IMPACT OF HIGH RUNOFF ELASTICITY ON WATER RESOURCES IN NORTH TEXAS UNDER CLIMATE CHANGE
Water availability in North Central Texas (DFW, Dallas-Ft. Worth) is remarkably sensitive to climate, and recent multi-decadal climate variations may provide an important preview of likely hydrologic consequences of long term climate change. Mean runoff elasticity for DFW is 2.8, representing a high average multiplier for runoff change in response to precipitation change. Two statistically distinct periods of anomalous precipitation illustrate this sensitivity, the 1950-57 drought of record (observed changes: -25% precip, -60% runoff, Mann-Whitney p≤0.001) and a multi-decadal wet period from 1983-1996 (+8% precip, +43% runoff, p=0.014). DFW depends on surface water for 94% of its supply, and average storage has declined 15–20% since 1996, while gated (flood) releases have decreased by 50%. The end of the wet period has doubled the monthly risk of encountering Stage 3 drought restrictions in DFW, albeit to 1.5%. Monthly risk of encountering Stage 1 or higher drought restrictions is 15%, seven times that experienced during the wet period. Net discharge from DFW via the Trinity River has decreased only 16% since 1996, as flow has been augmented by urbanization-related increases in runoff, increased water importation and reuse within the basin.
Projected climate change will serve to further reduce runoff, primarily through increased evapotranspiration in response to warming. Utilizing historical correlations of drought index (Palmer Drought Severity Index or PDSI) and stream discharge, increasingly negative future PDSI would be accompanied by up to a 40% reduction in average annual runoff by 2050. Probability of extended drought increases similarly, and probability of yearlong droughts increases to 30% from historical values of 4%. Three-year droughts, historically resulting in imposition of Phase III drought restrictions, increase in probability to nearly 10%.
Water resources plans must account for these effects of runoff elasticity, or risk coming up short.