Water Transport In Different Landscepes And Water Exchange Among Landscape Transtion Zone In The Middle Reaches Of Heihe River Basin

Water resources are oftentimes scarce and a restricting factor for vegetative growth in arid inland river basins, such as occurs in the middle reaches of Heihe River, Gansu, Northwest China. Water shortages in these areas have become an increasingly serious problem in recent years due to the overexploitation of water for agricultural irrigation within oases. These shortages have substantially changed the local hydrological cycles and caused degradation to the distinctive ecosystems. Consequently, it is important to mitigate the consequences of water deficits, increase the efficiency of the water use, and ensure oasis ecological security.In view of the realistic conditions in the middle reaches of Heihe River basin, such as the shortage of water resources, diversified landscapes, huge water for farmland irrigation, long freezing/thawing period, and fragmentized landscapes distribution. The eco-hydrological processes in different landscapes and among the landscape transition zone were studied based on the field monitoring, laboratory analysis, model simulation and theory analysis. The studies were mainly focused on the water balance of different landscapes, the differences of water consumption between the new and old oasis farmland, the effects of freezing and thawing processes on water circles, and the hydrological links and ecological effects among the landscape transition zone. Management strategies were put forward to improve the water use efficiency (WUE) and irrigation water use efficiency. The results are outlined as follow:(1) The soil water balance and vegetation water consumption in the farmland, forest and the desert were quantified and compared. Different soil water dynamics were identified in three landscapes, which were mainly attributed to the irrigation amount differences. The soil water balance items in the farmland and forest were greater than that in the desert, resulted by the larger water input in the farmland and forest. Deep percolation was observed in the farmland and forest, implied the present irrigation schedule was unreasonable in some degree. The evaporation and infiltration in the desert was relatively close, which meant that the deep soil water or the groundwater was the mainly water resource for the desert vegetation.(2) The water consumption in the wetland farmland, old oasis and new oasis were compared, and the optimized irrigation schemes were suggested. Obviously water balance differences were indentified in three farmlands, which were attributed to the soil type, groundwater level and irrigation schedule differences. Deep percolation and water stress were observed simultaneously in three farmlands, implied present irrigation strategy was problematic in some degree which was characterized by uniform irrigation. The reasonable irrigation schedule should consider about the crop water requirement and root distribution. Less water was used and no water stress was observed for the optimized irrigation, the soil evaporation and deep percolation were also reduced. In contrast, more groundwater was consumed in the wetland farmland.(3) The soil freezing and thawing processes in the farmland, forest and the desert were characterized. The greatest freezing and thawing processes were observed in the farmland, characterized with the deepest frost depths and longer continuous frost days, followed by the forest, and then the desert. The combination of Neutron Moisture Meter and Time Domain Reflectometry was successful to separate soil water in different phases, and water phase change was detected. Profiled water redistribution was obviously observed in the moist forest, but neither in the wettest farmland or in the driest desert. The soil frozen processes had a beneficial effect on soil water conservation with reduced evaporation and seepage, and consequently on the high water content maintained, which was important for plant germination in the following spring.(4) The main forms for water exchange among the farmland-forest-desert landscape transition zone was identified, and the water exchange resulted by single irrigation event was quantified. The hydrological links between the farmland and the forest was greater than that between the desert and farmland or the forest. The hydrological links was maintained by the lateral water flow resulted by the irrigation event and the extended root system from adjacent landscape. The forest would be recharged by a farmland irrigation event with the maximum recharged rate for about 25 mm lasted for about one week, but the desert would not. In contrast, the farmland and desert would be recharged by a forest irrigation event with the maximum recharged rate for about 20 mm and 200mm, and lasted for about 7 and 2 days, respectively. The differences of water recharge time and recharge rate were mainly resulted by the different soil types and initial groundwater levels.(5) The vegetation growth affected by the water exchange was quantified. There was a vegetation growth gradient (e.g., root, biomass, tree height) along the transition zone. At the farmland-forest interface, the impacts of extended tree roots reduced maize growth extended for 7-9 m into the farmland. In addition, the trees near the farmland grown better than those located far away from the farmland, which were attributed to the water recharge from farmland. The tree root distributions in the forest along the transition zone were affected by the soil water content and groundwater level. The study indicated that it would be more beneficial for irrigation water efficiency if the farmland and the forest were to be irrigated separately and the width of the forest was to be reduced to 15-20 m.(6) The water cycle model among the landscape transition zone was built, and the factors which would affect the water cycle were pointed out. The water cycles would be affected by the soil distribution, regional groundwater movement and local water pumping, except for the irrigation and vegetation effects. The water exchange between the farmland and the forest was enhanced by the aquifuge located in the farmland and forest, as the deep percolation was reduced obviously. The regional groundwater decrement and local water pumping resulted in the accelerated water move from farmland to the forest or the desert, and finally leaded to the decreased irrigation water use efficiency.