Reconciling limited water availability with an increasing demand in asustainable manner requires detailed knowledge on the benefits people obtainfrom water resources. A frequently advocated approach to deliver suchinformation is the ecosystem services concept. This study quantifies waterprovision as an ecosystem service for the 43 000 km2Pangani Basin in Tanzania and Kenya. The starting assumption that anecosystem service must be valued and accessible by people necessitates theexplicit consideration of stakeholders, as well as fine spatial detail inorder to determine their access to water. Further requirements include theuse of a simulation model to obtain estimates for unmeasured locations andtime periods, and uncertainty assessment due to limited data availability andquality. By slightly adapting the hydrological model Soil and WaterAssessment Tool (SWAT), developing and applying tools for inputpre-processing, and using Sequential Uncertainty Fitting ver. 2 (SUFI-2) incalibration and uncertainty assessment, a watershed model is set up accordingto these requirements for the Pangani Basin. Indicators for water provisionfor different uses are derived from model results by combining them withstakeholder requirements and socio-economic datasets such as census or waterrights data.Overall water provision is rather low in the basin, however with largespatial variability. On average, for domestic use, livestock, and industry,86–105 l per capita and day (95% prediction uncertainty, 95 PPU) areavailable at a reliability level of 95%. 1.19–1.50 ha (95 PPU) of farmland onwhich a growing period with sufficient water of 3–6 months is reached at the75% reliability level – suitable for the production of staple crops – areavailable per farming household, as well as 0.19–0.51 ha (95 PPU) of farmlandwith a growing period of ≥6 months, suitable for the cultivation of cashcrops.The indicators presented reflect stakeholder information needs and canbe extracted from the model for any physical or political spatial unit in thebasin.
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