Comparison of Measured and Modeled Evapotranspiration Rates for Turfgrass in North Dakota.

Limited research has been done to determine the actual evapotranspiration (ET) rate for turfgrass water requirement in North Dakota. With a higher water demand following the North Dakota oil boom and the increase in population, precision irrigation scheduling for turfgrass with an up-to-date crop coefficient (K_c) and reference ET method is urgently needed. In this project, an Eddy Covariance (EC) system and two sets of soil moisture (SM) sensors were used to measure actual ET rates at a cool grass surface at North Dakota Agricultural Weather Network (NDAWN) Fargo station site in 2011-2014. The NDAWN site is a grass area of 0.5 ha, surrounded by agricultural lands with various crops. Using reference ET estimated with Jensen-Haise (ET_JH) method and Penman-Monteith grass (ET_o) and alfalfa (ET_r) methods, new K_c values can be developed from the ratio of measured to reference ET values using the four year’s data. Comparing the measured ET rates (EC and SM methods) with the modeled ET rates (product of the newly developed K_c values and the three reference ET values) as well as the traditionally modeled ET rates (0.85ET_JH), the biggest ET difference was found in spring time when wind speed was high. Though the annual amount of turfgrass water demand is similar between the measured and the modeled ET methods, the seasonal ET variations can provide a better understanding of turfgrass water requirements and effective ways to conserve water in turfgrass irrigation.