Runoff and soil erosion on cultivated rainfed terraces in the Middle Hills of Nepal
Introduction
Soil and water are widely recognized as very important resources in the mountain kingdom of Nepal (HMG Nepal, 1988). Over the past 20 years, significant concerns have been raised over the degradation of the soil resource in the Middle Hills of Nepal as a result of the expansion of agricultural land and the increase in cropping intensity. These concerns, most notably expressed by Eckholm (1976) and the World Bank (1979), led to the formulation of the Theory of Himalayan Degradation. This ‘Theory’ has been reviewed critically by Ives and Messerli (1989). Its central tenet was that deforestation and land use change were causing accelerated runoff and soil erosion on the steep Himalayan hillslopes. Implicit if not explicit in the ‘Theory’ was the view that, if accelerated runoff and soil loss were occurring, the local cultivators were either oblivious to what was happening or, if aware, were somehow unconcerned. This would clearly be an unsustainable situation.
By the late 1980s these ideas were coming under increasing scrutiny; a number of detailed studies were suggesting that deforestation in the Middle Hills was not a recent phenomenon, and that it was currently occurring at rates of less than 0.1% per annum (Land Resource Mapping Project, 1986, Mahat, Griffin and Shepherd, 1986). The contribution of human activity to landsliding was also being questioned (Ramsay, 1985, Ramsay, 1986). Also some studies, such as that of Fleming (1983), were indicating that soil erosion on cultivated slopes was not as high as was thought. Above all, it was recognized that reliable and accurate data with which to test the various components of the ‘Theory’, both at the hillslope and catchment scales, were largely unavailable (Ives & Messerli, 1989). The data that did exist were frequently based on perceptions and observations over relatively short time periods (Gilmour, 1991), usually with a western, developed perspective, and were often contradictory and fraught with uncertainty (Thompson & Warburton, 1985). In terms of soil erosion, the paucity of data relating to rates, timing and processes was highlighted. Thus, most studies concluded that there was a clear need to evaluate carefully the ways by which human activity may be causing accelerated soil erosion through changing land use, land cover and land management practices.
This paper presents surface runoff and soil loss rates for nine rainfed agricultural terraces in the Likhu Khola basin in the Middle Hills of Nepal, measured during the 1992 and 1993 monsoon seasons. Terraces are the basic unit of cultivation and soil management, and the scale of this study reflects this. The findings are also compared to those obtained from the relatively small number of previous studies on similar agricultural terraces in the Middle Hills area of the Himalaya.
Section snippets
Study area
The study area comprises five well-defined subcatchments (of 1.14–4.23 km2 in size) and the valley slopes between them in the drainage basin of the Likhu Khola (Fig. 1). The Likhu Khola (river) is a tributary of the Trisuli, which flows from the High Himalaya, in the Langtang region, and ultimately into the Ganges. The drainage basin trends from east to west and is located entirely within the Middle Hills region immediately to the north of the Kathmandu Valley (27°50’N, 85°20’E). Altitude
Methodology
There have been few attempts to examine, in any systematic manner, the variability of water runoff and soil erosion on different terraces in one area. Also, relationships with independent variables, such as rainfall amount and intensity, and soil and vegetation characteristics, have only been investigated in a very rudimentary fashion. The methodology adopted in this study was designed to examine many of these factors. Following a pilot study of terrace shape and form and surface soil textures,
Runoff
Empirical relationships between total rainfall and runoff allow comparisons to be made between terraces. The relationships can also be used to produce estimates of total runoff and runoff coefficients over the pre-monsoon and monsoon periods. Rainfall/runoff relationships for all terraces in both 1992 and 1993 are shown in Fig. 7. The relationships are clearly non-linear and show that runoff is highly variable between the terraces. Runoff coefficients (water running off the plot as a percentage
Discussion
Few studies have measured runoff and soil losses on agricultural terraces in the Middle Hills region of the Himalaya. Comparisons with earlier work can only be based around estimates of gross annual soil losses as few of the published studies have examined the temporal patterns of loss. Comparisons are also hampered by differences in methodologies, plot size and configuration, soil characteristics, cropping cycle, and land management practices. These problems make it impossible to account
Conclusion
A monitoring programme in 1992 and 1993 on nine outward-sloping, rainfed cultivated terraces in the Likhu Khola drainage basin has provided a detailed dataset on runoff and soil losses in the sensitive Middle Hills of Nepal. The results show the spatial and temporal variability in such losses, as well as their overall magnitudes, on cultivated terraces under land management regimes that are traditional in all ways except in the use of chemical fertilizer. The philosophy behind the field
Acknowledgements
The project was funded by the UK Overseas Development Administration and was coordinated jointly by the Soil Science Division (Nepal Agricultural Research Institute), Royal Geographical Society and Institute of Hydrology (Natural Environment Research Council).
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