Water yield loss in the Upper Colorado River basin driven by dust radiative forcing in snow

Disturbance of desert soil surfaces in the southwest US beginning in the mid 1800s led to 5-fold increase in dust loading to the mountain snow cover of the Colorado River Basin (CRB). This vast increase in dust absorbs additional solar radiation through its decrease in snow albedo, and accelerates snowmelt, shortening snow cover duration by approximately one month. Present day dust concentrations in snow cause daily average radiative forcing of 25 to 50 W/m2 (global average anthropogenic greenhouse gas forcing is 1.6 W/m2) in the east central CRB over March/April/May, with instantaneous forcings reaching greater than 400 W/m2. However, the degree to which the modern level of dust loading to mountain snow has affected the water yield of the upper CRB relative to prior to disturbance has been unknown. Here we use the Variable Infiltration Capacity model with present-day and pre-disturbance albedo parameterizations to assess the perturbation of the naturalized hydrograph and yield at Lee’s Ferry, AZ. We find that peak runoff occurs over three weeks earlier under present-day dust loading than it would have prior to disturbance. Moreover, the increase in evapotranspiration due to the shorter snow duration decreases annual upper CRB water yield by 0.974 billion cubic meters or ~5% of annual average yield. For perspective, this amount represents about half of the US treaty obligation to Mexico, or half the annual overdraft of Lake Mead. This result has broad implications for land and water management in the Western US and in other regions such as the central Asian desert/mountain systems. Improved management to produce less dusty conditions could significantly lessen the overdraft by Lower Basin states of the Colorado River and reduce the probability of future delivery shortages. The promise of more persistent snow cover, slower runoff, and greater water resources of higher quality is likely to affect a new era of tension between land and water managers.