Simulating the transition of a semi-arid rainfed catchment towards irrigation agriculture

We investigate the effects of land-use change in the semi-arid Lerma basin (Ebro valley, Spain), which underwent a transition from rainfed towards irrigation agriculture. For four consecutive years, this transition of formerly uncultivated land to irrigated farmland was intensively monitored. We use the calibrated and validated, physics-based, 3-D fully-coupled model HydroGeoSphere to study the hydrological effects of the change for this unique site, where spatio-temporal data on cropping patterns, irrigation and fertilizer amounts, and the associated catchment response are available with comparatively high resolution. Validation results show that the physics-based model can simulate and predict the impact of the land-use transformation and irrigation on surface and subsurface flow dynamics with high accuracy. Sensitivity and correlation analyses about the calibrated model parameter vector indicate that the set of van Genuchten parameter values and hydraulic conductivities is identifiable and locally unique for the parameter zonation that was defined using information on lithological units and texture data. In order to indicate changes in the runoff generation process and catchment functioning, we analyze the evolution of the total stream length and the average infiltration capacity provided by the model. The results show that irrigation agriculture has raised the base level of the water table of the Lerma aquifer causing new portions of the drainage network to become perennial. Furthermore, we introduce an approximate infiltration capacity, analyze its evolution and study its effect on Hortonian overland flow. Due to the physics-based nature of the model we can obtain values for exfiltrating fluxes directly from the model and show that both, the approximate infiltration capacity curve and the contribution of exfiltration to stream flow are consistent in indicating a shift from Hortonian towards Dunne flow runoff generating processes triggered by the land-use change.