Downstream Dangers

Terracing, the practice of farming on graduated steps, is an effective method to prevent soil erosion and surface runoff on sloped terrain, such as these fields in Indonesia.

International Water Management Institute

“Don’t empty the water jar until the rain falls.”

Philippine proverb

Agricultural activities influence water quality in two ways. First, unsustainable farming practices in upland areas cause soil erosion. Particles wash downstream when it rains, causing downstream lakes, rivers and other bodies of water to fill with silt. This can cause flooding and prevent dams from working. Over time, nutrients become depleted and the upland soil structure damaged.

In a vicious cycle, the land becomes less productive and more soil washes away. If the erosion continues, the soil becomes unusable. This problem is growing as the world’s uplands are being cleared and intensively farmed. Population pressures, government policies, market demands and the spiral of land degradation are forcing farmers to move to ever higher lands.

The second impact of farming on water quality is from the use of fertilizers and pesticides. These chemicals can run off with rainwater and flow downstream via rivers into lakes and oceans. In Africa, where fertilizer is less often used, the additional nutrients are sometimes beneficial to farmers downstream. In Asia, however, where agrochemicals are subsidized, rivers can become overloaded with chemicals. This can lead to eutrophication: Algae or other plants grow excessively and disrupt ecosystems, depleting oxygen and killing fish. If fertilizers and pesticides enter groundwater used for drinking, human health problems can result.

The Management of Soil Erosion Consortium (MSEC), founded in the 1990s to address the lack of river catchment data across Southeast Asia, conducts research in this area. The consortium includes Indonesia, Laos, the Philippines, Thailand and Vietnam, along with the IWMI and the French Institut de Recherche pour le Développement (IRD). During a five-year project, MSEC assessed runoff and sediment from farming practices in 27 catchments (drainage basins) and subcatchments in the five countries. Findings show that sustainable land use practices can help prevent negative impacts on downstream communities.

Scientific Solutions

The analysis demonstrated that rapid land use changes were having an impact on catchments. For example, when farmers sought to increase earnings by intercropping rambutan plantations with cassava in one Indonesian catchment, the sediment increased from 2.9 megagrams (Mg) per hectare per year to 13.1 Mg.

The scientists identified farming practices that were causing erosion (for example, higher cropping intensity of growing maize instead of rice). They then identified practices that could help prevent erosion, such as planting fruit trees or sowing strips of native grass along land contours of the cultivated land.

Notable improvements in sediment amounts were seen when farmers adopted certain conservation practices. For example, when the Indonesian rambutan plantation that had been intercropped with cassava was replaced with grass for livestock, the annual sediment dropped from 13.1 Mg per hectare per year to 2.7 Mg. Similarly, farmers in Laos reduced sediment from 5.9 Mg per hectare per year to 0.1 Mg using a fallow system that increased the land’s recovery time.

Incentive to Change

Globally, farmers have been slow to adopt erosion reduction techniques. IWMI scientists believe farmers lack sufficient incentive. Scientists are assessing a plan to apportion benefits between upland farmers and hydropower companies operating downstream dams. The idea is that hydropower companies, which have a vested interest in maintaining silt-free reservoirs, would pay farmers to minimize soil erosion.

“This region has very high topography and, given the economic boom, they need power,” says Pay Drechsel, theme leader for water quality, health and environment at IWMI. “Many hundreds of dams are being planned.”

Using incentives to help reduce pollution from fertilizers and pesticides does not mean paying farmers to use fewer agrochemicals; it requires targeting subsidies more precisely. For example, if dangerous levels of phosphorus were found in an area, it would make sense to remove subsidies on phosphorus-containing chemicals or make them expensive, encouraging farmers to use them sparingly.

The International Water Management Institute is a nonprofit scientific organization and one of 15 research centers supported by the Consultative Group on International Agricultural Research (CGIAR). IWMI’s mission is “improving the management of land and water resources for food, livelihoods and the environment.” The Institute is working with overseas partners to help find ways to reduce the negative impacts of inappropriate land use and enhance water quality and livelihoods.