Effects of the implementation of the Alternate Wetting and Drying (AWD) irrigation strategy in an Italian rice district: lesson learned by applying a semi-distributed agro-hydrological model

The north-western part of the Padana Plain in northern Italy is the most important rice district in Europe and the second in the Mediterranean basin after Egypt (230,000 ha in Italy and 450,000 ha in Egypt). Traditionally, rice irrigation was based on wet seeding and continuous flooding until to approximately three weeks before harvesting. Recently, due to an increased frequency of water scarcity periods and competition for water among agricultural and non-agricultural uses, water saving techniques are being introduced. Although these techniques must be firstly tested on a field scale, it is important to estimate their effects on the overall water resources system. In fact, especially in rice areas characterized by shallow aquifers, the strong interaction between traditional irrigation methods and phreatic aquifer levels leads to a re-use of the irrigation water which contributes to water flows in rivers and irrigation networks thus increasing the overall irrigation efficiency of rice areas. An experimental platform was set up in the core of the Italian rice area (Castello D’Agogna, PV) to compare three rice irrigation management strategies over two agricultural years (2019-2020): wet seeding and traditional flooding (WFL), dry seeding and delayed flooding (DFL), and a ‘safe’ wet seeding and alternated wetting and drying irrigation (AWD-safe). Irrigation water use was monitored by the installation of flow meters, and all the other soil water balance components were quantified. At the field scale, water savings of AWD and DFL were found to be about 20% and 14% compared to WFL without penalizing rice production, while the temporal distribution of irrigation water needs and percolation fluxes changed as a function of the irrigation strategy. Results achieved in the experimental fields were used in the parametrization of a semi-distributed and physically-based agro-hydrological model aimed at stimulating the overall irrigation system efficiency of a rice district (about 1000 ha) located close to the experimental platform. The modelling framework consists of three sub-models: i) one for the agricultural area, based on the physically-based SWAP model; ii) one for the channel network percolation; iii) one for the groundwater level dynamics. After investigating the current water dynamics and irrigation system efficiency for the period 2013-2020, the modelling system was used to explore the effects on the water resources system of some ‘what-if scenarios’, such as the adoption of a AWD-safe rice irrigation strategy in the whole district. The AWD-safe technique after wet seeding seems to be a good compromise solution in terms of recharging groundwater and reducing the peak irrigation request for rice. This research was developed in the context of the MEDWATERICE (PRIMA-Section2 2018; https://www.medwaterice.org/) project.