The Effects of Forestation on Soil Hydraulic Properties in the Middle Hills of Nepal: A Preliminary Assessment

There is much international interest in the ecological condition of the heavily populated mountain region of the Nepal Himalaya. Claims are frequently made that deforestation in this heavily populated region has negative effects on downstream flood- ing and sedimentation. The implication is that deforestation results in compaction of the soil with a consequent decrease in infiltration capacity and an increase in surface runoff (and hence flooding potential). This study examines soil hydraulic properties of a typical area in the Middle Hills where a forestation programme has been in operation for about 12 years. Measurements of infiltration and permeability were made at a range of depths up to one metre in areas representative of the various forest land-use categories. The results indicate that in a site which has been deforested for probably a century, and very heavily grazed, the surface 10 cm was noticeably compacted. However, under the prevailing rainfall regime only 17 percent of the monsoon season rain-days included rainfall events for which the 5-minute intensity exceeded the surface infiltration rate. Five- and 12-year-old plantation forests established on such sites exhibited higher surface infiltration rates than the grazed area and this probably reflects an amelioration in the surface soil condition following plantation establishment and protection from grazing. The general conclusion is that infiltration of most of the monsoon season rainfall would not be impeded by the soils measured in this study and very little overland flow could be expected except in the heaviest of the monsoon season rain events. The findings are relevant for the humid sections of the Himalaya and do not necessarily apply to areas characterized by semi-arid conditions. The data refute the contention that deforestation necessarily results in an increase in large-scale flooding, and conversely that forestation would decrease the frequency or magnitude of such flooding.