The effect of site conditions on flow after forestation in a dryland region of China

Although forestation has been encouraged worldwide for the benefit of forest ecosystem services, it can lead to a reduction in runoff. As site conditions also strongly contribute to this runoff reduction, site selection has been suggested in order to mitigate this negative effect of forestation. Up to now it is not clear, however, which catchment parameters play key roles in the water budget and runoff responses to forestation. In this study, two plots within an experimental basin in the Liupan Mountains, NW China, were chosen, and the eco-hydrological model BROOK90 (Version 3.25) was used to quantify the effects of slope (gradient, aspect) and soil (thickness) parameters on water flow after forestation. The simulation showed that the annual flow in a larch plantation was strongly affected by soil thickness and lightly affected by aspect and gradient. When the soil thickness increased from 30 cm to 70 cm, the annual flow in the larch plantation rapidly declined from 91 mm/yr to 56 mm/yr. When the soil depth was greater than 70 cm, however, the annual runoff was no longer sensitive to soil thickness. With respect to a forestation strategy, this means that trees preferentially should be planted on sites with thinner soil so as to lessen the impact of runoff reduction after forestation and, thus, alleviate the forest-water conflicts in dryland regions. The trade-off between a possible reduction in tree-growth on thinner soil on the one hand and an increase in runoff on the other, however, remains an open question for future interdisciplinary research.