Abstract
Abstract. This work investigates the trade-off between increases in vegetation water use and rain water infiltration afforded by soil improvement after reforesting severely degraded grassland in the Lesser Himalaya of central Nepal. The hillslope hydrological functioning (surface and subsurface soil hydraulic conductivities and overland flow generation) and the evapotranspiration (rainfall interception and transpiration) of the following contrasting vegetation types were quantified and examined in detail: (i) a nearly undisturbed, natural broadleaved forest; (ii) a 25-year-old, intensively-used pine plantation; and (iii) a highly degraded pasture. Planting pines increased vegetation water use relative to the pasture and natural forest situation by 355 and 55 mm year−1, respectively. On balance, the limited amount of extra infiltration afforded by the pine plantation relative to the pasture (only 90 mm year−1 due to continued soil degradation associated with regular harvesting of litter and understory vegetation in the plantation) proved insufficient to compensate the higher water use of the pines. As such, observed declines in dry season flows in the study area are thought to mainly reflect the higher water use of the pines although the effect could be moderated by better forest and soil management promoting infiltration. In contrast, a comparison of the water use of the natural forest and degraded pasture suggests that replacing the latter by (mature) broadleaved forest would (ultimately) have a near-neutral effect on dry season flows as the approximate gains in infiltration and evaporative losses were very similar (ca. 300 mm year−1 each). The results of the present study underscore the need for proper forest management for optimum hydrological functioning as well as the importance of protecting the remaining natural forests in the region.
Highlights
The traditional view of forest hydrological functioning, in which wet season rainfall is readily absorbed and stored for subsequent gradual release during the dry season, has often been likened to a “sponge” (Hamilton and King, 1983)
As long as rainfall intensities remain below the surface Kfs threshold for overland flow to occur, soil water reserves are being recharged
For intensities above this threshold a major proportion of the rain is redirected laterally over the surface as overland flow and less water is available for soil moisture replenishment
Summary
The traditional view of forest hydrological functioning, in which wet season rainfall is readily absorbed and stored for subsequent gradual release during the dry season, has often been likened to a “sponge” (Hamilton and King, 1983). The forest sponge effect had been the subject of debate before (cf reviews of older literature by Andréassian, 2004; Bruijnzeel, 2004; Galudra and Sirait, 2009), the concept came under severe scrutiny in the early 1980s when Bosch and Hewlett (1982) reviewed the results from nearly 100 paired catchment experiments around the globe. They concluded that “no experiments in deliberately reducing vegetation cover caused reductions in water yield, nor have any deliberate increases in cover caused increases in yield”. Since the early reviews of Bosch and Hewlett (1982) and Hamilton and King (1983), many have emphasised the more “negative” aspects of forests, such as their high water use or inability to prevent extreme flooding (e.g. Calder, 2005; FAO-CIFOR, 2005; Jackson et al, 2005; Kaimowitz, 2005) rather than focus on the positive functions of a good forest cover, including the marked reduction of surface erosion and shallow landslip occurrence (Sidle et al, 2006), improved water quality (Bruijnzeel, 2004; Wohl et al, 2012), moderation of all but the largest peak flows (Roa-García et al, 2011; Ogden et al, 2013) or carbon sequestration (Farley et al, 2005; Malmer et al, 2010)
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