Determination of soil hydraulic properties and its implications for mechanistic simulations and irrigation management

Water retention theta(h) and hydraulic conductivity K(h) are mandatory soil hydraulic properties (SHP) for consistent hydrological modeling and for an efficient irrigation management. Most commonly, SHP are determined by conventional methods (CM), based on hydrostatic equilibrium and the independent measurement of saturated hydraulic conductivity, which is used as a matching point for K(h) function. Alternatively, inverse-modeling experiments allow simultaneous parameter estimation using data from transient water flow conditions. This study aims to investigate the implications of these two protocols on simulations of soil water content (theta) and crop evapotranspiration (ET), and how they affect irrigation management and scheduling for different irrigation systems and crops. The SHP obtained from CM and IM were used in simulations with the Richards equation-based SWAP hydrological model. ET and theta were simulated for passion fruit under high-frequency drip irrigation and for pasture under conventional sprinkler irrigation. The simulation performance was evaluated using measured theta and ET obtained with passion fruit under drip irrigation. Both methods (CM and IM) gave similar results in the wetter range, while in the drier soil, CM estimated higher theta than IM. These differences affected the simulated ET and irrigation scheduling. Regarding the ET and theta simulations, for the drip irrigation scenario, in which the water content in the root zone remains near saturation, both SHP determination methods produced similar results. On the other hand, for scenarios with larger irrigation intervals such as sprinkler irrigation, simulations were affected significantly, with CM likely biasing irrigation frequency and depth.