Research

Hydrological impact of reforesting degraded lands in the Middle Mountains of Nepal

PhD project

Since the soil degradation in the tropical parts of the world is increasing and the seasonal stream flow regimes is getting disturbed with excess flows and sedimentation during the rainy season and water shortages during the dry season, the need of land rehabilitation is ever increasing. Several recent research reports have stressed that forest tree plantations in an area invariably lead to diminished water yields of that area round the year as compared to regular crops, grass or scrubland. So, these recent reports seem to be at odds with the traditional view which claims that a good forest cover provides better stable seasonal flows.

To do a further research on how these forest plantations affect water yield in a particular catchment, I have taken a case example, some 40 km NE of Kathmandu, which lies in the middle hills of Nepal. Since the mid 1970s a substantial area of degraded land in the said location has been replanted with various pine species, the oldest stand of which should now have an age of 30-35 years. Gilmour et al in 1987 has presented the impact of reforestation age upon the soil's saturated hydraulic conductivity (Ksat) in the above said location.

My PhD research carried out within the collaboration between ITC and Free University of Amsterdam aims to re-examine the sites documented by Gilmour et al at the present context particularly: (i) to document to what extent Ksat values have differed then and now and (ii) to compare measured rainfall intensities in the area with measured (rather than inferred) frequency and magnitude of overland flow for the respective land-cover types. In addition, the rainfall interception and soil water uptake (transpiration) for an age series of pine stands in the research area will be measured and it will be compared with the parameters measured in the poorly developed grassland area in order to know the net hydrological impact of reforestation over time.