Prepared in cooperation with Fred Phillips Consulting

Literature-Reviewed Estimates of Riparian Consumptive 

Water Use in the Drylands of Northeast Arizona, USA 

Open-File Report 2020–1129

U.S. Department of the Interior

U.S. Geological Survey
Cover. Riparian trees along Comb Ridge, Navajo Nation. USGS photograph by Pamela L. Nagler.
Literature-Reviewed Estimates of Riparian Consumptive 

Water Use in the Drylands of Northeast Arizona, USA

By Pamela L. Nagler

Prepared in cooperation with Fred Phillips Consulting

Open-File Report 2020–1129

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

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Suggested citation:

Nagler, P.L., 2020, Literature reviewed estimates of riparian consumptive water use in the drylands of Northeast 

Arizona, USA: U.S. Geological Survey Open-File Report 2020–1129, 9 p., https://doi.org/10.3133/ofr20201129.

ISSN 2331-1258 (online)
iii

Acknowledgments

We would like to thank the reviewers of this report, Pradeep Wagle (U.S. Department of 

Agriculture) and Kul Khand (U.S. Geological Survey). We appreciate the data and map generated 

by our colleagues at the University of Arizona, Vegetation Index and Phenology Lab, Armando 

Barreto-Muñoz and Kamel Didan. 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. 
iv

Contents

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

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

Literature Review...........................................................................................................................................1

Conclusion.......................................................................................................................................................7

References Cited............................................................................................................................................7

Figure

1. Riparian river reaches on the lower Colorado River and its delta .......................................2

Tables

1. Tabulated potential and actual evapotranspiration for reference land cover types ........2

2. Evapotranspiration estimates for agricultural areas on the Lower Colorado River 

based on Moderate Resolution Imaging Spectroradiometer enhanced vegetation 

index from satellite sensors and ground measurements of potential 

evapotranspiration .......................................................................................................................3

3. Evapotranspiration estimates for agricultural areas on the Lower Colorado River 

based on Moderate Resolution Imaging Spectroradiometer enhanced vegetation 

index from satellite sensors and ground measurements of potential 

evapotranspiration .......................................................................................................................3

4. Phenology assessment metric evapotranspiration using Landsat-enhanced 

vegetation index (in millimeters per year) depicting year to year change from 2000 

to 2019 in seven riparian reaches and for the total riparian area of the Lower 

Colorado River delta.....................................................................................................................4

5. Phenology assessment metric evapotranspiration using Landsat-enhanced 

vegetation index depicting year to year change from the year 2000 to 2019 in five 

riparian reaches and for the total riparian area of the Lower Colorado River from 

Hoover Dam to Morelos Dam in the United States .................................................................5

6. Potential evapotranspiration, precipitation and actual evapotranspiration for natural 

vegetation, Atriplex canescens (Pursh) Nutt and Sarcobatus vermiculatus (Hook.) Torr, 

for a 318 hectare region on the Navajo Nation near Monument Valley for ten years 

between 2000 and 2010................................................................................................................6

7. The estimates of evapotranspiration, rainfall, net water requirement, area, and 

consumptive water use as reported for varying riparian vegetation density and 

potential evapotranspiration from various sites in Arizona as reported in 

the literature ..................................................................................................................................7
v

Abbreviations

ATCA2

ECOSTRESS

ET

ETa

ETo

EVI

METRIC

MODIS

SAVE4

SSEBop

USGS

Atriplex canescens (Pursh) Nutt

The ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station

evapotranspiration

actual evapotranspiration

potential evapotranspiration

enhanced vegetation index

mapping evapotranspiration with internalized calibration

Moderate Resolution Imaging Spectroradiometer

Sarcobatus vermiculatus (Hook.) Torr.

operational Simplified Surface Energy Balance

U.S. Geological Survey

Conversion Factors

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 0.4047 hectare (ha)

acre 0.004047 square kilometer (km2

)

Volume

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

)

acre-foot (acre-ft) 0.001233 cubic hectometer (hm3

) 

Flow rate

acre-foot per day (acre-ft/d) 0.01427 cubic meter per second (m3

/s)

acre-foot per year (acre-ft/yr) 1,233 cubic meter per year (m3

/yr)

million gallons per day (Mgal/d) 0.04381 cubic meter per second (m3

/s)

Literature-Reviewed Estimates of Riparian Consumptive 

Water Use in the Drylands of Northeast Arizona, USA 

By Pamela L. Nagler1

Abstract

This report provides the best estimates of riparian area 

evapotranspiration (ET) on the rivers and streams of the 

Navajo Nation by (1) quantifying the natural riparian vegetation water use within the Little Colorado River watershed 

using a literature search for comparable riparian ET estimates, 

and (2) in conjunction with the given area of stream-side plant 

cover on the Navajo Nation, provides the best estimate of 

consumptive use, the total water requirement (in acre-feet). 

This report includes riparian water use information only from 

the literature for riparian areas that are in similar dryland 

ecosystems in the Southwest, and not specific to the perennial 

tributaries and springs on the Navajo Nation within the Little 

Colorado River watershed. The report also includes any information found regarding the location of Navajo Nation weather 

station variables, such as where we can derive required data 

inputs from the Navajo Nation to estimate actual ET rates 

(in millimeters per day or millimeters per year). We provide 

estimates of annual riparian plant water use and calculations 

that include reference ET (potential ET or ETo), precipitation 

(in millimeters), and the calculations of consumptive water 

requirements of riparian vegetation. We cite our data sources 

and provide references used to determine the consumptive 

water requirement (acre-feet). 

Introduction

This U.S. Geological Survey (USGS) report provides an 

estimate of river and stream vegetation (riparian) water use 

within the Little Colorado River watershed in Arizona, and 

specifically provides an estimate of riparian plant area evapotranspiration (ET) using newly presented data and literature 

reported estimates of ET in similar dryland ecosystems such as 

those from other parts of Arizona as well as the Four Corners 

region of the southwestern United States.

Accurate estimates of natural plant area water use (ET, 

in millimeters per day [mm/day] and in millimeters per year 

[mm/year]) are important to acquire so that surface water, 

in-stream use can be partitioned for human and natural 

1

U.S. Geological Survey, Southwest Biological Science Center, Tucson, 

AZ, 85721 USA

environments. The grasses, shrubs, and trees that grow 

alongside rivers and streams are collectively called riparian 

vegetation and their leaves transpire water that is considered 

a loss to the ecosystem. Bare soil also loses water through 

evaporation. In the landscape, we quantify both losses as 

one variable, ET. A variety of users (land managers, water 

managers, water rights attorneys, research scientists, and 

so on) are interested not only in riparian plant ET, but also 

net water requirements or how much water is left after 

precipitation is subtracted from ET. To quantify the water 

requirement for a given area or the consumptive use in terms 

of acre-feet, the water requirement (after precipitation and 

soil moisture are subtracted from ET) is multiplied by the 

number of acres of the given riparian vegetation. This is 

normally quantified using an area measured by vectors on 

high-resolution, remotely sensed imagery and provides a 

measurement of acres of riparian zone vegetation. The riparian 

zone area multiplied by the net water requirement (ET minus 

precipitation and soil moisture storage) is the average annual 

total water requirement for an acre-foot.

Literature Review

Evapotranspiration in drylands consumes between 70 

and 90 percent of precipitation (Sun and others, 2019; Wilcox 

and others, 2017). Accurately measured riparian ET has rarely 

been included in water budgets because more accessible 

agricultural estimates have been used instead, such as crop 

coefficients. These thermal methods also include agriculturalcentered methods such as the Actual ET (ETa) that is produced 

using the operational Simplified Surface Energy Balance 

(SSEBop) model (USGS, 2015; Senay and others, 2011), 

mapping evapotranspiration with internalized calibration 

(METRIC; Allen and others, 2007), and more recently, the 

ECOsystem Spaceborne Thermal Radiometer Experiment on 

Space Station (ECOSTRESS; National Aeronautics and Space 

Administration, 2020). Water-use data describe (1) water that 

is withdrawn from a source (for example, groundwater or 

surface water and fresh or saline), (2) water that is delivered 

(for example, domestic homes), (3) water that is unavailable 

(for example, consumptive use and ET), and (4) water that 

is returned to a water resource (via for example, wastewater 
2 Literature-Reviewed Estimates of Riparian Consumptive Water Use in the Drylands of Northeast Arizona, USA

returns; Maupin and others, 2018). Along the Colorado 

River, water withdrawals are mainly for agriculture (78 

percent), but other uses exist, such as public supply, domestic 

use, commercial use, industrial use, livestock and mining, 

aquaculture, wastewater returns, and power. The portion taken 

by agriculture is in fact larger than 78 percent because much of 

the water withdrawal for crop irrigation is consumed via ET, 

and crops are not able to consume the entire portion of water 

applied. As a result, some form of return flow is provided, 

either as surface flow or groundwater recharge, which is then 

used by other downstream users.

All riparian plant water use is in reference and compared 

to potential ET (ETo), which is measured as a one-acre area 

completely covered by a foot-high, full-cover alfalfa or grass 

(Allen and others, 1998; Jensen, 1990). Weather stations report 

ETo because it is the maximum amount of water used by 

vegetation given the area’s weather conditions, which include 

temperature, vapor pressure deficit, and so on. These daily 

reports are the updated standard condition in near realtime. 

Based on the general definition surrounding consumptive 

use and measurement metrics (Water Education Foundation, 

2020), it is important to state some general findings from the 

literature that give values to alfalfa, grass, crops. and riparian 

areas. Remotely sensed optical data combined with weather 

station data has been a good method for estimating actual 

riparian ET (Nagler and others 2005a, b; Glenn and others 

2007, 2008a, 2010). We provide a list of water use estimates 

for the Lower Colorado River and delta (see Figure 1, the 

border between California and Arizona and Mexico) in table 1 

(Murray and others, 2009; Nagler and others, 2020).

The difficulty with providing natural plant ET estimates 

is that for decades, agricultural methods, such as crop 

Table 1. Tabulated potential and actual evapotranspiration, in 

millimeters per year (mm/year), for reference land cover types.

[Land Cover Source column lists general location and date of data. Values are 

from Murray and others (2009), Nagler and others (2020), and this study (for 

the Lower Colorado River, 2020)]

Figure 1. Riparian river reaches (R) on the lower Colorado River 

and its delta. Data for reaches in Mexico (R1–R7) are described in 

table 4. Data for reaches the United States (R3–R7) are described 

in table 5.

coefficients were used for riparian species and areas, but 

this method over-estimated riparian water use (Nagler and 

others, 2005a, b; Westenberg and others, 2006; Hunsaker 

and others 2007; Khand and others, 2017). Crop-coefficients 

were established in 1998 based on the best evidence 

available at the time (Allen and others, 1998; Jensen and 

others, 1990). However, subsequent ground and remotely 

sensed measurements of riparian ET have produced lower 

riparian water-use results compared to that from cropcoefficient estimations (Nagler and others, 2005a, b). From 

these remotely sensed measurements of actual ET that were 

validated with ground-based measurements in the Lower 

Colorado River riparian reaches, we produced the first 

accurate riparian water-use estimates for the Lower Colorado 

River (Nagler and others, 2005a, b, 2007, 2008a, b, 2009a, b; 

Murray and others, 2009). Based on a mean annual flow of 

1.8 million cubic meters (m3

) in the river, riparian vegetation 

consumes about 2.1 percent of the flow (Nagler and others, 

2009b). Tamarix spp. L. (saltcedar) dominated associations 

consume 1.0 percent of the flow, much lower than earlier 

estimates of riparian ET (Nagler and others, 2009b). The 

best estimates for ETo, Agricultural ET, and Riparian ET are 

provided in tables 2 and 3 (Murray and others, 2009).

Land Cover Type Land Cover Source Potential and Actual 

ET (mm/year)

Alfalfa HayDay Farms, 

2009

2,146

Grass Lower Colorado 

River, 2009

2,036

Crops Lower Colorado 

River, 2009

1,300–1,900

Riparian All Species Lower Colorado 

River, 2009

447–1,155

Riparian Gallery 

Trees

Lower Colorado 

River, 2009

1,123

Riparian All Species Lower Colorado 

River, 2020

777–1,163

Riparian All Species Colorado River 

Delta, 2020

654–1,130
Literature Review 3

Data from table 2 shows that ETo mean use is 

approximately 2,146–2,036 mm/year and uses three-times 

more water than riparian species, and ET from crops (mean 

use 1,486 mm/year) uses two-times more water than riparian 

estimates (mean of 854 mm/year). Recent findings in Nagler 

and others (2020) show ETo from the Arizona Meteorological 

Network (AZMET) Yuma Valley station to be 1,994.36 mm/

year average over the past 20 years, with range of 1,854 to 

2,142 mm/year (AZMET, 2020). Our estimates are based on 

an ET algorithm that incorporates Enhanced Vegetation Index 

(EVI) from the MODerate Resolution Imaging Spectrometer 

(MODIS) from Nagler and others (2005; 2013): 

ET (MODIS)= ETo (daily) × 1.65 (1-e−2.25EVI) −0.169 (1)

where

ETo potential evapotranspiration (ET) from Yuma 

AZMET; and

EVI enhanced vegetation index.

The enhanced vegetation index (EVI) was calculated as 

follows (Huete and others, 2002, 2010):

Table 2. Evapotranspiration (ET) estimates (in millimeters per year) for agricultural areas on the Lower Colorado River based on 

Moderate Resolution Imaging Spectroradiometer (MODIS) Enhanced Vegetation Index from satellite sensors and ground measurements 

of potential evapotranspiration (ETo) (Murray and others, 2009).

[ETo estimates are listed from three irrigation districts Mohave (MID), Palo Verde (PVID), Yuma (YID), and a privately owned farm (HayDay) along the 

Colorado River. Crops refer to ET estimates for specific irrigation districts. Mean and Standard Error (SE) are provided in the last row]

Year MID ETo PVID ETo YID ETo HayDay Farms MID crops PVID crops YID crops

2000 2,177 2,217 2,118 2,495 1,238 1,842 1,242

2001 2,062 2,062 1,920 2,234 1,172 1,888 1,327

2002 2,060 2,060 2,045 1,713 1,187 2,024 1,327

2003 1,885 1,877 1,941 1,580 1,317 1,803 1,333

2004 1,930 1,930 1,887 1,588 1,208 1,844 1,246

2005 2,019 1,951 1,854 omitted 1,347 1,622 1,286

2006 2,019 2,019 2,035 2,593 1,479 1,566 1,329

2007 2,108 2,108 1,999 2,612 1,373 1,886 1,300

2008 2,096 2,096 2,096 2,353 1,484 1,962 1,316

Mean (SE) 2,040 (30) 2,036 (35) 1,988 (31) 2,146 (159) 1,312 (40) 1,846 (29) 1,301 (12)

Table 3. Evapotranspiration (ET) estimates (in millimeters per year) for agricultural areas on the Lower Colorado River based on 

Moderate Resolution Imaging Spectroradiometer (MODIS) Enhanced Vegetation Index from satellite sensors and ground measurements 

of potential evapotranspiration (ETo) (Murray and others, 2009).

[Table headings represent ET values of riparian vegetation for the referenced year north of the Mojave Irrigation District riparian vegetation (MID-Rip); Havasu 

National Wildlife Refuge (HNWR), Bill Williams National Wildlife Refuge (BWNWR), Cibola National Wildlife Refuge (CNWR), Imperial National Wildlife 

Refuge (INWR), Mittry Lake Wildlife Area (MLWA) and the confluence of the Colorado and Gila rivers at Yuma, Arizona (CRG). Mean and Standard Error 

(SE) are provided in the last row]

Year MID_Rip HNWR BWNWR CNWR INWR MLWA CRG

2000 324 1,036 1,447 1,005 1,305 1,083 1,048

2001 437 1,019 1,172 961 1,211 969 953

2002 391 1,005 1,197 834 1,120 902 912

2003 448 1,004 1,056 876 1,141 901 941

2004 415 962 645 833 1,042 806 840

2005 619 963 943 904 1,060 713 778

2006 458 951 1,053 699 1,144 848 632

2007 446 1,002 1,120 608 1,129 801 542

2008 483 1,016 1,173 710 1,242 857 686

Mean (SE) 447 (26) 741 (16) 1,123 (51) 825 (43) 1,155 (28) 875 (36) 815 (56)
4 Literature-Reviewed Estimates of Riparian Consumptive Water Use in the Drylands of Northeast Arizona, USA

EVI = G (pNIR − pRed) / 

(pNIR + C1 × pRed − C2 × pBlue + L) (2)

where

G the gain factor (set at 2.5);

C1 correction for aerosol resistance (set at 6 );

C2 correction for aerosol resistance (set at 7.5);

L adjusts for canopy background (set at 1);

pNIR reflectance in the near infrared wavelength;

pRED reflectance in the red wavelength; and

pBlue reflectance in the blue wavelength.

Equations 1 and 2 were used by Jarchow and others 

(2017a, b) to estimate water use in the Lower Colorado River 

with values of ETo ranging from 1,996 to 2,036 mm/year and 

actual ET (ETa) from two different vegetation indices ranging 

from 75 mm to 108 mm/year for 2013, before additional 

surface water was added in 2014. After adding water, ET 

increased to 89–112 mm/year in 2014 (Jarchow and others, 

2017a). For this same region, they report ET between 10 and 

34 million m3 per year (Jarchow and others, 2017b). Jarchow 

and others (2018) adapted an algorithm based on eqs. 1 and 

2 for use with high-resolution (30 meter pixel size) Landsat 

data because riparian corridors are often very narrow, and 

MODIS lacks the ability to accurately predict ET for riparian 

areas based on its pixel size of 250 meters (m). Using this 

modified equation, recent work by Nagler and others (2020) 

show annualized differences in ET in mm/year (using Landsat 

EVI for the delta [table 4] and the Lower Colorado River 

between Hoover and Morelos Dams [table 5]) for 20 years 

covering seven reaches on the lower Colorado River delta 

and five reaches on the U.S. side of the river (fig. 1). Before 

the recent drought, water use for the riparian zone was about 

1 mm/day or 1,130 mm/year (year 2000), however, it is now 

as low as 0.8 mm/day or 654 mm/year (year 2019)—a loss in 

20-years of 476 mm/year or 1.61 mm/day in the delta (Nagler 

and others, 2020). The peak or growing season ET (mm/day) 

has also dropped over the 20-year period from 4.11 mm/day 

(2000) to 2.53 mm/day (2019) in the delta. The values for ET 

over the same 20-year period on the U.S. side of the river were 

similar to the findings in the delta. The annualized ET for the 

riparian zone on the U.S. side of the Lower Colorado River 

was 1,163 mm/year (year 2000) and is now 777 mm/year 

(year 2019), a loss in 20-years of 386 mm/year or 1.59 mm/day 

Table 4. Phenology assessment metric evapotranspiration (ET) using Landsat-enhanced vegetation index (in millimeters per year) 

depicting year to year change from 2000 to 2019 in seven riparian reaches and for the total riparian area (All) of the Lower Colorado 

River delta (from Nagler and others, 2020). 

[Average (AVE) and standard deviation (STDEV) are shown in the final two rows]

Year Reach 1 Reach 2 Reach 3 Reach 4 Reach 5 Reach 6 Reach 7 All

2000 1,041.2 975.5 1,058.3 1,220.4 1,379.2 1,178.9 1,058.1 1,130.2

2001 979.6 899.3 925.9 1,184.5 1,195.2 1,059.3 971.4 1,030.7

2002 1,015.8 936.2 979.9 1,255.3 1,231.6 1,082.0 947.1 1,064.0

2003 840.0 779.5 799.2 1,065.0 1,062.4 915.4 811.9 896.2

2004 780.3 700.5 719.2 953.0 937.7 813.2 741.8 806.5

2005 869.1 766.7 813.6 1,070.1 1,000.9 795.4 786.7 871.8

2006 915.3 781.2 804.6 1,109.0 1,041.2 849.9 781.3 897.5

2007 818.5 692.5 676.9 1,039.1 939.7 778.2 696.7 805.9

2008 886.4 705.1 680.0 1,173.6 1,059.5 897.3 792.5 884.9

2009 876.1 706.2 671.9 1,121.8 1,026.0 860.4 772.7 862.1

2010 988.2 790.0 778.1 1,154.7 1,132.6 939.2 834.7 945.4

2011 992.5 786.4 759.9 1,080.4 1,006.0 904.1 814.9 906.3

2012 835.0 602.5 588.9 1,056.6 929.5 768.5 685.1 780.9

2013 818.2 602.6 559.7 997.7 879.7 710.1 588.3 736.6

2014 999.8 770.1 674.3 1,117.9 1,000.6 833.9 643.5 862.9

2015 885.5 689.5 575.1 1,051.1 895.6 783.3 540.8 774.4

2016 816.9 666.3 549.2 1,035.8 787.9 702.7 454.0 716.1

2017 733.6 565.9 514.6 1,044.9 799.5 702.4 484.8 692.3

2018 691.5 539.1 493.5 945.5 734.4 643.6 444.7 641.7

2019 655.7 523.2 505.2 970.1 755.6 660.0 509.5 654.2

AVE 872.0 723.9 706.4 1,082.3 989.7 843.9 718.0 848.0

STDEV 109.0 124.2 160.3 85.6 164.5 142.4 174.7 131.2
Literature Review 5

Table 5. Phenology assessment metric evapotranspiration using Landsat-enhanced vegetation index (in millimeters per year) 

depicting year to year change from the year 2000 to 2019 in five riparian reaches and for the total riparian area (All) of the Lower 

Colorado River from Hoover Dam to Morelos Dam in the United States.

[Average (AVE) and standard deviation (STDEV) are shown in the final two rows]

Year Reach 3 Reach 4 Reach 5 Reach 6 Reach 7 All

2000 1,152.46 1,039.13 1,042.15 1,362.09 1,219.39 1,163.04

2001 1,060.54 969.88 1,010.93 ,1251.79 1,040.44 1,066.72

2002 1,204.82 1192.94 1,100.23 1,312.58 1,205.28 1,203.17

2003 1,065.54 969.64 884.77 1,178.69 964.01 1,012.53

2004 961.94 862.80 818.60 1,117.92 959.40 944.13

2005 1,144.67 1,122.55 1,022.95 1,277.23 1,015.84 1,116.65

2006 1,045.21 935.57 879.69 1,143.42 994.26 999.63

2007 914.14 775.63 771.07 950.22 873.05 856.82

2008 1,078.36 969.70 937.36 1,178.98 897.21 1,012.32

2009 1,105.82 979.64 916.57 1,067.73 984.99 1,010.95

2010 981.73 905.81 876.41 990.97 937.36 938.46

2011 897.96 859.10 776.81 932.31 838.39 860.92

2012 805.86 771.81 719.66 868.10 810.37 795.16

2013 919.02 918.44 820.68 914.67 867.51 888.06

2014 1,107.57 1,011.68 929.98 1,143.68 1,038.86 1,046.36

2015 896.62 868.28 781.72 1,005.67 886.24 887.70

2016 732.20 760.42 700.62 886.75 796.73 775.35

2017 797.89 925.15 891.19 1,029.75 922.12 913.22

2018 743.43 838.78 768.16 897.36 889.70 827.49

2019 742.09 797.36 737.58 787.73 819.20 776.79

AVE 967.89 923.72 869.36 1,064.88 948.02 954.77

STDEV 149.36 114.45 114.14 163.79 116.58 125.45

(table 1). The peak or growing season ET (mm/day) has also 

dropped on the U.S. side over the 20-year period, from 

4.79 mm/day (2000) to 3.18 mm/day (2019). These ET estmates are in line with the results (1,100 mm/year) found a 

decade earlier for the same region (Nagler and others, 2008). 

Jarchow and others (2020) produced estimates of ET for 

a riparian river reach on the San Juan River in New Mexico 

and showed ET ranged from 280 and 620 mm/year between 

2000 and 2018. These values are slightly lower than those in 

the delta of the Lower Colorado River because the dominant 

riparian species, saltcedar has been affected somewhat by Diorhabda Spp. (leaf beetle), whereas in the delta, the leaf beetle 

has not yet been found and therefore does not yet affect the 

water use of the riparian zone which contains saltcedar. 

Also on the Navajo Nation, near Monument Valley, 

projected measured transpiration from sap flow of two main 

dryland species, Atriplex canescens (Pursh) Nutt (ATCA2) 

and Sarcobatus vermiculatus (Hook.) Torr (SAVE4), was projected over the area for a site of 318 hectares (ha) near Comb 

Ridge by Glenn and others (2008). The average transpiration 

was 1.54 mm/day, ranging between 1.0 and 1.9 mm/day and 

116 mm/year, and ranged between 73 and 163 mm/year for 

years between 2000 and 2007 (Glenn and others, 2008b).Bresloff and others (2013) reported ETo, precipitation, and ET (in 

mm/year) for the same site and dryland species in Monument 

Valley (table 6). Their study reported that mean ET was 137 

mm/year between 2000 and 2004 and 186 mm/year be-tween 

2005 and 2010, mean precipitation was 139 to 

166 mm/year, and ETo was between 1,504 mm/year (2000–

2004) and 1,442 mm/year (2005–2010) (Bresloff and others, 

2013). They reported peak ET of native species was observed 

in July and ranged between approximately 1.18 and 1.52 mm/

day. Values of ET over 10 years ranged from 75 mm/year in 

2002 to 240 mm/year in 2010 and somewhat followed precipitation patterns (Bresloff and others, 2013).

ET for upland species on the Navajo Nation was much 

lower than riparian ET, which was approximately 150 mm/

year (Bresloff and others, 2013). We are citing this range 

between 137 and 186 mm/year from Bresloff and others (2013) 

as notable but we are not using it in our estimates as this was 

not based on riparian species, but upland species. For riparian 

species on Navajo Nation at Shiprock, New Mexico, estimates 

of actual ET ranged between 280 and 620 mm/year (Jarchow 

and others, 2020). Estimates from the literature for the riparian 
6 Literature-Reviewed Estimates of Riparian Consumptive Water Use in the Drylands of Northeast Arizona, USA

Table 6. Potential evapotranspiration (ETo), precipitation and actual evapotranspiration (ETa) for natural vegetation, Atriplex canescens 

(Pursh) Nutt (ATCA2) and Sarcobatus vermiculatus (Hook.) Torr (SAVE4), for a 318 hectare (ha) region on the Navajo Nation near 

Monument Valley for ten years between 2000 and 2010, with mean and (standard error [SE]) provided after five and six years.

[Data from Bresloff and others, 2013. Abbreviations: ha, hectare; mm/yr, millimeter per year]

Year ETo (mm/year) Precipitation 

(mm/year)

ETa Natural 

Vegetation

Inside Fence

ETa Natural

Vegetation

Outside SAVE4

ETa Natural

Vegetation

Outside ATCA2

ETa Natural

Whole Site, 318 ha

(mm/year)

2000 1573 144 (29) 189 146 123 144

2001 1499 142 (39) 145 149 122 136

2002 1482 148 (8) 13 99 99 90

2003 1508 112 (27) 183 191 147 169

2004 1461 151 (42) 185 159 129 146

Mean (SE) 1504(19) 139 (7) 161 (17) 148 (15) 124 (8) 137 (13)

2005 1463 217 (37) 282 220 196 195

2006 1452 78 (39) 206 157 110 143

2007 1465 167 306 235 199 200

2008 1421 193 259 248 160 162

2009 1432 107 193 184 114 150

2010 1419 234 310 356 242 268

Mean (SE) 1442(8) 166 (25) 259 (20) 233 (28) 170 (21) 186 (19)

vegetation on the Lower Colorado River (this study) and delta 

(Nagler and others, 2020) have a mean of 777 mm/year and 

654 mm/year, respectively, in the most recent year of 2019. 

The average 20-year mean values for the Lower Colorado 

River (this study) and delta (Nagler and others, 2020) were 

slightly higher, 955 mm/year and 848 mm/year, respectively. 

However, due to the declining trends in riparian vegetation, we 

used the most current year in our estimates. Thus, to calculate 

consumptive water use, we use the upper range values from 

Jarchow and others (2020) for the San Juan riparian vegetation, 620

mm/year, as well as the entire region of riparian 

vegetation from 2019 using the Lower Colorado River value 

of 777mm/year (this study) and delta value of 654 mm/

year (Nagler and others, 2020). An average of 684 mm/year 

was used as the mean value in our literature-based estimates 

from the Lower Colorado River data. This value converted to 

inches is 26.93inches or 2.24 feet; whereas Natural Resources 

Consulting Engineers (2017) reported 52.2 inches or 4.35 feet 

of actual ET for riparian vegetation cover and nearly 82 inches 

or 6.83feet for ETo. We then subtract 6.06 inches (average 

effective precipitation from Bresloff and others, 2013) from our 

value of 26.93 inches of actual plant ET, which results in a net 

water requirement value of 20.87 inches (1.74 feet). We multiply this by the total area of 14,598 acres, the region of interest 

within the Little Colorado River watershed on the Navajo 

Nation, to estimate the average annual total water requirement 

in acre-feet (and assuming negligible soil moisture change, 

this will also estimate the average annual depletion in acrefeet). We then multiply this area by 1.74 feet to determine the 

net water requirement, which is 25,387—the average annual 

depletion in acre-feet for the region of interest within the Little 

Colorado River watershed. 

Bresloff and others (2013) reported an ETo in this region 

of 1,473 mm/year or 58 inches or 4.83 feet/year as maximum 

water use. After this research and the reported 6 inches in 

rainfall over 11 years, we calculate 52 inches or 4.33 feet/

year as the maximum net water requirement. Using an area 

of 14,598acres multiplied by 4.33 feet for average riparian 

vegetation water use yields a maximum consumptive water 

use (using ETo), for the site on the Colorado Plateau that is 

near Comb Ridge on the Navajo Nation, equal to 63,258 acrefeet. This value is a literature-based estimate that reflects large 

averages across river reaches on the Lower Colorado River 

(Murray and others, 2009) containing a range of riparian 

species and cover by species. If the region of interest on the 

Navajo Nation was entire gallery tree species of cottonwoods 

and willows, for the entire 14,598 acreage, ET would be 

1,123 mm/year for these gallery trees or 44.20 inches. 

Subtracting mean rainfall of 6.06 inches yields a net water 

requirement of 38.14 inches or 3.18 feet. These calculations 

result in an estimation from the literature values of a consumptive water use of 14,598 acres times 3.18 feet of gallery forest water use yield a consumptive water use for trees 

of 46,397acre-feet. Potential ET using data from Bresloff 

and others (2013) results in an ETo of only 1,473 mm/year, 

about 75 percent of the Murray and others (2009) estimate of 

2,036mm/year from the study on the Lower Colorado River. 

Using Murray and others (2009), we applied the average of the 

three irrigation districts that reported ETo of 2,021 mm/year. 

This is reported as ETo Lower Colorado River (table 7). 
References Cited 7

Table 7. The estimates of evapotranspiration (ET), rainfall, net water requirement, area, and consumptive water use (acre-feet) as 

reported for varying riparian vegetation density and potential ET from various sites in Arizona as reported in the literature (Murray and 

others, 2009; Jarchow and others, 2017a, b, 2020; Natural Resources Consulting Engineers [NRCE], 2017; Nagler and others, 2020).

[Abbreviations: AVG, average; ETo, potential evapotranspiration; ft, feet; ft/year, feet per year; in/year, inch per year; mm/year, millimeter per year]

ET 

(mm/year)

ET 

(in/ year)

Rainfall (in/year)*

Bresloff and others 

(2013)

Net Water Requirement 

(in/year)

 (no soil moisture change)

ET

(ft/ year)

Net Water 

Requirement 

(ft)

Area:

14,598 acres

Consumptive 

water use 

(acre-ft)

AVG 2019 Cover San Juan River and Lower Colorado and Delta Reach Level

684 26.93 6.06 20.87 2.24 1.74 14,598 25,387

AVG 2000–2019 Cover Lower Colorado and Delta Reach Level

808 31.81 6.06 25.75 2.65 2.15 14,598 31,325

Riparian Gallery Trees Only

1,123 44.21 6.06 38.14 3.68 3.18 14,598 46,397

Navajo Nation ETo

1,473 57.99 6.06 51.93 4.83 4.33 14,598 63,258

Lower Colorado River ETo

2,021 79.57 6.06 73.51 6.63 6.13 14,598 89,486

NRCE Report

1,273 50.1 5.10 45.0 4.18 3.75 26.2 98.4

NRCE ETo

2,080 81.9 8.1 73.8 6.83 6.15 --- 108.2

Conclusion

Using literature estimates and newly presented data from 

this study, we calculate the net water requirements based on 

an ET for riparian vegetation between average species cover 

and entirely gallery trees between 20.87 and 38.14 inches or 

1.74 and 3.18 feet. We project consumptive water use to be 

between 25,387 and 46,397 acre-feet for the riparian vegetation along the streams in the area of interest within the Little 

Colorado River watershed on Navajo Nation. We report a 

range of potential ET as well. The estimates from a study 

done on the Navajo Nation by Bresloff and others (2013), 

Jarchow and others (2020) and Natural Resources Consulting 

Engineers (2017) were the closest in geographical proximity. 

Several studies with values for both ET and ETo that were 

larger than in Bresloff and others (2013) were not in the region 

(see the Lower Colorado River studies by Murray and others, 

2009; Jarchow and others, 2017a, b; Nagler and others, 2020).

The range reported for ETo is between 1,473 and 2,080 mm/

year, and a reported net water requirement is between 4.33 and 

6.15 feet.

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Menlo/Moffett Publishing Service Center, California 

Manuscript approved for publication November, 3, 2020

Edited by John Mark Brigham

Layout and design by Kimber Petersen
ISSN 2331-1258 (online)

Nagler—Literature-Reviewed Estimates of Riparian Consumptive Water Use in the Drylands of Northeast Arizona, USA—Open-File Report 2020–1129 November 2020