Transpiration and the water relations of barley

Measurements of evapotranspiration, leaf water potential and stomatal conductance were made with a barley crop over the spring and summer of 1979. From these measurements a model of transpiration from barley was developed. Predictions of the model were compared with Bowen ratio measurements of evapotranspiration on a selection of days whose results had not been used in the development of the models. Model predictions agreed well with measurements. It was found that leaf water potential could be predicted adequately with a simple model comprising a series resistor and zener diode, with a capacitor in parallel to account for changes in stored water within the plant. Inputs to the model are the transpiration rate and the effective value of soil water potential. Changes in water storage within the plant and around the roots cause changes in leaf water potential to lag behind changes in transpiration rate, and at times more than 50% of the flow within the plant was to or from storage. Stomatal conductance was found to have a linear dependence on net radiation, leaf water potential and vapour pressure deficit, increasing with the first two and decreasing with the last. An equation was found which predicted stomatal conductance from these three variables. When this was used in conjunction with Monteith's equation and an appropriate value of the effective leaf area index it predicted evapotranspiration well. The internal model was combined with the equation predicting stomatal conductance to give a comprehensive model of water flow through barley from the soil to the atmosphere. The model uses as inputs net radiation, ground heat flux, temperature, humidity, windspeed, leaf area index and effective soil water potential. Most of these are easily measured or estimated. The model predicts leaf water potential as well as transpiration and is potentially useful in the prediction of water deficits in plants and may be an aid to both growth prediction and irrigation scheduling.