| Large-scale study of ecological hydrology in arid area began with the study on rational use of water resources and ecological environment protection in the northwest region of the major projects of the National Science and Technology Research Program of the Ninth Five-Year Plan(1996-2000)in China.After years of research,a unified understanding of the hydrological cycle and ecological pattern in the arid regions has been formed.Rainfall production in mountainous areas,runoff consumption in plain areas,inland river flow recharges groundwater;vegetation pattern depends on the water table,which characterizes the landscapes of oasis,transition zone and desert,within different distances from an inland river.The development and utilization of water resources has changed the groundwater subsurface flow field,and the structure of the ecological circle has also changed.Therefore,the safety of the ecosystem depends on the stability of the groundwater subsurface flow field.The key is to maintain a certain transition zone between the oasis and deserts,so that the oasis and the desert maintain a safe distance.According to the theory of the ecological layer structure,this requires a transition zone to maintain a certain depth of groundwater.Oasis safety is the core of ecological protection in in arid areas.Irrigation is a common form of water resources development and utilization in arid areas.This is the process of water concentration in oasis,which may have a major impact on the ecological security of the oasis.On the one hand,due to the concentration of water resources to the oasis,the groundwater level at the outer edge of the subsurface flow field has decreased,resulting in the disappearance of vegetation degradation in the interlaced transition zone of the oasis,the reduction of the transition zone,and the desert to the oasis.On the other hand,the irrigation and drainage is not smooth,resulting in a sharp rise in the groundwater level inside the oasis,resulting in secondary salinization.For example,in the Heihe River Basin,compared with the 1970s,the area of the transition zone around the oasis decreased by 6,972 km2,the desert expanded by 14,281 km2 in the 1990s,and the transition zone area subsided.Compared with the 1990s,the area of arable land increased by 732 km2,and the transition zone with only a small area decreased by 4127 km2,and the desert expanded by 8280 km2.At the same time,the area of secondary salinization continued to expand,reaching a cumulative 722.22 km2.It is estimated that the total evaporation loss due to salinization and the amount of salinized water is about 692?822 million m3/y.This kind of ecological problem caused by desertification and salinization caused by changes in groundwater level(depth),both internally and externally,seriously threatens the ecological security of the oasis.The purpose of this paper is to explore a comprehensive technical method for systematically solving the problems of oasis ecological security in inland rivers and arid plains.Through theoretical analysis and empirical investigations,the in-depth study of the formation and evolution mechanisms of desertification and secondary salinization and their internal relations,By artificially disturbing the local groundwater level,the secondary salinization can be eliminated and the transition zone restored in order to curb the expansion of desertification.Based on the theory of ecosystem layer structure,this paper explains the oasis-transition belt-desert symbiosis system in the arid area of the arid area,and studies the connotation of the transition zone,and establishes the theoretical basis of the oasis ecological security.Analyzed the characteristics of phreatic evaporation in arid area,and the conceptual model of phreatic-gas-belt-surface was constructed.The characteristics of soil moisture structure inside the dam affected layer were further deepened,and the theoretical calculation formula of the thickness of the dam affected layer was optimized.Characteristics of the root layer of the vegetation community in the transition zone of the arid region.Based on the relationship between the Shallow Groundwater Active Layer and the vegetation roots and salt transport,the critical depth of the transitional desertification and salinization groundwater is defined,and the quantitative calculation formula is given.The shrinkage of groundwater subsurface flow field leads to desertification in the transition zone,and the groundwater depth is too deep.The salinization is the excess water resources in the local area of the oasis,and the groundwater depth is too shallow.Based on the concept of groundwater continuous subsurface flow field supporting the ecological layer structure,The idea and technical method of groundwater level regulation for synergistically solving salinization and transitional desertification in irrigation districts.In this paper,the plain area of the Heihe River basin is a typical area,and Luocheng Irrigation District is selected as a demonstration area.Through a large number of field investigations and research analysis,the application of oasis ecological security in inland river arid areas is carried out.The main research results are as follows:(1)Established the theoretical basis of oasis ecological security in arid areas.According to the principle of the ecosphere structure of the inland river plain in the arid region,by revealing the relationship between the oasis-transition zone-desert symbiosis system and the change of groundwater depth,it shows that the transition zone plays a decisive role in curbing the expansion of desertification and ensuring the ecological security of the oasis.Restoring the transition zone is the key to the overall ecological security of the oasis.Because the development and utilization of irrigation and water resources is a process of water resources concentration to oasis,the spatial distribution of groundwater levels will inevitably change:First,the groundwater level in the transition zone where diving evaporation is weakest will decline,and the result will be the degradation of the transition zone and The expansion caused the oasis to face the desert directly;secondly,the poor drainage of the irrigation area caused the groundwater level to rise unusually,leading to a large number of saline-alkali lands within the oasis,which directly threatened the oasis' own safety.Obviously,the use of water resources will inevitably trigger the degradation of the transition zone and the expansion of secondary salinization.The continuity of groundwater subsurface flow field makes both of them inevitable internal connection and complementarity,which provides theoretical basis for coordinated research.(2)A conceptual model of diving evaporation in arid plain was established to reveal the groundwater depth conditions of desertification and salinization from the mechanism.The plain below the inland river exit is a runoff dissipation area,and diving evaporation is its main form.Driven by the attraction of capillaries,shallow groundwater active layer is formed by attaching to the diving surface,and groundwater is connected with the surface ecology through the shallow groundwater active layer.Surface vegetation absorbs water from the shallow groundwater active layer through the root action layer.For the transition zone,when the groundwater level drops,the shallow groundwater active layer loses contact with the vegetation root action layer of the transition zone.Due to the loss of water supply,the vegetation in the transition zone disappears and desert invasion occurs.When the groundwater level rises,the shallow groundwater active layer contacts the surface,and the salt in the soil is continuously transported to the surface to precipitate crystals,forming secondary salinization.The concept of shallow groundwater active layer describes the distribution law of capillary water on the diving surface.The maximum rise of capillary water is defined as the thickness of the shallow groundwater active layer to summarize the range of soil capillary rise water activity.The vertical distribution of soil water is used to describe the soil water distribution structure in the shallow groundwater active layer.Using the self-derived formula for calculating the equivalent aperture of soil capillary tube,the value of the liquid surface tension in different ecological problems was discussed by analyzing the parameter value method,and the calculation method of the thickness of the shallow groundwater active layer was deepened.The empirical formula was used to fit the curve,and at the same time,the distribution structure of soil water in the shallow groundwater active layer was attempted from theory.(3)The key factors that affect the ecological security of the oasis are studied in depth.Through a large number of field investigations and research analyses,reasonable desert boundaries in the lower plain of the Heihe River Basin have been determined.Field survey and research analysis in the transition zone of the plain area of the Heihe River Basin,including vegetation succession,community composition,range of root systems,and groundwater depth,etc.,totaling 235 field vegetation sampling points,and investigation of the relationship between vegetation and groundwater in 8 typical sections of the river bank,68 groundwater observation points.The main types of vegetation communities in the transition zone of the plain region of the Heihe River Basin were Camel thorn,Haloxylon ammodendron,Nitraria sibirica,Zagua jujube,and Tamarix.The law of vegetation succession under changing water conditions was obtained.With the change of water conditions,the shrub and semi-shrub transitional vegetation communities in the transition zone gradually replaced the desert vegetation and even died out.Based on the field investigation data of the thickness of the root system and the depth of groundwater in different vegetation communities,the burial depth conditions for groundwater replenishment in the transition zone were analyzed,and the reasonable protection range of the transition zone in the lower plain area of the Heihe River Basin was determined.Borders prevent the expansion of desertification.(4)According to the mechanism of desertification and secondary salinization,the corresponding critical groundwater depth is defined and calculated as an important basis for governance and control.When the diving influence layer comes into contact with the root community interaction layer of the vegetation community in the transition zone,it starts to absorb water,and the groundwater can be stably recharged in the transition zone if there is cross groundwater between them.And defined as the critical burial depth of desertification groundwater.Under the effect of diving evaporation,the soil salt is dissolved and moves with the water.When the diving affected layer comes into contact with the ground surface,the carried salt will precipitate on the surface and crystallize.Therefore,the thickness of the diving affected layer is directly defined as the threshold of secondary salinization and groundwater Buried deep.Through the case analysis of Luocheng Irrigation District,the calculation shows that the critical depth of desertification groundwater in the transition zone of Luocheng Irrigation District,Haloxylon ammodendron+White thorn+Shah's Jujube,Tamarix chinensis,Camel thorn+Reed+Sabina,is 8.26m,11.26m and 13.26m,the critical buried depth of secondary saline-alkaline groundwater in the Oasis Irrigation District is about 1.29m,and a large number of field empirical analysis is carried out.(5)Based on the concept of continuous underground flow field supporting groundwater layer structure,the idea and technical method of groundwater level regulation for synergistic solution of desertification in transition zone and salinization in irrigation district are proposed.The cause of desertification in the transition zone is the lack of groundwater,and the depth of groundwater is too deep.Salinization is an excess of water resources in the local area of the oasis,and the depth of groundwater is too shallow.The groundwater subsurface flow field supporting the ecological layer structure is a continuum.In terms of water quantity,the two problems are highly complementary,so the transition zone desertification and secondary salinization can be solved synergistically to control the groundwater level.This paper identifies the desert boundary in the regulation of groundwater level and estimates the amount of secondary salinization excess water in the irrigation area.At the same time,it has a new understanding of the conditions of secondary salinization in the arid area and the waste of water resources.It is studied how to reduce the groundwater depth in the secondary saline-alkali zone to below the critical depth,and how to absorb and use the desert boundary area.Problems such as the amount of water and the amount of water delivered.The Luocheng Irrigation District was applied to the groundwater level control demonstration area,and the potential of the secondary salinization area within the Heihe River Plain was analyzed.According to the analysis,?the amount of water occupied by the salinization area in the Luocheng Irrigation District is 1.76×106?4.70×106m3,and the maximum additional evaporation is 1.99×107m3.Through regulation,the expected recoverable width is about 0.85?2.37km.It is a transition zone range of 25?235km2.?The secondary salinization of the Heihe River Plain uses about 0.43?1.73×108m3 of water resources,and the maximum additional evaporation is about 6.49×108m3,by transferring excess water from the secondary salinization zone to the transition zone.For the desert boundary,the area of the transition zone is expected to be about 287?2344km2. |
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