RUNOFF AND LAND USE IN THE DESCHUTES BASIN∗

Abstract

ABSTRACT The hydrologic consequences of land resource development are a greater concern in the Deschutes Basin than in most watersheds. The uniform natural flow of the Deschutes River is sustained by groundwater discharge from basaltic lava that underlies much of the basin. Regular streamflow is the principal water supply throughout the basin during the dry summer months, but forest and agricultural resource development are increasingly modifying the natural runoff process and are exerting growing demands on the summer water supply. Estimates of the hydrologic effects of complete timber removal and the maximum development of irrigated agriculture reveal contrasting and severe changes in the quantity and timing of summer runoff. Removal of timber increases runoff by an average of 560,000 acre-feet annually, but fifty-eight percent of the total computed runoff increase occurs during the wet season. Full development of irrigated agriculture decreases runoff by about 386,000 acre-feet annually, and all of the depletion is concentrated in the dry months. Timber removal increases summer runoff by only 234,000 acre-feet annually, and monthly runoff increases are insufficient to consistently abrogate monthly irrigation depletions. Maintenance of summer runoff at levels that satisfy diversion and instream water requirements is the most important hydrologic criterion for evaluating more environmentally sensitive resource development strategies.