For those of you interested in the sometimes arcane debates that occasionally break out on New Mandala on the relationship between forests and catchment hydrology, here is an interesting article. It examines the relationship between land cover, soil moisture and run-off in the Mekong basin. The results are based on a “macro-scale hydrologic model.” I won’t go into all the details – much too boring – but one of the main conclusions is worth quoting:
Simulated soil moisture shows an important relationship to vegetation type: It was in general highest for agricultural areas; and lowest for grassland and woodland areas-except when antecedent precipitation was high (as is the case immediately following the rain season), when soil moisture was the lowest for forested areas. Thus, the vast forest to agriculture conversion that took place in the second half of the twentieth century was likely accompanied by an increase in soil moisture levels. Such an increase is exacerbated in those agricultural areas that are irrigated or where runoff is retained by bunds. (page 1745, my emphasis)
Why is this important? Who cares about hydrological modelling?
It is important because farmers, especially farmers in upland areas, are often unfairly blamed for causing hydrological problems (water shortages in particular) when they clear forested areas for agriculture. There is a persistent mythology within the region (and elsewhere) that forest clearing produces desication. The findings of this particular study add to the large body of good scientific evidence that this a highly misleading interpretation of complex catchment processes. Sub-soils can be wetter in agricultural areas because many forms of agriculture use less water than forests. And, as the quote above indicates, farmers often use a range of practices to encourage water to soak into agricultural soils.
Blaming hydrological problems on deforestation is easy because it is consistent with conventional wisdom. But it is often inaccurate.