A framework for quantifying water extraction and water stress responses of perennial lucerne

A generic framework was developed and validated for predicting the water extraction and water stress responses of perennial lucerne (Medicago sativa) to improve existing crop models. Perennial forages have roots established throughout a soil profile so require a different approach to quantify water extraction patterns than annual crops. Two years of experimental data from two fields in New Zealand, each containing dryland and irrigated lucerne crops, were analysed to develop the theory of the water extraction framework. This showed that the temporal pattern of water extraction was consistent and each year commenced in the shallowest layer and progressed downward. Water extraction from each soil layer was quantified as the minimum of soil water supply and crop demand for that layer. For each soil layer, water demand was represented by transpiration demand (the product of potential evapotranspiration and crop cover) minus the sum of water extraction in overlying layers. This approach gave accurate descriptions of water extraction patterns over a range of rainfall and irrigation situations. Water supply from each soil layer (l) was quantified as the product of plant-available water and an extraction rate constant (kl(l)). The kl(l) of lucerne could not be calculated using the traditional curve-fitting procedure so kl(l) was calculated by integrating the water extraction framework described above with a soil water balance and fitting kl(l) to minimise residuals for water extraction predictions in each soil layer. This gave kl(l) values that decreased from 0.035/day in the 0-0.2 m layer of soil to 0.01/day in the deepest layer measured (1.8-2.3 m). The water extraction framework was validated against another 3 years of dryland and irrigated lucerne data and gave accurate predictions of water extraction patterns throughout the soil profile. Water stress was quantified from actual transpiration relative to transpiration demand (T/T-D). The most sensitive variable was leaf area expansion, which decreased from an optimum at T/T-D = 1 to zero at T/T-D = 0.2, followed by radiation-use efficiency, which decreased from an optimum at T/T-D = 1 to zero at a T/T-D of zero. The framework for quantifying water extraction and the techniques determined for identifying appropriate parameters to measure and characterise the framework are expected to be generally applicable to perennial forages in a wide range of environments.