NOTE : ETo is a Reference Evapotranspiration. --------- This is NOT equal to a Pan Evaporation value. ETo values can be considered equal to evaporation from a large body of water, such as a pond or lake. However, for smaller, shallower bodies of water this relationship does not apply. Many factors affect lake and pond evaporation including surface area, depth, water temperature, and turbidity. To date, not enough research has been done to determine the minimum body of water size at which ETo is still equal. In the southwest USA, evaporation pans do not accurately measure the amount of evaporation which is actually occurring. The sunlight heats the sides and bottom of the metal pan, which adds more energy to the water. This results in incorrectly high evaporation values. To get an approximate Pan Evaporation value from the ETo value use this 'rule-of-thumb' conversion: divide the ETo value by a conversion constant. In winter or cooler times of the year, divide by 0.7. During summer or warmer periods, divide by 0.6. || ETo Winter || ETo Summer ------- = Approx. Pan Evap. || ------- = Approx. Pan Evap. 0.7 || 0.6 || ETo units can be in 'English' (inches) or Metric (millimeters). Many factors can affect the rate of evaporation from an open body of water; depth of water, area of the water, temperature of water, turbidity of the water, topography and vegetation surrounding the body of water, etc. The rate of evaporation from a irrigated field (drip, flood, sprinkler) can be affected by the same factors as an open body of water (above). The soil type, texture, color and porosity will also influence evaporation.