Flow Persistence – How Local Populations Can Improve Drainage in River Catchments

Scientists from the World Agroforestry Centre in Indonesia and Wageningen University and Research have developed an approach in Indonesia which stimulates the local population to buffer the drainage in catchment areas of strongly fluctuating rivers. The approach involves using a new and relatively easy to determine property of the river to calculate remuneration for the local population for their active contribution to achieving a more stable discharge from the catchment area, making the area ‘healthier’.

Floods

In catchment areas where the land offers little buffering (‘sponge effect’), heavy rainfall has a direct and severe impact on the amount of rainfall that gets into the river within a day– with all the consequences thereof. The discharge (river flow) is much more gradual in catchment areas where the soils can absorb and retain lots of water, a far more desirable situation not least because of the way it reduces the risk of flooding.

Until now there was no proper but simple way of measuring the capacity of a catchment area to buffer rainwater. While the amount of forest present in an area is a commonly used measurement for the buffering function, it raises questions as to what happens outside the forest and what does or does not count as forest. According to Meine van Noordwijk, professor in agroforestry at Wageningen University and Research, these forests are a good example of the complexity of water buffering management. “Deforestation can indeed lead to reduced buffering. However, once the soil has compacted to such an extent that it cannot easily absorb water due to deforestation, any reforestation of the area has only a very slow positive effect on the buffering of rainwater over a period of some ten to twenty years.”

‘Flow Persistence’ studies and approach to flood prevention in South East Asia. Credit: World Agroforestry Centre in Indonesia and Wageningen University & Research

Controlling the buffering may be complex but improving buffering is extremely important in many catchment areas. Climate scientists indicate that rainfall will become heavier, which means that buffering large amounts of precipitation will only become more important in the future. It is a well-known fact that the local population can have a significant impact on water buffering. The Indonesian and Dutch scientists therefore looked for an approach which would stimulate the local population to contribute to a better water buffering and, therefore, more stable drainage by the river.

Proven Method

Earlier research and practice have shown that the population can become more involved in similar issues by choosing a simple measurable unit based on which they receive a financial reward for active efforts. Van Noordwijk: “For hydroelectric plants in river catchment areas, the amount of soil particles in the water – the sediment – can lead to significant costs. Sediment can damage the electricity plants and sink to the bottom of large basins, requiring removal. Scientists  from the same team used a way to easily measure the amount of soil particles. Consequently, the population in the catchment area of the power plants received remuneration for their active help in reducing the amount of sediment. This stimulated the residents to take action and reduced costs for the power plants. In addition, the reduced erosion increased the value of the soil in the catchment area.”

Encouraged by this success, Van Noordwijk and team searched for a property of the river or area that could serve as a good basis for determining the quality of the water buffering in catchment areas. Using large quantities of measurement data on rivers in four catchment areas in Southeast Asia, they came to a surprisingly simple solution: a single figure can describe the extent to which a river fluctuates and be a measurement for the buffering. Van Noordwijk: “The better the buffering, the lower the fluctuation in drainage. The fluctuation of the river can be measured much more easily than, say, the extent to which a certain plot is able to retain water. Moreover, the measurement of the fluctuation is also in line with the local population’s knowledge about the river. This increases the chance of a successful approach.”

The scientists call the measurement a ‘health indicator’ of a catchment area and named it ‘flow persistence’. The value can easily be determined based on measurements during a period of rainfall. The better the catchment area is able to buffer sudden rainfall, the higher its flow persistence.

Van Noordwijk: “The flow persistence is a value between zero and one. The closer to one this figure is, the better the buffering. Forests with properly permeable soils are at around 0.8, areas with a mix of forest and agriculture at 0.7, agricultural areas at 0.5, and urban areas even lower. Heavy rains enter the river directly in catchment areas with a flow persistence of nearly zero, which means the water level rises quickly and dangerously. In catchment areas with a high flow persistence it takes a long time before the heavy rain becomes visible in the amount of water a river must discharge, and the increase in the amount of rain water is much smaller.

Flow persistence makes it easier to measure the buffering improvement in catchment areas. This in turn makes it possible to give the local population a financial reward for their successful contribution to making the catchment area healthier.”

See the full studies:

Flood risk reduction and flow buffering as ecosystem services – Part 1: Theory on flow persistence, flashiness and base flow

Flood risk reduction and flow buffering as ecosystem services – Part 2: Land use and rainfall intensity effects in Southeast Asia

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