A Soil Water Balance Approach: Quantifying the Contribution of Shallow and Deep Roots On the Edwards Plateau.

On the karst Edwards Plateau in central Texas, woody plants, especially Ashe juniper and honey mesquite are invading grasslands and savannas. These plants typically have deeper roots than grasses and can potentially exploit more stable sources of water. Quantifying the water loss through tree roots to water uptake is crucial for understanding how much soil water eventually recharges groundwater. The objective of this study is to monitor soil moisture profiles and quantify contributions of shallow and deep roots based on a soil-water balance in a field plot. The experimental site was a savanna with ~50% woody cover on the Edwards Plateau. The soil is a gravelly loam with a high percentage of chert fragments overlying fractured limestone at a depth of ~1.5 m. The research used neutron scattering and time domain reflectometry (TDR) to measure water content profiles to a depth of 1.5 m.  Access tubes and TDR probes were installed on a 25 m x 25 m grid, with a 5-m node spacing (total of 36 access tubes and TDR probes). A soil water balance approach allowed us to calculate seasonal changes in water loss from shallow and deep layers. Spatial variation of rock density significantly affected water content profiles and the water loss. The amount of water loss from the water balance was compared with evapotranspiration obtained from eddy covariance.