| The redistribution of rainfall by vegetation canopies can change the spatial distribution patterns of water within plant community, thus affecting infiltration, runoff, evapotranspiration, and other hydrological processes. The replenishment of soil moisture at plant root zone by stemflow can be an active mechanism that plant has evolved to adapt to drought environment. Evapotranspiration is the main component of the soil – vegetation – atomosphere – transport(SVAT). The water-consuming characteristics of plants and their adaptive abilities to drought environment are basic informations that required for vegetation construction. The study of the patterns of rainfall redistribution by vegetation and their water-consuming traits is theoretically beneficial for uncovering plant's adaptive mechanism to drought environment, and is practically important for vegetation restoration in this area.This dissertation is focused on the key hydrological process of “rainfall?canopy rainfall partitioning?runoff?rainfall net input?evapotranspiration”. Two re-vegetated shrub species(i.e. Caragana korshinskii and Salix psammophila) that typically used on the Loess Plateau were chose as the study objects. Throughfall(TF), stemflow(SF), canopy interception loss(IL), and their percentages of gross rainfall(TF%, SF%, and IL%) and the influencing factors were quantified. The infiltration depth of SF and its replenishment of moisture in the soil at shrub bases were investigated using dye tracing experiment. A method was established to estimate shrub biomass of plot scales using easily measured morphological indexes(e.g. basal diameter and length) of branches and the scale expansion factor(e.g. number of standard branches). The dynamics of profile soil water storage(SWS) within 0-400 cm soil layer and their influencing factors were comprehensively analyzes. Shrub evapotranspiration at plot scales were finally estimated, according to the principle of water input and out balance, based on the determination of rainfall net input in the canopy partitioning process, runoff, and the budget of profile SWS. Meanwhile, the characteristics of water-consuming for shrubs at plot scales and the influencing factors were discussed. The main conclusions of this study are as follows:1. Good consistency was shown in the patterns of rainfall redistribution by the two shrub species. TF accounted for the largest proportion of gross rainfall for C. korshinskii and S. psammophila, followed by the proportion of IL, and by the proportion of SF. The average values of TF%, IL% and SF% were 73.0%, 16.6%, and 10.4% for C. korshinskii, and 67.1%, 25.7%, and 7.2% for S. psammophila, respectively. The SF funneling ratios(the ratio of the amount of the rainfall that delivered to the bases of plant to the amount of rainfall falling on an open area) were averagely 124.9 for C. korshinskii and 87.5 for S. psammophila, respectively.2. Rainfall amount was the most influential factor on rainfall partitioning, and the effect of rainfall amount showed good accordance for the two shrub species. As rainfall amount increased, TF, TF%, SF, SF%, IL, and SF funneling ratios increased in linear or exponential functions, while IL% decreased exponentially. Shrub morphological characteristics have an effect on the redistribution of rainfall, but this effect varied between the two shrub species, due to the complex nature of the morphology. Additionally, the impact of morphology on the patterns of rainfall partitioning also differed in different gradients of rainfall amount or rain intensity.3. In the process of rainfall partitioning, the net water input to the soil beneath the canopies of C. korshinskii and S. psammophila(i.e. throughfall plus stemflow) accounted for 83.6% and 74.0% of the gross rainfall that falling outside the canopy area, respectively. Rainfall is the most important factor affecting soil water net input in the partitioning process. As the increase of rainfall amount, the rainfall net input in the canopy partitioning process increased linearly, while its percentage of gross rainfall increased exponentially. The influence of shrub morphology on rainfall net input differed between shrub types, and the gradients of rainfall amount and intensity. The dye-tracing experiment indicated that stemflow infiltrated significantly deeper in the soil at shrub bases than the soil beneath and outside of the crown. Stemflow can effectively increase the soil water content at the bases of the two shrubs.4. Two easily measured morphological indexes of shrub branches, i.e. basal diameter(D) and length(H), can be used to estimate branch biomasses for C. korshinskii and S. psammophila with a good degree of accuracy. Compared across the four biomass models that employed, the allometric model which uses D2 H as the input variable was optimal in estimating branch biomass for the two shrub species, after transformed into its linear form. Meanwhile, the heteroscedasticity of the biomass data was greatly eliminated. This model contributed to a maximum value of coefficient of determination(R2), and meanwhile resulted in minimum values of mean error(ME), mean absolute error(MAE), total relative error(TRE), mean systematic error(MSE), and mean absolute percentage error(MPSE). The accuracy of this model in estimation basically meets the requirement in ecological study. Based on the established models, shrub biomass at stand or plot scales can be ultimately determined by investigating the scale espansion factor of biomass(i.e. the number of standard branches) and the morphological indexes of the standard branches.5. The investigation of profile soil water storage(SWS) in 0-400 cm soil layers indicated that annual rainfall and its distribution characteristics is an important factor affecting SWS. The coefficients of variation(CVs) of profile SWS in the plots of C. korshinskii and S. psammophila showed a decreasing trend with the increase of soil depth, indicating that the fluctuations of soil moisture decreased with soil depth. The distribution of soil particles(i.e. clay, silt, and sand) in profile is an important factor affecting the vertical distribution of soil moisture. In 0-400 cm soil layer, profile soil water content in the plots of C. korshinskii and S. psammophila increased linearly or exponentially with the increase of the clay, and silt content of soil. Slope position showed a significant influence on profile SWS, and the SWS were highest in the bottom of the slope, followed by that in the middle area, and by that in the top area. In vegetation factors, shrub density, shrub coverage, and shrub dry mass per area have an effect on profile SWS for C. korshinskii and S. psammophila plot.6. The results of shrub evapotranspiration on plot scales showed that, as the growing season advances, the evapotranspiration of C. korshinskii and S. psammophila both increased at first stages and then became to decrease. Shrub evapotranspiration was significantly affected by the conditions of weather, terrain, and soil. Within meteorological factors, rainfall amount was the most important factor, and showed a positive linear correlation with evapotranspiration. In topography factor, the plots with larger gradient had a small value of shrub evapotranspiration, which can be attributed to the decreased net water input that caused by increased runoff under large gradient conditions. In soil factors, the thickness of sand that covering on the plots decreased evapotranspiration significantly. As for vegetation factors, shrub density, shrub coverage(the percentage of total shrub canopy projection area to plot area), and shrub dry mass density had no significant effect on evapotranspiration.In conclusion, the redistribution of rainfall by shrub canopies during the rainfall process can significantly affect the net input of rainfall to the ground beneath their canopies, thus afftecting a series of hydrological processes. Stemflow can infiltrate preferentially into the soil at the bases of shrubs, and have an important contribution to the replenishment of soil water content at shrub bases. Shrub evaportranspiration on plot scales is greatly affected by rainfall, patterns of rainfall partitioning, topography, and soil textural properties. |
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