Water Resources Inventory Area 1 Watershed Management

Introduction to Water Use


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To estimate the amount of water that is available for future allocation in WRIA 1, all significant components of the water budget have to be known, including transfers of water by people. This includes water pumped from the ground-water system, water diverted from surface-water bodies, and water returned to the surface-water and ground-water systems. Estimating water transfers by people is difficult, because record keeping is limited.

Generally, water is withdrawn for a number of different purposes, including municipal, industrial, agricultural (for irrigation and/or livestock), and domestic. Water in these categories may be provided by public-supply systems, or industries, farms, and homes may have private systems. Of these, withdrawals by public supply-systems are estimated most easily, because such systems are monitored by the Washington State Department of Health to assure distributed water is of drinking quality.

Generally, withdrawals from private systems can only be estimated using indirect methods. For example, knowledge of the land use, the number of livestock, and electric usage could be used to estimate agricultural water use. Whether or not errors in those estimates are significant depends on what fraction of the overall water budget is represented by the water use. If water withdrawals by people are only a small fraction of the overall water budget in a watershed, even an error of 100 percent in the withdrawal estimate will have little impact on the overall water-budget estimate.

On this webpage, transfers of ground water and surface water are tabulated (Historical Ground-Water Withdrawals in the WRIA 1 Study Area for Public-Supply Systems with at Least 50 Connections and Selected Historical Surface-Water Diversions in the WRIA 1 Study Area, respectively) and primarily represent withdrawals by public-supply systems with at least 50 connections. Monthly volumes of transferred water were obtained from either system operators or the Washington State Department of Health. Smaller systems were not included in the data collection effort, because it is more cost effective to estimate their withdrawals by other methods.

The effort to estimate historical transfers of water in the WRIA 1 study area is incomplete and will be completed in the next study phase. What remains to be estimated are agricultural, industrial, and domestic water withdrawals, and return flows for all water-use categories. Return flow is water that is withdrawn and returned to ground- or surface-water bodies. Examples are seepage to the water table from irrigation and septic-system discharge and discharge of effluent from sewage treatment plants. Some information is provided on this webpage that could be used to start estimating agricultural withdrawals.