Let’s start thinking about water!

Risk-Based Decision-Support Groundwater Modeling for the Lower San Antonio River Basin

Surface water–groundwater interaction has been a growing area of interest (and controversy) in Texas because both systems are connected in the physical space, even if they aren’t in the legal space. With increasing demands and the potential of decreasing recharge due to a warming climate, effects on water levels in groundwater systems and flows in surface-water systems are becoming more of a concern because the use of private property rights in the subsurface affect the use of private property rights in the streamcourse.

Foster and others take a modeling look at this issue in the lower San Antonio River Basin while taking a stab at quantifying uncertainty. They included the part of the basin that encompasses the Carrizo-Wilcox and Gulf Coast aquifers. Foster and others found that low flows in the river could be reduced by 6-25% over an 8-year period if climate reduced recharge by 25% and pumping increased by 25% with “a fair amount of uncertainty.” The recharge reduction did not include the temporal dynamics of the unsaturated zone, so Foster and others did not consider any time delays associated with percolation to the water table.

Gradient Self-Potential Logging in the Rio Grande To Identify Gaining and Losing Reaches Across the Mesilla Valley

Ikard and others use a geophysical technique — a gradient self-potential logging survey — to assess surface water-groundwater interaction on the Rio Grande in the Mesilla Valley. Surface water–groundwater interaction in this area is the subject of a U.S. Supreme Court case between Texas and New Mexico over releases from Elephant Butte Reservoir and irrigators intercepting that flow through wells along the river. The survey began at Leasburg Dam in New Mexico (about 10 miles north of Las Cruces) and ended at Canutillo, Texas. The survey showed a net loss of water in the river between Leasburg Dam and the Mesilla diversion dam 20 miles downstream. After that, the river was neutral to gaining for 21 miles before transitioning to losing for the rest of the survey.

Climate Change Amplification of Natural Drought Variability—the Historic Mid-Twentieth-Century North American Drought in a Warmer World

We don’t need climate change for droughts. The record, including tree rings, shows droughts affecting North America long before elevated greenhouse gases became a concern. But the question remains: How might a warmer climate impact future droughts? Cook and others seek to answer that question through a 20-member climate model.

First, they assign blame for the 1950s drought to “cold tropical Pacific sea surface temperatures,” that is, multi-year La Niña conditions. When they repeat a similar situation in their climate models with enhanced warming (3° Celsius) of the climate, they find that cold-season rainfall deficits aren’t as bad in the Southwest and Southern Plains, but that warm-season rainfall deficits are worse across North America and intensified in the Southeast. Cook and others also found that runoff and soil moisture deficits are significantly worse across the South during the warm season due to rainfall deficits and increased evaporation from warming.

Modeling studies like this are helpful for getting a glimpse into what the future may hold. For example, if water supplies rely on winter precipitation in the Southwest, this is good news. If not, then this is not-so-great news. Ultimately, these modeling results will be tested after time and data reveal what the actual effects are.