A Simulation Study of Groundwater Withdrawals from the Upper Pawcatuck River Basin

This study examined the risk of low flow under conditions of extensive groundwater withdrawals in the Upper Pawcatuck River basin. The streamflow record of the Pawcatuck River at Wood River Junction from 1941-1968 was used in this study. The contribution of groundwater to streamflow (termed baseflow) was estimated from these records. There is a constant gravity drain or baseflow recession occurring in the groundwater reservoir. A recession constant relating the recession to an exponential decay process was found using the streamflow record. The recharge of the aquifer, resulting from the infiltration of precipitation to the aquifer, was estimated for every month in the record. A simulation model of the stream was then developed using recharge as a random variable and the recession equation as a deterministic component. Recharge was generated from empirical distributions on a monthly basis. The effect of pumping from wells near the stream (stream depletion) was found using Jenkins\u27 model of an idealized stream-aquifer system. The output of the simulation model under conditions of no pumping was compared with the historical records in order to validate the model. A search program was then used in conjunction with the simulation model in order to find the maximum withdrawal possible subject to a constraint on the maximum number of mean flows below a set minimum flow. An additional constraint was necessary to restrict the allowable range of the pumping rates. The simulation was then altered to reflect a more realistic situation: 3 wells with different stream depletion factors and fixed pumping rates. The combination of wells which would maintain a certain annual supply of water and which would deplete the stream the least was found. These results were related to the 1, 7, and 30 day minimum flow. This study did not present any new safe yield figure or single optimal pumping plan. Instead, the study demonstrated the effects of time and location of pumping on the risk of low flow in the Pawcatuck River Basin. It is possible that a more elaborate model could be used to determine the safe yield of the basin