Estimating the effects of a conversion from flooded to aerobic rice at the district scale

Several studies report a great improvement of the rice water use efficiency at the plot scale by replacing continuously flooded rice with the (less productive) aerobic rice cultivation. However, the effects of a massive conversion of larger domains are rather poorly investigated, even if several authors agree that a consequent increase of the groundwater depth (GWD) may have repercussions on the potential water savings due to an increased percolation and a reduced capillary rice contribution. In this context, the paper presents a modelling approach for estimating the effective water saving potential at the district scale in the case of a massive conversion from flooded to aerobic rice irrigation. A four-years study was conducted on the eastern part of San Giorgio di Lomellina district (about 500 ha) located in the middle of the most important Italian rice basin. The selected area is characterized by sandy soil and shallow GWD (about -1 m in summer). The area is nearly evenly cropped with flooded rice, maize, and poplars. The core of the proposed approach is an empirical regression-based relationship between the observed monthly GWD and the corresponding monthly percolation rates estimated for the present state conditions through a mixed mass balance and modelling (SWAP) approach. The obtained empirical relationship (R2= 0.89) was then used as groundwater law for dynamically defining the GWD in the land use scenario where all the flooded rice is converted to aerobic rice. The results obtained showed that an extensive shift from flooded to aerobic rice has a strong impact on the irrigation requirements of the other crops in the basin, due the increased GWD. As a consequence, significantly higher water supplies are required by maize and poplar trees in order to avoid crop stress, and the need to increase the frequency of the irrigation events (e.g. from 15 to 7 days) is likely to occur. Compared to the present situation, the overall reduction of the irrigation district’s water requirements is calculated to be about 40% when the effects of the increased GWD are taken into account, against 70% when they are not. The case study shows that, in presence of a shallow groundwater table, changes in irrigation methods may have strong impacts on the different water balance components, that cannot be forecasted on the basis of results obtained by plot-scale experiments, highlighting indeed the importance of studies accounting for the whole system.