Terrestrial Water Cycle Scheme In Heihe River Basin And Its Responses To Human Activities

Heihe River Basin is the most typical inland river basin in northwest China. Extensive attentions have been paid to its so many scientific and hot research issues, such as its specific hydrological processes in cold regions, complicated terrestrial water cycle processes, water affairs conflicts between provinces, fragile eco-environment problems, and so on. Its water resources, characterized by distinct regional features, is generated in mountain areas of the upper reaches; and then it is developed and utilized in the irrigated oases of the middle reaches; finally it is disappeared in the huge evapotranspiration of the lower reaches. Due to the large-scale and long-term developments of water resources, Heihe River Basin has became to a synthetical test site where human activities intervene and transform the relationships between surface water, soil water and groundwater. Based on the different characters of the upper, middle, and lower reaches of Heihe River Basin, this paper separately explains their terrestrial water cycle schemes and their responses to human activities by designing several models and numerical simulations.The upper reach at Qilian Mountain is the only runoff generating area of Heihe River Bain. Enough attention is paid to the main stream; many sub-basins of Heihe River catchment at Qilian Mountain are lack of enough concern because of limited observation data. Based on identifying the digital terrain of Qilian Mountain, the relationship of precipitation and runoff at more than 20 sub-basins is statistically analyzed from the view of average water balance, and then a statistical model used to estimate the runoff at the basin outlet under long-term average conditions is put forward. In this statistic model, the ratio of evapotranspiration to evaporation can be described as a power function of precipitation. By further study, the annual evaporation coefficient can be statistically described as a hyperbolic function of the ratio of annual precipitation to annual evaporation. In this paper, it is assumed that the monthly evaporation coefficient has the same function format with the annual evaporation coefficient. Based on this assumption, a kind of TANK Model is designed to simulate the monthly runoff processes for mountainous region of Heihe Mainstream River. 38 percent of annual precipitation is used to generate runoff, which contains directly surface runoff, snowmelt runoff and sub-surface runoff and their contributions to runoff are separately 52 percent, 11 percent, and 37 percent through the simulation. By selecting Zhangye Basin as the study area and based on the qualitative analysis of the interactions between surface water and groundwater in Zhangye Basin, an annual water balance model is put forward to explain quantificationally the mutual transformation between all kinds of resource. This annual water balance model contains a non-linear stream channel leakage model which emphasizes the leakage behavior of stream channel under the condition of stream flow timely changing and human activities. From the principle of groundwater dynamics and according to analyzing the relationship between groundwater levels in typical observation wells and the base flow of Gaoya cross-section (very important cross-section on the Heihe Mainstream River), a kind of monthly runoff model based on the observed groundwater level for Gaoya cross-section is brought forward. This model has fewer parameters and higher model accuracy and basically represents the variation mechanism of Gaoya cross-section. In the recharge area of groundwater of Zhangye Basin, thick vadose zone is widespread distributed under the ground surface, which effects on the time and amount of the recharge of groundwater. Simplified numerical model are carried out to investigate the general behaviors of leakage-recharge conversion through a thick vadose zone, and then a simple groundwater recharge model is presented. Two different schemes for simulating the runoff of Zhengyi Gorge are taken into account, combining with above simple recharge model, two monthly water balance model for Zhangye Basin are proposed, which can describe every details of the transformation process between surface water and groundwater.The water cycle in the lower reaches is characterized by vertical evapotranspiration. The growth and succession of vegetation are controlled by the level of groundwater, and the leakage from Heihe River channel is the main recharge component for groundwater. So, river, groundwater, vegetation, which feedback and confine each other, compose an integral ecological chain. After analyzing the geological and hydrogeological conditions of the lower reaches, considering the new situation of water allocation of the Heihe River, a groundwater model is set up to simulate the variation of the ecological water consumption area before and after the implement of water allocation by the software Visual Modfolw 4.0. It is shown that the area of ecological water consumption is significantly enlarged after water allocation, which predicts the vegetation will turn to be better.Based on the results calculated by the models developed for the middle and lower reaches, the effects of human activities on the water cycle processes are analyzed deeply. Irrigation return water needs approximate 5 months to arrive the groundwater level, which can contribute to the second recharge event for groundwater. The effects of the irrigation water diversion on the water cycle processes in the middle reaches have double nature. On hand, the runoff of Zhengyi Gorge is strongly negatively correlated with the ratio of the amount of the irrigation water diversion to the runoff of Yingluo Gorge. On the other hand, it is weakly positively correlated with the amount of the irrigation water diversion. Under the condition of confining the amount of the irrigation water diversion, the increment of the conveyance efficiency of canal water can decrease the leakages from surface water efficiently. After the implement of the water allocate of the Heihe River, for the middle reaches, except some local areas having an uptrend in the groundwater levels, the rest areas have a downtrend in the groundwater levels. For the lower reaches, the groundwater levels rise widespread due to the increment of leakages from the Heihe River.