Objective Soil moisture is a key factor linking atmospheric precipitation, surface water and groundwater, and plays an important role in eco-hydrological processes such as surface runoff, rainfall infiltration and vegetation transpiration. The analysis of dynamic response process and differences in soil moisture content of various covers under rainfall events in Bashang Plateau of northwestern Hebei Province of northern China will be helpful to explore the characteristics of replenishment effects of soil moisture content in this region, which is of great theoretical and practical significance to optimizing regional vegetation structure.
Method The grassland, Caragana korshinskii shrub land and poplar forest land in Zhangbei County, Hebei Province were selected as research objects. The dynamic response process and differences of soil moisture to different types of rainfall under various vegetation cover types were analyzed by monitoring continuous soil moisture content data and rainfall data from April to October, 2019.
Result (1) The main rainfall events in the study area were light rain and moderate rain, but the main amount of water was provided by heavy rain. The percentage of light rain and moderate rain occurrences was 81.58%, but they only contributed 42.66% to the total annual rainfall. The percentage of occurrences of heavy rain was only 18.42%, but it provided 57.34% of the total annual rainfall. (2) From April to October, the mean value of soil moisture in poplar forest land was significantly higher than that in grassland, and the fluctuation of soil moisture in Caragana korshinskii shrub land was the strongest. The mean soil moisture values under each vegetation cover types were poplar forest land (13.99 ± 2.04)% > Caragana korshinskii shrub land (12.63 ± 0.93)% > grassland (10.67 ± 2.77)%, and the mean soil moisture coefficient of variation was Caragana korshinskii shrub land (26.22%) > poplar forest land (20.51%) > grassland (13.89%). The coefficient of variation at each layer was at a moderate variation level. (3) The soil moisture content in 20−40 cm soil layer of the grassland was significantly higher than that in the other four soil layers (P < 0.05). There was no significant difference in soil moisture content between the 20−40 cm and 80−100 cm soil layers, as well as between the 0−20 cm and 40−60 cm soil layers in the Caragana korshinskii forest, and the soil moisture content in the 20−40 cm and 80−100 cm soil layers was significantly higher than that in the other three layers (P<0.05); the soil moisture content of the 80−100 cm soil layer in the poplar tree forest was significantly higher than that of the other four soil layers (P<0.05). (4) Under the effect of different rainfall amounts, the indicators of soil response to rainfall in shrublands were often significantly different from those of the other two vegetation types, except for some special cases. Among all types of rainfall, the mean values of soil moisture content response rate, recharge amount and recharge rate for each vegetation type cover were the greatest in Caragana korshinskii shrub land but the smallest in poplar forest land. Light and moderate rain can only respond to the 0−20 cm soil layer of the three vegetation types, with limited supply. Under heavy rain, the response of grassland to soil layer was 0−60 cm, while that of trees and shrubs was 0−80 cm. The response speed, peak reaching speed, replenishment amount, and replenishment rate of soil moisture content under three types of vegetation cover weakened with the increase of soil depth, and the response of upper soil to rainfall was always faster and greater than that of lower soil. Conclusion Under current rainfall conditions, the Caragana korshinskii shrub land responded the fastest and strongest to rainfall, but the poplar forest land responded the slowest and weakest. In addition, the deeper soil moisture under the three vegetation covers only responds significantly during heavy rain and allows the soil moisture to be replenished. The results of this study provide a scientific basis for changing the traditional view of emphasizing arboreal forest land but neglecting shrubs in the process of creating protective forests in the region in the future, and then reasonably adjusting the proportion of shrubs to achieve the overall improvement of eco-hydrological functions.
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