Soil-crop hydrology modelling approaches for conjunctively estimating recharge, crop demand and pumping in irrigated agriculture: a review

Soil-crop hydrology models are frequently used to estimate regional-scale groundwater recharge, crop water use, runoff and freshwater extraction rates to support irrigation demands. Because interactions between soils, crops and irrigation practices influence recharge, crop water requirements and pumping, it is important to apply models that can simulate these processes simultaneously. This review examines 16 soil-crop hydrology models and 13 case study applications in which recharge, irrigation demand and extraction rates (i.e., from surface water and/or groundwater) were estimated to support regional water accounting and water-resource investigations. Model functions for runoff, soil water movement, root water uptake, crop growth and irrigation were compared to identify differences between models and their limitations. This review showed that 13 of the 16 models incorporate crop growth simulation linked to soil water dynamics, while 7 also account for salinity stress. Twelve models allow irrigation to be triggered based on soil water content, with 4 also allowing irrigation to respond to crop stress factors. The reliability of soil-crop hydrology models for estimating pumping rates remains uncertain because pumping was validated against metered data in only 2 of the 13 case studies reviewed. In those studies, metered extraction data provided important constraints on model parameterisation. Although runoff was simulated, it was reconciled with field observations in only 1 of the 13 case studies. Soil-crop hydrology models remain important tools for estimating recharge and irrigation demand, however, the representation of conjunctive use of groundwater and surface water, and the resulting pumping estimates, are commonly imposed externally and remain weakly validated against field observations.