以局部穩定條件率定之土壤厚度估測模式

Abstract

土壤乃集水區地表的基質，土壤厚度更常為各種水文、邊坡模式所需的基本資訊，然而台灣地區關於土壤厚度的預測模式研究卻極少。由於地形扮演著控制各種邊坡作用的主要角色，地形與土壤化育的結果和表徵密切相關，因此本研究以地形演育概念下的作用模式為基礎，結合土壤生成函數（soil production function）與擴散模式（diffusion model）來建構土壤厚度的估測模式，並以陽明山國家公園七星山地區作為試驗區，透過數值高程模型（DEM）來進行土壤厚度的數值模擬。 本研究假設模擬其間地表形貌無明顯改變，且試驗區邊坡土壤的搬運能以簡單潛移（simple creep）概念下的擴散模式來描述，且搬運速率在稜線、山脊等坡頂處與土壤生成速率維持局部穩定平衡狀態（local steady state），並以此條件來率定模式演算所需之相關參數。模擬所得試驗區之土壤厚度分布在0至2公尺之間，土壤厚度在空間分布上有極高的變異性，主要由地形曲率控制。以野外實測之土壤厚度值來驗證模擬結果，發現模式估計的土壤厚度在凸坡、平緩處有較高的準確度，在坡度較陡、集水面積較大的區域有較大的估計誤差，原因可能是來自其他邊坡作用的影響，並非單純由土壤潛移作用所造成。整體而言，利用局部穩定平衡假設所率定之參數，可使模式有效估測部分邊坡的土壤厚度，唯其他類型之邊坡作用的影響，乃是將來應進一步釐清的對象，以期能提高邊坡土壤厚度估測模式的應用層面。

Soil thickness is one of the fundamental components in many hydrological and slope stability models. Since landform critically influences many slope processes, soil thickness which is related to various slope processes is also highly affected by topography. In this study, a soil production function is coupled with a simple diffusion model to form a soil-thickness prediction model. The Yang-Ming-Shan National Park is chosen as a study area to test the validity of the model. It is assumed that the topography has not had any drastic change during the simulation time period, the soil is moving downhill according to simple creep law, which sets the transport flux equal to a linear function of local slope gradient, and its transport rate is in equilibrium with the soil production rate at ridge lines. Model parameters are calibrated under this local steady-state assumption, and used to estimate the soil thickness within the study area. The results show that soil depth in the study area ranges from 0 to 2 meters, and spatial variation is controlled by topographic curvature. Field verification shows that the model performs well on convex, law gradient, and upper part of hillslopes. It is discovered that in those poorly modeled area, slope processes other than diffusion prevail which bring about the disparity in model prediction. Generally speaking, assuming local steady state facilitates parameter calibration and improves model’s applicability. However, in order to extend the model application to the entire hillslope, it is essential to incorporate other processes in the model.