Comparison of water extraction models for grain sorghum under continuous soil drying

Abstract

The aim of this study was to compare three contrasting models of water extraction, by comparing predictions of the models with water extraction observed under continuous soil drying from seven crops of grain sorghum in the field. Sensitivity of the model predictions to variation in evaporative demand, and soil and root characteristics were also assessed. The datasets covered soil types ranging from 68 to 252 mm of total extractable soil water (TESW), and levels of mean evaporative demand ranging from 1.8 to 4.9 mm d −1 . The first model was based on the simple function relating daily relative transpiration rate (RTR), the ratio of transpiration to demand, to the fraction of extractable water (FESW) in the maximum root zone of 200 cm, where RTR declined from 1.0 to zero as FESW fell from 0.3 to zero. The second model was the same as the first, except that RTR depended on FESW in the current root zone, where the current root zone was assumed to increase by 3 cm d −1 from emergence to reach 200 cm at anthesis. The third, more complicated model calculated daily transpiration as the lesser of potential extraction and transpirational demand, with potential uptake calculated in each soil layer from root-soil diffusivity and simulated root length density. Inputs of transpirational demand and extractable soil water contents were the same for all models. All models gave good predictions of the observed pattern of RTR and cumulative extraction in the seven crops studied, despite the differences in the level of detail specified within the models. The results indicated that in most situations, RTR will decline once FESW in the maximum rooting depth reaches around 0.3. Sensitivity analysis revealed that this threshold will change substantially where crops are growing on a low TESW soil under high demand, or on a high TESW soil under low demand. Of the soil and root characteristics assessed, the threshold FESW at which RTR declined was most sensitive to the downward rate of root front penetration, rather than the density of rooting or the root-soil diffusivity. This study confirms the robustness and validity of the simple relationship between RTR and extractable soil water currently used in many crop growth models. Use of the more detailed model showed under which situations the form of this relationship is likely to change.