| Since 1999,the Loess Plateau implemented large-scale ecological construction measures,such as Green for Grain project,check-dam and terrace construction,which significantly improved the ecological environment in the region.The monitoring and previous research indicated that changes of the underlying surface conditions on the Loess Plateau in the past 10 years was the most during the past century.Ecological construction altered the eco-hydrological process through the changes of the underlying surface conditions of the watershed.This paper selected typical ecological construction contrast watersheds on the Loess Plateau as research objects.Through field monitoring,laboratory experiment and simulation calculation,we examined the the spatial and temporal distribution characteristics of hydrogen and oxygen isotopes in different water bodies between the contrast watersheds.We also evaluated the relationship between hydrogen and oxygen isotopes in different water bodies and identified its influencing factors.We studied the spatial and temporal distributions of soil water content and clarified its relationship with hydrogen and oxygen isotopes.The variation law of water transmission time in different water bodies under the two contrasting watersheds were revealed.The sources of different water bodies and its seasonal replenishment characteristics were analyzed.Evaporation losses in different periods were quantified.The influencing mechanism of water transformation and transmission under ecological construction watershed were clarified.The present research could provide scientific basis for the deep understanding of the ecological construction recovery and the sustainable use of water resources in this region.The main conclusions of this paper were as follows:(1)The different water isotopes of ?D and ?18O in the ecological construction watersheds(Jiuyuangou and Wangmaogou)were more enrichment than that in natural watershed(Peijiamao).The fitting line of precipitation isotope of Jiuyuangou,Peijiamao,and Wangmogou were ?D=6.67?18O-0.63,?D=7.00?18O+1.45,and ?D=6.88?18O+0.24,respectively.The slope and intercept of precipitation lines in the ecological construction watersheds(Jiuyuangou and Wangmaogou)were smaller than that in natural watershed(Peijiamao).The isotope values were low in rainy season,and high in dry season.The surface water isotopes of ?D and ?18O in the ecological construction watersheds(Jiuyuangou and Wangmaogou)were more enrichment than that in natural watershed(Peijiamao).The isotopic values of the surface water and shallow groundwater in the three watersheds showed a dilution trend with the elevation.The isotope values firstly increased and then decreased on the monthly scale within the seasonal variation,with low value in spring and winter and high value in summer and autumn.(2)Soil water content of five land use types under ecological construction shown in the order of dam field>slope farmland>terrace>forest land>grassland.The variation of soil water content showed that up slope had lowest value,then middle slope and down slope had the highest value.The mean value of soil water content in the slope farmland increased with soil depth,and soil water content fluctuated with soil depth in check-dam field,terrace and forestland.Moreover,the surface soil water content was low in grassland,while the values were relatively stable in other soil layers.Under rainfall event conditions,the isotopes of soil water in the check-dam fields were first enriched and then depleted.The isotopes of soil water in the shallow layer of grassland first enriched,then depleted and then stabilized.The variations of soil water isotopes in forestland with depth were first enriched,then depleted,then enriched and then stabilized.The variations of soil water isotopes in terrace with depth were first enriched,then depleted,then fluctuanted,and then stabilized.The isotopes of soil water in sloped farmland first enriched,then depleted,and then stabilized.(3)Ecological construction can extend the water transmission time from precipitation to surface water.Based on the stable isotopes of hydrogen and oxygen,we found that the water transmission time from precipitation to surface water in Jiuyuangou was about 183 days,and that in Peijiamao was about 101 days.The water transmission time in Jiuyuangou was about 1.81 times larger than that in Peijiamao.Meanwhile,the water transmission time from precipitation to shallow groundwater in Jiuyuangou was about 935 days,and that in Peijiamao was about 888 days.The water transmission time in Jiuyuangou is about 1.05 times larger than that in Peijiamao.Water transmission time from precipitation to shallow groundwater was about 6 times larger than that from precipitation to surface water.The changes of water transmission time in different sections of Jiuyuangou and Peijiamao were in accordance with logarithmic function,which indicated that the water transmission time has scale effect.In the dry season,the precipitation and shallow groundwater recharge surface water in Jiuyuangou,Peijiamao and Wangmaogou watershed,but the conversion ratios were different in the three watersheds.The conversion ratio of shallow groundwater to surface water in dry season increased 4.5%?22.47%after ecological construction.During the wet season,the precipitation and surface water recharge shallow groundwater in Jiuyuangou,Peijiamao and Wangmaogou watershed,and the conversion ratios were different.The conversion ratio of precipitation recharge shallow groundwater in wet season decreased by 14.8%?1 8.46%after ecological construction.(4)The results indicated that temperature,altitude,ecological construction measures and slope were the most important factors,and then precipitation and air temperature,and finally aspect.The main factors that affected precipitation isotopes were temperature,precipitation,relative humidity and solar radiation.The main factors that affected river and shallow groundwater isotopes were temperature,altitude and ecological construction measures.The main factors that affected soil water isotopes were land use,soil depth,slope and aspect.The rank of influencing factors on precipitation isotopes were rainfall,air temperature,relative humidity and solar radiation.The rank of influencing factors on surface water and shallow groundwater isotopes were attitude,temperature and ecological construction measures.The rank of influencing factors on soil water isotopes were spect,slope,soil layer depth and land use. |
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