When Normal Isn't Good Enough: Surface Water and Runoff Along the Colorado River of Western and Central Texas

The period 2011-2014 has been a time of major drought along the Colorado River of western and central Texas. The water year 2010-2011 in particular was historically dry. Since then, streamflow levels and reservoir inflows have continued to remain near historically low levels, despite the return of near-normal precipitation to most of the region. As a result, reservoir storage is at or near historically low values.

Motivated by streamflow simulations and records that show that substantial runoff is produced only by intense rainfall events, we investigate decade-scale variations in the frequency of unusually intense rainfall events in and near the Colorado River drainage basin. We relate such unusually intense events to streamflow variations and reservoir levels. We show that while mean precipitation has remained high, extreme events have declined in recent years, leading directly to declines in surface water availability.

We also compare intense event frequencies in neighboring regions to determine whether these variations in extreme events are taking place on a spatial scale too small to be plausibly caused by long-term climate change.