Monitoring of Hydrologic Effects of Salt Cedar Control in the Upper Brazos River Basin, Texas, August 31, 2019

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2019

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Invasive saltcedar (Tamarix spp.) is prevalent throughout the upper Brazos River (UB) basin in the Southwestern Tablelands ecoregion of West Texas. The armoring of stream banks and sandbars by saltcedar may lead to a reduction in stream width, deepening of stream channels, and increased current velocities. Additionally, saltcedar has been suspected of being a relatively high user of alluvial groundwater and soil moisture. Consequently, saltcedar control has been considered as a potential strategy for conserving water, increasing streamflow, and restoring natural channel hydraulics and riparian habitat.

The goal of this study is to monitor soil moisture in the unsaturated zone, groundwater level, and conductivity in UB alluvial aquifers, along with precipitation, to characterize UB hydrology and evaluate the efficacy of herbicide treatment on saltcedar stands to alter water use. Specifically, this study focuses on surface and groundwater exchange in shallow alluvial aquifers, addressing questions such as: How does saltcedar abatement affect baseflow? When does the river gain water from or lose water to the aquifer, and how does this process vary depending on season and stream stage?

We evaluated river baseflow, storm flow, and total flow changes at upstream USGS gauges to assess subbasin-scale streamflow gain and loss. Additionally, we determined soil and alluvial aquifer water storage, groundwater flux, hydraulic gradient magnitude, and changes in groundwater flow direction. We estimated evapotranspiration (ET) during the study time by measuring diurnal soil-water content changes from the soil surface to a 50-cm depth, as well as by measuring diurnal groundwater table fluctuations using a method developed by White (1932).

Ultimately, this study enhances our understanding of soil water and groundwater fluxes at representative UB saltcedar stands.

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