Abstract
In the Upper Colorado River Basin (UCRB), the principal source of water in the southwestern U.S., demand exceeds supply in most years, and will likely continue to rise. While General Circulation Models (GCMs) project surface temperature warming by 3.5 to 5.6°C for the area, precipitation projections are variable, with no wetter or drier consensus. We assess the impacts of projected 21st century climatic changes on subbasins in the UCRB using the Soil and Water Assessment Tool, for all hydrologic components (snowmelt, evapotranspiration, surface runoff, subsurface runoff, and streamflow), and for 16 GCMs under the A2 emission scenario. Over the GCM ensemble, our simulations project median Spring streamflow declines of 36% by the end of the 21st century, with increases more likely at higher elevations, and an overall range of −100 to +68%. Additionally, our results indicated Summer streamflow declines with median decreases of 46%, and an overall range of −100 to +22%. Analysis of hydrologic components indicates large spatial and temporal changes throughout the UCRB, with large snowmelt declines and temporal shifts in most hydrologic components. Warmer temperatures increase average annual evapotranspiration by ∼23%, with shifting seasonal soil moisture availability driving these increases in late Winter and early Spring. For the high-elevation water-generating regions, modest precipitation decreases result in an even greater water yield decrease with less available snowmelt. Precipitation increases with modest warming do not translate into the same magnitude of water-yield increases due to slight decreases in snowmelt and increases in evapotranspiration. For these basins, whether modest warming is associated with precipitation decreases or increases, continued rising temperatures may make drier futures. Subsequently, many subbasins are projected to turn from semi-arid to arid conditions by the 2080 s. In conclusion, water availability in the UCRB could significantly decline with adverse consequences for water supplies, agriculture, and ecosystem health.
Highlights
The Colorado River is perhaps the most important source of water in the western United States, providing water to 30 million people, irrigating over 16,000 km2 of agricultural land, and producing over 8 billion kilowatt hours of hydroelectric power annually [1]
Decreased streamflow from changes in hydrologic components at the subbasin-level owing to climate change could have significant impacts on water resources in the Upper Colorado River Basin (UCRB)
A calibrated and validated Soil and Water Assessment Tool (SWAT) model of the UCRB including 46 unimpaired streamflow sites was used to investigate the effects of climate change using 16 General Circulation Models (GCMs) under the A2 emission scenario
Summary
The Colorado River is perhaps the most important source of water in the western United States, providing water to 30 million people, irrigating over 16,000 km of agricultural land, and producing over 8 billion kilowatt hours of hydroelectric power annually [1]. With water supply and demand already in a tenuous balance in the CRB, the ability of the United States Bureau of Reclamation (USBR) as well as other state and municipal agencies to meet future water-delivery requirements and basic ecological needs in the CRB is imperiled by both climatic variability and change, as well as rising human demands on Colorado River water. Periods of drought are part of the natural climatic variability in the region. The current drought, which started in 1999 and is ongoing through the time of this writing, has exacerbated concerns by the USBR [2,3,4,5]. From 2000 to 2011, estimated unregulated streamflow entering Lake Powell, which is located directly upstream of Lee’s Ferry, Arizona, was above average for only three years (2005, 2008, and 2011) [5]
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