Study On Characteristics Of Transformation Between Different Waters Under The Condition Of Ecological Construction In The Loess Hilly And Gully Regions

The Loess Plateau, which is characterized by weak ecological environments and scarce water resources, is located in the Northwest semi-arid sub-humid regions of China. Due to the development of economy and increment of population, this region has been faced with the increasing pressure of water resources shortage. In order to achieve the scientific allocation and efficient utilization of water resources in watershed, it is of great significance to conduct the investigations on characteristics of waters transformation in the area. With the implementation of extensive ecological construction in the Loess Plateau, which could have great influences in the underlying surface of the watershed, the characteristics of transformation between precipitation- soil water- surface water- groundwater have changed profoundly.Isotope hydrology, which was used by International Atomic Energy Agency(IAEA) for the first time in 1970, is now an important branch of hydrology. Hydrogen and oxygen isotope technique, as one of the most important techniques in isotope hydrology, plays huge advantages in studying the water vapor source of precipitation, transforming relationships between different water bodies, response process of runoff, plant water sources and calculating the evaporation of waters. Scientifically understanding to the characteristics of transformation between different waters under the condition of ecological construction in the loess hilly and gully regions of watershed has already become a key problem to be solved. As typical case for study, the Jiuyuangou Watershed and Zhifanggou Watershed, which are located in Suide County, Yulin City and Ansai County, Yan’an City of Shaanxi Province, were selected to investigate the characteristics of transformation between waters in the loess hilly and gully region. Precipitation samples, soil samples, surface water samples and groundwater samples were collected and tested to quantify the soil moistures and bulk densities of the soil samples and the compositions of the hydrogen and oxygen isotopes in each water body during March to September, 2015. The thesis systematically analyzed the temporal and spatial variation characteristics of the soil moisture and the hydrogen and oxygen isotopes in different waters, clarified the transformation relationship between precipitation-soil water-surface water-groundwater. Besides the mechanism of the effect of different underlyingsurface factors on the recharging process between precipitation and soil water was revealed.Major conclusions obtained are as follows:(1) The soil moisture in the 0—50 cm soil layer was affected by the seasonal variation in varying degrees, 0 — 15 cm soil layer was the active layer of water vapor exchange. The average soil moistures during March to September in the locust forest and waste grassland were almost equal, but the soil moisture in the waste grassland was less stable than that in the locust forest. The spatial variation of soil moistures at deferent slope positions was complex,which indicated a larger spatial heterogeneity. The average soil moisture below the slope was greater than that up the slope. With the increase of soil layer depth, the soil water content gradually decreased, and the rate of change tended to be gentle. The soil moistures in each soil layer in the locust forest were all larger than those in the waste grassland, the spatial variation was more stable. In general, the soil moisture in the northwest slope was larger and more stable than that in the southeast slope.(2) The Influence degrees of different underlying surface factors on the soil moisture in the Zhifanggou Watershed was: “soil layer depth(P=0.004)” >“slope aspect(P=0.018)” >“vegetation types(P=0.327)”>“slope position(P=0.495)”. The Influence degrees of different underlying surface factors on the hydrogen and oxygen isotope in the soil water was: “soil layer depth(P=0.001)” >“slope aspect(P=0.028)” >“vegetation types(P=0.812)”>“slope position(P=0.871)”. The differences of δ18O between different soil layers increased with the depth. When the depth differences exceeded 15 cm, the differences of δ18O tended to be extremely significant differences. It could conclude the depth caused by the evaporation was15 cm.(3) The evaporation line equation of gully-channel water in the Jiuyuangou Watershed was EL-1: δD = 5.40 δ18O- 20.34, and the evaporation line equation of the gully-channel water and well water was EL-2: δD = 5.33δ18O- 20.89. The local meteoric water line equation in the Zhifanggou Watershed is LMWL: δD = 6.71δ18O – 3.22, the evaporation line equation of the surface water is EL-1: δD = 6.77δ18O – 5.32, and the evaporation line equation of the soil water is EL-1: δD = 3.69δ18O – 32.96.The surface water and ground water in Jiuyuangou Watershed were both supplied by atmospheric precipitation. The weighted average value of hydrogen and oxygen stable isotopes in precipitation which could effectively recharge the ground water was δ18O =-11‰ and δD =-79.80‰ respectively. Furthermore, the main transformation relationship between the gully-channel water and well water was characterized by unidirectional recharge from the gully-channel water to the well water with a slow recharge rate, indicating that the retention time of the water in Jiuyuangou Watershed was longer. Based on the above conclusions, water loss caused by evaporation in thetransformation process between the gully-channel water and well water is also calculated using Rayleigh distillation model and regression equation between absolute temperature T and fractionation factor 18αw-v and 2αw-v with an assumption that the water vapor exchange is equilibrated. The estimated result shows that the gully-channel water will lose 7% during the transformation process.(4)The Influence degrees of different underlying surface factors on the unit recharge of the precipitation in the Zhifanggou Watershed was: “soil layer depth(P=0.029)” > “slope aspect(P=0.349)” >“vegetation types(P=0.943)”. The recharge effect of precipitation on soil water mainly concentrated in the surface 15 cm soil layer, and the average recharge was 0.52mm/cm, which tended to smaller and less stable with the depth increased. The unit recharge of the precipitation in the northwest slope was larger and more stable than that in the southeast slope. The unit recharge of the precipitation during August was the maximum value; the unit recharge during July was the minimum value. The total recharge of the precipitation in the locust forest was 2.67 mm larger than that in the waste grassland, and the total recharge rate of the former was 4% larger than the latter. In addition, the total recharge of the precipitation in the northwest slope was 3.04 mm larger than that in the southeast slope, and the total recharge rate of the former was 4.79% larger than the latter.