FAO56 Penman-Monteith Open Water Evaporation Estimates

The Penman-Monteith combination equation is used to estimate the rate of moisture transport away from a surface (i.e., evaporation rate from soil or water surfaces and transpiration rate from plant surfaces) since this parameter is only observed at a limited number of locations across the southeastern United States. The total moisture flux from the ground to the atmosphere plays an important role in the water and energy balance on earth’s surface and is of particular interest to agricultural and irrigation practices.

By modifying the Penman-Monteith combination equation for an open water surface and using the same methodology as in the Food and Agriculture Organization of the United Nations (FAO) paper number 56, an equation to estimate open water evaporation has been derived. Specific characteristics include an albedo of 0.05, height of 2mm, and a canopy surface resistance of zero since the surface is open water. Similar to the FAO56 Penman Monteith method, this technique requires physical atmospheric observations of solar radiation, temperature, relative humidity, and wind speed. The estimate is dependent on time of year and location so the equation also requires parameters of day of year, elevation, and latitude. For more detailed information on the FAO56 Penman-Monteith method, please refer to the UN FAO56 Penman-Monteith documentation.

Please note that solar radiation measurements are not readily available at all stations across the Southeast US. In these cases, the Hargreaves and Samani (1985) solar radiation estimate is utilized as an input to the modified Penman-Monteith equation. The Hargreaves and Samani (1985) technique is based on the assumption that daily temperature range varies indirectly with cloud cover and thus, is related to incoming solar radiation.

To obtain raw data values of daily estimated open water evaporation, please visit our CRONOS database, or contact our office for larger datasets.