Study On The Interaction Between Vegetation And Hydrological Cycle In The Non-humid Regions Of China

Analysis on the spatial and temporal relationships between vegetation dynamics and hydrologic cycle is one of the hot spots in ecohydrology. The growth of vegetation is controled by water availability, while vegetation growth also feeds back to influence regional water balance. A better understanding of the relationship between vegetation state and water balance would help explain the complicated interactions among climate, vegetation and the catchment hydrological cycle. In the present study, the non-humid regions in China are selected as the study area, including the Loess Plateau and the Tibetan Plateau in the Yellow River basin, some Inland River basins, and the Hai River basin. Based on the correlation analysis, the coupled water-energy balance principle, the ecohydrological model and the distributed hydrological model, this research is aimed to explore the mechanism of interaction between the vegetation and catchment hydrological cycle.Firstly, the correlation between vegetation and components of the catchment hydrological cycle is analyzed based on long-term climate, hydrological and vegetation data. The results indicate the vegetation coverage is higher in the relatively wetter environment, associated with a higher evapotranspiration. In the catchment with higher precipitation, the inter-annual variation of regional NDVI is lower. An increment of the vegetation coverage comes with an increase of the evaporation efficiency but a decrease of evaporation ratio. Higher vegetation coverage, ratio of the base flow to the total flow is higher. Soil moisture and the regional vegetation coverage show a positive correlation.Then based on the coupled water-energy balance principle, the impact of vegetation on catchment water balance is analyzed. The distribution of vegetation coverage on the Budyko curve helps to explain why the wetter environment has higher vegetation coverage associated with higher evapotranspiration efficiency. It is also suggested the regional long-term water balance should not vary along a single Budyko curve; instead it should form a group of Budyko curve owing to the interaction between vegetation, climate, and hydrological cycle. Vegetation coverage is successfully incorporated into an empirical equation for estimating the catchment landscape parameter in the coupled water-energy balance equation in order to improve the spatial and inter-annual simulation of the actual evapotranspiration.Based on the catchment ecohydrological model developed by Eagleson, from the water balance analysis for the vegetation growing season, the allocation of evpotranspiration flux among canopy transpiration, surface interception, and bare soil evaporation is analyzed. The quantitative relationship between the vegetation coverage and soil moisture is derived, which would be useful for predicting the change of vegetation coverage due to climate change and human activity. Coupling water-energy balance model and the ecohydrological model, a new model is developed for predicting the impact of climate and vegetation changes on the catchment water balance.The vegetation parameterization scheme in the catchment hydrological simulation is discussed. A distributed physically-based hydrological model (GBHM) and the water-energy balance model are used to predict actual evapotranspiration in the Luan River basin. From the analysis at different time scales through comparison of these two models, it is shown that catchment annual evapotranspiration is controlled mainly by the annual precipitation and potential evaporation, variability of soil water and vegetation become more important at a smaller time scale. It is also known that the relationship between potential and actual evapotranspiration shows a highly nonlinear relationship at the annual and catchment scale, but can be simplified to a linear relationship at hourly temporal and hillslope scales.This research is potentially useful for assessing the long-term effect of vegetation changes on catchment hydrology, and hence has implications to the water resources management in the non-humid regions of China.