Modelling water provision as an ecosystem service in a large East African river basin

Reconciling limited water availability with an increasing demand in a sustainable manner requires detailed knowledge on the benefits people obtain from water resources. A frequently advocated approach to deliver such information is the ecosystem services concept. This study quantifies water provision as an ecosystem service for the 43 000 km(2) Pangani Basin in Tanzania and Kenya. The starting assumption that an ecosystem service must be valued and accessible by people necessitates the explicit consideration of stakeholders, as well as fine spatial detail in order to determine their access to water. Further requirements include the use of a simulation model to obtain estimates for unmeasured locations and time periods, and uncertainty assessment due to limited data availability and quality. By slightly adapting the hydrological model Soil and Water Assessment Tool (SWAT), developing and applying tools for input preprocessing, and using Sequential Uncertainty Fitting ver. 2 (SUFI-2) in calibration and uncertainty assessment, a watershed model is set up according to these requirements for the Pangani Basin. Indicators for water provision for different uses are derived from model results by combining them with stakeholder requirements and socio-economic datasets such as census or water rights data. Overall water provision is rather low in the basin, however with large spatial variability. On average, for domestic use, livestock, and industry, 86-105 l per capita and day (95% prediction uncertainty, 95 PPU) are available at a reliability level of 95 %. 1.19-1.50 ha (95 PPU) of farmland on which a growing period with sufficient water of 3-6 months is reached at the 75% reliability level - suitable for the production of staple crops - are available per farming household, as well as 0.19-0.51 ha (95 PPU) of farmland with a growing period of >= 6 months, suitable for the cultivation of cash crops. The indicators presented reflect stakeholder information needs and can be extracted from the model for any physical or political spatial unit in the basin.