THE EFFECT OF RANDOMIZED CLOUD SEEDING ON RUNOFF DEPTH FROM A SMALL WATERSHED IN CENTRAL ISRAEL

The effect of randomized cloud seeding on daily depth of runoff, durin g 27 years of experiments in central Israel, is estimated through data for a small watershed. The estimation is carried out with respect to depths of precipitation over the watershed and at two control areas, D ouble ratio and linear regression models are applied for this estimati on. Distribution of rainfall depths by intensities and of seeding resu lts with respect to wind direction are also examined. Differing result s are obtained for the four individual experiments, each carried out o ver a period of 6 years. These reflect sampling variations in the prop erties of precipitation and runoff. A comparison of variables at the c ontrol areas, between seeded and unseeded allocations, indicates that an equipoise has existed over the entire experimental period. The resu lts for this period are a 19% increase in the number of runoff days un der seeded allocation, a similarity in the lower halves of the distrib utions of daily runoff depths along with marked differences in the upp er halves, a 15% increase in the mean daily depth of immediate runoff and a 36% enhancement of the total depth of runoff under seeded alloca tion. These values are considerably lower than those obtained in forme r studies on three small watersheds located in northern Israel. Volume tric ratio of runoff to rainfall on days with northwesterly winds is h igher than that on days with southwesterly winds, This reflects a rela tion between wind direction and properties of rainfall associated with generation of runoff. (C) 1997 Elsevier Science B.V.