Modelling freshwater quality scenarios with ecosystem-based adaptation in the headwaters of the Cantareira system, Brazil
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Abstract. Freshwater fluxes are influenced by the volume and quality of water at the headwaters of strategic river basins under change. Although hydrologic models provide hypothesis testing of complex dynamics occurring at river basin scales, freshwater quality modelling is still incipient at many river catchments. In Brazil, approximately only one in twenty modelling studies assesses freshwater nutrients, which limits the policies regarding hydrologic ecosystem services. This paper aims to compare freshwater quality scenarios under different land-use/land-cover (LULC) change, one of them related to the Ecosystem-based Adaptation (EbA) approach in subtropical headwaters. Using the spatially semi-distributed SWAT (Soil and Water Assessment Tool) model, nitrate and total phosphorous loads and sediments yield were modelled in Brazilian subtropical catchments ranging from 7.2 to 1037km2. Part of these catchments are eligible areas of the Brazilian PES-programmes called Water Producer/PCJ and Water Conservator in the Cantareira Water Supply System, which until the drought in 201315 had supplied water to 9 million people in the Sao Paulo Metropolitan Region. We considered freshwater quality modelling of three LULC scenarios, with no climate change, as: (i) recent past scenario (S1), with the historic LULC records in 1990, (ii) current land use scenario (S2), considered the LULC for the period 20102015 as the baseline, and (iii) future land use scenario (S2+EbA). The latter scenario proposed forest cover conversion with restoration through EbA in protected areas according to the Basin Plan of the Piracicaba-Capivari-Jundia (PCJ) watersheds by 2035. The three LULC scenarios were tested with the same records of rainfall and evapotranspiration observations in 20062014, which comprised the occurrence of extreme drought events. We propose a new index to assess hydrologic services related to the grey water footprint (greyWF) and water yield estimated. The Hydrologic Services Index (HSI), as a non-dimensional factor to compare water pollution levels (WPL) for referenced and unreferenced catchments, comprise water pollution levels for nitrate, total phosphorus and sediments. On the one hand, leaching simulations of nitrate and total phosphorous allowed for the regionalization of greyWF at different spatial scales under LULC changes. According to the critical threshold of reference catchments, HSI identified basins in less sustainable and more sustainable areas. On the other hand, conservation practices simulated through the S2+EbA scenario envisaged not only additional and viable best management practices, but also preventive decision making at the headwaters of water supply systems.
