Computer models show agroforestry can repair watersheds
Watersheds sustain life but many have been degraded. Computer modelling has shown that agroforestry can help rescue them, says David Wilson with Regine Joy P. Evangelista, at the Philippine’s First International Agroforestry Congress
Watersheds not only supply water for domestic use but also provide a multitude of ecological and cultural services, including water for irrigation and industry, shelter, habitats for biodiversity and, in very poor areas, sources of livelihoods.
Over the years, however, these same watersheds have suffered from intensive resource extraction and mismanagement. In countries like the Philippines, the quest for “progress” has turned previously healthy watersheds into severely degraded landscapes. Several watershed areas in the country are now considered alienable and disposable, permitting largely unregulated, and often agricultural, activity characterized by forest loss and very high rainfall runoff rates and erosion.
Given the present state of our environment and climate change, the question now is not why or when but how can we reverse the degradation of our watersheds.
To answer this, let’s look at the case of the Gabayan watershed in eastern Bohol, Philippines. The Gabayan watershed incorporates a heavily degraded, multi-use landscape covering over 5000 hectares. Dwelling within the watershed are several hundred smallholding farmers’ households whose principal livelihood is subsistence agriculture—particularly rice and maize—as well as livestock management and aquaculture. Mainly poor and marginalized communities, the lives of these farmers and their families are closely bound to the ecosystem services provided by the watershed.
The land here has been largely deforested and replaced by farms and unproductive grasslands. Farmers here have reported environmental problems, such as alternate flooding and drought periods, reduction in water quality, and an accelerated level of soil erosion as well as downstream sedimentation of irrigation networks. All of these negatively affect agricultural activities within and surrounding the watershed, leading to low productivity and very low incomes for farmers.
The problems raised by the farmers in the Gabayan watershed have been linked to unsustainable land management practices. To address them, alternative land-use practices, which continue to offer livelihoods’ benefits, need to be implemented as soon as possible before any irreparable damage is done.
To be able to recommend an effective alternative, a study was conducted by the World Agroforestry Centre that employed the Soil and Water Assessment Tool (SWAT) to model effects of different land-use practices on the ecosystem services provided by the watershed. SWAT was used to simulate the impacts on watershed hydrology of two scenarios: 1) current land-use practices; and 2) conservation agriculture with agroforestry in strategic locations.
Based on the land-cover-change analysis, ARC SWAT 2012 was used to simulate changes to existing land cover by adjusting the configuration and type of vegetative cover and management practices, including tillage. Ecologically acceptable and locally familiar agroforestry species were combined with conservation agriculture practices, which had been deployed in other areas of the Philippines to restore watershed functions, especially to reduce sediment yield and transfer to stream channels.
The model results showed a significant reduction in sediment yield (20%) and sediment concentration (35%)in the Gabayan watershed under agroforestry and conservation agriculture. The study was able to provide scientific evidence that agroforestry, combined with improved management practices, is an effective land-use strategy at the watershed scale.
It creates a more sustainably managed watershed, allowing it to continue to provide ecological services and sustain the lives of its human settlers.
This work is linked to the CGIAR Research Program on Forests, Trees and Agroforestry