Linking Soil Water to Water Resources in the Texas Hill Country.

The 2011 drought in Texas resulted in a total water storage deficit of 62 km³ with soil water accounting for potentially 20-100%. This uncertainty, stemming from model output, is difficult to overcome without observational data; yet, soil water monitoring networks are currently sparse across Texas. Although we can observe stream flow, reservoir capacity, and groundwater levels, soil water storage is more subtle. As drought continues in Texas, or occurs elsewhere, water managers are becoming aware of the soil water deficit and its link to reservoir and groundwater levels. To assist managers and to advance the science of monitoring soil water, we have two primary goals: (1) to serve as a calibration and validation point at 3, 9, and 36 km to support NASA’s Soil Moisture Active Passive (SMAP) satellite that launches later in 2014 and (2) to operationalize soil water into water forecasting models used by water authorities at state and county levels. Here, we focus on the implementation of our forthcoming Texas Soil Observation Network (TxSON): an intensively monitored 36 km cell (1300 km²) located near Fredericksburg, Texas, along the Pedernales River and within the middle reaches of the Lower Colorado River Authority. TxSON consists of over 40 soil water monitoring stations with sensors installed at 5, 10, 20 and 50 cm depths. The network will support SMAP’s Calibration and Validation Program by operating within the Equal-Area Scalable Earth (Version 2) Grid. Using a nested design, TxSON will replicate soil water measurements at 3, 9 and 36 km cell footprints corresponding to SMAP retrievals. The grid location was determined through temporal stability analysis of soil water time series data from the North American Land Data Assimilation System model across each Hydrologic Unit Code (HUC8) within the Middle Colorado drainage basin. Subgrid locations at 3- and 9-km were based primarily on land accessibility and secondarily on geomorphic settings including soil thickness, bedrock geology, and terrain. The end goal is to establish areally-averaged soil water (content and storage) estimates at each spatial scale through additional field measurement campaigns. Although the primary motivation for this study is to provide on-ground calibration for the SMAP program, our project goal is to assure that the data is valued by Texas water resource managers, by linking soil water to surface water and groundwater resources.