Document Type


Publication Date

January 2012


Introduction: The hydrologic cycle can be subdivided into three phases: 1)Humidity is transported within the atmosphere and becomes precipitation, and 2) Water moves downhill until eventually reaching the sea while all the time 3) Evapotranspiration returns exposed moisture to the atmosphere. During its movement, flowing water transports suspended sediments and dissolved minerals to reshape landforms and redistribute the mineral composition of the earth's surface throughout geologic time. It is during the second phase that flowing water can be diverted for uses that generally add to evaportranspiration (through consumptive use) and transport (through waste disposal). The water resources development that serves these uses adds a humanly managed phase to the hydrologic cycle. For simplification in our intiial analysis, we will index the size of a water development project by the amount of dependable flow diverted into the water use system. Opimization of the diversion design involves computing facility costs and estimating benefits for a range of sizes and identifying the project size that maximuzes benefits minus costs as illustrated on Figure 1. The seeming simplicity of the process disguises a multitutde of forecasting asumptions in forming the cost curve and need assumptions in forming the benefits curve. In both cases, a conservative, empirical approach in the face of uncertainty reduces the needs that can be fulfilled by a given water project. The theme of this paper is that scientific appraoches to hydrology and to needs estimation can be used to increase water supply efficiency greatly. Conversely, the research needed to accomplish these increases defines the mixture of contribution required from the traditional sciences to develop hydrologic sciences for water supply and water use. Both aspects are assessed individually below.