Water balance of rice plots under three different cultivation methods : first season results

In the last years rice cultivation methods have been the object of an intense research activity aiming to implement new irrigation methods in addition to traditional flooding, in order to reduce water use. This change has concerned also the traditional paddy-rice territories of the north-west of Italy, where rice has been traditionally cultivated as flooded and where paddy fields are a strong landscape landmark and represent a central feature in the Italian and European network for nature protection. The new techniques introduced in these territories consist in a dry seeding followed by field flooding after about one month (third-fourth leaf), and in a full aerobic cultivation with intermittent irrigations, similarly to standard irrigated crops. This paper presents the results obtained after the first year of a monitoring activity carried out at the Ente Nazionale Risi Experimental Station of Castello d’Agogna-Mortara (PV, Italy), where the main terms of water balance have been measured or estimated during the whole crop season. Because there is a substantial lack of data concerning the water balance related to the new water management techniques, the data are of wide interest despite this study covered only one season. The results here presented show that dry seeding-delayed flooding method required a rather similar amount of water respect to the traditional flooding method (2200 mm and 2491 mm, respectively), whereas the aerobic technique required one order of magnitude less water (298 mm), also due to the very shallow depth of the surface aquifer. Since evapotranspiration was nearly the same for the three methods (578 mm, 555 mm, and 464 mm, respectively for traditional flooded, dry seeding-delayed flooding and aerobic methods), percolation was very high in the case of the two flooded methods and very limited in the case of the aerobic cultivation with intermittent irrigations. These results suggest that, if the aerobic cultivation of rice represents a highly effective water-saving technique at the field scale, at the same time if applied on a large scale in traditional paddy areas, as the north-west of Italy, it could be a potential threat for groundwater dynamics, due to the dramatic decrease of groundwater recharge, and in general for traditional landscape conservation and nature protection.