The term Firm Yield has been relied upon for water resource planning and determining supplies for future development. As we face another year of brutal drought conditions and heat it is time to acknowledge this is an ESTIMATE at best. Far too much is at stake to continue relying on this as a planning tool.
Firm Yield is defined as the MAXIMUM amount a reservoir could supply under the most severe conditions known to date. The 1950’s Drought of Record is used as a benchmark. The word risky is an understatement as this uses a BEST CASE SCENARIO assuming reservoir supply will meet those expectations. If and when these estimated supplies fall short water resources will be inadequate to supply developments built years before.
Using the 1950’s drought as a benchmark for firm yield has been sharply criticized by hydrogeologists as unreliable due to climate change. According to data collected worldwide this first two weeks of July has been the hottest recorded in human history. These trends are expected to continue. Demands increase sharply and higher temperatures have a profound impact on reservoir evaporation losses. Data collected reveals that 114 reservoirs in Texas lost an average of 6.88 MILLION acre feet of water per year. Even a modest increase of surface water temperature (2 degrees Celsius) will increase these losses by 15%, an additional 1 MILLION acre feet annually. These losses will increase with temperature.
The Guadalupe River has had extremely low flow above Canyon Lake for over two years and is again disappearing entirely. This is a direct result of decreased groundwater levels. These conditions could continue indefinitely. This is not sustainable and will inevitably reduce the supplies from Canyon lake and downstream. Has this been accounted for? Are we prepared for the Guadalupe and other spring fed waterways to STOP supplying reservoirs?
The Trinity Aquifer recharges very slowly with less than 4% of rainfall becoming groundwater. Unlike the Edwards its geology is complex and layered. Water migrates, response to drought is unpredictable. Additional monitoring wells are needed to more accurately assess this resource. Twenty percent of the City water supply is from two Trinity wells.
All water resources are interconnected, depletion of one source impacts others.
Climate conditions are expected to become more severe. Water resource planning based on best case scenarios and estimates is increasingly risky. Forecasting water supplies is extremely complex involving data collection, statistical analysis and modeling. Even with these efforts there is always a degree of uncertainty. The drawbacks of using firm yield are obvious and need to be acknowledged. This is increasingly obvious as the ground bakes beneath us and regional rivers become bone dry.
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