Physiologically Dynamic Alternatives to Crop Coefficients for Use Under Deficit Irrigation Regimes.

Crop deficit irrigation is one alternative to relieve pressures on agricultural water resources in the western United States. Crop models provide a tool for designing strategies to quantify crop stress and water use under deficit irrigation. Most cop models, however, do not predict crop performance in response to soil moisture stress. Similarly, crop coefficients estimate evapotranspiration (ET) of a crop species from meteorological data by developing a relationship between ET of a well-watered reference crop relative to the predicted crop. A crop coefficient, however, fails to take into account changes in plant morphology and physiology, and is only applicable when soil water is adequate. The objective of this research was to assess the MAESTRA models accuracy for estimating Helianthus annus transpiration rates in response to a wide range of soil water deficits. MAESTRA is a mechanistic based plant model that has successfully forecast transpiration and carbon sequestration rates under irrigated and water-stressed conditions in woody perennials. However, we know of no report that uses it to predict herbaceous species transpiration. MAESTRA was parameterized with measured physiological and morphological plant traits. For example, weekly estimates of the maximum carboxylation rate and the maximum electron transport rate were calculated from measurements of gas exchange. In a glasshouse, well-watered and water stressed Helianthus annus were weighed daily to quantify water loss. Measured and modeled estimates were compared, where transpiration estimates in response to drought were modified with a general stomatal response to soil moisture stress. After model validation and calibration in the glasshouse, sap flow was used to further validate the performance of MAESTRA in a limited irrigation field study that included several levels of soil moisture deficit. Although MAESTRA does not predict yield, mechanistic based Helianthus annus transpiration estimates suggest a morphological and physiologically dynamic alternative to crop coefficients.