Seepage determinations from water stage and evaporation records with application to inflow hydrographs.

Abstract

Complete inflow hydrographs for inflow events to reservoirs cannot be determined from stage recorder records alone, unless seepage losses from the reservoir are known. Determination of the actual magnitude of the hydrograph and duration of the inflow recession requires knowledge of the changes of seepage rates with surface head in the reservoirs. This thesis develops three independent methods of determining seepage under saturated flow conditions from stage recorder and pan evaporation records: the application of Darcy's Law; the application of extrapolated curves of storage decline versus pan evaporation to zero pan evaporation; and the application of a three dimensional regression model. It is felt that the Darcy method produces the most accurate results, and that the extrapolation method produces the least accurate results. The extrapolation method requires several averaging methods contributing to its inaccuracy. The seepage rates obtained on Tank 2, Atterbury watershed ranged from 1.5 x l0^-4 cfs for two feet of surface head and a reservoir capacity of 0.60 acre-feet to approximately 1.5 cfs for nine foet of surface head and a reservoir capacity of 22 acre-feet. The application of the seepage curve derived by the Darcy method produced a summer inflow hydrograph recession of three and one-half hours, and a winter inflow hydrograph recession of four and one-half hours, as compared with durations of two and one-half and four hours respectively for the uncorrected summer and winter hydrographs.