On the effects of winter flooding on the hydrological balance of rice areas in northern Italy

Recent research suggests that groundwater overdraft can be mitigated by application of water onto agricultural lands for direct groundwater recharge. This technique is often called Agricultural Managed Aquifer Recharge (Ag-MAR) and is carried out in wintertime. Winter flooding of rice paddies could be considered as a form of Ag-MAR. Currently, it is implemented in Europe as an environmentally friendly practice, which could also bring many agronomic and hydrological benefits; however, especially the latter aspects have been poorly investigated in the past. Results of a hydrological study focused on winter flooding in three rice areas in northern Italy and lasting 18 months (winter seasons 2017–2018 and 2018–2019, and summer season 2018) are presented in this paper. The objective of winter flooding was to maintain higher groundwater levels till the beginning of the agricultural season in order to increase rice irrigation efficiency and protect some groundwater dependent ecosystems. Main findings are: (1) the percolation efficiency of winter flooding is very high (92–97% of the water volume applied on the fields); (2) local groundwater levels underneath the recharge areas increase at a similar rate as during the summer flooding and reach peak values that are close; (3) however, the rate of level decrease after the end of the flooding period is much faster with the winter flooding, when pre-flooding groundwater levels are re-established in 1 month, in contrast with at least 2 months with the summer flooding: this is due to the fact that the areas adopting winter flooding are less extended and more fragmented than the ones flooded during the agricultural season; (4) consequently, to maintain higher groundwater levels until the beginning of the cropping season, winter flooding should last longer and involve larger areas.