| The Loess Plateau of China has been susceptible to ongoing severe soil erosion.Among many controlling measures, vegetation restoration is the most economical andefficient. Soil moisture is the most critical factor affecting vegetation restoration on theLoess Plateau, and exerts major influences on vegetation growth, agricultural development,soil erosion, and solute transport. Soil moisture is an integrated response to climate,vegetation, topography, and soil properties, and is closely related to soil indexes such astexture, saturated hydraulic conductivity and bulk density. Therefore, knowledge of thespatial-temporal characteristics of soil moisture and related variables is of great importanceto soil water management and vegetation restoration.In connection to the spatio-temporal issues of soil moisture and related soil indexes,and based on a large number of in-situ measurement data and the use of classical statisticsand geostatistical methods, this dissertation mainly focuses on the following issues: thetemporal stability of soil water storage at the hillslope scale; the distribution ofspatio-temporal variability and temporal stability characteristics of water content withinsoil profiles (0-300cm); a feasibility analysis of the a priori prediction of temporalstability locations; the scaling of temporal stability for surface soil moisture (0-6cm); theinterpolation accuracy for seven soil properties at various sampling scales; and the spatialscaling of soil saturated hydraulic conductivity in a small watershed. The investigationswere all carried out at the hillslope scale except for the last one. The main results were asfollows:(1) The temporal stability of soil water storage in different soil layers (0-1,1-2, and2-3m) was strong at the hillslope scale. The temporal stability was stronger with increasesin soil depth based on either the Spearman correlation coefficient or the standard deviationof relative difference (SDRD) index. Furthermore, the closer two soil layers were within agiven profile and the deeper any two adjacent soil layers were, the more similar was thetemporal pattern. Using the relative difference method, representative locations wereindentified for each soil layer. More locations estimated the mean soil water storage of the study area accurately in deeper soil layers than in shallower layers. However, none of thelocations were able, individually, to represent the mean soil water storage for all threelayers. Temporal variability played a more important role than spatial variability indetermining the number of representative locations.(2) The soil water storage during this study was more heterogeneously distributed onthe studied hillslope under wetter than under dryer conditions. A linear equation coulddescribe well the positive relationship between the mean soil water storage and its variance(p <0.01). Furthermore, this dependency increased with increasing soil depth. Thedetermination coefficients between mean soil water storage and their variance, based onthe full dataset, were0.33,0.91, and0.97for the soil layers of0–1,1–2, and2–3m,respectively. The soil water content data were then analyzed at smaller sampling intervals:10cm increments between soil depths of0and100cm; and at20cm increments betweenthe100and300cm soil depths. The relationships between the mean soil water contentsand their variances were fitted slightly better by a power function than by a linear equation.The coefficients of determination did not consistently increase down the0-300cm soilprofile, but followed the pattern of decreasing between0and30cm, increasing from30to160cm, and being relatively constant below160cm.(3) Choosing200cm as the maximum soil sampling depth would be sufficient inareas similar to the study area when the spatio-temporal characteristics of soil moisture areto be studied. This was justified after identifying three soil sub-layers according to theprofile distribution of spatio-temporal variability and the temporal stability characteristics.Layer1, a complex layer (0-60cm), was considered to be the active root-zone in which thesoil water within the layer was also subject to the strongest effects resulting from climaticand topographical factors. The multiple influencing factors led to the diversity of thespatio-temporal characteristics of the soil moisture. Layer2was the steadily changinglayer (30-160cm), in which most of the spatio-temporal characteristics either increased ordecreased at an almost constant rate. This stable rate of change mainly occurred because ofthe effects of vegetation and rainfall on soil moisture, which steadily decreased withincreasing soil depth. Layer3(160to300cm) was the stable layer. In this soil layer,vegetation and rainfall had almost no effect on soil moisture. Thus, the variability of soilproperties became the most important factor to the spatio-temporal characteristics of soil moisture in this layer. The loessial soils have homogeneous soil profiles, which leads to thestability of the soil moisture spatio-temporal characteristics within this soil layer. Therefore,when spatio-temporal variability and temporal stability characteristics in soil moisture areinvestigated, it would be reasonable to choose200cm as the maximum soil samplingdepth.(4) Elevation and clay content of the soil were the dominant factors affecting thetemporal stability characteristics of soil water in the shallow soil layer (0-60cm). However,the a priori selection of representative locations based solely on soil properties andelevation was determined to be infeasible at the present time since predicted locationsdiffered greatly from those identified by measurement. Therefore, it is necessary tointroduce more variables or to use a more advanced method to obtain more reliablepredictions of the relationships between the indexes of temporal stability and the selectedvariables. Furthermore, the relationships between soil moisture and correlated variablesvaried in time and space, which limited the application of these empirical models.Therefore, we concluded that the a priori identification of representative locations ispresently infeasible, and that more work is needed.(5) The temporal stability characteristics of surface soil moisture (0-6cm) at thehillslope scale were scale-dependent. Sampling extent had a stronger effect on the temporalstability of soil moisture than sampling spacing.For most of the parameters, a logarithmicequation could express well the relationships between these parameters and samplingscales. The parameters changed at a greater rate when sampling spacing or sampling extentwas smaller. However, the specific patterns of scaling differed among parameters. Forexample, the mean values of the Spearman rank correlation coefficient did not significantlychange with sampling spacing (p>0.05), but they increased significantly with increasingsampling extent (p <0.01). The ratio of the number of sites under diverse dates withsignificant temporal stability, at both the0.01and0.05probability levels, to the totalnumber of datasets decreased with increasing sampling spacing or decreasing samplingextent; the range of mean relative difference (MRD) decreased linearly with the increase insampling spacing (p <0.01), and increased logarithmically with the increase in samplingextent (p <0.01); the mean values of the SDRD increased logarithmically with the increase in both sampling spacing and sampling extent (p <0.01), but the increase was moresensitive to changes in sampling extent.(6)Sampling scaling had an important effect on the data distribution types andinterpolation accuracy, as defined by G values. The interpolation accuracy was predictedbetter by the scaling index than by the classic index or by the geo-statistic index. For theseven soil properties (clay, silt and sand contents, bulk density, saturated hydraulicconductivity (KS), surface soil moisture content and soil organic carbon content) thesmaller the sampling extent or the greater the sampling spacing, the greater the probabilitythat the sample distribution would be normal or log-normal. For all the studied soilproperties, the interpolation accuracy increased with either increasing sampling extent ordecreasing sampling spacing. However, the mean interpolation accuracy varied greatlyamong the seven investigated soil properties. To obtain the greatest contribution rate (theratio of the G value to the number of samples) under the same sampling extent, sandcontent required the fewest number of samples while soil organic carbon content requiredthe most, and about the same number of samples was required for the other five soilproperties.(7)The statistical parameters (variance, correlation length and nugget-sill ratio) forsoil saturated hydraulic conductivity were scale-dependent in a small watershed, anddepended differently on the scale triplet, in terms of sampling spacing, sampling extent andsampling support. With increases in sampling spacing, apparent variance tended todecrease in a non-significant linear relationship (p=0.137); as sampling spacing increasedbelow1.1times the "true" correlation length (i.e. below80m), the apparent correlationlength decreased slightly but, as spacing increased above80m, it notably increased; thenugget-sill ratio decreased logarithmically with the increase in spacing (p <0.01). Thethree parameters all increased with increasing sampling extent but with different patterns.When the sampling support increased, apparent variance and nugget-sill ratio decreasedand correlation length increased. The mean coefficient of determination of the fittedmodels between the three parameters and sampling spacing, sampling extent and samplingsupport were0.53,0.96and0.83, respectively. Thus, for the soil property, KS, upscaling ordownscaling was more reliable when based on sampling extent than on spacing or supportin this study. Consequently, distributing limited sample locations in a sub-area of the main study area at a higher sampling density is an alternative sampling method, especially in amore homogeneous study area.Based on a large number of field measurements of soil moisture and related variables,a series of issues concerning the temporal stability of soil moisture and the spatial scalingof related variables were explored in a small watershed on the Loess Plateau. The findingspresented in this dissertation add to the knowledge about the spatio-temporalcharacteristics of soil moisture and related variables in semi-arid environments. They are ofbenefit to the application of the temporal stability concept to ecological construction andagricultural production in the Loess Plateau region. They can also add to the data related tospatio-temporal variability at multiple scales. Moreover, the findings can also be usefulwhen designing optimal sampling strategies for similar research work. |
