Study On The Soil Water Response Associated With Ecological Construction On The Loess Plateau

Global warming has accelerated the water cycle,which in turn has caused changes in regional atmospheric precipitation processes.Precipitation is one of the direct sources of soil water of the Loess Plateau and also one of the crucial links in the water cycle.The soil water acts as a momentous link among precipitation,plants and groundwater.Its spatial and temporal distribution is also the crucial factors to the growth of plants in the ecosystem of the Loess Plateau In order to solve the key problems of ecological construction and comprehensive utilization in water resources on the Loess Plateau,this paper applies for multidisciplinary in theories and methods,and combines stable isotope tracer as long as "3S" technologies with the field investigations and soil/water samplings to depict both the spatial and temporal variation and the effect of precipitation isotope and the trajectory of moisture source in the researched area.This paper also illustrated the spatial and temporal regularities of distribution of soil water movement on typical watershed of the Loess Plateau,and identified the relationship between the profile of isotope and soil water movement.The study aims to provide reliable scientific basis for comprehensive utilization of water resources,ecological construction and restoration of the Loess Plateau.The main conclusions of this paper are as follows:(1)This study clarifies the distribution of hydrogen and oxygen isotopes in the atmospheric precipitation of the Loess Plateau.The results of studying both hydrogen and oxygen isotopes and environmental factors show that the equation of Local Meteoric Water Line of the Loess Plateau is:?2H=6.92?18O+0.71.The spatial distribution of ?2H and ?18O values in the Loess Plateau is characterized by high in the northwest and low in the southeast.However,the spatial distribution of ?2H and ?18O in different seasons is significantly different.Among them,spring and winter are characterized by low northwest and high southeast.The summer and autumn trends are the opposite.The hydrogen and oxygen isotopes of precipitation show that the "temperature effect" of hydrogen and oxygen isotopes of precipitation are obvious in the northwest and north of the Loess Plateau.The "precipitation effect" of the hydrogen and oxygen isotopes of precipitation are obvious in the southeast of the Loess Plateau.However,the "rainfall effect" of the hydrogen and oxygen isotopes of precipitation are extremely obvious during the rainy season in the north,east and south of the Loess Plateau.The relationship between temperature and ?2H and ?18O values of precipitation in the northwestern Loess Plateau is:3.13‰(?2H)or 0.44‰(?18O)/?.The relationship between northern temperature and ?2H and ?18O values of precipitation is:1.17‰(?2H)or 0.19‰(?18O)/?.The relationship between rainfall and ?2H and?18O values of precipitation in the east is:-0.21‰(?2H)or-0.03‰(?18O)/mm.The relationship between rainfall in the south and ?2H and ?18O values of is precipitation:-0.26‰(?2H)or-0.04‰(?18O)/mm.The relationship between precipitation in the southeast and ?2H and ?18O values of precipitation is:-0.16‰(?2H)or-0.01‰(?18O)/mm.The results of d-excess value of precipitation on the Loess Plateau show that there are not only extreme drought in the north and northwest of the Loess Plateau,but also strongly unbalanced evaporation and secondary evaporation in the precipitation process.(2)This study clarifies the moisture source of precipitation on the Loess Plateau.The study of the trajectory of moisture source in precipitation on the Loess Plateau shows that are very different channels and distribution of spatial and temporal in the trajectory of moisture source.In the spring,the moisture from northwest is the main channel and source for the moisture of the west,north,east,south and southeast of the Loess Plateau,with the average contribution ratios as follows:70.87%,74.91%,76.14%,60.54%and 55.45%,respectively.In the summer,the moisture from northwest is the main channel and source for the moisture of the west,east and south of the Loess Plateau,with the average contribution ratios as follows:72.73%,58.75%and 56.64%,respectively.And the moisture from northeast is the main channel and source for the moisture of the south and southeast of the Loess Plateau,with the average contribution ratios as follows:47.10%and 51.96%,respectively.In the autumn,the moisture from northwest is the main channel and source for the moisture of the west,north,east,south and southeast of the Loess Plateau,with the average contribution ratios as follows:65.46%,71.47%,70.98%,52.87%and 62.75%,respectively.In the winter,the moisture from northwest is the main channel and source for the moisture of the north,east,south and southeast of the Loess Plateau,with the average contribution ratios as follows:92.59%,98.26%,61.47%and 66.82%,respectively.And the moisture from southeast is the main channel and source for the moisture of the west of the Loess Plateau,with the average contribution ratios as follows:57.57%.(3)This study reveals the complementary mechanisms of precipitation to soil water use in different land uses.The sloping farmland is more favorable on reserving soil water under a single rainfall event,while the forestland is most unfavorable on keeping soil water under a single rainfall event.The tendencies of soil water content of forestland,grassland and sloping farmland change from decrease to increase firstly then to increase gradually with a time node respectively on the 5th,3rd and 0th day after a single rainfall event.Present study demonstrated the ?18O was more sensitive than the soil water content for tracing the maximum infiltration depth of event water and recharge mechanisms.Forestland,grassland and sloping farmland have preferential flow during the infiltration process in the 80-120 cm soil layer.The isotopes of soil water in grassland,forestland and sloping farmland are all formed the maximum enriched layer in the surface layer of 0-20 cm,and the ratio gradient of heavy isotope to light isotope occurs along the soil profile.Present study demonstrated the ?2H and ?18O was more sensitive than the soil water content for tracing the maximum infiltration depth and recharge mechanisms on event rainfalls.The implementation of ecological construction primarily affects the infiltration process of forestland and grassland above the depth of 160 cm in soil layer.The acts of "preferential flow"of forestland are more prominent than that in grassland and sloping farmland.The soil water infiltration process not only in 20-40 cm in grassland but also in 20-40 cm and 60-80 cm of sloping farmland was dominated by piston flow,while the soil water infiltration process in forestland was dominated by preferential flow.(4)The equation of Local Meteoric Water Line of the Wangmaogou watershed is:?2H=7.99?18O+7.81.The relationship between precipitation amount and ?2H and ?18O values of precipitation is:-0.40‰(?2H)or 0.04‰(?18O)/mm.Under continuous rainfall conditions,the hydrogen and oxygen isotopes of soil water of grassland,forestland,terraced and sloping farmland formed the largest enrichment in the 20-40 cm soil layer.The degree of evaporation and fractionation of soil water is sorted as follows:terraced>grassland>forestland>sloping farmland>dam land at the same time.Forestland has the highest degree of "preferential flow",and grassland and forestland can significantly extend the path of "preferential flow".The hydrogen and oxygen isotopes of soil water of grassland,forestland and terraced were relatively stable below the lines of 160 cm,200 cm and 200cm soil layer,respectively,in soil.(5)This study reveals the temporal and spatial variation of soil water in different land uses in the Wangmaogou watershed.Under the ecological construction,the soil water of the sloping farmland is more stable in summer and autumn than in winter and spring,while the soil water of the dam land is the most stable in autumn.And the soil water of the forestland is more stable in winter and spring than in summer and autumn.The relationship between soil water content and time(month)is:?=0.65T+12.43(sloping farmland),?=0.34T+15.92(damland),?=0.10T2-0.92T+13.32(terraced),?=0.31T+9.01(forestland),?=0.12T2-1.18T+16.65(grassland).The soil water of the terraced reached a relatively stable below 180 cm depth.The soil water of the forestland also reached a relatively stable below 100 cm depth.The soil water in the depth of 0 to 100 cm of sloping farmland,grassland,dam land,forestland and terraced are obvious volatility,in which the deepness is the primarily area of soil water consumption and compensation Moreover,the depth of soil water from 100 to 160 cm of sloping farmland,grassland,forestland and dam land are mainly stability,in which is the major depth of the soil water compensation The results of the study in soil water consumption and compensation of the watershed show that the different land use types have differences on consumption and compensation of soil water.This study reveals the variation of inter-annual soil water consumption and compensation balance for different land use types.On the annual scale,the dam land and sloping farmland are the main types of land use that compensate soil water,with the "compensation effect" the average soil water content increased by 5.52%and 3.93%,respectively.The forestland is the major land use type that consumes soil water,and with the "consumption effect" the average soil water content decreased by 1.28%.In conclusion,the dam land and sloping farmland are more beneficial in increasing soil water resources of the watershed,and the expansion of forestland will accelerate the consumption of soil water resources.Furthermore,the dam land increases soil water content of the soil profile from the depth 160 to 240 cm of the watershed,and the Grain for Green Project played a "negative effect" at the depth for 0-240 cm in the watershed.