Potential soil water extraction by sunflower on a range of soils

A conceptual framework of the time course of soil water depletion under water limiting conditions was used to quantify water extraction by sunflower (Helianthus annuus L.) on a wide range of soil types. The framework takes account of the maximum plant-available soil water content (MAWC) in each layer, the rate at which the soil water extraction front descends through the soil profile (EFV) and the rate of water extraction within each soil layer (kl). Total plant-available soil water for the profile (TPAW) is defined as the sum for all layers of the difference in volumetric soil water between the drained upper limit (theta(u), determined in a separate infiltration experiment) and the lower limit (theta(l), determined at the end of the experiment after water extraction by the crop). To quantify parameter values, soil water content was measured frequently in sunflower crops grown entirely on stored soil moisture. Measurements were made in an environment with high temperature and vapour pressure deficit to determine the potential rate of water extraction. The parameters theta(u), theta(l) and kl were determined for each 20-cm depth increment for each of five soil types. The rate of progression of the soil water extraction front through the profile (EFV) and time at which the extraction front commences its descent (t0) were also determined for each soil type. The framework described actual water extraction on each soil type well