Spatial And Temporal Variations Of Soil Water Content And Modeling Of Soil Physical Properties In Arid Regions Of Northwest China

Fragile ecological environment leads to serious problems in in arid regions of Northwest China,such as degeneration of vegetation,land desertification and soil erosion severely.Soil moisture is one of the key limiting factors controlling the growth of plants in arid regions.A better understanding of the temporal and spatial variations of soil moisture in arid regions is important for unraveling the role of soil moisture in the dryland ecosystems and will alo provide scientific value for the ecosystem maintenance and warer resource management.This study selects the lower reaches of Tarim River and North-South Transect of Loess Plateau as the study area,through the methods of combining field observation and laboratory analysis,typical statstatistic analysis and scale expansion,in order to investigate the temporal and spatial variations of soil moisture and its influencing factors,and to model and predict soil physical properties(bulk density,saturate soil hydraulic conductivity)at different spatial scales.These will have important values for evaluating the limiting factors of vegetation restoration and ecosystem reconstruction.The main results are as follows:(1)The soil moisture in the studied stand ranged from 5%~23%(g/g)and reavealed middle variability.Except in the layer close to the goundwater table,soil moisture in other layers varied little over time.Soil water contents were highly related to the soil texture,and soils exhibited higher water content when they had a higher clay fraction.The soil moisture profiles were similar even they had different groundwater depths,and there is no obvious relationship between soil water content and groundwater depth.The special water use strategies of phreatophytic species in the hyper-arid climate caused unique temporal soil moisture processes.We suggest that water flows in groundwater-soil-plant-atmosphere systems in hyper-arid regions are in steady or quasi-steady states.As a result,soil moisture generally remains constant over time,and the amount of water in the soil is mainly determined by soil texture in hyper-arid regions.(2)With the increase of the distance to the river channel,the groundwater level logarithmicly declined,and vegetation cover exponentially decreased,respectively.Soil textures had significantly impacts on soil water retention.The field capacity of silty loam(0.32 g g-1)was more than 3 folds of that of sandy soil(0.10 g g-1),and the effective water content of the former was 1.6 folds of the latter.The vegetation cover was controlled by the groundwater level and soil water content together.Populus euphratica and Tamarix has the ability to adapt to hyper-dry environment,but the water use strategies of them were different.Roots of Populus euphratica directly took up groundwater and soil water,and had the obvious hydraulic lift during the growing season.However,Tamarisk roots mainly absorbed groundwater and the saturated soil water in the layer near the groundwater level.(3)There was no significant linear relationship between the soil moisture and the distance to the river channel in the lower reaches of the Tarim River.The soil moisture content and vegetation cover in transects decreased gradually along with the direction of the river flow.The ?positive texture effect? was found in the lower reaches of the Tarim River,which meant higher vegetation cover occurred in soils with higher clay content.(4)The investigation of soil moisture content within 0~5 m profile across the North-South transect(86 sampling sites)on the Loess Plateau showed that soil desiccation had evident latitudinal zonality.The index of soil desiccation demonstrated higher variability before the rainy season than that after the rainy season.The water supply from precipitation alleviated soil dried layer,and this effect is more evident in the middle Loess Plateau than that in the south and north.The difference of soil desiccation index distribution in the soil profile befor and after rainy season occurred maily in the shallow layer(0~2.3 m),which is attributed to the water supply in the shallow layer by precipitation.(5)The investigation of spatial distribution of soil bulk density(BD)at different depths(0~10,10~20,20~40cm)across the North-South transect(86 sampling sites)on the Loess Plateau showed that the magnitude of BD variability in the 0~20 cm layer was moderate according to the coefficient of variations,while the BD variability in the 20~40 cm layer was weak.The key factors affecting the spatial distribution of BD differed among different soil depths.Soil organic carbon,clay and sand content were the key factors in the 0~10 cm layer;soil organic carbon,clay,sand and precipitation were the key factors in the 10~20 cm layer;while in the 20~40 cm layer,clay,sand,precipitation and land use were the key factors to affect the spatial distribution of BD along the transect in the study.State-space models were consistently more effective than multiple step wise regression functions and pedotransfer functions for estimating spatial distribution of BD.State-space model that included clay,sand,precipitation and land use showed the best simulation result,and the combination of such variables explained 92.3% of the total variation of BD.State-space models are recommended for studying spatial relations between soil bulk density and other variables on the Loess Plateau.(6)The investigation of spatial distribution of satuated hydraulic conductivity(Ks)in a layer with depth of 49 m at a vertical profile scale(96 sampling sites)in the southern Loess Plateau through classical statistics and first order autoregressive state-space modelling showed that Ks had evident spatial variability and the magnitude of Ks variability was strong.The degree of spatial dependence was moderate with range of 15 m.Ks,bulk density,organic carbon content,silt content,and clay content were auto-correlated at lag distance of 8,4,8,1,and 2 respectively.Ks had positive cross-correlations with organic carbon content and clay content,but had negative cross-correlation with bulk density and silt content.Organic carbon content and silt content were the key factoes affecting the spatial distribution of Ks on the sloping land.A combination of organic carbon content and soil particles was the best to explain the spatial variation of Ks(R2 > 0.6).The first order autoregressive state-space model is suitable for simulating the Ks distribution at the profile scale in the field.