Abstract
Abstract. Although agriculturally accelerated soil erosion is implicated in the unsustainable environmental degradation of mountain environments, such as in the Himalaya, the effects of land use can be challenging to quantify in many mountain settings because of the high and variable natural background rates of erosion. In this study, we present new long-term denudation rates, derived from cosmogenic 10Be analysis of quartz in river sediment from the Likhu Khola, a small agricultural river basin in the Middle Hills of central Nepal. Calculated long-term denudation rates, which reflect background natural erosion processes over 1000+ years prior to agricultural intensification, are similar to present-day sediment yields and to soil loss rates from terraces that are well maintained. Similarity in short- and long-term catchment-wide erosion rates for the Likhu is consistent with data from elsewhere in the Nepal Middle Hills but contrasts with the very large increases in short-term erosion rates seen in agricultural catchments in other steep mountain settings. Our results suggest that the large sediment fluxes exported from the Likhu and other Middle Hills rivers in the Himalaya are derived in large part from natural processes, rather than from soil erosion as a result of agricultural activity. Catchment-scale erosional fluxes may be similar over short and long timescales if both are dominated by mass wasting sources such as gullies, landslides, and debris flows (e.g., as is evident in the landslide-dominated Khudi Khola of the Nepal High Himalaya, based on compiled data). As a consequence, simple comparison of catchment-scale fluxes will not necessarily pinpoint land use effects on soils where these are only a small part of the total erosion budget, unless rates of mass wasting are also considered. Estimates of the mass wasting contribution to erosion in the Likhu imply catchment-averaged soil production rates on the order of ~ 0.25–0.35 mm yr−1, though rates of mass wasting are poorly constrained. The deficit between our best estimates for soil production rates and measurements of soil loss rates supports conclusions from previous studies that terraced agriculture in the Likhu may not be associated with a large systematic soil deficit, at least when terraces are well maintained, but that poorly managed terraces, forest, and scrubland may lead to rapid depletion of soil resources.
Highlights
The loss of soil by erosion can present an environmental challenge with potentially grave social and economic consequences, including decreased agricultural productivity (Pimentel et al, 1995; Montgomery, 2007), damage to hydropower infrastructure and reduction of reservoir life spans (e.g., Harden, 1993), and increased flooding hazards (e.g., Costa, 1975)
We focus on the Middle Hills of the Nepal Himalaya, where there is a rich literature on soil erosion (e.g., Gardner and Jenkins, 1995; Shrestha et al, 1997; Blaikie and Sadeque, 2000; Merz, 2004; amongst many others) and where the concept of dwindling Himalayan soil resources and the associated theory of Himalayan environmental degradation (Eckholm, 1975; Ives and Messerli, 1989) have been widely discussed (e.g., Asia Development Bank and ICIMOD, 2006; Sitaula et al, 2005)
To shed additional light on the problem of soil degradation associated with agriculture in the Himalaya, here we present new cosmogenic 10Be in quartz (10Beqtz) data from well-studied agricultural catchments in the Nepal Middle Hills, and we compare these data with previously determined sediment yields, soil loss rates from plots, and first-order estimates of mass wasting fluxes
Summary
The loss of soil by erosion can present an environmental challenge with potentially grave social and economic consequences, including decreased agricultural productivity (Pimentel et al, 1995; Montgomery, 2007), damage to hydropower infrastructure and reduction of reservoir life spans (e.g., Harden, 1993), and increased flooding hazards (e.g., Costa, 1975). Restoration of dense vegetation has been shown to remediate erosional losses (Vanacker et al, 2007), but population pressure and intense competition for land use means that permanent re-vegetation is not always possible or sustainable for rural communities Techniques such as terracing and low-till agriculture can mitigate erosional losses (Chow et al, 1999; van Dijk and Bruijnzeel, 2003; Gardner and Gerrard, 2003; Inbar and Llerena, 2000; Montgomery, 2007; Morgan, 2005), but significant questions remain about the extent to which specific agricultural practices prevent or exacerbate erosion and land degradation (Bai et al, 2008). Answering these questions is critical to optimizing the strategy for preventing unsustainable soil loss in mountain regions
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