During this time, the team collected high-resolution data on rainfall, streamflow, soil moisture and evapotranspiration using a variety of instruments operating in a coordinated manner. Using long-term data from these sensors, Schreiner-McGraw identified that large amounts of the incoming rainfall, especially during wet monsoons, was not being lost to the atmosphere via evapotranspiration nor from the channel system as streamflow. Instead, runoff was being lost as percolation in small channels not more than two feet in width — an unexpected finding.    

Simulating where the water was going — an improved hydrological model

By tracking the fate of monsoon rainfall, the research team set out to explain how hillslopes and channels of the piedmont slope might lead to groundwater recharge.

“Soils on hillslopes are very different than those in the channels,” Schreiner-McGraw said. “They are compact and do not absorb water very quickly, and they also have calcium carbonate layers about 12 to 20 inches below the surface that limit infiltration. Channels, on the other hand, have coarse and permeable sediments that can absorb water much more quickly.”  

This information was used to modify a hydrological model of the instrumented watershed, originally developed during Vivoni’s graduate studies at the Massachusetts Institute of Technology. Based on their field work, the research team tested the model against a suite of long-term data, including evapotranspiration, soil moisture, streamflow and percolation.

“It is uncommon to have a hydrologic model tested so thoroughly,” Vivoni said. “By performing iterations of field observations and model developments, we demonstrated the value of long-term research.”

The research team then used the numerical model to isolate two important factors affecting the percolation process: the infiltration properties of hillslopes and of channel reaches. Simulations indicated diverging effects of these factors on the proportion of rainfall that recharges groundwater systems. These findings are applicable to arid piedmont slopes anywhere on Earth. “Understanding the groundwater recharge process in arid regions can help us sustainably manage groundwater use in these climate settings,” Schreiner-McGraw said.

With water becoming an increasingly precious resource, a better understanding of how groundwater is recharged could help communities across the globe.

“Groundwater is much like a bank account,” Vivoni said. “Underground aquifers can store water delivered from surface systems which can be subsequently extracted under periods of water scarcity.”  

Effects of vegetation change through the land-water connection

The Chihuahuan Desert, like many areas in the southwestern U.S., has experienced a transition in vegetation communities from grasslands to shrub lands.

“We have historically used large open areas of the western U.S. and northern Mexico for livestock grazing,” Vivoni said. “As a result, many grasslands have disappeared and been replaced by desert shrubs.”

In addition, drought and fire suppression have contributed to the conversion of grasses to shrub lands. 

An open question remains as to whether this transition has impacted the groundwater recharge process in piedmont slopes.

“We have examined how the instrumented watershed contributes to groundwater recharge under current conditions,” Schreiner-McGraw said. “The next step in the research is to determine how these contributions would be altered under different plant communities.”

Here, the hydrological model will be employed as a numerical laboratory to determine how vegetation changes alter groundwater recharge, for instance under a historical grassland scenario or a case with desertification and the absence of vegetation.

“The future of water resources for humans and wildlife is uncertain,” said John Schade, director of the National Science Foundation’s Long-Term Ecological Research program, which funded the research. “Studies like this are essential to proper water management in the face of rapid environmental change, especially in arid lands where water is scarce. This study is an example of the critical role long-term research plays in uncovering what controls the availability of freshwater. It advances our ability to forecast how freshwater availability will change in the years and decades to come.”

Karin Valentine

Media Relations & Marketing manager, School of Earth and Space Exploration