A Research On The Cumulative Effect Of Forest Changes On Flow Regimes In Large Watershed

The Poyang Lake, the largest freshwater lake in China, is a research hot spot due to its great role in flood control, wetland and aquatic ecosystem protection. The problems which current studies mainly focus on are the impacts of climate change and hydraulic projects on hydrology and lake ecosystem, rather than flow regimes changes caused by large-scale forest changes in the upstream rivers flowing into the Poyang Lake and the associated potential threats to the lake. The Meijiang River watershed as an important headwater of the Poyang Lake, is situated in the upper reach of the Ganjiang River. Forest coverage in this watershed was reduced by 11% from 35.9% in 1950 to 24.7% in 1972,and then jumped from 24.9% to 71.0% between 1983 and 2006 (an increase of 35%). The forest change pattern is very representative as many watersheds in China have similar forest disturbance histories (both deforestation and reforestation). This study used the Meijiang watershed as an example to investigate the impact of long-term large-scale forest changes on flow regimes. It is significant to adaptive water and forest resources management and the protection of aquatic and wetland ecosystems in the Poyang Lake.At first, the daily runoff simulation of Meijiang River Watershed was carried out by using the SWAT model. Among that, we use the Anusplin program to interpolate meteorological data to improve the accuracy of watershed simulation. In this study, the watershed was divided into 35 subbasins and 160 hydrological response units, and we used the SWAT-CUP program to calabrate the parameters and analyse the sensitivity of the results. Then we chose the coefficient of determination R2 and Nash-Sutcliffe model efficiency coefficient Ens to assess the simulation. According to the result, the model grouped with the best patameters, which were selected to construct the ecological hydrological model of the watershed. The R2 and Ens values were 0.63 and 0.62 respectively in calibration period (1982-1997) and were 0.68 and 0.60 in validation period(1998-2006). The evaluation results of calibration and validation both satisfied the requirements and so the watershed could be better simulated by the SWAT model.Next, according to the characteristics of the watershed, we established 4 kinds of forest change scenarios and 20 kinds of climate change scenarios and then predicted the change of the watershed daily runoff in different forest and climate scenarios. Based on the theory of river ecosystem protection, the changes of flow regimes (magnitude, timing,duration, frequency and variability) in different scenarios were quantitatively calculated.In this paper, the changes of the flow regimes in different scenarios were presented in the Box-plot, and we discussed and analysed about the reasons. The main conclusions were as follows:(1) In the forest change scenarios, the evaporation of the watershed and the transpiration of the vegetation would be enhanced when the bare land and farmland converted to forest land. At the same time, the litter layer of the newly added forest would conserve some of the precipitation and the water content of soil layer would also increase.Eventually leading to annual runoff, wet season runoff and dry season runoff reduction.The forest in the Meijiang River watershed had obvious spatial difference. In dry season,increase forest of riparian had greatly reduced the surface water due to the forest consumed a large number of riparian soil moisture and groundwater, which interfered with supplemental water for surface river.(2) The increase of forest coverage in non-riparian zone could significantly reduce high flow runoff, variability and duration. The forest changes in the riparian had little effect on the high flow regimes. The increase of rainfall could increase high flow runoff and duration. For per 5% increase in rainfall, the high flow runoff could increase by about 8% and the effect of temperature change on high flow is opposite to that of precipitation.(3) In the non-riparian zone, per 10% increase of forest coverage could make about 7%-8% increase of low flow runoff, 0.5-1 day decrease of low flow duration, and 2%-3%decrease of low flow variability. The hydrology effect of forest increase in riparian zone is opposite to that of the non-riparian zone. Changes in rainfall and temperature had almost the opposite effects on low flow regimes. For per 5% increase of rainfall, low flow runoff increased by 20%-30%, and the temperature increase of 0.5 ?, low flow runoff decreased by 4%-8%.