The effects of introducing non-native vegetation on hydrological partitioning in a tropical catchment

Background/Question/Methods

Current global population growth and economical development accelerates the land cover conversion in many parts of the world and compromises the natural environment. The impacts on the hydrologic cycle at local to regional scales are poorly understood. The present study investigates the hydrologic implications of land use conversion from native vegetation to rubber (hevea brasiliensis) in Southeast Asia. Rubber was introduced in China in the mid 1950’s, and since then, native vegetation (mainly primary and secondary forest) has been substituted by rubber plantations at a breathtaking rate. Rubber is not native in China, and therefore is not adapted to the local climatic conditions, resulting in distinct rates and timing of water consumption than the native vegetation it replaces. Introduction of non-native species with different water consumption possibly affects the partitioning of hydrological fluxes and regional water balance. In this paper, we propose a novel approach to understand and predict changes in the hydrologic cycle due to the introduction of rubber in an experimental catchment (69 km²) in Southwest China. The study area is divided into four dominant land covers: tea, secondary forest, grassland and rubber. Continuous records of soil moisture profiles (up to 2m deep) in each of these land covers and surface radiation data in tea and rubber canopies, help understanding vegetation phenology and timing of water demand.

Results/Conclusions

Results show that the water demand of most of the native vegetation is controlled by water availability (adapted to distinct wet and dry seasons), whereas rubber water demand is controlled by the increase in day-length. Flushing of rubber leaves with the subsequent root water uptake happens during the spring equinox, weeks before the first precipitation events. In contrast, native vegetation mostly activates with the arrival of the first monsoon rainfall. These observations and field measurements are used to better describe the root zone, surface and subsurface flow characteristics for these four land covers. Furthermore, a root zone water balance model is derived from these observations for each of the main land covers in the basin. The water and energy balance models are applied to single hillslopes and their integrated hydrologic response compared for different land covers. Finally, the response of individual hillslopes is routed through the channel network to represent the basin. Results from the model are compared to measured catchment-scale water and energy fluxes.