Research Of Deep Soil Hydrological Processes And Its Variability In The Loess Tableland

The Loess Plateau is an important element for the development strategy of the "Belt and Road" of China.In order to guide the scientific and rational development and utilization of water resources in the region and build a beautiful and harmonious ecological environment,the research of deep soil hydrological process and its variation in the Loess Plateau are of important theoretical and practical significance.Taking the semihumid Changwu area as an example,the Community Land Model(CLM)is used to simulate the deep soil hydrological processes in the Loess Plateau under the land covers of winter wheat,summer corn and the 15-year-old apple tree between 1901 and 2015.At first,through the first order moving average model and autoregressive model,two different data sources are used to make the atmospheric forcing dataset of China with a spatiotemporal resolution of 6-hour and 0.1-degree as the meteorologic input of CLM.It includes seven meteorologic variables of precipitation,air temperature,shortwave radiation,longwave radiation,atmospheric pressure,specific humidity,surface wind speed and its time range is from 1901 to 2015.At second,the soil layers were divided according to its texture,and the root distribution pattern corresponding to the soil layers was made dependent on the measured data of the root system.Then inputting them into the CLM as external variables and calculating the root distribution pattern at any time between the two measured dates of root based on the interpolation of time.And the drainage and groundwater table calculation schemes were modified in CLM.At last,based on the new atmospheric forcing dataset of China,the deep soil hydrological processes in the Loess Plateau were simulated with the modified CLM under the different land covers.Soil water dynamic of 100 m soil layer,precipitation infiltration depth,vertical distribution of soil water activity,deep percolation process and the needed recharge time of soil water at the depth of 75 m were studied in the semihumid area of the Loess Plateau during the 115 years.To explore the recharge mechanism of deep soil water and groundwater in the Loess Plateau,the impact of vegetation types including the plant's growth period and root depth,precipitation conditions and climate status on the deep soil hydrological processes in the Loess Plateau have been analyzed in the study.The main results and conclusions of the study are as follows:(1)The Atmospheric Forcing Dataset of China has the advantages of high spatial and temporal resolution and long time range.As far as climate conditions in the Changwu area are concerned,the seven meteorologic variables of the new dataset have been revised in terms of mean level and volatility compared with the initial meteorologic dataset,which is basically consistent during the 115 years of 1901-2015.Therefore,using the new meteorologic dataset as the input of CLM is reliable and reasonable to simulate the deep soil hydrological processes in the Loess Plateau.(2)The modified CLM has more flexible and efficient features,and its applicability extends to the hydrological process simulation of deep soil and groundwater level as well as landuse types of deep-root plant.It can fully consider the spatial variation of soil properties and root growth,also it can simulate the deep drainage and ensure the consistence of change between the simulated groundwater table and soil water profile.The simulated groundwater table is basically consistent with the measured values and is controlled into a rational range during the whole 115 years.At the same time,the simulated soil water profile of 100 m agrees well with the measured values.These declare that it is feasible to use the new drainage and groundwater table schemes to control the simulated groundwater level,and it is reasonable and reliable to simulate the deep soil hydrological processes based on the modified CLM.(3)The growth period and root depth of plant both affect the infiltration depth of precipitation in the Loess Plateau,and the effect of root depth is relatively larger than that of plant's growth period.For monthly scale,heavy precipitation with monthly amount over 160 mm is the main recharge source of deep soil water and groundwater below the 75 m depth for the winter wheat with shallow root system growing in the non-rainy season,the occurrence frequency of recharge is only 4.3%.For the summer corn with shallow root system growing in the rainy-warm season,usually heavy precipitation with monthly amount over 180 mm can recharge to the depth below 75 m,with an occurrence frequency of about 2.8%.For the 15-year-old apple tree with deep root system,recharge to depth over 75 m occurs when the monthly amount of heavy precipitation is larger than 230 mm(in wet period)or its annual amount is over 1000 mm,and their occurrence frequencies are less than 1%.Therefore,it is very difficult to recharge deep soil water and groundwater for the most precipitation in the semihumid Changwu area.Instead,their recharge is mainly by slow and steady infiltration of soil water.And it can be judged that,heavy precipitation is the main cause of the dramatic changes of deep percolation at the 75 m depth and groundwater table.(4)Soil water activity is primarily controlled by the infiltration depth of precipitation and the root depth of plant,but relatively small by plant's growth period.Since the infiltration depth of most precipitation is about 6 meters in the semihumid Changwu area,as well as the shallow root systems of winter wheat and summer corn principally distibute in the depth less than 5 meters,the active layer of soil water is above 6 m depth in the winter wheat and summer corn field.For the 15-year-old apple orchard with a root depth of about 20 m,the active layer of soil water is mainly affected by its root depth and over the depth of 25 m.(5)Vegetation types and climate are the main controlling factors of deep percolation in the Loess Plateau.According to the different evapotranspiration levels on land surface,the deep percolation process was analyzed by the three divided periods of 1901-1926,1927-1963 and 1964-2015.The average annual deep percolation is inversely proportional to the evapotranspiration level for the shallow-root plant and is largely affected by the antecedent soil moisture of unsaturated zone for the deep-root plant.For the averages of annual deep percolation in the three preiods,their values are about 165,246 and 115 mm in the winter wheat field;the values are nearly 135,234 and 89 mm in the summer corn field;the values are around 23,238 and 54 mm in the 15-year-old apple orchard,respectively.Hence,the plants that have the growth period of rainy-warm season or deep root system can reduce the recharge of deep soil water and groundwater in the Loess Plateau.In addition,the proportions of additional deep percolation caused by heavy precipitation to total deep percolation amount are all less than 30% in the above three periods for different vegetation types.It illustrates that the slow and steady infiltration of deep soil water is the primary recharge source of deep groundwater in the Loess Plateau.(6)The recharge time to deep soil water and groundwater is mainly affected by the vegetation types,precipitation conditions,climate status,and antecedent soil moisture.During the three periods of 1901-1926,1927-1963 and 1964-2015,the recharge times to the 75 m depth are proportional to the evapotranspiration levels on land surface for the shallow-root plant,which is controlled by the climate status.But they are largely affected by the antecedent soil moisture of unsaturated zone for the deep-root plant.Including,the recharge times in the above three periods are about 134,91.2 and 185.6 years in the winter wheat field;nearly 162.1,95.3 and 235.2 years in the summer corn field;and around 876.2,94 and 370 years in the 15-year-old apple orchard.Thus,except for the extremely humid climatic conditions where remarkly low evapotranspiration level is,the plants that have the growth period of rainy-warm season or deep root system can increase the recharge time to deep soil water and groundwater in the Loess Plateau.In addition,the recharge time of fast infiltration caused by heavy precipitation to the soil layer of 75 m depth is about 10 years in the semihumid area of the Loess Plateau.It is mainly affected by the amount of heavy precipitation and antecedent soil moisture of unsaturated zone.Through this study,combining the Community Land Model with the experiment to make full use of the model's advantage of long time and the experiment's advantage of large depth,namely that research duration of 115 years(1901-2015)and deep loess layer of 100 meters(reaching to the groundwater table).Then soil structure,root distribution,drainage scheme and calculation of groundwater table in the CLM have been made important modifications and then it is successfully applied to the simulation of deep soil hydrological processes in the Loess Plateau.The results of this study clarify that heavy precipitation is the main recharge source of deep soil water and groundwater with fast infiltration and is indispensable for maintaining the balance of ecological environment in the Loess Plateau,as well as demonstrate that the dry soil layer in the region has some retardance effect on the vertical infiltration of precipitation,but it does not completely block the recharge of deep soil water and groundwater by precipitation.Under the state of extremely heavy precipitation or relatively humid climatic conditions,precipitation can infiltrate through the dry soil layer and recharge the deep soil water and even groundwater in the region.It is worth pointing out that,under the shallow-root plant,most of the precipitation can infiltrate through the root layer to recharge the deep soil water and eventually recharge the groundwater in the semihumid area of the Loess Plateau,which is an important way of the groundwater recharge in the Loess Plateau.Based on the conclusions of this study,we have deepened our understanding of the recharge mechanisms of deep soil water and groundwater in the Loess Plateau,which is useful to guide the rational exploitation of natural resources and scientific protection of ecological environment and then promote the long-term sustainable development of the economy and society in the region.