Streamflow Depletion by Well - Understanding and Managing the Effects of Groundwater Pumping on Streamflow

The effect that a pumping well has on surface water is of concern to water users and managers, especially when water is scarce. Numerical modelling has been using to simulate groundwater flow for managing aquifers and risk assessment. Uncertainty in numerical modelling arises from various sources, including lack of data to characterize the system of interest, incomplete process representation, errors in the specification of initial and boundary conditions, and spatial variability of hydraulic properties within hydrogeologic units. The objective of this study is to examine how the decision styles of stakeholders impact their preferred well designs with and without hydrologic uncertainty. We defined two utility functions for two opposing stakeholder interests. The town interest is to maintain the stream volume. The farm interest is to grow crops, that is, the extraction of a higher amount of water in a short period leads to get more profit. The Town utility depends on the streamflow depletion while the Farm utility depends on the annualized pumping rate. Consequently, we used Hantush's analytical solution to estimate streamflow depletion caused by pumping wells. This solution conceptualizes a stream that fully penetrates an aquifer with streambed resistance. The study is designed to illustrate these relationships for the simplest conditions that still lead to significant hydrologic uncertainty. We evaluated the performance of one analytical streamflow depletion function to quantify the sensitivity to the choice of changing the hydrologic conditions and well designs over five years of simulation, assessing 16,384 outcomes by using different ways of quantifying the different decision styles. The estimation of the streamflow's spatiotemporal response based on sixteen pumping stresses with 1,024 different hydrologic conditions allowed us to demonstrate the potential range of impacts of groundwater management on stream depletion. The scenarios provide significant insights into spatial and temporal scales of measures and potential venues needed to understand existing conflicts between stakeholders represented as a local farm and town. The results determined that if the Town makes their decision based on the average streamflow depletion in time, all models led to an acceptable design for the Town. However, if the Town makes their decision based on the maximum streamflow depletion in time, the Pareto optimal designs that the Town would prefer all have the maximum distance from the stream, but any pumping rate (equivalently, pumping duration) would be acceptable. Furthermore, we found that the analytical depletion function is most sensitive to the well distance, pumping rate, and aquifer specific storage, under the conditions studied. Storage is an essential factor in hydrology because it represents the physically-available water in the soil that can be extracted before the cone depression reaches the water resource of the stream. In other words, increased storage leads to reduced impact on the stream because most of the water is pumped from the groundwater. This approach provides a relatively expeditious and accurate assessment of a stream water budget in response to various stresses imposed upon the flow system caused by a well. This is an important tool for decision-making, allowing managers to evaluate proposed water management scenarios in greater detail and specific evaluation of risk