Water balance implications of switching from continuous submergence to flush irrigation in a rice-growing district

Studies conducted at the field scale report significant reductions in the irrigation requirements of ricewhen continuous submergence (CS) is replaced by less water-demanding regimes such as flush-irrigation(FI, i.e. intermittent irrigations of rice growing in non-submerged soils). However, the effects of theirextensive application in paddy areas with shallow groundwater is much less studied. We present a sce-nario analysis investigating the impacts on irrigation requirements induced by a shift from CS to FI inan irrigation district of Northern Italy where rice is the main crop, followed by maize and poplar. Thearea is characterised by a shallow water Table whose depth fluctuates between two meters (in winter)and less than 1 m (in summer). We applied a three-stage procedure, where we first analysed presentstate conditions using the SWAP (Soil, Water, Atmosphere, Plant) model to simulate irrigation deliver-ies and percolation fluxes. Then, we calibrated an empirical relationship between estimated percolationfluxes and measured depths to groundwater. Finally, we applied this relationship, in combination withthe SWAP model, to predict the variation of district irrigation requirements due to a widespread shiftfrom CS to FI. Results show that neglecting the feedback between groundwater recharge due to irrigationand groundwater depth led to overestimating the reduction of irrigation requirements of rice, whichdecreased from around 80% when no feedback was considered to around 60% when it was accountedfor. Moreover, increased groundwater depths resulted in higher irrigation requirements for maize withan estimated growth of more than 50% due to the need of shortening the irrigation turn. These resultsdemonstrate the importance of considering the impacts on the hydrological processes at larger scaleswhen planning the conversion of CS into more efficient field irrigation methods.