U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2006–3027

May 2006 Printed on recycled paper

Prepared in cooperation with the

BUREAU OF LAND MANAGEMENT

Hydrologic Requirements of and Evapotranspiration by

Riparian Vegetation along the San Pedro River, Arizona

In 1988, Congress designated portions of the San Pedro River in 

the Upper San Pedro Basin of Cochise County, Arizona, as the San 

Pedro Riparian National Conservation Area (SPRNCA; fig. 1). This 

conservation area is home to an ecologically diverse and important 

riparian system. The basin also hosts growing populations of residents 

in areas hydraulically upgradient from sensitive reaches of the riparian 

system. The largest communities are Sierra Vista, Bisbee, Tombstone, 

Huachuca City, and the U.S. Army’s Fort Huachuca. The total 

population of these communities and surrounding rural developments 

is about 75,625 (Arizona Department of Economic Security, 2003). 

Residents of several local communities have responded to the need for 

water planning by forming the Upper San Pedro Partnership (USPP), 

a consortium of 21 agencies and organizations with a primary goal of 

ensuring long-term water needs are met, both for the area residents 

and for the riparian vegetation and wildlife along the river within 

the SPRNCA. As part of the overall planning strategy, the USPP 

assembled a multidisciplinary team of researchers and began a study to 

characterize the water needs of the SPRNCA’s riparian system.

The characterization of riparian water needs was done by 

determining the (1) annual water-use rates for riparian species and

open-water evaporation, (2) areal distribution of riparian species and, (3) 

relation between hydrologic factors, water-use rates, and the distribution

of riparian species. These characterizations not only helped define the 

current water needs for the SPRNCA’s riparian system, but also provided 

information about how the ecological structure of the system and its 

water needs could change under different hydrologic conditions. The

study was a collaborative effort by the U.S. Geological Survey, Arizona 

State University, and the U.S. Department of Agriculture-Agricultural 

Research Service with assistance from the U.S. Army Corps of 

Engineers, the University of Wyoming, and the University of Arizona. 

The San Pedro River within the SPRNCA was delineated into 

14 reaches that were internally homogenous in terms of streamflow 

hydrology (spatial intermittence of streamflow) and geomorphology 

(channel sinuosity and flood-plain width). A total of 26 sites (fig. 1) 

within the SPRNCA were selected for collection of vegetation data 

from three primary streamflow regimes: perennial, intermittentwet, and intermittent-dry (fig. 2). These regimes were defined 

by observed ranges of ground-water and streamflow conditions 

(table 1). Hydrologic data were collected at 16 of these sites (called 

biohydrology sites); water-use and water-source data were collected 

at a subset of 5 sites. Vegetation data were also collected at sites in the 

Lower San Pedro Basin. Data were collected from 2000 to 2003. 

The term “riparian” refers to transitional areas between terrestrial 

and aquatic ecosystems that depend on the existence of surface or 

subsurface water flows. The riparian corridor in the SPRNCA is a 

band along the river that encompasses low-flow channel sandbars, 

streambanks, and post-entrenchment flood plains, as well as preThis report summarizes analyses of riparian system 

hydrologic requirements and ground-water use detailed 

in U.S. Geological Survey Scientific Investigations Report 

2005–5163, “Hydrologic requirements of and consumptive 

ground-water use by riparian vegetation along the San 

Pedro River, Arizona,” compiled by J.M. Leenhouts, J.C. 

Stromberg, and R.L. Scott.

0 5 MILES

0 3 KILOMETERS

EXPLANATION

Biohydrology study site 

Supplemental riparian study

 site

Evapotranspiration study site 

Intermittent-dry—class 1

Intermediate-wet—class 2

Wet—class 3

Condition classes:

R

B ari

abocom

i ver

110o30’

San Pedro

Riparian

National

Conservation

Area

San

UNITED STATES

MEXICO

ARIZONA

SONORA

Sierra Vista

Palominas Bisbee

BENSON SUBWATERSHED

SIERRA VISTA SUBWATERSHED

Tombstone

Fort Huachuca P

de ro R vi er

Base from U.S. Geological Survey

digital data, 1:100,000, 1982

Universal Transverse

Mercator projection,

Zone 12

31o30’

32o

45’

92

90

82

Benson

15’ 110o

Tombstone

streamflowgaging

station

Huachuca City

Boquillas

Palominas 

Fairbank

10

80

Study area

Tucson

ARIZONA

Contention

Moson

1

2

3

4

5

7

6

8

9

11

10

12

14

13

Figure 1. Boundaries of the Sierra Vista Subwatershed and locations 

of study sites and 14 reaches of the San Pedro River in the San Pedro 

Riparian National Conservation Area, Upper San Pedro Basin, Arizona.
entrenchment terraces at a higher altitude than the current flood plain 

(fig. 3) that are vegetated, in part, by phreatophytic plants that use 

ground water from the stream alluvium. 

The hydrologic factors measured included ground-water depth 

and streamflow. Riparian vegetation measurements included biomass 

structure, abundance of common riparian plant species, and abundance 

of plants classified into functional groups. Riparian evapotranspiration 

(ET) and free-water evaporation were measured by using a combination 

of sap-flow, eddy-covariance, and meteorological techniques.

Distribution Patterns, Effects of Hydrologic 

Conditions, and Water Use by Riparian Vegetation

Fremont Cottonwood (Populus fremontii)-Goodding Willow 

(Salix gooddingii) Forest 

Cottonwood-willow forests are present mainly in the active flood 

plain; a few old cottonwood trees are present on the pre-entrenchment 

terraces. The forests were dense and multi-aged where maximum 

ground-water depths averaged less than about 3 meters, streamflow 

permanence was greater than about 60 percent, and intra-annual 

ground-water fluctuation was less than about 1 meter (Lite and 

Stromberg, 2005), but declined in abundance and age-class diversity 

where water availability was less (fig. 4). Cottonwood-willow forests 

gave way to tamarisk stands as site-average ground-water depths 

across the flood plain exceeded 3 meters. Conditions were too dry at 

intermittent-dry streamflow regime sites to allow for establishment of 

cottonwood and willow seedlings.

Cottonwood sap flow was measured during most of the 2003 

growing season to estimate transpiration along a perennial and an 

intermittent-wet reach (table 2). Cottonwood transpiration at the 

perennial reach was about 20 percent greater on a per canopy area 

basis than previous estimates (Goodrich and others, 2000). Cottonwood 

transpired considerably less at the intermittent-wet reach than at the 

perennial reach, and had greatly reduced rates of transpiration as 

ground-water levels declined in the premonsoon season. Reduced 

transpiration likely was a result of physiological stress. Roughly 40 

percent of the cottonwood forests in the SPRNCA were classified as 

being on intermittent reaches. Work by Snyder and Williams (2000) 

suggests that cottonwood transpiration was fully derived from ground 

water at perennial reaches and about 85 percent derived from ground 

water at intermittent reaches.

Table 1. Observed hydrologic conditions of the three riparian condition classes

Condition class Streamflow regime class Streamflow permanence

(percent)

Average flood-plain 

ground-water depth in dry 

season, in meters

Annual ground-water 

fluctuation,

in meters

3 Perennial 100 Less than 2.5 Less than 0.5

2 Intermittent-wet Between 100 and 60 Between 2.5 and 3.5 Between 0.5 and 1.0

1 Intermittent-dry Less than 60 Greater than 3.5 Greater than 1.0

Figure 2. Active channel of the San Pedro River in summer. A, Perennial Moson site; B, Intermittent-wet Contention site.

8

VERTICAL DISTANCE, IN METERS 6

4

2

0

125

WEST EAST

DISTANCE FROM SAN PEDRO RIVER, IN METERS

100 75 50 25 0 25 50 75 100

Preentrenchment

 terrace

Preentrenchment

terrace streambank

Left Right

Sandbars

Riparian corridor–phreatophytic plants

Postentrenchment

flood plain

Postentrenchment

flood plain

San Pedro River

Figure 3. Cross section of stream-channel morphology, San Pedro 

Basin, Arizona.

Table 2. Total growing-season evapotranspiration of specific vegetation types and annual evaporation from open-water surfaces per unit 

land area

[---, indicates no data; ET, evapotranspiration; ET is reported per unit land area 

occupied by each vegetation type; ET is the total water use by the vegetation, 

including ground- and surface-water sources]

Vegetation type Water use, in millimeters

2001 2002 2003

Mesquite woodland 694 638 670

Mesquite shrubland --- --- 565

Cottonwood – perennial reach --- --- 966

Cottonwood – intermittent-wet reach --- --- 484

Sacaton --- --- 554

Open-water evaporation --- --- 1,156

Seepwillow --- --- 819

A. Moson B. Contention
water, depending in part on seasonal availability. Some plant groups, 

including wetland annuals, upland annuals, and xerophytic perennials, 

increased sharply in abundance following the summer monsoonal rains 

and floods; this relation likely indicates primary reliance on monsoon 

water sources.

The ET of sacaton in this study (table 2) was substantial and was 

partially derived from ground water; this result differs from a previous 

study that reported that sacaton did not use ground water (Scott and 

others, 2000). The likely reason for this discrepancy was shallower 

ground water depths (less than 3 meters) at sacaton sites in the current 

study. High-resolution topographic measurements suggest that about 

30 percent of the total sacaton within the SPRNCA likely grows where 

depth to ground water is less than 3 meters.

Tamarisk (Tamarix ramosissima)

Currently, there is a relatively small amount of tamarisk in the 

SPRNCA, mainly restricted to areas north of Fairbank. In contrast to 

the patterns of cottonwood and willow, tamarisk abundance increased 

at dry sites, likely due, in part, to reduced competitive interactions 

with cottonwood and willow trees (Sher and Marshall, 2003). This 

study also found that the relative abundance of tamarisk to that of 

cottonwood-willow along the San Pedro River has been stable at many 

sites but has increased over time at other sites, including those in the 

Palominas and Boquillas-Fairbank areas and north of Fairbank. Recent 

increases in tamarisk abundance may be indicative of decreasing site 

water availability.

For the water-use estimates, ET rates and ground-water use 

patterns of tamarisk were assumed to be similar to those of mesquite, 

in part, because mesquite and tamarisk have a similar stand 

structure and are able to acquire ground water at deeper depths than 

cottonwoods.

Seepwillow (Baccharis salicifolia) Shrublands 

Seepwillow is an evergreen shrub that forms thickets on low-lying 

flood plains adjacent to the stream channel. Seepwillow was most 

abundant on surfaces where the annual maximum (2002 data) groundwater depth was less than about 2 meters. Seepwillow had very sparse 

cover at the intermittent-dry sites.

Seepwillow transpiration was measured using sap-flow methods as 

a preliminary step toward quantification. Because available cover data 

indicate that total seepwillow area is small in the SPRNCA, groundwater use by seepwillow was likely small and was omitted from the 

water-use budgets.

Streamside Wetland Vegetation and Open Channels

Among all herbaceous plant groups analyzed, the wetland 

perennials showed the strongest correlation with surface-water 

availability. Rushes, bulrush, and other wetland perennial herbs 

depend on shallow, inflowing ground water and had high abundance 

only at sites with perennial or near-perennial streamflow; abundance 

declined sharply as flows became intermittent. The channel-bar 

vegetation became sparser and less diverse, and species composition 

shifted towards more mesic species such as bermuda grass (Cynodon 

dactylon) as sites became increasingly intermittent (along spatial 

gradients and through time). Wetland herbaceous plants (perennials 

and annuals) varied in abundance between years; they were 

particularly abundant in 2001 in response to a large flood in October 

that increased streamflow in the SPRNCA for several months. 

Wetland vegetation ET was not quantified owing to the relatively 

small area that the vegetation occupies. The evaporation from the river 

surface (table 2) was determined using potential evaporation and a 

reduction factor to account for the effect of shading and entrenchment 

that would reduce this rate relative to the rate for unsheltered open 

water.

TAMARISK BASAL AREA,

IN CUBIC METERS

PER HECTARE

30

40

GOODDING WILLOW

BASAL AREA, IN CUBIC

METERS PER HECTARE

0

20

10

80 100

STREAMFLOW PERMANENCE (2002 WATER YEAR), IN PERCENT

0 20 40 60

8

10

0

6

4

2

FREMONT COTTONWOOD

BASAL AREA, IN CUBIC

METERS PER HECTARE

40

50

0

30

20

10

EXPLANATION

Biohydrology sites Lower basin sites

 (Lite, 2003)

Figure 4. Basal area of Fremont cottonwood, Goodding willow, and 

tamarisk in relation to streamflow permanence of San Pedro River 

sites, Arizona. Each data point represents a site. Data shown for 

the 16 biohydrology study sites in the San Pedro Riparian National 

Conservation Area, and for 12 sites in the Lower San Pedro Basin, 

Arizona (Lite, 2003).

Mesquite (Prosopis velutina) Woodland and Shrubland 

Velvet mesquite is deep-rooted and can survive in areas where it 

does not access ground water, but forms denser stands when in riparian 

settings. It was widely distributed in the SPRNCA, being abundant at 

dry and wet sites and on flood plains and terraces. Growth form and 

abundance of velvet mesquite was related to site elevation (with trees 

larger and more abundant at downstream sites) and to average flood 

intensity but not to site water-availability factors.

Mesquite is the most abundant vegetation type within the SPRNCA. 

Multiyear ET observations from a mature mesquite woodland and a 

mesquite shrubland indicate that (1) both used substantially more water 

than previously estimated, and (2) their water use was nearly equal on 

a per unit canopy area basis (table 2). Stable-isotope measurements 

(δ18O) revealed considerable seasonal variation in the proportion 

of mesquite transpiration derived from ground water between sites. 

Mesquite used a combination of surface (recent precipitation), groundwater, and deep (1 to 10 meters) vadose-zone sources depending on the 

availability of each source through the season. There was a tendency 

toward proportionally less ground-water use in mesquite stands that 

had comparatively less access to ground water (deeper water table). 

Nevertheless, mesquite at all isotope sample sites used substantial 

quantities of ground water.

Sacaton (Sporobolus wrightii) and Other Herbaceous Vegetation 

The perennial grass sacaton occupies more area on the Upper 

San Pedro River flood plain than any other herbaceous plant species 

and is also abundant on terraces. Bermuda grass and johnson grass 

also have high cover on flood plains. These common flood-plain 

plants show some seasonal variance in cover and likely use a variety 

of water sources, including ground water, precipitation, and flood 
River, 11,840,000 to 14,867,000 cubic meters per year (9,600 to 12,050 

acre-feet per year) of ground water was consumptively used within the 

riparian corridors of the Sierra Vista Subwatershed for 2003. This is 25 

to 57 percent greater than the 9,498,000 cubic meters per year (7,700 

acre-feet per year) estimate of Corell and others (1996) owing in part to 

a large disparity between the two studies’ estimates for the Babocomari 

River. It is important to recognize that interannual climatic variability 

will result in different water-use values year to year (as much as 30 

percent for mesquite).

—James M. Leenhouts (U.S. Geological Survey) , Julie C. 

Stromberg (School of Life Sciences, Arizona State University), 

and Russell L. Scott (Southwest Watershed Research Center, 

Agricultural Research Service, U.S. Department of Agriculture)

References

Arizona Department of Economic Security, 2003, July 1, 2003, 

population estimates for Arizona’s counties, incorporated places and 

balance of county areas, accessed November 17, 2005, at http://

www.workforce.az.gov/admin/uploadedPublications/1003_EEC03.pdf

Corell, S.W., Corkhill, F., Lovvik, D., and Putnam, F., 1996, A 

groundwater flow model of the Sierra Vista Subwatershed of 

the Upper San Pedro Basin, southeastern Arizona, in Modeling 

Report No. 10: Phoenix, Arizona Department of Water Resources, 

Hydrology Division, 107 p.

Goodrich, D.C., Scott, R.L., and 17 others, 2000, Seasonal estimates of 

riparian evapotranspiration using remote and in-situ measurements: 

Agricultural and Forest Meteorology, v. 105, p. 281–309. 

Leenhouts, J.M, Stromberg, J.C., and Scott, R.L., 2006, Hydrologic 

requirements of and consumptive ground-water use by riparian 

vegetation along the San Pedro River, Arizona: U.S. Geological 

Survey Scientific Investigations Report, 2005–5163, 154 p. 

Lite, S.J., 2003, San Pedro River riparian vegetation across water 

availability and flood disturbance gradients: Tempe, Arizona State 

University, Ph.D. dissertation, 166 p. 

Lite, S.J., and Stromberg, J.C., 2005, Surface water and ground-water 

thresholds for maintaining Populus - Salix forests, San Pedro River, 

Arizona: Biological Conservation, v. 125, no. 2, p, 153–167.

Scott, R.L., Shuttleworth, W.J., Goodrich, D.C., and Maddock, T., 

2000, The water use of two dominant vegetation communities in a 

semiarid riparian ecosystem: Agricultural and Forest Meteorology, 

v. 105, no. 1–3, p. 241–256.

Sher, A.A., and Marshall, D.L., 2003, Seedling competition between 

native Populus deltoides (Salicaceae) and exotic Tamarix 

ramosissima (Tamaricaceae) across water regimes and substrate 

types: American Journal of Botany, v. 90, no. 3, p. 413–422. 

Snyder, K.A., and Williams, D.G., 2000, Water sources used by riparian 

trees varies among stream types on the San Pedro River: Journal of 

Agricultural and Forest Meteorology, v. 105, no. 1–3, p. 227–240.

Riparian Condition Class

A riparian condition index was developed, on the basis of fieldmeasured vegetation traits, to diagnose and monitor riparian vegetation 

condition change as related to changes in streamflow and ground-water 

conditions. Site index scores allow for categorization into condition 

classes 1 through 3, each associated with particular ranges for site 

hydrology, vegetation structure, and ecosystem functional capacity. 

Condition class 3 indicates ecological conditions reflecting the highest 

water availability, class 2 indicates intermediate availability, and class 

1 indicates the lowest water availability presently occurring within 

the SPRNCA (table 1). Overall, 39 percent of the SPRNCA riparian 

corridor fell within class 3, 55 percent in class 2, and 6 percent in 

class 1 (fig. 1). 

At class 3 sites (and reaches), flood-plain vegetation is characterized 

by tall, dense, multi-aged cottonwood-willow forests and woodlands 

with intermixed areas of other vegetation types. Drought-tolerant, 

deep-rooted pioneer species such as tamarisk are subdominant in the 

forests. The stream channel is lined by dense and diverse herbaceous 

cover. At class 2 sites, cottonwood and willow are the dominant 

pioneer trees in the flood plain, but tamarisk presence is increased, and 

cottonwoods and willow trees undergo dry-season declines in water 

use and productivity. Major changes in the herbaceous vegetation occur 

between classes 2 and 3. Streamside cover of wetland plants is reduced, 

owing to loss of perennial streamflow. Many of the wetland perennial 

herbs have been replaced by perennials that are more drought tolerant, 

such as bermuda grass.

In the transition from class 2 to class 1, major changes occur 

in woody vegetation composition and structure in the flood plain. 

Hydrologic thresholds for cottonwood and willow survivorship have 

largely been exceeded and only a few age classes of these species 

persist in favorable locations. Deep-rooted phreatophytes, typically 

tamarisk, have replaced shallower-rooted species. Structurally, the 

flood plain is dominated by shrublands with little upper canopy cover. 

Streamside herbaceous cover is sparse in the summer dry season.

The vegetation characteristics of the three condition classes 

provide some measure of the changes in vegetation structure and 

composition that might occur in response to future changes in base 

flow and ground-water availability. For example, if streamflow 

became more intermittent and depth to the alluvial ground-water 

table increased, herbaceous species such as bulrush and rushes would 

decline in abundance, and streamside-zone species composition would 

shift towards species such as bermuda grass. Across the flood plain, 

cottonwood-willow recruitment rates would decrease and mortality 

rates would increase; cottonwood-willow forests could give way to 

tamarisk shrublands. A reverse scenario would occur if streamflows 

became more permanent and ground water in the alluvium became 

shallower and more stable.

In all three condition classes, periodic floods of varying size and 

timing increase diversity by providing the physical disturbance that 

allows for establishment of a wide range of disturbance-adopted and 

pioneer plant species.

Scaling-up Water Use Estimates to the System Scale

Vegetation cover maps were used to scale species-specific ET 

measurements to the system scale. Mesquite ground-water use was 

the dominant component of the riparian water use, owing to high 

abundance, followed by cottonwood-willow, open water, sacaton, and 

tamarisk. Riparian ground-water use in 2003 along the San Pedro 

River from the international boundary to the USGS streamflowgaging station near Tombstone, 09471550, (within the Sierra Vista 

Subwatershed) was calculated to be 9,065,000 to 11,112,000 cubic 

meters per year (7,350 to 9,010 acre-feet per year), 12 to 37 percent 

higher than the total that Goodrich and others (2000) estimated for 

1997. Combining totals for the Babocomari River and the San Pedro 

For further information, contact:

James M. Leenhouts

U.S. Geological Survey

Arizona Water Science Center

520 North Park Avenue, Suite 221

Tucson, Arizona, 85719-5035

Email: leenhout@usgs.gov or visit home page

http://az.water.usgs.gov