Using a Weighing Lysimeter to Determine a Crop Coefficient for a Green Roof to Predict Evapotranspiration with the FAO Standardized Penman-Monteith Equation

Evapotranspiration (ET) has been studied extensively by the agricultural sector for irrigation purposes. The Food and Agriculture Organization (FAO) and American Society of Civil Engineers (ASCE) published standardized Penman-Monteith equations with coefficients for crop specific characteristics and water stress conditions. The crop coefficient is based on the stomatal resistance of the leaf and the aerodynamic resistance through the foliage of the plant. The FAO suggests using the average number of days between rainfall events as a measure of potential water stress conditions. Stormwater Best Management Practices (BMPs) are designed to capture rainfall. Evapotranspiration may be a large part of the outflow from BMPs. Preliminary investigation of a green roof's performance, using a weighing lysimeter, in 2009 and 2010, indicates that soil moisture conditions play a large role in the rate and quantity of ET. Crop and water stress coefficient were developed to predict ET.