The Role of Groundwater Withdrawals on River Regulation: Example from the Columbia River Basin

The availability of water resources in the western United States (US) is indispensable to a wide range of users in agriculture, power generation, municipal, and industrial sectors. However, such resources are always stressed by increasing demands as a result of growing population and recurrent droughts. Following a fully-appropriated policy for surface-water supplies in the western US, many sectors, especially domestic and agricultural, replenish their needs from groundwater. In this study, we evaluated the sensitivity of regulated flow to groundwater withdrawal in the Columbia River Basin (CRB). The CRB is a heavily regulated basin (with more than 250 dams) that includes subbasins with significant dependence on groundwater withdrawals. For instance, more than 25% of the total agricultural water withdrawals are pumped from groundwater wells. It is therefore crucial to re-construct a unified water budget that accounts for groundwater withdrawals within predictive hydrologic models for this basin. Neglecting contribution to withdrawals in modeling efforts could result in inaccurate simulations and forecasts of the reservoir operation and consequently regulated flow.

This study constitutes our initial attempt to quantify and understand how groundwater pumping could modulate the spatiotemporal patterns of regulated streamflow in the CRB using a modified version of the Variable Infiltration Capacity (VIC) model integrated with a water management component (VIC-IRR). The VIC-IRR integrates irrigation with a reservoir module for dam release optimizations. Yet, the model does not consider groundwater availability when allocating water for irrigation, which could lead to biased simulations in regions relying on groundwater for irrigation. In this study, we improved the VIC-IRR to include a groundwater withdrawal component which allows for more supply options to irrigation demands (VIC-GIRR hereafter). The VIC-GIRR simulations were performed in the daily time step at 1/16° over the CRB basin. Information on groundwater withdrawal is compiled from USGS estimates, available every five years at a county scale, which was spatially disaggregated to the modeling grid based on ancillary groundwater well information such as the number of wells and well’s casing diameter. Three scenarios are analyzed based on the VIC-GIRR simulations: 1) A baseline scenario that only simulates the naturalized flow without considering any water management activities; 2) A surface water only scenario in which the irrigation demands are met with the available surface water from local streamflow and upstream reservoirs; and 3) A surface water + groundwater scenario that combines the surface water and groundwater withdrawals with reservoir operation to mimic reality. To account for impacts of extreme climatic conditions, we considered the reservoir operation under dry and wet episodes to identify potential water management alternatives with the availability of both surface and ground- water. We aim to demonstrate that it is crucial to explicitly connect groundwater withdrawals, regulated streamflow, and reservoir operation to manage a multi-reservoir system, improve its forecasting skill, and achieve the optimization of multiple objectives of improving agriculture productivity, energy productivity (e.g., hydroelectric power generation), and sustainable water management for both surface and ground- water.