For services rendered? Modeling hydrology and livelihoods in Andean payments for environmental services schemes
Introduction
World population and commodity demand is growing rapidly, placing increasing pressure on ecosystem functions, including watershed services such us sediment retention and streamflow regulation (Kremen, 2005). One alternative is ecosystem conservation or restoration through payments for environmental services (PES), including watershed protection (Asquith and Wunder, 2008). In Latin America, PES schemes are popular, though few possess all stylized ‘ideal’ PES criteria of conditionality, voluntariness, transactions between at least one buyer and one seller, and an adequate definition of the services being paid for (Wunder, 2005). This article will deal with the last assumption: hydrological services being traded in watershed PES systems are normally inadequately defined and quantified, yet widely accepted in a pragmatic way (Quintero and Estrada, 2006). Much work exists on ecosystem services threats and valuation (e.g. Daily, 1997), but the relation between incremental area conserved or restored and marginal ecosystem service gains has received much less attention (Dasgupta et al., 2000). Hence, it is difficult to know how much, and where in the landscape, land should be protected or land uses be changed, in order to deliver ecosystem services.
Desired watershed services in the Andes are mostly enhanced dry-season streamflow and sediment retention (Celleri, 2009). Biophysical complexity across watersheds is high, with large altitude variations (1000–5000 m.a.s.l) within small distances, generating a mosaic of soils, precipitation, vegetation types, and land uses. Hence, management interventions have highly variable impacts across the landscape. When PES resources are scarce, spatial prioritization becomes essential (Wünscher et al., 2008). Yet, when services are neither spatially determined nor quantified, more informed economic analysis is precluded.
The concept of a “service-providing unit” in watersheds refers to relatively homogenous spatial entities determining e.g. seasonal water yield, sediments, etc. (Kremen, 2005, Houlahan and Findlay, 2004). Once critical service-providing units have been determined, one can establish which are needed to safeguard a target level of ecosystem service provision. Combining such biophysical data with socioeconomic analysis can then help estimating landowners’ opportunity costs of introducing desired land uses in these “service-providing units”.
Lumped hydrological models use basin-wide averages, assuming uniformity across the basin in estimating total basin streamflow (HEC, 2000, Johnson et al., 1997, Shah et al., 1996). Lumped models consider a catchment as one complete unit, characterized by a relative small number of parameters and variables (Refsgaard, 1997). In contrast, hydrological distributed models establish specific parameters values for the different spatial subunits of a watershed (Beven, 1985). Thus, they can identify “service-providing units” and also distinguish complex physical functions determining watershed services (Jayakrishnan et al., 2005), and are thus arguably more suitable under conditions of high spatial heterogeneity within watershed. However, lack of data often hinders the applicability of distributed models. In response, the Soil and Water Assessment Tool (SWAT) is a model with less complexity, and yet powerful in data generation (Arnold et al., 1999, Heuvelmans et al., 2005). SWAT is a continuous-time model where modeled catchments are subdivided into sub-basins and hydrologic response units (HRU), which are spatially explicitly parameterized to capture the impacts from different topography, soils, and land covers (Eckhardt et al., 2005, Di Luzio et al., 2005). HRUs contribute to the subwatershed with specific streamflow and sediment yields (Haverkamp et al., 2005). Thus SWAT spatially identifies units that are crucial for delivering watershed services (retention of sediments and production of water). This may also provide strategic spatial information to PES scheme designers.
We will present two small-scale municipal case studies to illustrate how SWAT, combined with an economic optimization model, can spatially predict effects on dry season flows, sediment yields, and socioeconomic impacts from different land-use alternatives. Our approach may serve as a relatively low-cost predictive tool for the spatial allocation of PES interventions.
The article is structured as follows. Section 2 will briefly describe study areas and methods applied to quantify the environmental services and the analysis of opportunity costs. Sections 3 Results, 4 Discussion will describe and compare the results for both sites. Section 5 summarizes conclusions and recommendations.
Section snippets
Study areas
The Pimampiro PES scheme, Ecuador Pimampiro, a town of 13,000 people, is located in Imbabura Province (northern Ecuador), in the eastern Andes (2150 m.a.s.l.). It relies on surface sources for drinking water and irrigation. The Palahurco micro-watershed, a main source, is part of the Pisque watershed and extends over 13.17 km2, at 2900–3900 m.a.s.l., with mean annual precipitation of 965 mm and mean annual temperature of 11.8 °C. The principal native vegetation there is cloud forest and páramo
Hydrological analysis
We defined eight sub-watersheds, encompassing 31 HRUs. The obtained flow-duration curve indicates that our simulated streamflow compares well with the reported data. Streamflow exceeding 75 l s−1 occurs in the watershed with a probability of 95%, which is comparable to the average streamflow reported of 60 l s−1 (Fig. 1). For both sedimentation and infiltration, some HRUs have a disproportionate impact. The HRUs under potato-based systems contributed most to sedimentation, especially those located
Palahurco
From a hydrological viewpoint, our results show that PES-compensated forest and páramo conservation is preventing much sediment production that would significantly affect water quality under the baseline of continued conversion to crops and pastures (Table 5). Conservation reduces total water yield, but this still slightly favors infiltration that feeds lateral flow and groundwater, thus marginally increasing seasonal flows. Similar effects have been obtained using instead the RAINRUN model by
Conclusions and recommendations
In Pimampiro (Ecuador), our hydrological modeling confirmed that protecting natural forest and páramo cover in the upstream Palahurco watershed from gradual conversion to pastures and crops has cost-effectively prevented a projected dramatic tripling in sedimentation (thus safeguarding water quality), and, to a minor extent, protected lateral/groundwater flows (thus stabilizing dry-season water quantities) from decreasing by 0.5% over a decade. However, the SWAT analysis clearly revealed that
Acknowledgements
We acknowledge the Andean Watersheds Project (CONDESAN–GTZ) and the CGIAR Challenge Program on Water & Food for support in implementing the Peruvian study, and CIFOR, MacArthur Foundation and the EU for the Ecuadorian one. We also thank the many contributors to our analysis: Natalia Uribe (CIAT) for managing and adapting GIS and climatic information, Montserrat Albán and Macarena Bustamante for gathering field data in Ecuador, EcoCiencia (Ecuador) for providing hydrological and land-use maps,
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