DOES RECHARGE MATTER? HOW GROUNDWATER RESOURCES IN TEXAS RESPONDED TO THE STATEWIDE DROUGHT OF 2011–2015

The drought of 2011–2015 in Texas ended as the second worst statewide drought of record, right behind the drought of the 1950s. As measured by surface-water reservoir storage, drought in west and far west Texas started in the mid-1990s and continues to today, contemporaneous with the ongoing drought in the southwestern United States.

The response of aquifers in Texas to the drought of 2011–2015 depended broadly on how those aquifers respond to climate and the amount of pumping, primarily from agriculture. Recharge in climate-responsive aquifers, such as karstic limestones (like the Edwards Aquifer) and highly permeable clastics (like the Seymour Aquifer), show near-instantaneous water-level responses due to sustained rainfall deficits. However, most aquifers in Texas are not responsive to drought or similar short-timescale changes to recharge. For example, recharge to the Ogallala Aquifer is not immediately effected by episodic periods of drought because of the time required for water to reach the water table (decades to centuries) and because what does reach the water table over a geographic area is a broad integration of climatic conditions over time. The response of the confined clastic aquifers of the state (Gulf Coast Aquifer, the Carrizo-Wilcox Aquifer, and Trinity Aquifer north of the Colorado River) is independent of recharge, in large part because these systems are permeability limited rather than recharge limited.

Pumping, especially for irrigation, is directly linked to drought: less rain means more pumping, and more pumping means larger water-level declines. Municipal pumping also increases, especially if surface-water supplies become unreliable. The median water-level decline in the state’s recorder network for the Ogallala Aquifer was -1.9 feet between 2010 and 2011 whereas the decline was 0.5 feet for the wetter 2014 to 2015 period. The confined aquifers of the state also showed larger declines in water-levels during the drought in the recorder network. For example, Trinity Aquifer north of the Colorado River had a median decline of 16.7 between 2010 and 2011. Between 2013 and 2014, when the drought was less intense, the decline was 2.4 feet. Once rainfalls returned to normal to above normal conditions, water levels rose 8.2 feet, reflecting an artesian pressure response to decreased pumping.