Laserfiche WebLink
<br />VII. 2 <br /> <br />001209 <br /> <br />ShOuld the reservoir be on a reach of stream that lost water <br />naturally (influent ground water), in general the seepage loss probably <br />-would increase within the reservoir but would change little downstream <br />from the dam. Again, if the effect was overlooked, an inflow-outflow <br />budget would indicate too large a water supply in the reservoir. <br /> <br />Volume changes due to seasonal variation in water temperature <br />usually are relatively small unless the reservoir or lake is uncommonly <br />extensive, and cannot be determined with assurance unless all water- <br />budget items can be measured with uncommon accuracy. For eXalllple, <br />assume that stage of the reservoir rises 10 feet due to inflow of snow- <br />melt having a temperature near that of maximum water density;....that is, <br />39~ F. If by midsummer that same water warms to a mean temperature of <br />70 F., it will expand to a depth of 10.02 feet. Such expansion would <br />amount to 2,000 acre-feet over a water-surface area of 100,000 acres. <br /> <br />Even though small in percentage of aggregate reservoir content, <br />these seasonal volume changes may be sufficient to indicate false values <br />of monthly inflow pr evaporation, or to preclude close accounting of <br />reservoir storage. Errol' so caused can be minimized by ascertaining <br />temperature gradients in the reservoir and adjusting the apparent volume <br />to a standard temperature. For example, in Pine Flat Reservoir on Kings <br />River, California, an intensive temperature survey by the Corps of <br />Engineers indicated nearly horizontal isotherms. Accordingly, the vertical <br />temperature gradient measured down the upstream face of the dam llas <br />reasonably representative of the mean gradient in all the reservoir. <br />Apparent volume was reduced readily to standard temperature. <br /> <br />It should be reiterated, however, that such volume adjustment is <br />warranted only if reservoir stage, inflow, and outflow can be and have <br />been measured precisely; also, only if the stage-storage relationship <br />is known with commensurate accuracy. <br /> <br />Records of storage <br /> <br />Capacity tables <br /> <br />. <br /> <br />Storage in a reservoir or lake is recorded either as aggregate <br />contents at sOllie regular interval of time, or change in contents du;ri~ <br />each such interval--commonly daily, monthly, or yearly. Either form of <br />record is derived from a "capacity table" and fluctUations of water- <br />surface stage. <br /> <br />