Today, forests and their role in regulating water supply and water quality remains hotly debated both in the scientific and public policy arena. More research and accurate communication of research findings is needed to ascertain what trees can and cannot do to meet the challenges a hot and crowded planet will face with regard to water supply and water quality.
For nearly a century, public education, forest and water policies were based on the assumption that under any hydrological and ecological circumstance, forest is the ideal land cover to maximize water yield, regulate seasonal flows and ensure high water quality.
Following this assumption, forest conservation and reforestation in upland watersheds was deemed the most effective measure to guarantee sustained water supply for domestic, agricultural and industrial use as well as protecting downstream communities from ravaging floods.
It is not surprising therefore that public policy making in environment and conservation has been based solely on the notion of more trees more water, and in some cases, this has been stretched to the belief that more trees would lead inevitably to more rain.
Some of these earlier assertions are counter intuitive. For instance, forested landscapes are in fact major users of water. Tree canopies reduce groundwater and stream flow through interception of precipitation, evaporation and transpiration.
Previous generalizations about the benefits of upstream forest cover on downstream annual and seasonal flows have been found to be misleading.
Both natural and plantation forest use more water than either pasture or cropland. There is therefore no doubt that even partial forest removal will increase downstream water discharge. Removal of heavy water demanding forest cover could be a rational course of action especially in arid areas, as a means of mitigating the effects of drought on dry season stream flow.
It has been shown that extensive reforestation in monsoonal climates can lead to severely diminished stream flows during the dry season that may engender a suite of other problems, potentially offsetting any advantages gained by planting trees. However, there are significant tradeoffs. Deforestation decisions must therefore be weighed against loss of important ecosystem functions besides water supply.
“Forests and Floods: Drowning in Fiction or Thriving on Facts?” , a report by Food and Agriculture Organization (FAO) and the Centre for International Forestry Research (CIFOR) published in 2005 concluded forests can play a role in minimizing runoff that causes localized flooding.
The FAO/CIFOR report further notes that even at the local level, the flood-reducing effects of forests are heavily dependent on soil depth and structure, and saturation levels, not exclusively on the presence of the trees. Impacts of deforestation are evident only at the micro level and in association with short-duration and low-intensity rainfall events.
Hence, the protective role of upstream forest cover against seasonal downstream floods has often been overestimated. This is especially true in connection with major events affecting large river basins.
In Kenya, there is little empirical evidence to demonstrate that recent trends in deforestation in the upland watershed of the Nyando River have exacerbated flood frequency or severity in the Kano plains. We can not demonstrate that the 1997/98 floods in the Nyando River basin were more severe than floods of 1961/62.
The ritual flooding in the Kano plains result largely from a combination of simultaneous discharge peaks of river Nyando and its tributaries, high runoff from the adjacent hills (Nandi escarpment, Nyabondo plateau and Sigowet escarpment), high groundwater tables, lateral river embankments and a lack of storage areas in the lowlands.
Partial or complete removal of tree cover may accelerate water discharge and increase flood risk during the rainy season. But as rainfall duration or intensity increases, or as distance of the rainfall area from the watershed increases, the influence of tree cover on flow regulation decreases.
There are many good and great reasons for reforesting the Nyando and Nzoia river basins or other flood prone basins elsewhere, (e.g. reducing soil loss, keeping sediments out of streams, maintaining biodiversity, carbon sequestration), flood risk reduction or even control is not one of them. Reforestation to prevent or reduce floods and associated land degradation may be effective only at the scale of Katuk Kodeyo gulley basin in Nyakach, Western Kenya.
The scientific debate on the role of trees in the landscape rages on.
In a study published in 2007 by Bradshaw et al., in the journal Global Change Biology, researchers from Charles Darwin University in Australia and the National University of Singapore claimed that a 10 percent increase in deforestation results in a 4-28 percent increase in flood frequency in the countries modelled. These findings were based on data from 56 developing countries.
However, the coarse scale country statistics utilized in this study may mask variation flood frequency and severity due to climate, soils, land use, population density land management and hydrological conditions making it difficult to draw solid conclusions about cause and effect.
Evidence from the Amazon and Yangzte river basins show that reduced infiltration and increased sedimentation contributed to flooding. But deforestation per se did not contribute to reduced infiltration and sediment generation and subsequent flooding. We know that sedimentation and reduced infiltration can be enhanced through better soil and land use management. Again reforestation alone does not necessarily reduce flooding if soil structure and river bed morphology does not change.
The tragedy is that what will grab the attention of policy makers and environmental activists is the claim, without solid empirical evidence, that deforestation amplifies flood risk and severity. A paper published in the international weekly journal of science, Nature, considered the paper by Bradshaw and his colleagues ‘a landmark study that offers strong evidence that forest do reduce frequency and severity of floods.
In Kenya the debate on the link between forests and water resources is perhaps even more intense in the public arena than it is in the scientific arena. All too often, what should a dispassionate debate is mired by stakeholder self interests. The debate is distorted for electoral/political expediency, public policy or for environmental activism.
We all remember too well the state-led humiliation of Nobel laureate Prof. Wangari Maathai in the heady days of her crusade against forest excisions motivated by land grabbing.
“Threats” to the Mau began with the land fragmentation and settlement program of the 1970s when large-scale forest excision began. It would help to look at time-series flow data for all the major rivers that flow from the Mau and relate these to spatial and temporal patterns of deforestation in the Mau Forest Complex.
Today evictions from Kenya’s Mau Forest is based on the notion that uprooting nearly 50,000 smallholder farm families will somehow save Sondu-Miriu hydropower plant and resuscitate Lake Victoria, especially the Winam Gulf, from ecological comatose.
It is for the sake of these voiceless and powerless citizens that we must act now to narrow the gap between research and public policy making and communicate with clarity, the linkages between forests and water to policy makers.
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