Quantifying the Influence of Long-Term Climate Trends on North Texas Water Yield, Demand, and Return Flows

Access to reliable sources of water is essential to urban areas, particularly those where precipitation is highly variable. Texas regularly experiences droughts severe enough to require restrictions on water usage to ensure that there is enough water available for primary purposes, interspersed with heavy precipitation events and record-breaking wet spells. As climate change is expected to increase in the severity and frequency of both future droughts and heavy precipitation events, the importance of incorporating these trends into planning for future water management is becoming evident.
The North Texas Municipal Water District, together with Freese & Nichols, Inc. and Atmos Research & Consulting, are collaborating on a study to quantify the influence of long-term climate trends on future water supply, demand, and return flow for two reservoirs, Lavon Lake and Jim Chapman Lake, in North Texas that provide part of the supply for over 1.7 million people north of Dallas and in the surrounding area. The study uses long-term observations of temperature, precipitation and evaporation as well as supply, demand and return flow, and combines these with future simulations generated by 23 CMIP5 global climate models, statistically downscaled to local weather stations. Two climate futures are analyzed: one where the global mean temperature increase is limited to 2°C, following the Paris Agreement, and another where global mean temperature increases to 4°C. The resulting projections are then used as input to water inflow-, demand-, and return flow models, and provide a clear message: long-term climate trends impact supply significantly, demand moderately, and return flow only minimally, compared to the impact of other factors. In terms of supply, during periods of drought, estimated reservoir yields for the 2°C and 4°C futures are considerably below those of historical droughts. Demand, however, continues to increase, both due to anticipated population increases as well as slighter higher per capita demand for outdoor watering during more frequent periods of drought. Average annual return flows appear to be less affected by climate change, which was indicated by the hydrological regression models being more sensitive to the selected input variables than the long-term climate trend. Average annual return flows may be more impacted by indoor water use patterns and water restrictions.
The study highlights the importance of NTMWD and the North Texas region being prepared for a future with periods of less available water and the district can now begin to plan and build a water supply portfolio more resilient to the impacts of long-term climate trends.