RISK OF SCARCITY OF MONTHLY PRECIPITATION AND STREAMFLOWS IN SEMIARIDREGIONS

In semiarid regions, periods of one to several consecutive months of s carcity of precipitation or low flow rates in rivers and streams frequ ently occur. In these regions, scarce precipitation considerably affec ts grazing activities od rangelands, irrigation agriculture, and flora and fauna of surface stream environments. This paper is concerned wit h estimating probabilities of occurrence for such events and associate d risks by using statistical information provided by monthly precipita tion and streamflow series. Conventional return period and risk concep ts as customarily used in hydrology are applied, and events of interes t such as meteorological droughts are considered by using monthly prec ipitation as a time-related series of supply, thereafter addressing hy drological droughts in terms of streamflow supply. For these two kinds of events, periodic situations of time-independent phenomena are cons idered for the case of precipitation, with time-dependent phenomena co nsidered for streamflows. The method is applied to the semiarid portio n of Chile to calculate probabilities of occurrence of specific number s of consecutive months with no significant precipitation, or now rate s insufficient to meet stream ecological needs.