What Integrated Hydrologic Modeling Reveals about Freshwater Availability in the Upper Colorado River Basin: A Survey of the Past and a Glimpse of the Future

The Upper Colorado River Basin (UCRB) is a mountainous 280,000 km² watershed located in the western United States that supplies water to over 40 million people in North America—including seven U.S. states, two Mexican states, and 28 Tribal communities. Groundwater plays a critical role in basin hydrology; approximately half of all streamflow is derived from groundwater. Because of the river’s snowmelt-dominated water supply and the tens of millions of people that rely on it for water and hydropower, sustainable water management, especially in a warmer future, is paramount in this basin.   

Due to the societal importance of the UCRB, numerous hydrologic and statistical models have been developed to study the basin’s underlying hydrology and projected changes to it due towarming temperatures. However, many of these models, especially at the basin scale, lack physical representation of groundwater flow processes despite groundwater’s importance to basin hydrology. To address this gap, we deploy a physics-based integrated hydrologic model (ParFlow-CLM) of the UCRB that simulates terrestrial hydrology from the bedrock to the top of the canopy. We leverage this model to highlight the importance of groundwater-inclusive hydrologic modeling in two case studies related to the basin’s water supply.   

First, we combined this integrated model with satellite-based water storage trends from GRACE to study the relationship between groundwater pumping and water loss during the 2000-2020 millennium drought—a relationship that is not yet fully constrained. To accomplish this, we ran basin-wide pumping and irrigation scenarios to ascertain how simulated groundwater pumping adjusted water storage trends relative to GRACE. Second, we explored the sensitivities of basin hydrology and the buffering capacity of groundwater under historical and projected climate change. We forced the model with a range of historical and projected temperatures spanning from 1900 to 2100—capturing the evolution of the basin’s full water balance and responses to precipitation drought over two centuries. Both case studies illustrate the utility of a holistic modeling approach—one that jointly incorporates groundwater, surface water, and land surface processes—to more completely quantify hydrologic feedbacks in a sensitive headwaters region.