Determining Surface Water Availability in Oregon By Richard M. Cooper, PE Abstract The Oregon Water Resources Department (Department or OWRD) limits appropriation from Oregon streams to assure new applicants use of surface water a reasonable amount of time and to minimize regulatory conflict. The standards for new appropriation of water are: (1) consumptive use from allocations for out-of-stream uses can total no more than the 80-percent ex-ceedance natural stream flow, and (2) allocations for in-stream flows can be no more than the 50-percent exceedance natural stream flow. OWRD has created and maintains a database of the amount of surface water available for appropriation for most waters in the state. This database is used to evaluate applications for new uses of water. Water availability (WA) is obtained from natural stream flow (QNSF) by subtracting existing storage (ST), out-of-stream consumptive uses (CU) and in-stream demands (IS). WA = QN -ST-CU-IS Ideally, water availability would be calculated for every watershed above a point of diversion or in-stream demand. Practically, the number of watersheds must be limited. The watersheds selected for analysis are called Water Availability Basins (WABs). Stream flow can be highly variable, and it is useful to characterize it in some way, usually by a statistic, e.g., a monthly or annual mean. For water availability, it is important to know how often water is available. The appropriate statistic in this case is exceedance stream flow. This statistic tells us how often to expect a given rate of stream flow to occur. Exceedance stream flows are determined directly from gage records, or for ungaged streams, by estimation through modeling. When determined from gage records, the exceedance flows must be corrected to a common base period, and then, to natural stream flow. When determined through modeling, the exceedance flows are estimated from statistical models that relate watershed characteristics to natural stream flow. The models are derived by multiple linear regression. Storage is water retained in a reservoir. It is debited from water availability when the water is stored. It diminishes availability both upstream and downstream of the point of diversion. Consumptive use is divided into three major categories: irrigation, municipal, and all others e.g., domestic, livestock. These uses are less than 100 percent consumptive. It is assumed the non-consumed part of a diversion is returned to the stream from which it was diverted. Consumptive use from irrigation is from estimates made by the US Geological Survey (Portland). Consumption from other uses is based on the associated water rights. In these cases, consumptive use is obtained by multiplying the maximum diversion rate allowed for the water right by a consumptive use coefficient. Consumptive use diminishes availability both upstream and downstream of the point of diversion. There are two types of in-stream demands: in-stream water rights and scenic waterway flows. In-stream demands diminish availability upstream only. Because they are non-consumptive, they do not diminish stream flow downstream as do consumptive uses. Water availability has been calculated for over 2500 WABs. In general, the calculation of water availability at one WAB cannot be considered in isolation from other WABs in the same stream system. For water to be available at any given upstream point, it must be available at all points of calculation downstream.