Abstract
Abstract. 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 km2 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 pre-processing, 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.
Highlights
Water is becoming increasingly scarce in many, especially arid and semi-arid, regions of the world (IPCC, 2007b; Millennium Ecosystem Assessment, 2005; UNESCO-WWAP, 2003)
In this paper we present an approach to quantifying water provision as an ecosystem service in the East African Pangani Basin according to the above-mentioned requirements using the Soil and Water Assessment Tool (SWAT) hydrological model (Andersson et al, 2009; Arnold et al, 1998; Betrie et al, 2011; Easton et al, 2010; Gassman et al, 2005)
SWAT (Soil and Water Assessment Tool; Arnold et al, 1998; Gassman et al, 2005) was chosen as hydrological simulator due to its comprehensiveness in simulating watershed processes, its flexibility in spatial discretization, which provides for detailed assessments at plot scale as well as more generalized continental-scale applications (Neitsch et al, 2005; Schuol et al, 2008), the availability of tools for uncertainty assessment (Abbaspour et al, 2007; van Griensven and Meixner, 2006), open access to the source code, and based on the fact that it has been successfully applied in other studies in data-limited environments, on the African continent (Betrie et al, 2011; Easton et al, 2010; Schuol et al, 2008; Ndomba et al, 2008)
Summary
Water is becoming increasingly scarce in many, especially arid and semi-arid, regions of the world (IPCC, 2007b; Millennium Ecosystem Assessment, 2005; UNESCO-WWAP, 2003). From that definition follows that only components and processes of ecosystems that are perceived and valued by humans as benefits can be regarded as ecosystem services; and for people to obtain the benefit, it must be accessible in time and space (Notter, 2010). On one hand, this makes the explicit consideration of stakeholders and their demands a critical input to any quantification approach. 3. Quantitative indicators for water provision for different uses in the Pangani Basin around the year 2000 were derived from model results and socio-economic data, based on criteria of valuation and accessibility by stakeholders
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