Assessing recent declines in Upper Rio Grande runoff efficiency from a paleoclimate perspective

Recent decades have seen strong trends in hydroclimate over the American Southwest, with major river basins such as the Rio Grande exhibiting intermittent drought and declining runoff efficiencies. The extent to which these observed trends are exceptional has implications for current water management and seasonal streamflow forecasting practices. We present a new reconstruction of runoff ratio for the Upper Rio Grande basin back to 1571 C.E., which provides evidence that the declining trend in runoff ratio from the 1980s to present day is unprecedented in context of the last 445years. Though runoff ratio is found to vary primarily in proportion to precipitation, the reconstructions suggest a secondary influence of temperature. In years of low precipitation, very low runoff ratios are made 2.5-3 times more likely by high temperatures. This temperature sensitivity appears to have strengthened in recent decades, implying future water management vulnerability should recent warming trends in the region continue. Plain Language Summary Since the 1980s, major river basins in the American Southwest such as the Rio Grande have experienced droughts, declining streamflow, and increasing temperatures. More importantly, runoff ratiothe portion of precipitation that ends up in the river each year, rather than evaporatinghas been decreasing as well. For water managers, it is important to know whether these trends are exceptional or are merely patterns that have occurred throughout history. We use long reconstructions of historical climate based on tree rings to estimate, for the first time, the paleo runoff ratio of the Upper Rio Grande. This new record indicates that the recently observed trends in runoff ratio are unprecedented in the 445 year record. Together with precipitation, high temperatures have an important influence, making very low runoff ratios 2.5-3 times more likely. These findings suggest that runoff ratio could decrease further if warming in the region continues, which may present challenges for water management in the river basin.