Developing a soil moisture Decision Support Tool to quantify the occurrence of flash droughts and saturated soil conditions for pasture grasses in the southeast of the United States

Extended droughts are known to cause severe damage to crops. Short-term droughts of two to three weeks that occur in areas with high evapotranspiration demands and soils with low water-holding capacity can also significantly affect crop yields although their impact has not been well quantified. These short-term droughts are sometimes referred to as flash droughts. The timing of flash droughts likely has a major impact on whether or not they result in significant yield losses. An ongoing project funded by the U.S. National Oceanic and Atmospheric Administration (NOAA) is quantifying the effect of flash drought on rainfed agronomic crops and pasture grasses in the southeastern U.S. The project is also developing tools to forecast when flash drought periods result in significant yield losses. This paper reports on the development of a tool for estimating daily crop water use and soil water content for three commonly used pasture grasses of the southeastern U.S. – Bermudagrasses (Cynodon dactylon and C. dactylon´ C. nlemfuensis), Bahiagrass (Paspalum notatum), and Tall Fescue (Lolium arundinaceum). Five rainfed farmer-managed fields in which these grasses are grown for hay were instrumented with capacitance-type soil moisture sensors to continuously measure volumetric water content in 12 cm increments to a depth of 60 cm. These data are used to estimate daily crop water use / daily crop evapotranspiration (ETc) which in turn is used to estimate daily crop coefficient (Kc) values using Penman-Montieth evapotranspiration (ETo). ETo is calculated from the University of Georgia Weather Station Network weather stations located near the fields. The final product is a decision support tool that helps farmers quantify the duration of periods of low soil moisture content. The effect on the yield of these flash droughts is quantified by using the DSSAT CSM-CROPGRO-Perennial-Forage crop simulation model.

Keywords: remote sensing, evapotranspiration, crop coefficient, smart irrigation.