Comparison of actual and crop evapotranspiration from grape vines

Maximum crop evapotranspiration (ETc) represents the evapotranspiration rate for a crop with little or no reduction in transpiration due to a water limitation. However, ETc is dependent on the irrigation method and management because of the influence on soil evaporation. When the soil surface evaporation is minimal, differences between actual evapotranspiration (ETa) and crop evapotranspiration (ETc) result from plant control of transpiration, which depends mainly on environmental conditions. Therefore, the ratio of ETa to ETc may provide a good measure of water stress when there is minimal soil evaporation. To investigate this idea, experiments were conducted over grapevines near Villasor, Italy. ETc was calculated as the product of ETo calculated using the Penman-Monteith equation and crop coefficient (Kc) values from FAO Irrigation and Drainage Paper 56. Big changes in the ratio ETa/ETc occurred following an irrigation, but the ratio tended to stabilize once the soil surface had dried. Typically, ETa and ETc rates were similar until just before noon when the ETa dropped below the ETc rate For the remainder of the day. The hourly and daily differences between ETa and ETc were attributed to both soil evaporation and physiological responses of the plants to water deficit.