地形指数的物理意义分析

地形指数模型 ( TOPMODEL)用地形指数在流域中的空间格局来确定流域饱和缺水量的空间分布和产流区的空间位置与范围 ,物理意义明确。本文介绍了地形指数的物理基础及其与土壤水分的关系。分析了地形指数空间变化与汇流面积 a及局地坡度空间变化的关系 ,a范围内有效汇流面积的变化 ,提出了一种确定 a上限值的方法。

Relationship of Upslope Contribution Area and Soil Water Content in TOPMODEL

Topography-based hydrological models (TOPMODEL) make use of topographic information and the spatial distribution of topographic index (ln a /tan β ) to represent the soil water storage deficit below saturation and spatial variability of contributing area. In this paper, the physical basis of the index and its relationships with soil water storage are introduced. The effects of spatial change of drainage area ( a ) and local slope β on the spatial change of the index are analyzed. The changes of effective upslope drainage area with soil water condition are discussed. Finally, by using a 100 m×100 m DEM and single flow direction algorithm, the index distribution functions are calculated for Sumo basin on upper reaches of the Yangzi river under different channel initiation threshold. An assumption of TOPMODEL is that there is a saturated zone in equilibrium with a steady recharge rate over an upslope contributing area a . This makes TOPMODEL appropriate only for humid areas. To break this assumption, the authors argue that two concepts, i.e., effective contributing area and maximum contributing area, should be distinguished. The effective contributing area indicates the area ( a , the maximum contributing area) with soil water content over field capacity according to the theory underlying the model. The authors also propose a method to calculate Channel Initiation Threshold (CIT) in order to derive a reasonable distribution of topographic index in a watershed. The CIT is usually not easy to define because of the variation of slope length with terrain type. The authors suggest that the CIT should be defined according to a GIS analysis of statistical distribution of slope length in the study watershed.