Reference evapotranspiration (ETo) rates are used extensively with crop coefficient (Kc) values to estimate crop water requirements for water resources planning and for irrigation scheduling. ETo is approximately equal to the evapotranspiration of a large expanse of unstressed cool-season grass that is between 0.10 and 0.15 m in height. The tall grass crop is used as a reference because it is assumed that the ETo rate varies only with changes in evaporative demand and it is not affected by management practices (e.g., irrigation or mowing).
Net radiation (Rn) is the main component of the equations that are typically used to estimate reference evapotranspiration for use in water balance calculations. Unfortunately, the sensors used to measure net radiation are expensive and delicate. Consequently, Rn over grass is often estimated from other weather variables coming from more durable sensors. Hourly calculation of Rn requires the use of complicated equations for estimating extra-terrestrial radiation, transmissivity through the atmosphere, cloud cover, emissivity from the sky, etc. In this paper, we will discuss Rn calculation methods and we will recommend a simple procedure for estimating hourly Rn over grass.