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.

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