An intensive study of the effect of grazing on the hydrologic and biotic characteristics of small drainage basins on the Colorado Plateau was begun in the fall of 1953. This report presents data obtained during the first 13 years of the proposed 20-year study.
For the period of record 1954-66, runoff from grazed watersheds has averaged about 33 acre-feet per square mile per year. Runoff from ungrazed watersheds averaged from 71 to 76 percent of that from grazed watersheds. During the last 6 years of the period, however, ungrazed watersheds produced 69 to 71 percent as much runoff as grazed watersheds. The sediment yield frown grazed watersheds during the same period was about 3 acre-feet per square mile per year. Sediment yield from ungrazed watersheds ranged from 51 to 75 percent of that from grazed watersheds and averaged 66 percent. The largest change in these relations occurred about 2 years after livestock were excluded from certain watersheds. The causative factors for changes in the runoff and sediment yield relations are not entirely clear. At the end of 13 years, a significant change had occurred in the amount of bare soil and rock. in the ground-cover index, and in the litter and moss on the grazed watersheds. These items remained essentially unchanged on ungrazed watersheds. The changes in ground-cover factors were not of large magnitude and did not occur at the same rate as the changes in runoff and sediment yield. A large part of the difference appears to have been caused by a change in the structure of surface soil. which was brought about by the elimination of trampling by livestock.
Deer mice were the most common rodent present on the experimental watersheds, but even their population was not great enough to affect the composition of range vegetation. Deer mice populations remained comparable on grazed and ungrazed watersheds during the study. Other rodents were not present in sufficient numbers to allow their comparison on grazed and ungrazed range. Desert cottontail rabbits and black-tailed jackrabbits were more plentiful in ungrazed watersheds but were not present in large enough numbers to affect range vegetation.