WaterSubject

The actual irrigation water demand in a district in Sicily (Italy) was assessed by the spatially distributed agro-hydrological model SIMODIS (SImulation and Management of On-Demand Irrigation Systems). For each element with homogeneous crop and soil conditions, in which the considered area can be divided, the model numerically solves the one-dimensional water flow equation with vegetation parameters derived from Earth Observation data. In SIMODIS, the irrigation scheduling is set by means of two parameters: the threshold value of soil water pressure head in the root zone, h_m, and the fraction of soil water deficit to be re-filled, ?. This study investigated the possibility of identifying a couple of irrigation parameters (h_m, ?) which allowed to reproduce the actual irrigation water demand, given that the study area was adequately characterized with regard to the spatial distribution of the soil hydraulic properties and the vegetation conditions throughout the irrigation season. The spatial distribution of the soil and vegetation properties of the study area, covering an irrigation district of approximately 800 ha, was accurately characterized during the summer of 2002. The soil hydraulic properties were identified by an intensive undisturbed soil sampling, while the vegetation cover was characterized in terms of leaf area index, surface albedo and fractional soil cover by analysing multispectral LandSat TM imageries. Irrigation volumes were monitored at parcel scale.

A reference scenario with h_m = ?700 cm and ? = 50% (corresponding to a mean actual to potential transpiration ratio of 0.95) allowed to reproduce the spatial and temporal distribution of the actual irrigation demand at the district scale. The spatial variability of the crop conditions in the considered area had much more influence to assess the irrigation water demand than the soil hydraulic spatial variability. The proposed approach showed that, under the agro-climatic conditions typical for the Mediterranean region, SIMODIS may be a valuable tool in managing irrigation to increase water productivity.

The issue of efficient water use for irrigation is of utmost importance in the semi-arid Mediterranean regions, where water scarcity often causes severe damages in the fragile agro-ecosystems. In the last two decades, this evidence has induced the development of physically based models for simulating the process of exchange of mass and energy in the soil–plant–atmosphere (SPA) system. In particular, deterministic models have been proposed to simulate all the components of the water balance in great detail, including crop growth, irrigation and solute transport. These models have been developed for site-specific applications but they have seldom been applied to large areas, due to the complexity in the acquisition of all required input data, often characterized by spatial and temporal variability. To overcome this problem, techniques have been proposed which involve the use of geostatistical approaches to spatialize the measured soil hydraulic properties integrated with data derived from Earth Observation to gather quantitative information on the temporal and spatial distribution of selected vegetational parameters.

The knowledge of the spatial distribution of the soil and vegetation parameters is of crucial importance for the assessment of crop water use and for the definition of management strategies aiming at improving the efficiency of irrigation. The spatial distribution of the irrigation water use is influenced by two components: a first one is deterministically linked to the physical processes involved, which themselves are depending on the spatial and temporal variability of climate, soil and vegetation; a second stochastic component is represented by the farmers’ behaviour in the irrigation practice.

At temporal scales larger than a few days, i.e. one week, and in presence of an on-demand irrigation scheduling, the first component is largely predominant on the second one. On the condition that the spatial variability of climate can be neglected compared to that of soil and vegetation, the irrigation scheduling can be described by means of two parameters: the threshold value of soil water pressure head in the root zone, h_m, and the fraction of soil water deficit to be re-filled with irrigation, ?. The first parameter, h_m, is crop dependent, since it describes the crop tolerance to water stress; the second, ?, is related to the soil hydraulic properties. Both parameters, to a certain extent, are site-specific, in the sense that they reflect the average perception of farmers to the soil water dynamics and their attitude to induce or not water stress for quantitative and qualitative production issues. This aspect has particular relevance when determining irrigation strategies for vineyards where drought is controlled during fruit ripening in order to improve the wine quality. Applications of agro-hydrological models to simulate the soil water balance for grapes and other fruit-tree crops were carried out by several authors.

Assessment of h_m and ? represents the basis for the simulation model SIMODIS (SImulation and Management of On-Demand Irrigation Systems), developed by. In this study, the SIMODIS procedure was applied to simulate the spatial and temporal variability of the water demand of vineyards covering an area of approximately 800 ha located in the south-west coast of Sicily (Italy). The increasing diffusion of the wine-specialized, high-income grape cultivation in this area emphasized the need of irrigation strategies aimed to optimize available water during drought periods. Available water consumptions for the season 2002 allowed to compare the spatial and temporal distributions of the simulated and measured irrigation volumes for the considered area.