Study On Changes Of Rainfall Infiltration Recharge Under Conditions Of Variable Environment In Songnen Basin

Groundwater is an important component of water resources in Songnen Basin. Compared with surface water which is getting polluted seriously, groundwater has better quality and is widely distributed. Since 1994, the portion of groundwater in total water supply in Songnen Basin has been increasing year by year. Rainfall infiltration recharge, which takes 80.37% of the natural groundwater resources in the Basin, is the main source of shallow groundwater. Changes of rainfall infiltration recharge have an important effect on the groundwater resources in Songnen Basin.To use the lumped parameter model to calculate the quantity of rainfall infiltration recharge in the large-scale area, it usually can't reflect the changes of rainfall infiltration caused by spatial and environmental variation. Additionally, to solve the spatial variability of rainfall infiltration recharge under the conditions of variable environment, this thesis studies the distributed evaluation method about the rainfall infiltration recharge in large area. First, whether the environment changed is diagnosed. Second, combined with GIS and RS, the distribute estimate model, which cover all the factors to the rainfall infiltration recharge, are used to estimate the rainfall infiltration recharge in large area with environmental changes.WetSpass model, which is a distributed model to calculate the rainfall infiltration recharge, can respectively consider the factors which can obviously affect the result (influencing factors), such as climate, land use/cover, soil texture and so on. Thus, in this evaluation, WetSpass model is introduced into the calculation of the rainfall infiltration recharge in Songnen Basin and its groundwater sub-systemsAfter calculation, the correlation analysis method of spatial distribution of map data put forward by Whitman in 1999 is used to analyze the relationship between evapotranspiration, local runoff yield, land use/cover, soil texture and precipitation infiltration recharge in Songnen Basin. The result indicates that there are very distinct spatial correlation between rainfall infiltration recharge, evapotranspiration, local runoff yield and soil texture & land use/cover. Though there are differences in soil texture & land use/cover, changes of rainfall infiltration recharge present an obvious law in different climate zones—while the climate changes from humid, semi-humid to semi-arid zones, the rainfall infiltration recharge is gradually decreasing. Infiltration recharge ability of different kinds of soil texture and land use/cover in Songnen Basin is studied by changing inputs of the WetSpass model, and the results show that infiltration ability of different soil texture is descending in this order: sand, sandy loam, silt sand, silt loam, and sandy clay. Taking the types of land use/cover into account, the order is: dry land, low-covered grass, mid-covered grass, high-covered grass, open forest land, forest land, resident land, saline-alkaline land, paddy field, wet land, and water area. Generally, factors like climate, soil texture, and land use/cover are all important impact issues on rainfall infiltration recharge in Songnen Basin. Precipitation has been in a relatively stable state of dynamic changes with little swing while temperature has increased obviously in the past 50 years.Calculation results of quantities of rainfall infiltration recharge under different climate conditions in Songnen Basin shows that rainfall infiltration recharge, interception, local runoff yield, and evapotranspiration all increased with the descend of guarantee probability. As the descending of guarantee rate, percentage of rainfall infiltration recharge in total precipitation increases gradually with the largest relativity. Thus it can be seen that precipitation increase/decrease impacts its infiltration recharge greatly. Since the difference of climate, soil texture, and land use/cover in groundwater subsystems, precipitation infiltration recharge presents different variable characteristics under different guarantee rate. Analysis results reveal that as the climate conditions changes, quantities of precipitation infiltration recharge would increase greatly if subsystems'conditions of soil texture and land use/cover are favourable for infiltration, and contrarily there would be little influence to the groundwater.As land use/cover change, quantity and spatial distribution of precipitation change accordingly. In the past 10 years, conditions of land use/cover have changed obviously. Generally speaking, area of dry land, paddy field, and resident land has increased while that of forest land and high-covered land decreased. There are two sides in the impacts about the change of land use/cover on rainfall infiltration recharge in Songnen Basin. On one hand, the dry land increase makes the rainfall infiltration recharge increase accordingly. And on the other hand, increase of paddy field and resident land makes against it. Therefore, it's hard to determinately analyze it will increase or decrease under variable conditions of land use/cover. Results of the model show that in the 10-year period from 1995 to 2005, average rainfall infiltration recharge per year had increased more or less under variable conditions of land use/cover in Songnen Basin. Of all subsystems in Songnen Basin, rainfall infiltration recharge in the subsystem—the High Plain in Nenjiang Watershed,had increased the most from 61.33mm to 63.53mm per year since conditions of land use/cover changed while that of the Songnen Trunk had decreased the most—from 57.85mm to 54.34mm and that of the Low Central Plain had changed the least—from 5.60mm to 5.63mm.As conditions of climate and land use/cover in Songnen Basin will change in future, this study predicates the conditions of land use/cover in Songnen Basin from 2018 to 2027 with CA-Markov model and the average quantity of precipitation infiltration recharge per year from 2013 to 2022 and from 2023 to 2032 combined the study result of future climate in Northeast China predicated by Mr. Wenci Zhao who works at NMIC. While change of climate conditions leads to increase of rainfall infiltration recharge under variable environment conditions in future in Songnen Basin and its subsystems, but the impacts of land use/cover conditions to every subsystem is different. Hereinto, that in High Plain in Songhuajiang Watershed & Mountain Front Plain in Nenjiang Watershed leads to decrease, in High Plain in Nenjiang Watershed, Low Central Plain & Songnen Trunk leads to increase.Results of this study are of great practical significance in definitude of the impacts of environmental changes on precipitation infiltration recharge, in direction of groundwater evaluation & management, and in achievement of sustainable development; and of great academic significance in studies on formation & control mechanism of groundwater resources in large area.