Traditional Irrigation in the Po Plain: Inefficient Practice or Key Contributor to Groundwater Recharge?

Traditional surface irrigation remains the most widespread practice in the Po Plain, supporting one of Italy’s most productive agricultural regions. The system relies on large river withdrawals that are distributed through an extensive network of unlined, often centuries-old canals supplying agricultural fields. Although frequently criticized for inefficiency, surface irrigation generates hydrological effects that extend well beyond direct crop water supply. Indeed, percolation from irrigated soils and seepage from open channels contribute substantially to groundwater recharge, thereby moderating seasonal fluctuations in surface water availability and alleviating drought stress. Despite its hydrological relevance, irrigation-induced groundwater recharge in the Po Plain remains poorly quantified. The region exhibits a highly complex hydrological system, dominated by strong surface water–groundwater interactions, while detailed data on irrigation practices, water deliveries, land use information and dynamic shallow water table fluctuations are limited. Robust estimates of the contribution of traditional irrigation to aquifer replenishment are therefore essential for sustainable water resources management, particularly in case of relevant climatic variability. Within the MidAS-Po project, a comprehensive methodological framework was developed to quantify groundwater recharge across the entire Po Plain, accounting for contributions from both percolation from irrigated fields and seepage from unlined irrigation channels. The approach integrates two components. First, the distributed agro-hydrological model IdrAgra was applied to simulate the daily soil water balance and estimate percolation from agricultural soils. Second, recharge associated with canal seepage was assessed using a simplified methodology based on the estimation of the channel distribution efficiency. The results indicate that recharge of aquifers induced by traditional surface irrigation accounts for between 50% and 65% of total recharge. The lower percentage is obtained when considering the role of the saturated soil zone in limiting percolation flows, and allowing root uptake from the capillary fringe in those areas of the Po Plain, where the aquifer is shallow; the higher percentage reflects the results of a simulation in which free drainage conditions were assumed at the base of the rooted soil volume. Given the considerable limitations in the knowledge of the spatial distribution of the phreatic aquifer depth, it is currently impossible to say which of the two estimates is more accurate. These findings challenge conventional views on traditional irrigation efficiency, highlighting that water “losses” from irrigated fields and open channels are not merely waste but represent an important source of aquifer recharge. Further refinements are needed to improve these estimates. In particular, shallow water table depth data are currently not sufficiently accurate in terms of spatial resolution and are inadequate for capturing the seasonal and inter-annual fluctuations. Enhanced data on shallow water table depth would strengthen the model performance, improving estimates of crop irrigation requirements and of groundwater recharge.