Safe Yield for Jointly Operated Reservoir System and Examination of ENSO Impacts
Determination of safe yield of a water source is a basic aspect of water supply planning. In this report, the safe yield is defined as the maximum constant release from a reservoir that is possible during a selected drought period. The yield depends on drought magnitude and duration controlled by nature and ability to manipulate the releases through man made controls in the form of impoundment structures and regulations. A water supply system with two reservoirs in series and one in parallel in Spotsylvania Countyâ the Hunting Run Reservoir, the Motts Run Reservoir (in series), and the Ni River Reservoir is considered to demonstrate the yield calculations.
When several reservoirs are considered, the critical periods (defined as the period from full storage to empty condition) may not coincide and the system must be analyzed for the binding critical duration. A zero-one linear integer programming formulation is proposed to compute the system yield. The formulation accommodates the various storage and river flow dependent instream flow requirements. It is found that the water treatment plant capacity, instream flow requirements, and flows themselves limit the yield.
Inflows to the reservoir are very important factor in determination of safe yield for any system of reservoirs. Changes in the precipitation hence inflows may cause a significant effect on the operation of reservoir. El Nino and La Nina phenomena, which occur due to changes in the atmospheric condition over the equatorial Pacific region, are found to affect the global climate in different studies. To examine the changes in the precipitation / streamflows due to El Nino and La Nina events on the safe yield, studies are done on the streamflows in the study area and four regions across the world during El Nino and LA Nina events. Lag correlation studies and descriptive analysis of the streamflows in the study region in Northern Virginia fail to show any pattern in the streamflow changes due to El Nino and La Nina events, based on the available data. However, this observation is not conclusive and further research if needed.