Agricultural Intensification in the Alentejo Region: Effects on Water Quality and Water Suitability for Irrigation

Abstract

Agriculture is an important sector in southern Portugal, intensified in recent years by the construction of the Alqueva Dam on the Guadiana River. The Alqueva Multipurpose Project was designed and built to supply the driest region in Portugal, including the global irrigation system. This global change in the agricultural system (i.e. from dry to irrigated land) led to a significant increase in olive grove areas. Since 2009, olives have been the most representative crop, exceeding 50% in the entire irri gated area, and since 2013 Portugal has become self-sufficient in the olive oil sector. The Alqueva irrigation perimeter is expected to increase in the next years to 170,000 ha, with a predicted olive grove increase, if no restrictive measures are implemented at a national level. In this context, a temporal analysis of water quality for irrigation data is carried out (2010–2019), based on both the Portuguese regulations (DL 236/98) and the FAO guidelines. Therefore, the present work aims (i) to verify the effect of the land use change on the aquatic ecosystem integrity and (ii) to evaluate soil salinity risks and potential yield losses due to salinization of the most representative crops grown. Our results show a possible degradation of the aquatic ecosystem integrity, given that persistent non-compliances were identified since 2011, for total phosphorus, pH, dissolved oxygen and chlorophyll a (biomass indicator), according to the Water Framework Directive guidelines. Furthermore, the breaches for bentazon, total phosphorus and chlorophyll a were more persistent in the last years (2018–2019), probably related to the olive grove super intensification in the irrigation blocks. Regarding the suitability of water for irrigation, most values were below the threshold of maximums recommended, even though some non-compliances were detected, namely, in the electrical conductivity, pH and bicarbonates.