Net precipitation, infiltration and overland flow production in three types of community-managed forest in the Mid-hills of East Central Nepal

Abstract

It is widely believed that community forest management concurrently improves the social, ecological and economic conditions of local populations. Accordingly, large areas of Nepal's Mid-hills’ forests are used and managed by local Community Forest User Groups (CFUGs). The CFUGs plant seedlings as well as thin and prune forest trees at varying intensities as determined by local forest conditions and the need for forest products (mainly timber, firewood, livestock fodder and compostable litter). However, the hydrological effects of these activities are uncertain. We present results of throughfall (Tf), stemflow (Sf) and overland flow (OF) measurements made between June 2015 and December 2016 (including two consecutive rainy seasons) in three types of community-managed forests subject to different levels of community usage in the Kavre district (East Central Nepal): (i) regenerating natural broad-leaved (BF), (ii) predominantly-planted pine (PF), and (iii) mixed pine-broad-leaved (MF). The BF and MF were used more intensively than the PF. Overall Tf fractions were 72.0%, 73.7% and 77.5% of incident precipitation (P) for the BF, MF and PF, respectively, with corresponding Sf fractions of 1.6%, 1.3% and 0.6%. Overall rainfall interception fractions were 26.4%, 25.1% and 21.9% for the BF, MF and PF, respectively, and reflected the trend in leaf area index. Total amounts of OF constituted 11.4%. 9.8% and 4.7% of net precipitation (Tf+Sf) inputs to the forest floor in the BF, MF and PF, respectively and broadly followed the relative intensity of forest usage. Observed amounts of OF in the study forests represent ca. 50–115 mm of foregone infiltration opportunity per year under average rainfall, implying reductions in the recharge of soil and groundwater reserves of ∼6% in the PF to 12–14% in the MF and BF. Our results thus show that regular harvesting of litter, fodder and firewood combined with surface compaction by human traffic constitute important determinants of hillslope hydrological functioning, particularly regarding amounts of foregone infiltration and runoff as OF in the more intensively used community-managed forests of Nepal's Mid-hills.