| China's northwest arid area has abundant land, adequate light and heat resources, while subject to long-term draught and water shortages, large tracts of arable land are desolate. Along with rapid growth of Chinese population, land resources in northwest arid area are exploited in a fast way, especially with the development of electric water pumping equipment and irrigation technology, many high-pumping irrigation zones are built in Northwest China. From the ecological perspective, the emergence and development of these irrigation zones are large area of artificial oasis building over the natural desert, creating huge economic, ecological and environmental benefits. In the meanwhile, due to extensive irrigation without appropriate discharge, adverse ecological and environmental problems arise consequently, such as soil salinization, natural vegetation decay, and deterioration of groundwater quality. These consequent environment evolution and changes have constrained the irrigation district's economy sustainability, stability and healthy development to certain extent. Therefore, it is of great significance to study the law of irrigation water transport in pumping area, grasp the dynamic migration of water and salt in soil, and also research the coupling between water-salt transport and irrigation water changes. These researches would play an important role in sustainably developing and scientifically managing irrigation area.The project, which focused on JingDian Electricity Pumping Irrigation District(hereafter JEPI) in Gansu Province, analyzed field investigation and environment monitoring data of the soil changes in this area within 40 years. Afterwards, I intensively studied the law of water-salt transport in irrigation area, simulated dynamic of ground water in different hydrogeological units, calculated the situation of water-salt balance, and studied the impact of artificial pumping irrigation on environmental changes. In the research, I combined the transform model of surface water with transport model of ground water, introducing several methods, such as theoretical research, field test and numerical simulation and so on. The conclusion reached from this project is as following:(1) JEPI is located in arid inland of Northwest China, where it is rich in land resources, sunlight and heat, and lack of rainfall and water resource. The unique geographical and climate conditions make the water resources, mainly irrigation water, the most critical factor of agriculture industry. Long-term irrigation caused regional changes in soli and water resources and environment, which is a typical case for researching the impact of water-salt transport on soil resources.(2) Pumping irrigation in arid areas has both positive and negative effects on transformation of local soil and water resources. On one hand, it improves local ecological environment and living environment, by building a large plateau area of oasis through electronic pumping irrigation in drought land. On the other hand, inappropriate irrigation system and long-term irrigation without reasonable drainage process leads to regional water-salt evolution and migration, accompanied part of secondary land salinization and soil degradation.(3) The transformation between surface water and groundwater in JEPI has remarkable features, with the water cycling system in irrigation area experiencing the concentrated transformation process of cross-regional water-irrigation infiltration-groundwater-subsurface transport or dissipate. Infiltration of irrigation water is the main recharge source of groundwater, accounting for 75.9%of groundwater recharge, while meteoric accounts 5.38%of total recharge, subsurface flow account for 15.3%; runoff recharge accounted for 3.42%.(4) The actual level of water and salt in the migration process is mainly affected by water infiltration, and evaporation from soil. The thesis established water movement model in saturated soil-non-saturated soil conditions, based on the unsaturated flow theory under infiltration and evaporation conditions. In the meanwhile, it established hydrodynamic dispersion equation of soil solute transport in saturated-unsaturated zone, and water quality model of soil solute dispersion.(5) Infiltration process of irrigation water in soil is quite complex, affected by soil structure, irrigation frequency, irrigation capacity and other factors, with soil salinity transport in the process of irrigation water infiltration. In case of perennial irrigation infiltration, the land surface are tends to be desalinated, the salinity is gradually transformed into deep water salt during the infiltration of irrigation water, and continually moving with the groundwater transport.(6) Water-salt transport in closed-type of hydrogeological unit shows the feature of water run-off stagnant and water-salt accumulation. In general, it gradually forms a relatively smooth irrigation infiltration zone, solute migration zone from the periphery to the center of basin, and gradually turns into a lagging irrigation infiltration zone with accumulative water and salt. Level of groundwater in migration zones changes regularly with stages of irrigation, and the changes within one year is the same with that during the irrigation peak period (May-October), the largest amplitude is up to 3.6m. The trend of changes in solute transport zone is similar to lagging change of irrigation infiltration zone, in the range of 0.6-1.8m, but change of groundwater level in water catchment-poly salt zone is small. According to the monitoring data of groundwater during 1994-2008, in closed-type of hydro geological unit, groundwater level showed an upward trend year by year in the water-salt transport zone, especially significant in the water catchment-poly salt zone, with largest increasing level by 22.6m. In the solute transport zone, the monitoring cumulative increase of groundwater level is 5-8m.(7) Water-salt transport in open-type of hydrogeological unit is smooth with groundwater flow,.but stagnant with solute transport. The open-type unit has also formed infiltration runoff zone, solute transport zone which gradually moving to drainage zone, with its flows coinciding with the terrain changes. For years, the cumulative rise in groundwater level ranges from 1.2-4.6m, the groundwater level changes with infiltration of irrigation water, ranged from 0.5-2.6m last year.(8) The process of groundwater chemical evolution differs between open-type hydrogeologicaunits and closed hydrogeological unit. In the area of irrigation water infiltration and solute transport, calcium carbonate type of groundwater with low salinity is transiting to type of 2.1-5.2g/ L of SO42-—Cl-(K++Na+)—Ca2+or SO42-—Cl-(K++Na+)—Mg2+. While in the salt-congregating drainage district of closed unit, groundwater salinity is up to 273.15g/L, with head water chemistry type of SO42-—Cl-—Ca2+—(K++Na+).(9) The data of water-salt balance in irrigation district shows that average annual irrigation water from Yellow River is 389 million m3 from 1994 to 2008, bringing to the area extra 15,560 tons of salt annually. The water discharged from the drain rises from 700 million m3 in year 1978 to 23.7 million m3, with average 16,800 tons of salt taking away annually, which means the irrigation is in the condition of desalination. From the perspective of export-oriented water composition, irrigation from channel is the main source of water, accounting for 76.3%, followed by rain water brought about 18.6%, and 5.1%from other process. Among the way of water exclusion and consumption, agriculture and forest take the largest share of 64.31%, followed by 15.37%from wasteland evaporation,13.37 from gutters and underground discharge, and the remaining 6.95%of others.(10) Due to water-salt evolution in the JEPI, the storage of groundwater increases by about 035.7 million m3 per year, with dramatic differences of groundwater quality among regions. The water-salt transport also affects the spatiotemporal transformation among different land types, e.g. the transformation ratio between irrigated land and desert is 71.72%. In addition, water-salt transport has resulted in 6,700 hm2 of secondary salinizated land, and another 12,000 hm2 arable land under threat of salinization. The previous desert landscape before introduction of irrigation gradually switches to oasis land.The results above are expected to provide science basis for the integrated management of irrigation, rational allocation of land resources, utilization and protection of water resources, and sustainable development model for irrigation area. Besides, it advanced several alternative references for administration departments to formulate relative policy or decision, in order to better serve economic construction and sustainable development in Pumping Irrigation District.
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