Hydrologic modeling to determine the effect of small earthen reservoirs on ephemeral streamflow

Abstract

Due to the concern of downstream water users, the effect of a large number of small stock water reservoirs on streamflow in North- Central Arizona was studied. The U. S. Department of Agriculture Agricultural Research Service's hydrologic watershed model -- USDAHL-74 Revised Model of Watershed Hydrology, developed by H. R. Holtan, N. C. Lopez, and others -- was used. The 49 square mile study watershed, Red Tank Draw, on the Beaver Creek Experimental Watershed, contains 27 small earthen reservoirs with a total storage capacity of 124 acre-feet which control runoff from 32 percent of the watershed. Average annual runoff for 14 years of record totals 4,192 acre-feet, with a range from 32 to 13,420 acre-feet. Approximately two-thirds (or 2,994 acre-feet) of the total occurs in the winter and spring runoff seasons. Results indicate that during the winter and spring, when the majority of runoff occurs, streamflow at the mouth of the watershed was reduced 2.6 to 10.7 percent for the four years studied. These results are consistent with the results of previous research on other watersheds in which reductions in streamflow due to small reservoirs ranged from 2 to 33 percent. The watershed model, as used in this study, was unable to adequately simulate runoff in low water yield years and during the summer runoff season. The model worked well in simulating the winter and spring runoff periods. Based on the findings of other studies, it did a reasonably good job in evaluating the effects of the reservoirs on streamflow.