Application of the transpirative deficit Index in a pilot study area of the Po river plain (Northern Italy)

In the Po River Plain, irrigated crops cover more than 70% of the agricultural land, massive amounts of water are diverted from rivers for irrigation, and surface irrigation methods are extensively applied. Additionally, domestic and industrial activities require significant water volumes. Nowadays, the region is not a water scarce basin, but short-term water shortage can occasionally occur in some areas, especially in years of drought. However, a recent EU report states that, due to the effects of global changes on water resources, by 2030 water scarcity will affect many areas of Europe, including the Po River Plain. In Europe, the monitoring and assessment of droughts is entrusted to the European Drought Observatory (EDO), which generates and distributes maps of drought indicators using both remote sensing and hydrological modelling. Simulations with hydrological models are conducted in absence of irrigation inputs, and products have a spatial resolution of 5 km. However, in the countries of southern Europe the presence of irrigation can compensate many agronomic droughts, therefore the irrigation supply needs to be considered when the objective is to assess the water scarcity conditions of an area. Moreover, to provide directly applicable products to end-users in areas where physiographic and agricultural features are highly variable in space, the spatial scale of EDO products needs to be increased. In this context, a research has been started at the DiSAA-UNIMI to develop a set of indicators for Water Scarcity and Drought (WS&D) suitable for the assessment of the state of irrigated agriculture in Northern Italy, to be used by regional policy makers, agricultural water managers and farmers. Indicator maps have a spatial scale of interest for end-users (250 m). This paper presents the implementation and the application to a pilot study area within the Po River Plain, of the SPI (Standardized Precipitation Index), a well-known meteorological drought indicator, and of the DTI (Transpirative Deficit Index). DTI is an agricultural drought index based on the transpiration deficit (calculated as the difference between potential and actual transpiration computed by a spatially distributed hydrological model, IDRAGRA in our study). In the study, DTI is used both as an agricultural drought indicator (simulations run in absence of irrigation) and as a water scarcity indicator (simulations consider the irrigation water availability and its distribution over the territory). The results for two years, respectively characterised by drought (year 2003) and abundant rainfall (year 2002), are presented and analysed.