Effects of huisache removal on rangeland evapotranspiration in Victoria County, south-central Texas, 2015–18

The U.S. Geological Survey and Desert Research Institute, in cooperation with the Natural Resources Conservation Service, Texas State Soil and Water Conservation Board, Victoria County Groundwater Conservation District, Victoria Soil and Water Conservation District, and the San Antonio River Authority, evaluated the hydrologic effects of Vachellia farnesiana var. farnesiana (huisache) removal on rangeland evapotranspiration in Victoria County, Texas. Measurements of evapotranspiration, rainfall, and related properties were made at two sites during March 2015 through August 2018. One site was predominantly grassland. The other site was dominated by dense huisache vegetation that was removed about halfway through the study period. The resulting evapotranspiration data were examined for differences between the locations and differences between the pre-removal (2015–16) and post-removal (2017–18) periods to assess the effects of huisache removal on evapotranspiration. Evapotranspiration measurements were made using the eddy-covariance technique and were supplemented by remote-sensing estimates of evapotranspiration derived from thermal and optical satellite images. A map of remotely sensed evapotranspiration was generated for the area surrounding the study sites for 2015 and demonstrates the capability of remote sensing to evaluate land-management effects on evapotranspiration for larger scale areas, such as a county or stream-basin area.

During the pre-removal period (March 2015–December 2016), evapotranspiration was greater at the huisache site than at the grassland site. Evapotranspiration at the grassland site (average of the eddy-covariance evapotranspiration and average remotely sensed evapotranspiration) was 87.6 millimeters per month (mm/mo) and at the huisache site was 100.8 mm/mo, with the differences in evapotranspiration rates being attributed to the difference in site vegetation. After huisache was removed in January 2017, evapotranspiration at the huisache site was substantially lower than at the grassland site, the changes in evapotranspiration rates being attributed not only to removal of huisache vegetation but also to possible disruption of soil runoff and infiltration characteristics. During the post-removal period (February 2017–August 2018), evapotranspiration was 88.5 mm/mo at the grassland site and 72.9 mm/mo at the huisache site (average of the eddy-covariance and average remotely sensed evapotranspiration).

The monthly differences in evapotranspiration between the grassland and huisache sites, determined by eddy-covariance and remote-sensing methods, were statistically significant between the pre-removal and post-removal periods. Also, the pre-removal period provided the best conditions to evaluate the differences between huisache site and grassland site evapotranspiration. During the pre-removal period, evapotranspiration from the huisache site as measured by the eddy-covariance method was, on average, 10.7 mm/mo greater than evapotranspiration measured at the grassland site. As determined by the average of the remotely sensed methods, huisache site evapotranspiration was 15.8 mm/mo greater than grassland site evapotranspiration. These average differences in evapotranspiration rates by the two methods indicate that evapotranspiration at the grassland site was, on average, 13.2 mm/mo less than that at the huisache site during the pre-removal period. This average difference in evapotranspiration rates also indicates potential increased groundwater recharge and (or) surface-water runoff at the grassland site.