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Northwest A&F University (2017)

Spatial and Temporal Variations of Soil Water Content and Modeling of Soil Physical Properties in Arid Regions of Northwest China

易小波

Titre : Spatial and Temporal Variations of Soil Water Content and Modeling of Soil Physical Properties in Arid Regions of Northwest China

Auteur : 易小波;

Grade : Doctoral Dissertation 2017

Université : Northwest A&F University

Résumé
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.

Mots clés : Arid region of Northwest China; the lower reaches of Tarim River; Loess Plateau; soil moisture; temporal and spatial variations

Présentation (CNKI)

Page publiée le 10 janvier 2018