Land use scenario development as a tool for watershed management within the Rio Mannu Basin

The Rio Mannu River Basin (Sardinia, Italy) is undergoing a process of agricultural intensification. Like many Mediterranean areas, this basin is characterized by water shortages and diffuse pollution from agricultural sources. Hence the objective of this study was to develop possible land use and land management scenarios that could constitute an alternative to the current watershed management. Several land use and land management scenarios were formulated and analyzed with local stakeholders, and two were selected and simulated as realistic in consideration of the socio-economical aspects of the study area. Scenario 1 involves agricultural practices that include a reduction in fertilizer use to meet the Water Framework Directive requirements for “good” status of water bodies. Scenario 2 introduces rapeseed cultivation, replacing durum wheat in a small area, to investigate the impact of biofuel plant cultivation on water quality. Each option was assessed by considering the effects on water quality, crop yields and economic benefits. The Soil and Water Assessment Tool (SWAT) model was applied to simulate hydrological processes and evaluate current and future nutrient loads. This model requires adequate streamflow data for calibration and validation. However, as is the case for many Mediterranean basins, insufficient data were available. Therefore, a methodology was developed and tested to calibrate hydrological processes based on the transposition of a parameter set from a gauged catchment located in the same region. This study suggests that a sound use of fertilizers could substantially reduce the amount of nutrients flowing into surface waters, although the effects of such a policy on crop yield and farm income would be negative in some cases. Moreover, the results clearly predict that the replacement of durum wheat with rapeseed (a biofuel crop), could offer a margin of profit, but would have a negative impact on water quality due to increased nutrient losses. Consequently, this option is unsuitable for this area. Furthermore, it can be inferred from these results that the promotion of the use of energy from renewable sources may have a negative impact on the objectives of the EU Water Framework Directive. Clearly, this process needs to be regulated, taking into account environmental and socio-economical aspects.