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Warmer temperatures cause decline in runoff ratio and water supply

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Finding could improve water supply forecasting

May 11, 2017 National Center for Atmospheric Research/University Corporation for Atmospheric Research  (from ScienceDaily)

…The study paints a detailed picture of how temperature has affected the runoff ratio — the amount of snow and rain that actually makes it into the river — over time, and the findings could help improve water supply forecasts for the Rio Grande, which is a source of water for an estimated 5 million people. The study results also suggest that runoff ratios in the Upper Rio Grande and other neighboring snow-fed watersheds, such as the Colorado River Basin, could decline further as the climate continues to warm.

“The most important variable for predicting streamflow is how much it has rained or snowed,” said NCAR scientist Flavio Lehner, lead author of the study. “But when we looked back hundreds of years, we found that temperature has also had an important influence — which is not currently factored into water supply forecasts. We believe that incorporating temperature in future forecasts will increase their accuracy, not only in general but also in the face of climate change….

Current operational streamflow forecasts depend on estimates of the amount of snow and rain that have fallen in the basin, and they assume that a particular amount of precipitation and snowpack will always yield a particular amount of streamflow.

In recent years, those forecasts have tended to over-predict how much water will be available, leading to over-allocation of the river…”The effect of temperature on runoff ratio is relatively small compared to precipitation,” Lehner said. “But because its greatest impact is when conditions are dry, a warmer year can make an already bad situation much worse.”…

Flavio Lehner, Eugene R. Wahl, Andrew W. Wood, Douglas B. Blatchford, Dagmar Llewellyn. Assessing recent declines in Upper Rio Grande River runoff efficiency from a paleoclimate perspective. Geophysical Research Letters, 2017; DOI: 10.1002/2017GL073253

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