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United States Department of Agriculture

Agricultural Research Service

Experimental Watersheds
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Research conducted at the Southwest Watershed Research Center is supported by outdoor laboratories at the Walnut Gulch Experimental Watershed near Tombstone, Arizona (since 1953) and at the Santa Rita Experimental Range south of Tucson (since 1975). The Walnut Gulch Experimental Watershed is the most densely gaged and monitored semiarid rangeland watershed in the world and is critical to improving scientific understanding of semiarid ecosystems.

The upper end of the Walnut Gulch Experimental Watershed looking toward the Dragoon mountains

Suggested publication:

Renard, K.G., Lane, L.J., Simanton, J.R., Emmerich, W.E., Stone, J.J., Weltz, M.A., Goodrich, D.C., Yakowitz, D.S. Agricultural impacts in an arid environment Hydrological Science and Technology 9:145-190 Published 1993.

Walnut Gulch Experimental Watershed

Arizona map

Studies on the Walnut Gulch Experimental Watershed at Tombstone, Arizona are part of the comprehensive research initiated in 1951 by the Research Division of the Soil Conservation Service. After considerable screening of prospective areas in Arizona, New Mexico, and Colorado, active research was begun in 1953 on the Walnut Gulch Experimental Watershed.

In 1954, the research and personnel were transferred to the Agricultural Research Service; and in 1961, the Southwest Watershed Research Center was established with headquarters in Tucson.

Research is being conducted in cooperation with the Natural Resource Conservation Service, the local Soil Conservation Districts, and the ranchers who own the land of the watersheds.

Walnut Gulch enters the San Pedro river at Fairbank, Arizona. The study area comprises the upper 150 sq. km of the drainage basin. There is evidence that much of the area was grassland less than 100 years ago; but now shrubs dominate about 2/3 of the watershed. Creosote, tarbush, mortonia, and whitethorn are the most common shrubs. The remaining 1/3 is still grassland, dominated mainly by black grama, curly mesquite grass, and tobosa grass.

Walnut Gulch

For more information about the Walnut Gulch Experimental Watershed, view our BROCHURE (5MB PDF)

Santa Rita Experimental Watersheds
Roads & Pastures of the Santa Rita Experimental Watershed

The SWRC currently maintains eight subwatersheds within the Santa Rita Experimental Range (SRER), the oldest continuously maintained experimental range in the western United States (est. 1903). Established in 1975, these eight small watersheds are set up as paired watersheds.

Each watershed is instrumented with one recording raingage and one flume. The watersheds range in size from 1.05 ha to 4.01 ha. Four of the watersheds (#1 - 4) are in the shrub-dominated lower portion of the experimental range; the remaining four watersheds (#5 - 8) are located in the grass dominated upper portion of the experimental range.

The Santa Rita Experimental Range is currently administered by the University of Arizona, the SRER serves as a primary research location for members of the SWRC working in cooperation with the University

GIS Data Sources

GIS coverages have been created in cooperation with the University of Arizona's Advanced Resource Technology lab. A small-scale GIS that covers the entire SRER is administered by the University's College of Agriculture . The SWRC maintains a large-scale GIS database that is focused on the 8 small subwatersheds administered by SWRC staff.

Published Data Sources

Data currently available for the Santa Rita Experimental Watersheds: (location #76 in the ARS Water Data Center database)

meterological data
 Landsat image of the Santa Rita Experimental Watershed

Raingage 81
Analog System

Since 1953, precipitation has been recorded at the Walnut Gulch Experimental Watershed in Universal weighing-type gages.  Within the protective housing, precipitation is collected in a bucket that rests on a weighing platform and frame, which are suspended from a spring.  As precipitation collects in the bucket, the load on the spring is increased, which lowers the platform and frame.  The deflection is proportional to the amount of precipitation collected.  The movement of the frame is mechanically transmitted to a pen, which marks a graduated revolving chart.

Digital System

Digital raingages were installed on the Watershed to provide a continuous data stream.  1999 was the first complete year of operation.

Lh3 Flume

Channel runoff is measured on the watershed at several locations.  Small waterhseds (up to approximately 25 acres) are instrumented with runoff measuring flumes and traversing slot sediment samples.

Traversing slot

Flume 6
Flume 6 on the Walnut Gulch Experimental Watershed

Flow in the main channels on the Walnut Gulch Watershed is measured at 11 gaging stations.  Eight of the main tributaries are gaged, and three flumes measure flow in the main Walnut Gulch channel.

Flumes on Main Channel

Flume 1 Flume 2 Flume 6
capacity 640 cms
(22,500 cfs)
520 cms
(18,400 cfs)
425 cms
(15,000 cfs)
drainage area 150 sq km
(57.6 sq mi)
109 sq km
(43.2 sq mi)
93.6 sq km
(36.1 sq mi)

Last Modified: 9/4/2012