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

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.

Advancing ecohydrology in the changing tropics: Perspectives from early career scientists

“…Burning and grazing diminish infiltration, not only by contributing to the formation of soil crusts (Poulenard, Podwojewski, Janeau, & Collinet, 2001) but also by facilitating the development of soil hydrophobicity (Golchin, Baldock, Clarke, Higashi, & Oades, 1997). When deforested lands are grazed, the animals' trampling of the soil destroys roots and causes loss of macroporosity, loss of topsoil organic matter and associated soil fauna, and exposure of the surface to erosive precipitation (Bonell, 2010;Ghimire, Bruijnzeel, Lubczynski, & Bonell, 2014). These alterations of vegetation and soil properties lead to greater water losses, by reducing infiltration into the vadose zone and increasing run-off.…”

“…However, degraded or plantation forests having ET levels comparable to those of natural primary forest may not recover their original infiltration capacity (Bonell, 2010). In these cases, lower fieldsaturated hydraulic conductivity can contribute to increased quickflow during storms and to decreased run-off in the dry season (Ghimire et al, 2014;Krishnaswamy et al, 2013). The net effects of both increased ET and increased infiltration with afforestation can depend on variables such as soil texture and geology, which have yet to be explored empirically across tropical landscapes (Malmer, Murdiyarso, Bruijnzeel, & Ilstedt, 2010).…”

Land Use‐Dependent Preferential Flow Paths Affect Hydrological Response of Steep Tropical Lowland Catchments With Saprolitic Soils

“…Model simulation results demonstrate that fast drainage through lateral PFPs in the early wet season and high flow in vertical PFPs to recharge deep groundwater in the late wet season contribute to the observed differences in peak storm runoff and the "forest sponge effect" during the dry season. This study provided insights to the mechanism by which reforestation may help to restore ecosystem services and water security in tropical settings.While direct field observations can assess the hydrological impact of LUCC (Bruijnzeel, 1989;Ghimire et al, 2014;Ogden et al, 2013), modeling studies can provide additional insights and flexibility in addressing the CHENG ET AL. 5551Key Points:• Fast lateral drainage in early wet season and high vertical groundwater recharge in late wet season explain land use effects on runoff • Core-scale infiltrability and plot-scale infiltration capacity allow estimating a number of vertical macropores that fully penetrate root zone • A physically based model allows causal attribution in land use/land cover effects to preferential flow…”

Watershed services in the humid tropics: Opportunities from recent advances in ecohydrology

“…Key insights in the tropics relate to the hydrological functions of undisturbed soil (Bruijnzeel, 2004) and their evolution with land-use change (Molina, Govers, Vanacker, Zeelmaekers, & Cisneros, 2007; Roa-García, Brown, Schreier, & Lavkulich, 2011). Direct implications for land management can be drawn from empirical data, for example, on the impact of land-use change on run-off (Ghimire, Bruijnzeel, Lubczynski, & Bonell, 2014;Tobón, Bruijnzeel, Frumau, & Calvo-Alvarado, 2010). In addition, plot-scale studies have been used to understand soil-vegetation interactions in tropical ecosystems.…”