RN-36 : Vegetation management for increased water yield

Abstract

Studies done in India and elsewhere have indicated that the vegetation management practices have a great potential in increasing water yield from a watershed. The vegetation management practices include various types of forest cutting or removal, or changes of forest cover from one tree type to another, or more drastic changes in which a forest cover is replaced by another type of vegetation. The increase in water yield is attributed to variation in ET losses associated with manipulation of vegetation or change in land use. Evaporation processes generally account for a significant portion of the annual precipitation input on most watersheds. Consequently, the potential to increase water yield by decreasing evapotranspiration is attractive. It has been observed that a little change in evapotranspiration causes a significant increase in stream flow runoff.

The report presents a summary of water yield improvement experiments done in India and elsewhere. The studies have indicated that a reduction in the densities of forest over stories and other vegetative cover types can increase water yield, however, it is difficult to predict response of vegetation management on water yield as the results obtained in various studies have been of diverse nature and suggest the complexity of the hydrologic factors that are involved. Moreover the studies have been mainly done on small experimental watersheds or runoff plots. The results of studies have been critically examined in this note. Based on results of various studies efforts have been made to quantify water yield from various land uses in terms of rainfall-runoff ratio. In general, the results indicate that vegetation management has a potential for increasing water yields from upstream catchments. The practices of partial cutting/thinning and forest conversions seem to be promising for improving water yield without causing appreciable soil erosion and at the same time maintaining substantial yields of forest products. Experiments need to be done on large catchments as the results obtained from runoff plots or small experimental watersheds may not give good results when extrapolated to larger catchments.