Evaluating the impact of irrigated agriculture on a coastal lagoon in a semi-arid catchment in southeast Spain

Coastal lagoons are biodiversity hotspots, but often affected by human pressure like high nutrient and sediment loads from intensive agriculture and negative impacts from tourism and urban expansion. The Mar Menor (southeast Spain) is the largest salt water lagoon of Europe, however, in recent years the lagoon has been subject to a substantial loss of its biodiversity due to increasing human pressure from the surrounding Campo de Cartagena catchment. The Campo de Cartagena is characterized by a semi-arid climate with long dry spells and infrequent, but extreme, precipitation events. Since the opening of the inter-basin water transfer Tajo-Segura in 1979, the area of intensive irrigated agriculture has progressively increased, from around 10% in 1977 to 45% in 2020. The aim of this study is to evaluate the impact of these historical land use changes on a range of hydrological and soil erosion indicators. These indicators include irrigation water demand, plant water stress, soil erosion, sediment yield, among others. We also evaluated an adaptation strategy by creation of a 1.5-km green buffer surrounding the Mar Menor lagoon consisting of wetlands with the aim to catch sediments before they can enter the Mar Menor. We applied the hydrology-soil erosion model SPHY, which simulates the most relevant hydrological and soil erosion processes, including irrigation and sediment transport. The model results show that historical land use change caused a substantial increase of irrigation water demand (+209%), sediment yield (+42%) and flood discharge (+115%), which negatively impact the scarce water resources, the water quality of the Mar Menor lagoon and flooding of urban areas. The Green buffer scenario has the potential to decrease the impact on the Mar Menor through a significant reduction in sediment yield towards the lagoon. This work has been financed by ERDF/Spanish Ministry of Science, Innovation and Universities—State Research Agency/Project PID2019-109381RBI00/AEI/10.13039/501100011033 (XTREME) under the National Program for Research, Development and Innovation focused on the Societal Challenges and the Horizon 2020 project COASTAL (Collaborative land-sea integration platform, grant agreement 773782).