides Landsat imagery every 8 days (National 

Aeronautics and Space Administration, 2019b). Cloud cover 

during the time of image collection reduces the total number 

of usable images. The study area was contained entirely within 

path 26 row 40 of the Worldwide Reference System–2 used 

by the Landsat program (National Aeronautics and Space 

Administration, 2019c). Images used in the METRIC model 

were manually selected, with images being rejected when the 

study area was obscured by clouds or cloud shadows. 

Reference evapotranspiration, surface temperature, 

albedo, vegetation indices, and land cover were primary 

inputs for the METRIC model, whereas surface temperature, 

air temperature, and reference evapotranspiration were 

primary inputs for the SSEBop model. METRIC has a built-in 

calibration method that involves the selection of cold and hot 

calibration pixels that represent conditions where maximum 

evapotranspiration and minimum evapotranspiration are 

occurring, respectively; this built-in calibration method is 

referred to as Calibration Using Inverse Modeling at Extreme 

Conditions (CIMEC) (Allen and others, 2013). Iterative 

selection of cold and hot pixels and calibration of METRIC 

was done manually, with initial selections based on the results 

from the CIMEC automated calibration method (Allen and 

others, 2013; Western States Water Use Program, 2019). 

SSEBop has no calibration process and was executed on 

Google Earth Engine by using batch scripts and suggested 

properties and datasets outlined in Senay (2018).

Daily ETr was used in both the METRIC and SSEBop 

models to calculate ETrF (where ETrF = ET / ETr) for each 

Landsat image and to perform temporal interpolation of ETrF

and time integration of evapotranspiration between Landsat 

image acquisitions. ETrF is analogous to the commonly used 

crop coefficient in the agricultural engineering literature (Allen 

and others, 1996) but is representative of actual evaporative 

conditions instead of well-watered conditions inherent in the 

traditional crop coefficient approach.

Monthly and annual remotely sensed evapotranspiration 

values were estimated as 

ET ET RS i F ET n   m r i r i   ( ) 24 (8)

where 

ETRS is the cumulative evapotranspiration for a 

period beginning on day m and ending on 

day n, in millimeters; 

ETrFi is the fraction of daily reference 

evapotranspiration (ETr) for day i

(dimensionless); and

ETr 24i is the 24-hour daily reference 

evapotranspiration total, in millimeters per 

day for day i.
Effects of Huisache Removal on Evapotranspiration 17

Temporal per-pixel linear interpolation of ETrF in 

between satellite image dates was performed to estimate 

daily values of ETrF. Because the satellite imagery only 

provides data for the day the image was acquired, daily ETr

is multiplied by the daily interpolated ETrF. The use of ETr

accounts for daily and seasonal variations in atmospheric 

water demand, whereas ETrF accounts for plant phenological 

stages, stress, and management such as harvest, cuttings, and 

fallowing. A minimum of one cloud-free ETrF image per 

month is desired to track plant phenology and any changes 

in cover, but some months did not have cloud-free imagery. 

Therefore, linearly interpolated values of ETrF were used. 

ETr was derived by using spatially distributed 

meteorological variables from gridMET (Abatzoglou, 2013; 

University of Idaho, 2019) and applying the standardized 

American Society of Civil Engineers standardized 

Penmen-Monteith reference evapotranspiration equation 

to the dataset (Allen and others, 2005). Site-specific daily 

ETr was estimated by multiplying gridMET-derived ETr

by a bias-correction factor. The bias-correction factor was 

calculated as the ratio of ETr (determined from meteorological 

data collected from the grassland site) to gridMET ETr. The 

average bias-correction factor for the study period (that is, the 

sum of study period grassland site daily ETr divided by the 

sum of study period daily gridMET ETr) was 0.85.

Bias-corrected gridMET ETr was multiplied by METRIC 

and SSEBop per-pixel daily interpolated ETrF to develop

per-pixel daily evapotranspiration estimates for each site 

(eq. 8). Monthly values of remotely sensed evapotranspiration 

(ETRS) are available in Slattery (2020).

Effects of Huisache Removal on 

Evapotranspiration

The effect of huisache removal on evapotranspiration was 

evaluated by comparing the difference in evapotranspiration 

rates between the grassland and huisache sites during the 

pre-removal and post-removal periods. Differences in 

evapotranspiration between the sites were evaluated by 

analyzing monthly ETC and remote sensing evapotranspiration 

(ETRS) estimated by METRIC and SSEBop techniques. 

The differences in evapotranspiration rates between the two 

sites are considered estimates of the differences in the water 

available for surface runoff and groundwater recharge between 

the two sites, as well as possible increased water availability, 

that can be realized through huisache removal.

Table 6 summarizes annual rainfall and pre- and 

post-removal period ETC and ETRS (METRIC and SSEBop). 

Figure 11 shows a plot of monthly ETC and ETRS (METRIC 

and SSEBop) at the two sites over the study period. Monthly 

values of ETC and ETRS are available in Slattery (2020).

Table 6. Summary of rainfall and evapotranspiration totals for the grassland and huisache study sites, March 2015–August 2018.

[mm/mo, millimeter per month; ETC, corrected eddy-covariance evapotranspiration; ETRS, remotely sensed evapotranspiration; METRIC, Mapping 

Evapotranspiration With Internalized Calibration; SSEBop, Operational Simplified Surface Energy Balance]

2015 2016 2017 2018 Pre-removal Post-removal

(Mar.–Dec.) (Jan.–Dec.) (Jan.–Dec.) (Jan.–Aug.) Mar. 2015–

Dec. 2016

Feb. 2017–

Aug. 2018

Rainfall (mm/mo) 122.9 89.4 112.6 71.6 104.6 100.5

ETC, grassland site (mm/mo) 89.9 86.4 100.0 90.6 88.0 99.2

ETC, huisache site (mm/mo) 104.2 94.2 75.6 75.9 98.7 77.8

ETRS (METRIC), grassland site (mm/mo) 97.0 77.9 72.7 106.8 86.6 89.3

ETRS (METRIC), huisache site (mm/mo) 105.6 99.1 61.7 96.1 102.0 77.2

ETRS (SSEBop), grassland site (mm/mo) 97.4 79.5 61.0 68.7 87.6 66.2

ETRS (SSEBop), huisache site (mm/mo) 107.9 100.3 54.1 61.6 103.8 58.9

Average METRIC and SSEBop ETRS, 

grassland site (mm/mo)

97.2 78.7 66.9 87.7 87.1 77.8

Average METRIC and SSEBop ETRS, 

huisache site (mm/mo)

106.7 99.7 57.9 78.8 102.9 68.0

Average of eddy covariance and average 

ETRS, grassland site (mm/mo)

93.6 82.5 83.4 89.1 87.6 88.5

Average of eddy covariance and average 

ETRS, huisache site (mm/mo)

105.4 96.9 66.8 77.3 100.8 72.9
18 Effects of Huisache Removal on Rangeland Evapotranspiration in Victoria County, South-Central Texas, 2015–18 Monthly evapotranspiration, in millimeters Monthly evapotranspiration, in millimeters

0

20

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60

80

100

120

140

160

180

200

0

20

40

60

80

100

120

140

160

180

200

Mar.

Apr.

May

June

July

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Sept.

Oct.

Nov.

Dec.

Jan.

Feb.

Mar.

Apr.

May

June

July

Aug.

Sept.

Oct.

Nov.

Dec.

Jan.

Feb.

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June

July

Aug.

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Oct.

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Aug.

2015 2016 2017 2018

Mar.

Apr.

May

June

July

Aug.

Sept.

Oct.

Nov.

Dec.

Jan.

Feb.

Mar.

Apr.

May

June

July

Aug.

Sept.

Oct.

Nov.

Dec.

Jan.

Feb.

Mar.

Apr.

May

June

July

Aug.

Sept.

Oct.

Nov.

Dec.

Jan.

Feb.

Mar.

Apr.

May

June

July

Aug.

2015 2016 2017 2018

A. Grassland site

B. Huisache site

Evapotranspiration

Corrected eddy-covariance measurement

Remotely sensed estimate, by model

METRIC

SSEBop

EXPLANATION

Evapotranspiration

Corrected eddy-covariance measurement

Remotely sensed estimate, by model

METRIC

SSEBop

EXPLANATION

METRIC, Mapping Evapotranspiration With Internalized Calibration model (Allen and others, 2007)

SSEBop, Operational Simplified Surface Energy Balance model (Senay, 2018)

Figure 11. Comparison of corrected eddy-covariance monthly evapotranspiration and remotely sensed (METRIC and 

SSEBop) monthly evapotranspiration at A, the grassland site and B, the huisache site, Victoria County, Texas, March 

2015–August 2018.
Effects of Huisache Removal on Evapotranspiration 19

Figure 11 indicates good agreement between monthly 

ETC and ETRS estimates at both sites. For the overall study 

period ETC was greater than the average METRIC and 

SSEBop ETRS by about 11 percent at the grassland site and 

about 2 percent at the huisache site. The average grassland site 

ETC for the pre-removal period (March 2015–December 2016) 

was 88.0 millimeters per month (mm/mo), compared with 

87.1 mm/mo determined by the average of the remote-sensing 

methods (METRIC and SSEBop) (table 6). The average 

huisache site ETC for the pre-removal period was 98.7 mm/mo, 

compared with 102.9 mm/mo determined by the average 

of the remote-sensing methods (table 6). The pre-removal 

period evapotranspiration at the grassland site based on the 

average of ETC and the average ETRS was 87.6 mm/mo. The 

pre-removal period evapotranspiration at the huisache site 

based on the average of the ETC measurement and the average 

ETRS estimate was 100.8 mm/mo (table 6).

During the post-removal period (February 2017–

August 2018), the average grassland site ETC was 

99.2 mm/mo, compared with the average ETRS of 

77.8 mm/mo. The average huisache site ETC for the 

post-removal period was 77.8 mm/mo, compared with the 

average ETRS of 68.0 mm/mo (table 6). The post-removal 

period evapotranspiration at the grassland site based on 

the average of the ETC measurement and the average 

ETRS estimate was 88.5 mm/mo. The post-removal period 

evapotranspiration at the huisache site based on the average 

of the ETC measurement and the average ETRS estimate was 

72.9 mm/mo (table 6).

The monthly cumulative difference in evapotranspiration 

rates between the huisache and grassland sites (ET huisache – ET 

grassland) are shown in figure 12. The cumulative difference in 

evapotranspiration rates is shown for ETC, the average ETRS

(METRIC and SSEBop), and the average of ETC and the 

average ETRS. 

In figure 12, both evapotranspiration methods show 

that during 2015–16 (pre-removal period) the cumulative 

difference between huisache site evapotranspiration and 

grassland site evapotranspiration (ET huisache – ET grassland) is 

increasing—ET huisache is consistently greater than ET grassland. 

Around October–November 2016, the cumulative difference 

between evapotranspiration rates levels off, indicating that the 

evapotranspiration rates at the two sites are about the same, 

likely because the vegetation at both sites is, to some extent, 

entering dormancy and evapotranspiration rates at both sites 

are relatively low. During January 2017, huisache vegetation 

was removed at the huisache site. By mid-May 2017, the 

cumulative difference in evapotranspiration was observed 

to decrease, indicating that the evapotranspiration rate at the 

huisache site was less than that at the grassland site. From 

December 2017 to March 2018 (huisache dormant season), 

both methods show relatively little or no change in the 

cumulative difference in evapotranspiration. Starting around 

April 2018, the cumulative difference in evapotranspiration 

again decreases through the growing season until the end 

of August 2018, indicating that the evapotranspiration at 

the grassland site was greater than the evapotranspiration 

measured at the huisache site. 

Monthly cumulative difference in evapotranspiration

between the huisache and grassland sites, in millimeters 

−200

−150

−100

−50

0

50

100

150

200

250

300

350

400

Mar.

Apr.

May

June

July

Aug.

Sept.

Oct.

Nov.

Dec.

Jan.

Feb.

Mar.

Apr.

May

June

July

Aug.

Sept.

Oct.

Nov.

Dec.

Jan.

Feb.

Mar.

Apr.

May

June

July

Aug.

Sept.

Oct.

Nov.

Dec.

Jan.

Feb.

Mar.

Apr.

May

June

July

Aug.

2015 2016 2017 2018

EXPLANATION

Corrected eddy-covariance evapotranspiration

Average remotely sensed evapotranspiration

Average of corrected eddy-covariance evapotranspiration

and average remotely sensed evapotranspiration

Figure 12. Monthly cumulative difference in evapotranspiration between huisache and grassland sites, by eddycovariance and remotely sensed evapotranspiration methods, Victoria County, Texas, March 2015–August 2018.
20 Effects of Huisache Removal on Rangeland Evapotranspiration in Victoria County, South-Central Texas, 2015–18

Selected statistics for the monthly difference in 

evapotranspiration between the huisache site and the grassland 

site (EThuisache – ETgrassland) before and after huisache removal 

are listed in table 7. Statistics are provided for both ETC

and ETRS. The pre-removal period was March 2015 through 

December 2016. The post-removal period was February 2017 

through August 2018. Huisache vegetation was being removed 

from the huisache site during January 2017. So, January 2017 

data were excluded from the statistical analysis (table 7). 

During the pre-removal period, the trend of monthly 

ET huisache – ET grassland was similar for both the EC and RS 

methods. The average monthly ET huisache – ET grassland for the 

EC method during the pre-removal period was 10.7 mm/mo. 

The average monthly ET huisache – ET grassland for the RS 

method during the pre-removal period was 15.8 mm/mo. The 

95-percent confidence interval of the average difference was 

4.7 to 16.8 mm/mo for ETC and 9.7 to 21.9 mm/mo for ETRS

(table 7). 

During the post-removal period, the average monthly 

ET huisache – ET grassland (ETC) was −21.4 mm/mo. The average 

monthly ET huisache – ET grassland (ETRS) was −9.8 mm/mo. The 

95-percent confidence interval of the average difference 

was −30.8 to −12.0 mm/mo (ETC) and −15.2 to 

−4.3 mm/mo (ETRS) (table 7). For both evapotranspiration 

methods, one-sided, Kruskall-Wallis tests indicate 

statistically significant differences (alpha level of 0.05) in 

evapotranspiration rates (Helsel and Hirsch, 2002) between the 

pre-removal and post-removal periods. The p-values (Helsel 

and Hirsch, 2002) for the tests were less than 0.01.

It might be expected that after huisache removal the 

evapotranspiration rates at the huisache site would be similar 

to those at the grassland site because both sites were then 

grassland sites. After huisache removal, native grasses 

emerged very quickly at the huisache site, as can be seen 

from figures 5 and 7. However, the grass density and root 

development were likely at a less mature stage compared with 

the original grassland site. Therefore, the emergent grasses 

at the huisache site were not able to utilize as much water for 

evapotranspiration as those at the original grassland site. Also, 

the physical removal of huisache likely resulted in alteration of 

the soils, possibly affecting infiltration and runoff rates, which 

could affect evapotranspiration. As a result, evapotranspiration 

at the huisache site was actually lower than that at the 

grassland site during the post-removal period. Therefore, the 

pre-removal period likely provides the best characterization 

of the difference in evapotranspiration between the huisache 

and grassland sites. During the pre-removal period, huisache 

ETC was greater than grassland ETC by an average of about 

10.7 mm/mo as measured by EC method. As determined by 

the average of the METRIC and SSEBop remote-sensing 

techniques, huisache ETRS was greater than grassland ETRS by 

an average of 15.8 mm/mo during the pre-removal period. 

Rainfall was above average during each calendar 

year of the study, especially during the pre-removal period 

(Slattery, 2020); therefore, the data collected during the 

study might be inadequate to describe evapotranspiration 

differences between the huisache and grassland sites during 

extended dry periods. During some months of low rainfall 

(for example, July 2015, July 2016, and October 2016), 

monthly evapotranspiration exceeded monthly rainfall because 

residual soil moisture from previous months was available 

for evapotranspiration (Slattery, 2020). As a result, there 

is poor correlation between monthly rainfall and monthly 

differences in evapotranspiration between sites. During dry 

periods, evapotranspiration rates for both sites would likely 

be low because of lack of available water, and differences in 

evapotranspiration between sites could be small. 

One of the anticipated changes of huisache removal 

is increased water availability for groundwater recharge 

and surface-water runoff to streams because of reduced 

Table 7. Selected statistics for the monthly difference in evapotranspiration between the huisache site and grassland site 

(ET huisache – ET grassland) before and after huisache removal.

[SSEBop, Operational Simplified Surface Energy Balance; METRIC, Mapping Evapotranspiration With Internalized Calibration; pre-removal period is 

March 2015–December 2016; post-removal period is February 2017–August 2018; mm, millimeter]

Statistic

Corrected eddy-covariance 

evapotranspiration 

(ETC) 

Average of SSEBop and METRIC 

remotely sensed evapotranspiration 

(ETRS) 

Pre-removal Post-removal Pre-removal Post-removal

Number of monthly values 22 19 22 19

Sum (mm) 236.1 −407.0 347.3 −185.3

Average (mm) 10.7 −21.4 15.8 −9.8

Median (mm) 8.5 −19.9 12.5 −7.7

Standard deviation (mm) 13.6 19.5 13.7 11.2

Variance (mm) 184.7 379.7 188.3 126.4

Minimum (mm) −5.4 −67.1 −5.7 −29.7

Maximum (mm) 49.2 3.3 45.5 5.9

95-percent confidence interval about the average (mm) 4.7 to 16.8 −30.8 to −12.0 9.7 to 21.9 −15.2 to −4.3
Mapping Evapotranspiration by Remote Sensing 21

evapotranspiration. Groundwater recharge and surface-water 

runoff were not measured as part of this study, and the 

distribution of available water for groundwater recharge 

or surface-water runoff cannot be determined, but a simple 

estimate of the possible difference in available water for 

groundwater recharge and surface-water runoff between 

the huisache and grassland sites can be calculated as the 

difference in evapotranspiration rates between the sites 

(ET huisache − ET grassland) as shown in figure 12. The average 

monthly differences in evapotranspiration rates for pre- and 

post-removal periods, as calculated from ETC, average ETRS, 

and the average of the ETC and average ETRS, are summarized 

in table 8. 

During the pre-removal period, the average ET huisache

– ET grassland was 10.7 mm/mo, as estimated by the EC 

measurements. The average ET huisache – ET grassland was 

15.8 mm/mo, as estimated by the average of the METRIC and 

SSEBop remote-sensing techniques. The average monthly 

difference in evapotranspiration, determined by an average 

of the EC and RS methods, was 13.2 mm/mo (table 8). This 

value represents the best estimate of the increase in water 

available for groundwater recharge or surface-water runoff at 

the grassland site, compared to the huisache site. 

During the post-removal period, because the huisache 

evapotranspiration was consistently less than the grassland 

evapotranspiration, the trend is reversed, and compared 

with the huisache site, less water was available at the 

grassland site for groundwater recharge and surface-water 

runoff by an average of 15.6 mm/mo (table 8). Because 

the pre-removal period represents a period where the sites 

could be characterized as mature, undisturbed grassland 

and huisache environments, the pre-removal period likely 

provides the most reasonable estimate of long-term differences 

in evapotranspiration rates and potential for water savings 

through huisache removal. Also, because rainfall conditions

during the study were average to above average and did 

not include an extended dry period, the observed effect of 

huisache removal on evapotranspiration and water availability 

is possibly different during extended dry periods. 

Mapping Evapotranspiration by Remote 

Sensing

A map of remotely sensed annual evapotranspiration 

for 2015 for an approximately 400-km2 area surrounding the 

study area is shown in figure 13 (Slattery, 2020). The remotely 

sensed evapotranspiration shown is the average of model 

outputs from the METRIC and SSEBop techniques. The map 

demonstrates the capability of remote sensing to evaluate 

land-management effects on evapotranspiration for larger scale 

areas, such as a county or stream-basin area. 

Lower evapotranspiration rates are shown as yellow 

and orange cells that generally represent grassland, cropland, 

bare ground, and developed areas. Map cells of higher 

evapotranspiration rates tend toward shades of green and 

generally represent areas of denser vegetation, flood plains, 

and water bodies. The difference in evapotranspiration 

between the grassland site (yellow) and huisache site (green) 

during 2015 is apparent in figure 13. Evapotranspiration 

within the map extent ranges from 225 mm per year 

to 1,690 mm per year. The map resolution is 30 m by 

30 m, although the true resolution of the map is limited 

by the resolution of the Landsat thermal imagery data, 

which is 100 m by 100 m. The map illustrates relatively 

high-resolution evapotranspiration data, which can be 

used to analyze and evaluate land-management effects on 

evapotranspiration for large areas. 

Table 8. Average monthly difference between huisache site and grassland site evapotranspiration 

before huisache removal (pre-removal period) and after huisache removal (post-removal period) for 

eddy-covariance and remote-sensing estimates.

[ET, evapotranspiration; ETC, corrected eddy-covariance evapotranspiration; ETRS, remotely sensed evapotranspiration]

Differences in 

evapotranspiration 

Measurement 

type

Average monthly differences 

in evapotranspiration

Pre-removal 

Mar. 2015–

Dec. 2016

(millimeters 

per month)

Post-removal 

Feb. 2017–

Aug. 2018

(millimeters 

per month)

ET huisache – ET grassland ETC 10.7 −21.4

ET huisache – ET grassland ETRS 15.8 −9.8

ET huisache – ET grassland average of ETC and ETRS 13.2 −15.6
22 Effects of Huisache Removal on Rangeland Evapotranspiration in Victoria County, South-Central Texas, 2015–18

Guadalupe River Guadalupe River

VICTORIA

COUNTY CALHOUN

COUNTY

REFUGIO

COUNTY

Huisache

site

Grassland

site

97°05' 97°00' 96°55'

28°40'

28°35'

EXPLANATION

Remotely sensed evapotranspiration, in millimeters

1,690

225

Huisache Evapotranspiration site and identifier

site

High

Low

Base modified from U.S. Geological Survey 1:100,000 scale digital data

Texas South-Central State Plane coordinate system

North American Datum of 1983

0 1 2 3 4 KILOMETERS

0 1 2 MILES

Area enlarged

TEXAS

Figure 13. Remotely sensed 2015 annual evapotranspiration rates for the study sites and surrounding area, Victoria County, 

Texas.
Summary 23

Summary

Much of the historical oak-grassland landcover across 

Texas has undergone woody-plant encroachment attributed 

to fire suppression and overgrazing. One of the encroaching 

species is Vachellia farnesiana var. farnesiana (huisache), 

a woody plant that can grow 5–8 meters tall and uses an 

extensive root system to access both shallow soil water and 

deeper groundwater. Huisache is a prolific seed producer 

and can overtake a grassland prairie in a generation. Once 

established, it can readily outcompete other native grasses 

for space, often resulting in areas with diminished grazing 

benefits. Because of its prolific spread and potential water 

uptake, huisache removal could affect groundwater recharge 

and/or surface-water supply. 

By removing woody vegetation such as huisache and 

allowing native grasses to reestablish, the hydrologic budget of 

the watershed might change to increase groundwater recharge 

and surface-water supply. One method of documenting 

changes to the hydrologic budget resulting from differences in 

vegetative cover is by measuring evapotranspiration rates. 

The U.S. Geological Survey and the Desert Research 

Institute, in cooperation with various Federal, State, and 

local agencies, studied the effects of huisache removal on 

evapotranspiration and the hydrologic budget on rangeland 

in Victoria County in south-central Texas during 2015–18. 

Federal, State, and local agencies that cooperated on the study 

were the Natural Resources Conservation Service, Texas 

State Soil and Water Conservation Board, Victoria County 

Groundwater Conservation District, Victoria Soil and Water 

Conservation District, and the San Antonio River Authority.

Measurements of evapotranspiration, rainfall, and 

other related properties were collected at two sites during 

March 2015 through August 2018. One site was predominantly 

grassland. The other site was dominated by dense huisache 

vegetation that was removed about halfway through the 

study period. The resulting evapotranspiration datasets were 

examined for differences between the locations and differences 

between the pre-removal (2015–16) and post-removal 

(2017–18) periods to assess the effects of huisache removal 

on evapotranspiration. 

Evapotranspiration data were collected by two methods 

and used to evaluate differences in evapotranspiration 

between the grassland site and the huisache site. First, 

eddy-covariance stations were installed at each of the study 

sites, and evapotranspiration and energy-balance data were 

collected from March 2015 through August 2018. Second, 

remotely sensed data (satellite optical and thermal imagery) 

were processed to estimate monthly evapotranspiration by 

using the Mapping Evapotranspiration at High Resolution 

With Internalized Calibration (METRIC) and Operational 

Simplified Surface Energy Balance (SSEBop) models. 

After evapotranspiration data were collected at the 

grassland and huisache sites through December 2016, the 

huisache site was cleared of huisache in January 2017. 

Evapotranspiration data collection continued at both sites 

through August 2018.

Corrected eddy-covariance evapotranspiration (ETC) 

correlated well with remote sensing evapotranspiration (ETRS) 

on a monthly basis. However, for the overall study period, 

average ETEC was greater than the average ETRS by about 

11 percent at the grassland site and 2 percent at the huisache site. 

During the pre-removal period, evapotranspiration 

was greater at the huisache site than at the grassland site. 

Grassland site evapotranspiration (average of ETC and ETRS) 

was 87.6 millimeters per month (mm/mo), and huisache 

site evapotranspiration was 100.8 mm/mo. After huisache 

was removed from the huisache site in January 2016, 

the post-removal period (February 2017–August 2018) 

evapotranspiration was 88.5 mm/mo at the grassland site and 

72.9 mm/mo at the huisache site. 

The monthly differences in evapotranspiration between 

the grassland and huisache sites were compared between 

the pre-removal and post-removal periods and exhibited 

statistically significant differences at the 95 percent 

confidence level. The average monthly difference in huisache 

site evapotranspiration and grassland evapotranspiration 

(huisache site evapotranspiration minus grassland site 

evapotranspiration) during the pre-removal period was 

10.7 mm/mo (ETC) and 15.8 mm/mo (ETRS)—the huisache site 

exhibited greater evapotranspiration. During the post-removal 

period, the average monthly difference in evapotranspiration 

was −21.4 mm/mo (ETC) and −9.8 mm/mo (ETRS)—the 

huisache site evapotranspiration was less than grassland site 

evapotranspiration during this period. 

It might be expected that after huisache removal the 

evapotranspiration rates at the huisache site would be similar 

to those at the grassland site because both sites were then 

grassland sites. After huisache removal, native grasses 

emerged very quickly at the huisache site. However, the grass 

density and root development were likely at a less mature 

stage at the huisache site compared with the grassland site. 

Therefore, the emergent grasses at the huisache site were 

not able to utilize as much water for evapotranspiration as 

were the grasses at the grassland site. Also, the physical 

removal of huisache likely resulted in alteration of the soils, 

possibly affecting infiltration and runoff rates, which could 

affect evapotranspiration. As a result, evapotranspiration at 

the huisache site was actually lower than that at the grassland 

site after huisache removal. Therefore, the pre-removal 

period provides the best characterization of the difference in 

evapotranspiration between the huisache and grassland sites. 

During the pre-removal period, huisache ETC was greater 

than grassland ETC by an average of about 10.7 mm/mo 

as measured by EC method. As determined by the average 

of the METRIC and SSEBop remote-sensing techniques, 

huisache ETRS was greater than grassland ETRS by an average 

of 15.8 mm/mo during the pre-removal period. The average 

monthly difference in evapotranspiration, determined by an 

average of the ETC and ETRS methods, was 13.2 mm/mo. 

This value represents the best estimate of the difference in 

evapotranspiration between the grassland and huisache sites 

and potential difference in increased water available for 

surface-water runoff or groundwater recharge at the grassland 

site compared with the huisache site.
24 Effects of Huisache Removal on Rangeland Evapotranspiration in Victoria County, South-Central Texas, 2015–18

Rainfall conditions during the study were average to 

above average and did not include an extended dry period. As a 

result, differences in evapotranspiration between grassland and 

the huisache sites might be less pronounced during dry periods.

A map of remotely sensed annual evapotranspiration 

for 2015 was generated for an approximately 

400-square-kilometer area surrounding the study area. The 

map demonstrates the capability of remote sensing to evaluate 

land-management effects on evapotranspiration for larger scale 

areas, such as a county or stream-basin area.

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For more information about this publication, contact

Director, Oklahoma-Texas Water Science Center 

U.S. Geological Survey

1505 Ferguson Lane

Austin, TX 78754–4501

For additional information visit 

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Slattery and others— Effects of Huisache Removal on Rangeland Evapotranspiration in Victoria County, South-Central Texas—Scientific Investigations Report 2020–5008

ISSN 2328-0328 (online)

https://doi.org/10.3133/sir20205008