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HYDROLOGIC BUDGET <br />• A Piydrologic budget is a quantitative expression of the balance <br />betwen ttie total water gains and losses of a basin over a certain <br />period of time. The hydrologic budget for a basin should con- <br />sider both surface and groundwater, entering and leaving, or <br />stored within the basin, <br />Within the Orchard Valley t9ine area precipitation, including rain <br />and snow, is the source of water entering the basins and is the <br />only water gain considered in the hydrologic budget, Water leav- <br />ing the basins include streamflow, evapo-transpiration, and <br />groundwater underfloor, Groundwater is stored in the unconsoli- <br />dated sediments of the streambed fill. Changes in the storage of <br />water in the soils are reflected in changes of the soil moisture, <br />and changes in groundwater levels. A hydrologic budget, for a <br />given basin and a given period of time, is expressed in the fol- <br />lowing equation: <br />P = R + ET + U ± QSs ± ~ S9 <br />where: P = precipitation <br />R = streamflow <br />ET = evapo-transpiration <br />U = groundwater underfloor <br />Q Ss = change in soil moisture <br />• ~Sg = chagne in groundwater storage <br />Streamfiow P. consists of a surface run-off component, Rs, and a <br />groundwater run-off component, Rg. <br />Evapo-transpiration can be divided into two components: surface <br />acid soil evapo-transpiration, including evapo-transpiration from <br />surface water bodies, soil moisture, and vegetation, ETs, and <br />groundwater evapo-transpiration, including evapo-transpiration <br />from the yroundwater table, ETg. <br />It is very difficult to estimate evapo-transpiration because it <br />depends on meteorological factors, available soil moisture, type <br />of soil, vegetation, and depth to the groundwater table. In the <br />mountains of Colorado, evapo-transpiration is very small during <br />tt~e winter, increases rapidly in the spring, reaches a maximum <br />generally in July, and decreases rapidly in the fall, Evapo- <br />transpiration is typically greater than precipitation during the <br />summer. <br />Measurement of seasonal changes in evapo-transpiration during a <br />year is very difficult, especially because of problems with con- <br />tinuous measurement of soil moisture. However, the annual change <br />in soil moisture is very small, Evapo-transpiration can he <br />• <br />