Assess the utility of a regional aquifer system groundwater model to inform the USGS National Hydrologic Model

Science Center Objects

The Issue: In Washington State, groundwater (GW) inflow to streams, or baseflow, is essential for maintaining aquatic habitats, and for out-of-stream uses such as irrigated agriculture during the typically dry summers. However, the National Hydrologic Model (NHM) currently is most suited to predicting total daily streamflow. 

This project aims to assess the NHM’s performance in simulating baseflow and to seek possible paths forward in improving this performance. If adjustments could be made to the NHM for simulating baseflow, it could be used as a predictive tool for critical seasonal dry periods and for a variety of climatic forecasts and other scenarios of water management and use. For high flow periods, such a tool would assist resource managers in estimating GW use in relation to GW discharge to streams. Complex GW models commonly are constructed for these purposes, but the NHM might address some of these needs at a lower cost to the public, if some improvements could be made. Further, the USGS has invested a large effort in building regional GW models. This project aims to extend the benefits of these GW models by exploring their potential to inform and improve the NHM.

Hydrography of the Columbia Plateau

Hydrography of the Columbia Plateau

(Public domain.)

Problem: The National includes a simplified groundwater GW component that may limit its performance in simulating and predicting the GW component of streamflow. Over recent decades, the USGS has constructed multiple regional-scale GW models to assess groundwater availability for principal aquifer systems in the Nation.  Although these regional GW models simulate the physics of GW discharge into streams, they are not coupled with the NHM. 

Objectives: The objectives of this project are to assess the utility of a regional GW model for the Pacific Northwest to inform and improve the NHM’s capability in simulating and predicting baseflow and to evaluate approaches for future research and development of the NHM. 

Relevance and Benefits: This project will make progress toward fulfilling several goals of the Washington State government and the USGS Water Mission Area (WMA). The Washington State Legislature has tasked the Washington State Department of Ecology (Ecology) with (1) issuing water supply reports, (2) finding “new uninterruptible supply of water for those whose rights are curtailed on the Columbia mainstem when minimum flows are forecast to be unmet,” and (3) to make “water available for instream benefits when needed most.”    Improved streamflow forecasting would have value in water supply reports and is required for the second task listed above. For the third task, forecasting could be used in planning when instream benefits will be most critical. 

According to Ecology, climate change is “likely to increase the frequency and severity of droughts and cause declines in mountain snow pack, affecting water supplies.” Ecology is “committed to meeting the challenges and working creatively and collaborative to address current and future water needs of people, farms, and fish.”   The NHM is an appropriate model for simulating the effects of drought and declines in mountain snow pack, because these atmospheric processes are accounted for. Further, the NHM has potential application for prediction of all streams in Washington. 

In addition, the State of Washington is in the process of modernizing their drought statutes to more effectively prepare for and respond to drought emergencies by allowing the prediction (rather than the onset) of drought conditions, such as low baseflows, to be used to begin mitigation of drought impacts. The predictive capability of the NWM ideally fits into this modernization.

Approach: The regional GW model to be used in this project is the Columbia Plateau Regional Aquifer System (CPRAS) model. To understand differences in how the CPRAS model and NHM simulate hydrologic processes, a comparison of individual water budget components for the two models will be made. For example, simulated baseflow for the NHM will be compared to that simulated by the CPRAS model for basins upstream from selected streamflow gages. Ungaged basins will be included where necessary to represent the range of hydrologic conditions in the study area. Although baseflow is the focus of this study, a thorough comparison of all water budget components will provide a better context for understanding of how baseflow is driven in the NHM, as compared to CPRAS. This is pivotal in determining how GW models like CPRAS can potentially improve NHM. The CPRAS model runs and generates water budgets on an annual basis, and therefore, the comparison will be on an annual basis.