REGIONAL WATER BALANCE FOR MANAGEMENT OF DEFICIT IRRIGATION SYSTEMS Giuseppe MENDICINO, Giancarlo PRINCIPATO Department of Soil Defence, University of Calabria Ponte Pietro Bucci, Cubo 41 b - 87036 Arcavacata di Rende (CS), Italy e-mail: [email protected] ABSTRACT Irrigation and agriculture projects interest large geographic areas, involving a large number of users and requiring a great amount of investments. Improvements in irrigation management and agricultural practices are essential for increasing crop production and reducing the unbalance due to excess irrigation in some places and water deficiency in other places. These problems seem to be worsened by the progressive increasing of drought observed in the last years in southern Europe. In this context, numerous initiatives have been promoted by the European Community to face the effects induced by drought on the member States. Among these, the main initiative is represented by the INTERREG IIC Programme, which has permitted the actuation of several trans-national planning programmes, and includes among its objectives a strategy of prevention and co-operation in facing the reduction of water resources due to drought. Specifically, in southern Italy, which is a region characterised by a meta-stable climatic regime with strong intermittences of the water balance, an integrated monitoring system for the analysis and forecast of the effects produced by prolonged water deficit periods has been realised. The system is based on an embedded coupling of GIS and models connected with a Data Acquisition System aimed at storing real time data recorded by the National tele-metering hydro-meteorological network managed by the Servizio Idrografico Mareografico Nazionale (SIMN). In this system tele-metering data is constantly updated and returned according to the input required by the simulation models. These models, for the whole southern Italy, allow the estimate of spatially distributed hydrological quantities, such as solar radiation, potential evapotranspiration and water deficit. In this study, the system functionalities have been used to analyse water stress conditions and actual water requirements of the main agricultural southern Italy areas. A monthly water balance model has been applied to each agricultural district with the aim of spatially determining the irrigation deficit during the drought period 1995 – 2002. For the same period, the deficit was compared with the water resources available on the region, verifying both the adopted irrigation interventions and the management strategies carried out during the extremely intense drought phenomenon. 1 INTRODUCTION Alternation of intense precipitation and drought periods observed in the last years in southern Europe, and more markedly in the southern regions of Italy, underlines the necessity of a rational use of water resources, mainly in the agricultural sector which specifically absorbs the higher amount of the water resources. The meteorological conditions in southern Italy show a decrease in precipitation during the autumn-winter period followed by an unusual increase in rainfall during the late spring that often does not allow an efficient filling of the reservoirs. As a consequence, a sensible water supply limitation is involved, both in agriculture and in other productive sectors. In the case of agriculture, water resources are also worsened by the reduced efficiency of the irrigation distribution systems. In southern Italy, specifically in Calabria, irrigation has a relevant importance due to ancient traditions; it has been characterised by a slow initial development and by a very limited local diffusion. Only in the last decades, by means of extraordinary Government actions, large areas have been made irrigable through the realisation of important irrigation systems. In Calabria public irrigation is carried out by 15 agencies (over a total number of 17), aimed at managing 83 irrigation systems on the whole region (Fig. 1). In the year 2000, 125’346 hectares were dominated by public irrigation systems; among these 92’129 hectares resulted irrigable and, only 46’998 hectares were effectively irrigated. Such values cannot be compared with those forecasted by national projects (Progetto Speciale 26 - CASMEZ, 1984), that specifically estimated for the year 2016 about 260’000 irrigable hectares (Principato, 2000, 2001). Figure 1. Calabrian irrigation systems. In this paper the problem concerning the management of the Calabrian irrigation areas has been faced through two different phases: the former based on a hydrological approach; the latter comparing the hydrological deficit with the water supplied in each of the considered districts. Initially, the analysis regarded the definition of the soil water balance during the drought period 1995 - 2001. By means of a spatially distributed approach, only the meteorological effects and their interaction with the soil has been considered, without taking into account the water exchanges due to irrigation, reservoirs, wells or rivers (Mendicino e Versace, 2000, 2001). Following, water supply of each district has been compared with the corresponding irrigation deficit, with the aim of determining the critical conditions during the analysed period. From these results, in the final part of the paper, suitable irrigation interventions together with water resources management strategies are proposed. 2 DATA ACQUISITION AND SPATIAL DISTRIBUTION Hydrological analyses have been carried out starting from the following data: − period 1999 – 2001: daily information recorded by hydro-meteorological stations managed by the Servizio Idrografico Mareografico Nazionale (SIMN) relative to 103 thermometers, 103 rain gauges, 17 hygrometers, 12 radiometers, 11 barometers, 9 anemometers, 4 soil moisture sensors and 4 evaporimeters; − period 1995 – 1998: daily information concerning measurements of 103 thermometers and 103 rain gauges; − period 1995 – 2001: data recorded by 7 stations managed by the Aeronautica Militare Italiana and by the Rete Agrometeorologica Nazionale; − land use derived by satellite images (1:100’000 scale) relative to CORINE (CoORdination of INformation on the Environment) Land Cover Project; − geo-lithological information derived by the Geological Map of Italy (1:250’000 scale); − monthly NDVI (Normalised Difference Vegetation Index) images, derived from NOAA-AVHRR satellite, for the space-time estimate of the vegetation stress. Hydro-meteorological spatial distributions have been achieved, in some cases, directly from the observed data using spatial interpolation procedures (precipitation, temperature); in other cases, because of a smaller spatial density of the gauges, theoretical models locally calibrated with direct observations have been used (solar radiation, evapotranspiration). 3 WATER DEMAND On the examined region, irrigation water deficit is monthly evaluated using a spatially distributed water balance model. This model follows the original approach suggested by Thornthwaite and Mather (1955) and simulates soil moisture variations, evapotranspiration and runoff on single grid cells using data sets that include climatic drivers, vegetation and soil properties. This model does not consider horizontal motion of water on the land surface, or in the soil. In southern Italy a uniformly distributed high-resolution precipitation station network exists. Therefore, spatially distributed rainfall estimates are obtained just using interpolation techniques based on bi-dimensional splines. Analysis carried out on rainfall data has allowed the estimate of monthly spatial distributions during the years 1995–2001. The reduced number of evaporation stations involved crop evapotranspiration estimates based on simulation models. Specifically, two different spatially distributed models were considered: during the years 1999–2001 the approach suggested by Penman (1948) and modified by Monteith (1965) was used, while for the period 1995–1998 the phenomenon was described through the relation proposed by Mendicino et al. (2002). 3.1 Crop evapotranspiration The evapotranspiration rate from a cropped surface can be directly achieved, through mass transfer measurements, through energy balance methods, or through experimental plots on which detailed soil water balance studies are carried out. The potential evapotranspiration from a cropped surface can be also estimated starting from meteo-climatic data and information depending on the analysed cultivation (Allen et al., 1998). The most reliable evapotranspiration method was proposed by Penman (1948) and modified by Monteith (1965). This method is mainly based on the hypothesis that the phenomenon is limited by the availability of solar energy and by the possibility of air to circulate. Penman (1948) formulated a “combined” equation in which the evaporation rate was estimated as a weighted average of the energetic and aerodynamic components. Subsequently Monteith (1965) proposed a modified equation in which the concept of resistance (aerodynamic and superficial) was introduced, to make explicit the dependence of evapotranspiration from wind and vegetation cover. In its original form, the formula is given by: 1 ∆()Rn − G + 4.86 ρ cp ()ea − ed / ra ETPM = (1) λ +∆ γ ()1+ rc ra -1 where ETPM is the potential evapotranspiration rate (mm d ), 86.4 is a conversion factor, ρ is the -3 -1 -1 atmospheric density (kg m ), cp is the specific heat of moist air (kJ kg °C ), (ea-ed) is the deficit of vapour pressure (kPa), ∆ is the vapour pressure gradient