As the dry season draws on, claims are often made that low flows in the region’s rivers and stream are a result of deforestation. Recently the Lao prime-minister got in on the act:

“If we don’t protect the trees, water in rivers will dry up. As we can observe in every dry season, many rivers now dry up,” PM Bouasone said. He said that protecting watershed areas was vital. “If we don’t do it, in the future we will have only concrete dams with no water,” he said. He added that Laos wanted to be the electric ‘battery’ of Asia . “If we don’t our protect forests, our objective of becoming Asia ‘s battery might not come true,” he said… Logging has a massive impact on the biology and water level of rivers.

This thinking is often informed by the view that forests are very effective catchment “sponges” – storing water from the wet season rains and then releasing it slowly during the dry season. It’s a nice theory, but the hydrological reality is a lot more complex.

A few years ago I undertook a detailed review of the literature (focussing on Thailand but also looking at international studies) on the relationship between forests and water. Here is a copy of the full paper [walker-2002-forests-and-water.pdf]. The key conclusions are:

Analysis of rainfall data suggests that there is no clear evidence of a long-term regional decline in rainfall, despite significant reductions in forest cover over this period.

Numerous international hydrological studies, and some studies undertaken in Thailand, show that forest clearing has the effect of increasing annual stream flow, given that clearing forest lowers the percentage of rainfall that is lost to the atmosphere in the form of ET [evapotranspiration].

A limited number of regional studies suggest that forests have higher rates of infiltration than cultivated land covers (though the actively cultivated portions of the cultivated landscape may have rates of infiltration similar to that of forests). Infiltration in cultivated landscapes is reduced by soil compaction and by the presence of hard surfaces such as roads, pathways and settlements. However, relatively accessible areas of forest (which are also probably the areas most likely to be cleared) also appear to be subject to a degree of compaction and reduced infiltration.

Hydrological studies and modelling exercises suggest that the higher rates of run-off in cultivated landscapes tend to alter the seasonal pattern of stream flow with a greater percentage of annual stream flow occurring in the wet season.

Hydrological studies and modelling exercises suggest that while clearing of forest for agriculture may change the pattern of stream flow, the absolute level of dry season flow does not necessarily decline and it may increase. This arises from the fact that the level of annual flow is higher given the reduction in ET. The positive impact on dry season flow of reduced ET will be outweighed by the negative impact of reduced infiltration only when runoff reaches relatively high levels.

These may seem like rather technical hydrological arguments. But the politics of hydrology is important. All too often upland farmers are blamed for destroying lowland water supplies. The reality is much more complex and those complaining about the loss of the forest “sponge” tend to ignore the fact that many lowland farmers are using much more dry season irrigation water than they ever did before.