Improvements to Water Use and Water Stress Estimates with the Addition of IR and Net Radiometers to Weather Stations.

Evapotranspiration (ET) is often estimated with the Penman-Monteith (P-M) equation. Net radiation (R_n) is a major component of the surface energy balance and an input to the P-M equation, but it is challenging and expensive to measure accurately. For these reasons, most weather stations do not include net radiometers and a model is used to estimate R_n for evapotranspiration (ET) calculations. Estimates of R_n from the sub-model used in routine application of the P-M equation were compared to R_n measurements made with four-way net radiometers over turfgrass and alfalfa canopies (the two defined reference surfaces for application of the P-M equation). Data indicate R_n estimates from the sub-model tend to be high, and that more accurate R_n data for use in ET estimation can be obtained by adding a net radiometer or infrared radiometer to weather stations running the P-M equation. Routine application of the P-M equation requires use of a defined reference surface representing well-watered (no stress) conditions. Thus, estimates of ET using a reference surface represent a reference ET. Actual water use for the crop of interest is then calculated by scaling this reference ET by an empirical crop coefficient. Measurement or estimation of surface temperature with an IR radiometer or net radiometer allows for application of a two-source (canopy and soil) energy balance model from which actual crop water use can be calculated. Actual crop water use can then be compared to reference crop water use as way to estimate water stress. A two-source energy balance model was verified over a well-watered alfalfa crop where ET was measured with a weighing lysimeter. The two-source model was then used to estimate water stress in other crops.