Assessing the Nitrogen and phosphorus Retention Ecosystem Service and Strategies for Sub-Watershed Management (Zanjan Province)
Document Type : Research Paper
Authors
1 Department of Environmental Studies, the Institute for Research and Development in the Humanities (SAMT), Tehran, Iran.
2 Department of Environmental Assessment and Risks, Research Center of Environment and Sustainable Development, Tehran, Iran.
3 Faculty of Governance, University of Tehran, Tehran, Iran,
Abstract
Non-point source pollution from nitrogen and phosphorus poses a global challenge, threatening ecosystem health. Zanjan Province, with its diverse land uses, represents a prominent example of regions affected by such pollution. This study was conducted with two primary objectives: First, spatial modeling of nutrient loading, export, and retention capacity, and then identifying critical areas for strategic management interventions. The research methodology employed the Nutrient Delivery Ratio (NDR) model within the InVEST software framework, utilizing datasets including a Digital Elevation Model (DEM), Land Use/Land Cover (LULC) map, and annual precipitation data. According to the findings, the provincial sub-watersheds were classified into three categories: Critical sub-watersheds (5 and 7) were identified with the highest export-to-loading ratio. Sub-watershed 5, with a recorded negative value for nitrogen retention, was recognized as a "net source" of pollution. Sub-watersheds with high retention efficiency (3 and 6), despite very high loading, contributed the most to nutrient retention. Sub-watersheds with improvement potential (1, 2, and 4) exhibited relatively low loading and export alongside satisfactory nutrient retention performance. The annual nutrient retention capacity within the province exceeds 59,700 tonnes. The innovation of this research lies in high-resolution ecosystem service mapping and the integration of both surface and subsurface nitrogen export pathways. While simulating seasonal variations was not feasible, its output provides a valid spatial prioritization for management. Sustainable management in the province requires the decisive focus on critical sub-basins (especially 5 and 7), which were identified as high-risk pollution transfer hotspots.
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