Effects of Huisache Removal on Evapotranspiration

Science Center Objects

USGS Texas Water Science Center scientists are evaluating potential changes in the hydrologic budget, specifically evapotranspiration, as a result of brush management (applied to huisache) as a conservation practice. 

Huisache

Huisache (Acacia farnesiana) is a woody plant that can grow 15-20 feet tall and uses an extensive tap root system to access both shallow and deeper groundwater sources.

Huisache is a prolific seed producer and can overtake a grassland prairie in a generation. Once established, it can readily out-compete native grasses for space, often resulting in areas with low wildlife or grazing benefits.

Source: Smith, H.N., end C.A. Rehentbin. 1964. Grassland restoration. The Texas brush problem. USDA, SCS, Unnumbered publication. Temple, Texas.

Summary

Measurements of evapotranspiration by eddy covariance will be used to evaluate changes in the hydrologic budget before and after brush management has occurred. In addition, the use of remote sensing will be evaluated for applicability to estimate evapotranspiration rates at a regional scale. These data will help inform future water resources management decisions, as well as potentially be incorporated in future hydrologic watershed models.

Project

Many of the traditional oak grassland savannahs across Texas have experienced woody encroachment which has been attributed to fire suppression and overgrazing. Anecdotal reports from ranchers as well as some scientific studies, suggest that by removing woody vegetation and allowing native grasses to reestablish in their place (hereinafter referred to as brush management), the hydrology in the watershed might change – specifically in regard to the evapotranspiration rates.

For example, at Honey Creek State Natural Area where brush management occurred (in that case, ashe juniper were removed and native grasses were allowed to reestablish), hydrologic data collected during pre- and post-brush management (USGS SIR 2011-5226) exhibited a reduction in evapotranspiration in the treatment watershed after brush management occurred. Potential changes in the hydrologic budget as a result of huisache removal needs to be quantified to better evaluate the effects of vegetative land-cover changes at a watershed scale.

The U.S. Geological Survey, in cooperation with The Natural Resource Conservation Service, Texas State Soil and Water Conservation Board, Victoria Ground Water Conservation District, and The McFaddin Ranch, will evaluate the effects of brush management (in this case, huisache removal) on the hydrologic budget using a paired site approach.

Evapotranspiration and rainfall will be measured on two adjacent plots of land, each approximately 200 acres. The paired sites are located near the town of McFaddin, Tex., between the San Antonio River and Guadalupe River in Victoria County. The first plot will be representative of a managed grassland area, and the second plot will be representative of a huisache dominated area. Evapotranspiration and rainfall will be measured at both sites for two years. After two years, the huisache site will be treated (i.e., remove the huisache), and native grasses will be allowed to re-establish, thus transforming it to resemble a grassland plot. Evapotranspiration and rainfall data collection will then continue at both sites for another 2 years.

To measure ET, each site will be equipped with an eddy-covariance system. This instrumentation includes an infrared gas analyzer, a three-dimensional sonic anemometer, a net radiometer, air temperature and humidity sensors, soil heat flux plates, and soil temperature and soil moisture sensors. The eddy-covariance instruments will be placed on (and near) a 40 foot tower at the huisache site and a 10 foot tower at the grassland site. To measure rainfall, each plot will be equipped with a 12-inch tipping bucket type rain gauge placed near the meteorological tower.

In addition to site-specific meteorological measurements, the use of satellite imagery (also known as remote sensing) from Landsat 8, Landsat 7, and MODIS will be evaluated for applicability of estimating evapotranspiration rates on the monthly to annual scale at the study area. Evapotranspiration (via remote sensing) will be calculated for one year during the pretreatment period and one year during the post-treatment period for both sites.

Real-time Data

USGS site 283526096594800 - ET Gage (treatment site) nr McFaddin, TX

USGS site 283534097002100 - ET Gage (reference site) nr McFaddin, TX