| Soil water is the demanded facter for plant growth in ecosystem, and it is also the most active part which affects plant growth, ecological construction and reasonable distribution and effective utility of water resource. Most areas of the loess plateau are located in semi-arid and semi-humid area. The soil water reservoir constructed during characteristic soil formation is an irreplaceable factor in the ecological construction. Soil water storage decreased as the vegetation reconstruction developed. So, the continuous utilization of soil water is an urgent problem in ecological construction. In this research, temporal and spatial variations and deficits of soil water was studied under different land uses in Yangou catchment located in loess hilly region. Combined with stable isotope technique, the isotope composition of precipitation, soil water, river and ground water in this area was investigated, and the main conclusions were drawn as follows:(1) Yearly precipitation pateern had definite effects on the seasonal variation and profile distribution of soil moisture. In normal year, soil moisture in dry farmland had a gentle seasonal variation; in dry year, it decreased slowly before rainy season but increased markedly after rainy season; while in rainy year, it had an overall increase and the increment was remarkable after rainy season. The soil moisture in R. psendoacacia forestland, Hippophae shrubland, and B. ischaemun grassland decreased as a whole in normal year. In dry year, soil moisture in Hippophae shrubland decreased first and increased then, while that in R. psendoacacia forestland and B. ischaemun grassland varied in"W"type, with the minimum in June and August. In rainy year, the seasonal variation of soil moisture in R. psendoacacia forestland and Hippophae shrubland presented"V"type, and that in B. ischaemun grassland fluctuated markedly, with the minimum in August. In dry farmland, the active and sub-active layers of soil moisture were deeper in dry year than in normal year, and the sub-active layer disappeared in rainy year. In R. psendoacacia forestland and B. ischaemun grassland, the active layer of soil moisture was deeper in dry and rainy years than in normal year; while in Hippophae shrubland, this active layer was shallower in dry and rainy years than in normal year.(2) Landuses made soil moisture in vertical layer quite different. In terrace land, the variational trend of soil moisture in different layers was accordant, that is, the fluctuation of soil moisture in vertical layer was little. Under different landuses, the variation of soil moisture between surface layer (0-30cm) and middle layer (30-100cm) was accordant, and landuse, in terms of the accordant degree, ranks in the descendant order of terrace land, pasture land, forestland, and shrub land. Except shrub land, the grey relational grade of the landuses between surface land (0-30cm) and deep layer (100-200cm) was least, which means that soil moisture varied remarkably. The grey relational grade of soil moisture in each month was different. In shrub land, except December, the variational trend of soil moisture was similar. By contraries, grass land had the different trend, and the fluctuation of soil moisutre in growth season was great. It can be seen that the different landuses resulted in the difference of dynamic characteristics of soil moisture.(3) No matter before or after rainy season, soil water storage under different measures of soil and water conservation was deficient and then increased in July. The increment for crop land and terraced land was the highest. In August, soil water storage deficit in surface layer increased. After rainy season, soil water storage deficit was restored by precipitation in October. Soil water storage deficit degree of crop land decreased with increased soil depth, while for fish-scale-pit land, it increased with increased soil depth. Soil water storage deficit degree of terraced land and narrow level-belt land was the highest in the 100-200 cm soil layer, the second place in the 0-50 cm, and the lowest in the 50-100 cm. The compensation effect of precipitation on soil water storage of crop land and terraced land was positive and the compensation depth was 160 cm. The compensation effect of precipitation on soil water storage of narrow level-belt land was negative at the depth of 90 cm and below 160 cm. The compensation effect in fish-scale-pit land was negative just at the depth of 30 cm and the compensation depth was at the depth of 100 cm. In the whole 0-200 cm soil layer, land types were in the descendant order of crop land, terraced land, fish-scale-pit land, and narrow level-belt land, in terms of compensation degree of soil water storage deficit. That is to say, soil and water conservation measures can influence the seasonal and vertical variations of soil water storage and the compensation impact of precipitation on soil water.(4) Compared with the global and Chinese meteoric water lines, slope and intercept of meteoric water line in loess hilly region was lower due to the lower individual rainfall, lower air humidity, inland location and strong evaporation of rainfall during the descent. The variation of soil water isotope composition was far lower than precipitation. TheδD andδ18O of soil water located at the right-down of the local water line. It showed a remarkable fractionation due to the strong imbalance evaporation before precipitation compensated for the soil water. The hydrogen and oxygen isotopic composition of soil water in different layers existed significant differences, which in 0-30cm layer was affected more by precipitation while in deeper layers by soil water evaporation driving more. Average value and standard deviation of hydrogen and oxygen isotope of ground water were all lower than precipitation. The relationship of hydrogen and oxygen isotope of ground water showed that slope and interception of which was lower than local water line's, and the values were mostly at the right-down of the line approach to the local water line closely. This indicated that river stream water came from the precipitation affected by the imbalanced evaporation less. Hydrogen and oxygen isotopic composition of ground water affected bigger by the precipitation, which was related to the former composition and fluxes. There was an increasing trend of isotopic value from upriver to lower river after the mixing of different water resource, despite the different isotopic values of varied part of river which was lead to by the different imbalanced evaporation. Variation of hydrogen and oxygen isotopic composition of well water and spring water were both lower than precipitation and soil water,δD andδ18O values of which were at the right-down of local line approach to the line closely, and the two values were close each other. This indicated that well and spring water were all come from precipitation and isotopic fractionation was less caused by the imbalanced evaporation in the course of compensation of precipitation. Replenishment of precipitation to spring water lagged about 30 days. The lagged time of precipitation replenishment to well water and the compensation of soil water to ground water demand for further investigation.
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