USING MULTI-PARAMETER RUNOFF ELASTICITY TO DISCERN LOCAL PARAMETER IMPACTS ON WATER SUPPLY UNDER A CHANGING CLIMATE

Water management in the face of variable climate must be implemented at the basin scale, and must account for the effects of local variability in climatic and hydrologic parameters. Determination of multi-parameter runoff elasticity (change in runoff relative to those parameters) is a valuable tool for quantifying these effects. The Dallas-Ft. Worth area (DFW) is entirely dependent on surface water, and lies in a mid-continent band of high precipitation elasticity (proportional change in runoff to change in precipitation), with ε_P = 2.64 and temperature elasticity ε_T= -0.41. Much of this elasticity derives from amplification of climate effects by Vertisols. Contour plots of DFW runoff departure vs. precipitation and temperature departure exhibit unusual concave patterns, indicating variable influence of additional parameters. The most influential local parameter in DFW is unexpected reduced summer (JJA) runoff in cooler wetter years. Those years exhibit increased JJA precipitation fraction, but widespread cracking soils in DFW minimize JJA runoff, yielding negative ε_P-JJA = -0.68. A comparative basin near Houston shows positive ε_P-JJA = 0.14, reflecting the local impact of tropical cyclones and lesser abundance of cracking soils. Both basins exhibit positive elasticity to one-year previous precipitation (e.g. DFW ε_P-1 = 1.24), reflecting the influence of soil moisture storage. Only DFW exhibits negative elasticity to two-year's previous precipitation (ε_P-2= -0.65) reflecting multi-year influence of vegetation growth and increased evapotranspiration.

Using the multi-parameter elasticities for DFW , analysis of historical multi-decadal climate departures indicates the 80% decrease in runoff during the 1950-7 drought of record was primarily caused by reduced precipitation. Runoff 56% above-normal during an unprecedented 1986-98 wet period was primarily caused by increased precipitation. This century, the primary influence has been temperature. Since 2000 runoff has fallen to 20% below-normal caused by ~1ºC warming, despite slightly elevated precipitation and reduced summertime precipitation fraction. Future warming will exacerbate this trend, and future precipitation droughts can be expected to exhibit magnified declines in runoff compared to historical experience.