Water management in race cultivation under climate and socio-economic pressures: a perspective from Italian experiences

In order to improve the understanding of the effects of the adoption of flush irrigation (FI) as an alternative to continuous submergence (CS) in rice irrigation, we carried out both field-scale experiments and district-scale investigations in a large traditional rice area in Northern Italy. During the agricultural seasons 2012 and 2013 we conducted an intensive field monitoring activity at the experimental site of the ENR (Ente Nazionale Risi – National Rice Agency) in Castello d’Agogna, with the aim of evaluating the water balance terms and the water use indicators of two parcels under CS and FI water management. Results show that the net amount of irrigation applied is much higher for CS than FI (more than 2,000 mm against about 700 mm), with corresponding water application efficiencies (evapotranspiration divided by rainfall plus irrigation) of approximately 20% and 50%. Rice yield, however, is more than 20% higher for CS (form 10 t ha-1 to about 7,5 t ha-1), but the water productivity (weight of grains divided by cumulative weight of water inputs) is higher with FI (0.9 kg m-3) than with CS (0.5 kg m-3), due to the higher irrigation amount of the latter. Besides the differences in the performances of the two treatments, a point that emerged very clearly from the results was the high inter-annual variability for the same treatment, which was determined primarily by fluctuations of the groundwater depth. Indeed, it is well known that the values of field scale application efficiency and water productivity are influenced by the fluctuations of groundwater depth, which, in turn, are affected by the management of irrigation. When the shift from CS to FI takes place over large areas, a decline of the aquifer recharge and hence an increase of groundwater depth can be expected. Therefore, we investigated the consequences of the shift from CS to FI in a pilot district, with an area of about 5 km2, where rice is the main crop. Results show that considering the reciprocal feedbacks between the amounts of water applied for irrigation and the fluctuations of groundwater depth may significantly downgrade the expected reduction of the district irrigation requirements. In our study, the reduction of the seasonal volume of irrigation drops from 50% when neglecting feedbacks to slightly more than 20% when fully considering the effects of groundwater drawdown.