Hourly Reference Evapotranspiration Estimates for Alfalfa in North Dakota

When many places in the world are facing water scarcities, the Devils Lake basin in northeastern North Dakota has received above normal inflow due to excess rainfall since 1993. A research project is being conducted to determine the extent to which irrigation can utilize some of the excess surface waters in the basin without impairing the productivity of soils, many of which are classified as only conditionally irrigable. Understanding of the evapotranspiration (ET) process is critical for irrigation application, but ET research is generally lacking in this region. The sub-humid continental climate in North Dakota has high variations in hourly, daily, seasonal, and annual ET rates. The soil normally stays wet in the early growing season. Irrigation is needed in the mid-growing season when crops are also in a critical stage for supplemental water. In this study, hourly reference evapotranspiration rates for alfalfa were examined by the ASCE EWRI Penman-Monteith method. The weather data used in the calculations were collected from a 42-ha alfalfa field located in Benson County, North Dakota. Thirty minutes values, including air temperature, relative humidity, solar radiation, and wind speed were measured and average hourly values were used in the reference ET calculations. The evaluation and comparison among the daily and sum of 24 hour reference ET provided a good assessment on the highest amount of water that could be potentially used by agricultural crops in the Devils Lake area.