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Modeling Nutrients in the Rio Mannu River Basin: Scenario Analysis
Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan www.asabe.orgCitation: 21st Century Watershed Technology: Improving Water Quality and Environment Conference Proceedings, 21-24 February 2010, Universidad EARTH, Costa Rica 701P0210cd.
Authors: Anna Maria De Girolamo, Antonio Lo Porto
Keywords: Watershed management, future scenarios, modeling, water quality, SWAT Model
The Rio Mannu River Basin (Sardinia, Italy) is experiencing an intensification of agriculture, and correspondingly, irrigation areas are increasing. Like many Mediterranean areas, this basin is characterized by water shortages and diffuse pollution from agricultural sources. It is than important to design, evaluate and adopt adequate management policies to prevent further water quality impairment. The Soil and Water Assessment Tool (SWAT), a model for agricultural watersheds, was applied to simulate hydrological processes and evaluate current and future nutrient loads. Several alternative scenarios were developed through discussions with stakeholders, and four of them were selected and simulated as realistic in consideration of the socio-economical situation in the study area. Scenario A involves agricultural practices that assume a reduction in fertilizer. Scenario B regards the reuse (in three sub-basins) of treated wastewater in fert-irrigation of olive trees instead of discharging it into the river system. Scenario C regards the partial substitution of durum wheat with sugar beet in a subset of the catchment. Scenario D introduces rapeseed and sunflower replacing durum wheat in an other small area. We assessed each option by considering the effects on water balance, water quality, crop yields and economic benefits. The model was able to simulate the hydrologic cycle and water quality notwithstanding an insufficient availability of measured data. This study suggests that a better use of fertilizers could substantially reduce the amount of nutrients flowing into surface waters, but the effects of such a policy on crop yield and farm income are in some cases negative. The reuse of treated effluent can results in a moderate reduction in nutrient load but in a larger reduction in nutrient concentration; it is more efficient in summertime and on soluble compounds (NO3, NH3 and min P); furthermore an application at larger scale of such scenario will result in very great results. The sugar beet scenario is the less competitive scenario: it features slightly higher loads and large increment in water resources usage. In the last scenario the release of several nutrient will increase; maybe a competitive scenario in terms of revenues for farmers coupled with a 6% reduction of water use in irrigation.