Using National Weather Service Forecasts and Model Output Statistics to Forecast Evapotranspiration

Competition for water is increasing because of increased demand and reduced supply due to drought in the Great Plains for the past 5 yrs. Availability of water and its economic use are dominant factors governing irrigation and food production. If irrigators apply all available water, they may overuse water without adding benefit to the crop. A better understanding of the water requirement of a crop can yield greater benefits if irrigators can use just the needed amount of water which will protect yield while reducing water use. This better understanding can be achieved through estimating crop water use. Evapotranspiration (ET) helps in understanding crop water use. ET will help irrigators determine their potential water requirement and prevent them from over-irrigating or under-irrigating and improve the quality of the yield. All over the world, ET has been calculated from weather data acquired from weather stations or from lysimeters. Usually, current or archived weather data is used to calculate ET. Various weather parameters (temperature, wind, relative humidity, solar radiation) and methods (Hargreaves-Samani, classic Penman, etc.) are used to calculate ET with the help of computer models for different types of crops. But little effort has been made to forecast ET, which in tum will help irrigation scheduling. In this study, we have made an effort to forecast evapotranspiration for different crops. This information can be used by irrigators and agronomists, and others to schedule irrigation and for other purposes, such as tracking soil moisture. Although many methods are available to calculate ET, we have used the FAO Penman-Monteith method (Allen et al., 1998) to estimate forecast ET because this method is the most accepted method to calculate ET. This method uses temperature, relative humidity, wind speed and solar radiation as its variables to calculate ET. A computer application is developed to forecast ET for 1-3 days in advance. Daily ET values are estimated for three locations in South Dakota, Brookings, Pierre and Caputa, to compare the forecast ET to daily estimated ET from the weather data. If we can forecast ET for these three stations successfully, then ET can be evaluated for any other station in the state or for that matter in the country. The forecast weather information was used from National Weather Service (NWS) and Model Output Statistics (MOS) to evaluate forecast evapotranspiration. Forecasted temperature, relative humidity and wind speed are the required forecasted variables to be taken from NWS and MOS. As solar radiation is not readily available in these forecasts, we use three types of solar radiation to determine the best one for forecasting ET, a constant solar radiation, solar radiation from M-H model (Mahmood and Hubbard, 2002) and solar radiation from sky cover percentage forecast, in the Penman-Monteith method. This application absorbs forecasted weather information from NWS and MOS using PHP scripting language and is stored in a MySql database. This stored information is extracted using a PHP program with forecasts being calculated using three different types of solar radiation methods. These programs run three times a day for NWS data and once a day for MOS data to make use of forecasts which are updated several times a day. Forecasted ET values from both the sources are compared to estimated ET from automatic weather station data to determine the statistical accuracy of these estimates using a regression trend line in MS-Excel