Hydrological impacts of community forests in the mid-hills catchments of Kavre district, Nepal

Abstract

The widely held perceptions that forest development improves catchment streamflow regulation have led to the implementation of many forestation programs worldwide. This is despite several studies showing forestation to generally reduce catchment water yield. Importantly, however, these studies poorly represent the tropical and subtropical regions, because they do not incorporate the effects of local forest use as found in Nepal’s mid-hills catchments. Forests of the mid-hills catchments are extensively used by the local community forest user groups (CFUGs), mainly to collect firewood, fodder and litter. During the 1970s, degradation of these forests allegedly caused widespread eco-hydrological problems, which subsequently led to the implementation of large-scale reforestation programs, largely to the species of pine. At present, CFUG-managed broadleaf and pine forests dominate the mid-hills’ catchments that are increasingly important for downstream water use. However, hydrological effects of this forest development or CFUG practices are essentially uncertain. This study aimed to examine these effects, particularly in view of the expanding community forest areas of the mid-hills catchments and Nepal’s recent focus to scientific forest management. This study employed a mixed method of scientific research. It combined local perceptions of forest-water relationships with experimental results measuring hydrological components of various community forests; a natural broadleaf (BF), planted pine-dominated (PF) and mixed broadleaf-pine forest (MF) in Roshi Khola catchment of Kavre district that pioneered community forestry. The BF and MF were used extensively by the CFUGs, while the PF only occasionally because it had limited supply of the traditional forest products. The survey showed a perception gap about forest-water relationships among sections of the Nepalese community, including the CFUGs’ disproval for pine forests as they supposedly reduced water quantity. Furthermore, soil properties of the research sites, particularly soil organic carbon (SOC), bulk density (BD) and saturated hydraulic conductivity (Ks) at various depths were compared with a minimally used religious forest (RF). SOC values of the RF were significantly higher (P < 0.05) and BD significantly lower than the PF and MF, mainly at 0-50 cm depth. The median Ks values (16-98 mm hr-1) were generally higher for the less used PF and RF. Subsequent comparison of the Ks values with the prevailing rainfall intensities showed PF to favour vertical percolation with possible greater contributions to subsurface storage. The overland flow production (OF) during June 2015 - December 2016 for the BF, MF and PF was 8.4%, 7.3% and 3.7% of incident rainfall, respectively. While the lower OF in the PF is likely explained by the site’s higher Ks values, overall results highlight the critical role of forest use related disturbance on water retention and runoff amounts of the mid-hills catchments with implications for downstream water use.