Use a computer to choose the best way to manage your land
Governments can use a computer model to see the effects of different land uses. We tested one model in a watershed in the Philippines, saving time and money, say Leo Kris M. Palao, Moises M. Dorado, Kharmina Paola A. Anit and Rodel D. Lasco
Local governments often have the responsibility of managing the way land is used in their jurisdiction. Assessing the effects of different policy choices on watersheds that provide important services can be time-consuming and expensive guesswork. Computer models can reduce costs to government planners and increase the accuracy of the different scenarios generated by different land-use policies.
We tested a well regarded model—the Soil and Water Assessment Tool (SWAT)—in the Layawan watershed, Mindanao, the Philippines. The software, which is in the public domain, was jointly developed by the USDA Agricultural Research Service and Texas A&M AgriLife Research, part of the Texas A&M University System. SWAT is a small-scale, watershed-to-river-basin model that simulates the quality and quantity of surface and ground water and predicts the environmental impact of land use, land-management practices and climate change.
Modeling the quantity and concentration of sediment is particularly important to help policy makers, environmental managers and development agencies predict the impact of different activities on soil and water quality. Erosion and sedimentation are big problems in many watersheds. Erosion reduces the area of arable land and removes nutrients and sedimentation clogs streams, dams and hydropower turbines, increases the cost of filtration, pollutes habitats of fish and other water creatures and lowers usability for irrigation and other purposes. Any change in the quantity and quality of soil and water will have an impact on the communities within a watershed and also those downstream. Different land uses will increase or decrease erosion and sedimentation. Rather than try out different systems in the field, which would take a lot of time and cost a lot of money, we used SWAT to assess the transfer of soil sediment in the watershed and to identify critical sub-basins.
Agriculture is the mainstay and the primary livelihood of the people living in the watershed. In 2003, the population was 26 041, some of whom lived in the upper watershed at the headwaters and engaged in agriculture. Moreover, a group of indigenous people, the Subanens, had been granted rights to 6610 hectares (Oroquieta Ancestral Domain Claim), a portion of which is in a strict protection zone inside the Layawan watershed. This situation is one of the major challenges in protected area management: how to integrate inhabitants’ land use with conservation.
The first scenario we tested with SWAT included information from a community-based watershed management planning workshop we conducted with farmers from downstream to upstream in the watershed. This scenario involved stabilizing the riverbanks, planting agroforests on grassland, conserving existing forests and not expanding coconut plantations any further.
The second scenario involved the logging and exploitation of forests in the critical sub-basins that the SWAT model also helped us identify, that is, ones with already high erosion and sediment in their waterways. In this scenario, we assumed that there was no conservation and there were no funds to fully implement the watershed management or protected area projects.
The results showed that the first scenario would see a 4% reduction in sediment. This scenario was feasible to implement since it came from the community-based watershed management workshop and had the ‘buy in’ of local communities.
The second scenario—clearing the forests—showed a 106% increase in sedimentation. About the same quantity of water would flow through the sub-basins but its quality would be lowered because of higher water turbidity and pollution, which would likely have a negative impact on the various freshwater species in the Layawan River, which are consumed locally. The coastal fisheries would also be affected, especially in the primary impact zone—the plume of sedimentation spreading out into the sea from the river mouth—because of increased transport of soils carrying pollutants.
The implications of the modelling are that if the local government wants to reduce environmental costs and maintain or improve the long-term sustainability of the watershed and the services it provides then they need to work coherently, consistently and concertedly with NGOs, academic institutions and central government agencies to avoid further degradation of the critical sub-basins. They should consider creating an integrated watershed management plan that includes the establishment of a community-based watershed management council, reforestation, eco-tourism, assistance for improved farming systems (such as agroforestry) and the development of land-use plans in the buffer zones around the protected area. A ‘payments for ecosystem services’ scheme could help to find the funds to implement these activities.
Through the use of the SWAT model we were able to identify the critical sub-basins in the Layawan watershed. This is important for environmental managers and development workers to guide future development plans and conservation initiatives. The likely changes in sediment yield and concentration for the two land-use scenarios could not have been easily assessed without the aid of SWAT. With this modeling tool, government agencies and private organizations may now precisely target critical locations in the watershed and have a more certain (quantifiable) impact assessment of possible land uses.
Edited by Robert Finlayson
Read the article
Palao LK, Dorado MM, Anit KP, Lasco RD. 2013. Using the Soil and Water Assessment Tool (SWAT) to assess material transfer in the Layawan watershed, Mindanao, Philippines and its implications on payment for ecosystem services. Journal of Sustainable Development 6(6):73-88.
This work links to the CGIAR Research Program on Forests, Trees and Agroforestry