| Popis: |
Unlike the Penman and Penman-Monteith methods, recursive calculation of crop evapotranspiration (ET) is a combination method that makes no assumption regarding the temperature and saturation humidity at the evaporating surface. Our objective was to experimentally verify a recursive calculation of crop ET. It was tested on an hourly and daily basis using data from Bushland, Texas, and on a daily basis with data from Tempe, Arizona, both on a well-watered alfalfa crop. In both locations, ET was measured half-hourly with lysimeters accurate to 0.05 mm. In Bushland, hourly amounts of ET were calculated from the hourly weather data using the recursive method with a root mean squared difference (RMSD) of 0.05 mm averaged over four days; and daily ET calculations using daily data for 26 days resulted in an RMSD of 0.75 mm. The sum of 26 daily-calculated values of ET was found to be within 7 mm of the measured amount. In Tempe, daily ET was also calculated with the same recursive formula from daily weather data for a period of 20 days after a flood irrigation that kept the crop well watered. The RMSD of daily ET was 0.8 mm, and the calculated total amount of ET over 20 days was within 2 mm of the measured value. We conclude that an iterative formulation provides good accuracy for calculating ET of well-watered alfalfa and that similar studies with other irrigated crops grown in hot and dry climates are needed. |
