Evaluating the In-Situ Soil Water Budget at USCRN Sites Using Soil and Climate Data

Accurately formulating the energy and water transfer to and from land vegetation is needed to provide a useful quantification and understanding of the water budget at the U.S. Climate Reference Network (USCRN) sites. Here a simple, semi-empirical model of soil water budget is developed by combining the energy and water transfer above the vegetation with dynamic processes in the soil beneath, so that evapotranspiration (ET) loss above the vegetation and soil water drainage and infiltration are predicted on the basis of the available net radiation, biophysical controls of the vegetation cover and the physical controls by the soil. Input climate data for the model is obtained from the USCRN observations largely consisting of air temperature, solar radiation, wind speed, humidity, and precipitation; and input data required for the soil consist of bulk density, texture, and soil water at wilting point and field capacity. The model is formulated to enable the water exchange and heat conduction at the soil surface to be calculated by linking the aboveground equations of energy and water transfer to equations of soil water transport and heat conduction. Fine tuning of the model is ongoing to develop generic model parameters applicable across a wide range of different vegetation-climate sites in the continental U.S. Preliminary tests conducted at USCRN sites in Texas and Illinois suggest that with adequate specifications of site characteristics data of the soil and the vegetation the model is able to quantify the surplus, sufficiency and deficit of the soil moisture observations at the various USCRN sites.