Reforesting severely degraded grassland in the Lesser Himalaya of Nepal: Effects on soil hydraulic conductivity and overland flow production

Abstract

[1] Severely degraded hillslopes in the Lesser Himalaya challenge local communities as a result of the frequent occurrence of overland flow and erosion during the rainy season and water shortages during the dry season. Reforestation is often perceived as an effective way of restoring predisturbance hydrological conditions but heavy usage of reforested land in the region has been shown to hamper full recovery of soil hydraulic properties. This paper investigates the effect of reforestation and forest usage on field-saturated soil hydraulic conductivities (K fs ) near Dhulikhel, Central Nepal, by comparing degraded pasture, a footpath within the pasture, a 25 year old pine reforestation, and little disturbed natural forest. The hillslope hydrological implications of changes in K fs with land-cover change were assessed via comparisons with measured rainfall intensities over different durations. High surface and near-surface K fs in natural forest (82-232 mm h À1 ) rule out overland flow occurrence and favor vertical percolation. Conversely, corresponding K fs for degraded pasture (18-39 mm h À1 ) and footpath (12-26 mm h À1 ) were conducive to overland flow generation during medium-to high-intensity storms and thus to local flash flooding. Pertinently, surface and near-surface K fs in the heavily used pine forest remained similar to those for degraded pasture. Estimated monsoonal overland flow totals for degraded pasture, pine forest, and natural forest were 21.3%, 15.5%, and 2.5% of incident rainfall, respectively, reflecting the relative ranking of surface K fs . Along with high water use by the pines, this lack of recovery of soil hydraulic properties under pine reforestation is shown to be a critical factor in the regionally observed decline in base flows following large-scale planting of pines and has important implications for regional forest management.Citation: Ghimire, C. P., M. Bonell, L. A. Bruijnzeel, N. A. Coles, and M. W. Lubczynski (2013), Reforesting severely degraded grassland in the Lesser Himalaya of Nepal: Effects on soil hydraulic conductivity and overland flow production,

Interactions between payments for hydrologic services, landowner decisions, and ecohydrological consequences: synergies and disconnection in the cloud forest zone of central Veracruz, Mexico

“…Forest conversion to more intensive forms of land use is often accompanied by soil compaction and degradation, which can reduce infiltration rates and increase rainfall-runoff responses (Molina et al 2007, Zimmermann and Elsenbeer 2009, Bathurst et al 2011, Ghimire et al 2013). At our Veracruz site, despite generally higher topsoil bulk density and much lower saturated hydraulic conductivity under pasture compared to regenerating and mature cloud forests (0.49 ± 0.06, 0.45 ± 0.11, and 0.25 ± 0.17 g cm -3 , and 30 ± 14, 615 ± 690, and 777 ± 931 mm h -1 , respectively) (Muñoz-Villers et al 2015), prevailing rainfall intensities at this elevation rarely exceeded topsoil infiltration capacities to generate flood-producing rates of overland flow (Muñoz-Villers and McDonnell 2013).…”

Negative trade-off between changes in vegetation water use and infiltration recovery after reforesting degraded pasture land in the Nepalese Lesser Himalaya

“…Depth to bedrock within the 50 m × 50 m plot was ca. 2.3 m. Further information on changes in soil characteristics with depth at this and the other two research sites is given by Ghimire et al (2013).…”

“…For the more open pine forest (LAI, 2.2), one expects E s to be somewhat higher than in the nearby natural forest. However, the pine litter is typically harvested by local people after the main leaf-shedding period (Ghimire et al, 2013(Ghimire et al, , 2014a, reducing amounts of litter present and thus its moisture retention capacity. Further, a substantial fraction of the rainfall in the pine forest runs off as overland flow (see Results below), reducing E s even further.…”

“…For surface K fs , 10 (pine forest) to 17 (degraded pasture) replicate measurements were made whereas for subsurface K fs , 45-80 auger holes were used per site. For a detailed description of the measuring procedures and sampling strategy, the reader is referred to Ghimire et al (2013).…”

“…3, Table 2). Marked reductions in surface-and near-surface K fs after converting tropical forest to grazed pasture have been observed in many cases (Alegre and Cassel, 1996;Tomasella and Hodnett, 1996;Deuchars et al, 1999;Zimmermann et al, 2006;Molina et al, 2007;Tobón et al, 2010) and the Himalayas are no exception (Patnaik and Virdi, 1962;Gilmour et al, 1987;Gerrard and Gardner, 2002;Ghimire et al, 2013Ghimire et al, , 2014a. The low surface and near-surface K fs reported for grazing conditions is mostly the result of destroyed macroporosity through trampling by cattle and by the much diminished soil faunal activity after forest clearing and burning with the associated loss of topsoil organic matter and surface exposure with erosive precipitation (McIntyre, 1958a, b;Lal, 1988;Deuchars et al, 1999;Colloff et al, 2010;Bonell et al, 2010).…”