Agroforests protect watersheds
Agroforests can help preserve watersheds but land use must be carefully managed to avoid altering drainage areas and potentially causing environmental and human damage, says Elis Hayati
A study by researchers from the World Agroforestry Centre conducted at the Bialo watershed in South Sulawesi province, Indonesia, has found that it remained ecologically healthy for more than two decades thanks to the agroforests that covered much of its area.
Any significant changes in land cover could decrease the amount of water that evaporates into the environment or enters the soil and increase the amount flowing across the surface, which would heighten the risk of erosion and flooding.
Indonesia has had a program in place since the 1970s to regreen its watersheds,which are crucial for drinking water, irrigation and other activities that support many of the country’s poorest communities. Agroforestry has played a key role in watershed management.
However, the watershed conservation program has had mixed success and damage remains a severe problem: when land conversion results in degradation, supply and quality of fresh water can be affected, while the frequency of water-caused disasters can increase, according to previous research by the World Agroforestry Centre. In places like the tin-producing island of Bangka, drainage basins have been stripped bare by mining, while pollution and degradation of the Citarum watershed in West Java has placed electricity supply for much of Java at risk. Land conversion and pollution at the Ciliwung watershed have heavily polluted the water of the greater Jakarta area’s main drainage basin, contributing to severe environmental problems and flooding in the capital city.
Centre scientists Lisa Tanika, Chandra Irawadi Wijaya, Elissa Dwiyanti and Ni’matul Khasanah used a Generic River Flow hydrological model to log water levels and storage in the landscape at Bialo, where more than 58% of the land is covered with agroforests.
They found that in the 21 years between 1989 and 2009, hydrological functions at Bialo remained stable and healthy, with evapotranspiration rates averaging 717.4 millimeters a year, accounting for 42.3% of precipitation, while quick surface soil flows accounted for 287.7 mm, or 17%. Slow flows—water that infiltrates into the soil and slowly seeps to the river—were 694.3 mm or 40.9%. Total precipitation rates varied between 1142 and 2668mm. Breaking down the data, evapotranspiration between 1989 and 2009 decreased by 2.6% and surface soil flows rose 2.4%, while slow flows tended to be relatively stable.
The scientists predicted that those trends would stay relatively intact if ground-cover ratios remained as they were between 2010 and 2020—a ‘business as usual’ scenario—with evapotranspiration falling 3.3%, surface flows increasing 9.5% and slow flows falling 1.7%.
However, under a scenario in which 50% of the agroforests were converted to undergrowth by 2020, surface flows would rise 26.8%, evapotranspiration would fall by 7.7% and slow flows by 6.9%, creating an elevated risk of flooding, erosion or other water-caused disasters. If all the land under agroforests were converted to scrub, surface flows would increase by more than half, and slow flows would fall by 14.2% and evapotranspiration by 15.8%.
Land managers should bear these results in mind when considering vegetation that best preserves the functions of the watershed and delivers benefits to communities.
Edited by Robert Finlayson
This work is related to the CGIAR Research Program on Forests, Trees and Agroforestry