陆面过程模型中垂直非均匀土壤的水分传输及相变的模拟

土壤湿度在陆气相互作用中的重要性体现在它既能影响陆地和大气之间水循环的速率, 又能改变地表的能量分配。本文针对陆面过程模型中描述土壤湿度变化的方程进行了理论分析, 指出在非均匀土壤和冻土中采用土壤水势梯度描述垂直非均匀土壤水分流动的合理性。基于描述土壤内部水热传输的统一土壤模型, 并利用推广的表征土壤水分特征的Clapp-Hornberger关系式, 研究了非冻结和冻结的土壤湿度对于垂直非均匀土壤的敏感性。结果表明, 由土壤质地决定的土壤水势和导水率对土壤湿度的模拟有重要的影响。具体地, 在决定土壤性质的Clapp-Hornberger关系式中, 与土壤质地有关的饱和水势、饱和导水率以及土壤孔隙大小分布指数B, 对土壤湿度的模拟起到了关键作用。参数B的重要性尤为突出, 它的增加会引起导水率的大大下降, 从而对水分在土壤中的垂直分布产生重要影响。饱和水势的绝对值和参数B的增加会使得土壤水势绝对值增加明显, 使土壤的结冰(融化)过程延迟, 土壤温度因为没有结冰(融化)释放(吸收)的潜热加热(冷却)而持续下降(上升), 因此在冻融时期土壤温度会比观测值振幅偏大。上述结果揭示了考虑土壤垂直非均匀性并采用有效的土壤特性参数对于陆面过程模型的重要性。

The Simulation of Soil Water Flow and Phase Change in Vertically Inhomogeneous Soil in Land Surface Models

Soil moisture plays an important role in land-atmosphere interaction because it not only has effects on the water cycle between the land and atmosphere, but also the surface energy distribution. This work theoretically analyzes the equations for soil moisture change in land surface models, and shows the rationality for the equations in which the soil water potential is used to describe the soil water flow in vertically inhomogeneous soil and frozen soil. Based on a Simple Unified Soil Model (SUSM) and the Clapp-Hornberger relationship, the sensitivity of soil moisture in frozen or unfrozen soil to the vertical heterogeneity is also investigated. The results show that soil water potential and hydraulic conductivity have crucial effects on the simulation of soil moisture. The parameter B (soil porosity distribution) in the Clapp-Hornberger relationship is the most important because, when it increases, it leads to decreasing hydraulic conductivity, which results in the soil water vertical distribution. Meanwhile, increases in both the saturated soil water potential and parameter B also lead to a higher absolute value of soil water potential. When soil begins to freeze, this effect would delay soil water from freezing (melting), meaning the soil temperature cannot persistently decrease (increase) due to a lack of release (absorption) of latent heat fluxes originating from the soil freezing (melting) process. Therefore, the amplitude of soil temperature when freezing or melting is larger than observed. The results of this study reveal the importance of soil heterogeneity and the use of effective parameters for soil characteristics in land surface models.