Hsin Fu Yeh

Soil water balance model for precipitation-induced shallow landslides

Precipitation infiltration is one of the most significant triggering factors for slope failure occurrence in many places around the world. Knowledge of the mechanisms leading to precipitation-induced slope failures is of great importance to the management of landslide hazard. In this study, a soil water balance model is developed to estimate soil water flux during the process of infiltration from rainfall data, with consideration of storm periods and non-storm periods. Two important assumptions in this study are given: (1) instantaneous uniform distribution of the degree of saturation and (2) a linear relationship between evapotranspiration and the related degree of saturation. For storm periods, the Brooks and Corey model estimates both the soil water retention curve and soil water parameters. The infiltration partition is employed by an infinite-series solution of Philip in conjunction with the time compression approximation. For none-storm periods, evapotranspiration can be derived for the moisture depletion of soil water. This study presents a procedure for calculating the safety factor for an unsaturated slope suffering from precipitation infiltration. The process of infiltration into a slope due to rainfall and its effect on soil slope behavior are examined using modified Mohr–Coulomb failure criterion in conjunction with a soil water balance model. The results indicate that the matric suction, which is closely related to slope stability, is affected by the degree of saturation controlled by rainfall events.

Estimation of groundwater recharge using water balance model

The main purpose of this paper is to apply a water balance concept with two models in the Ching-Shui watershed to describe the groundwater recharge. First of all, a soil moisture budget model is established to estimate the infiltration, runoff, evapotranspiration, and groundwater recharge in the watershed, where the moisture content of the soil is tracked through time. Secondly, the groundwater recharge was also estimated by the model of the base-flow-record estimation, with the assumption that groundwater evaporation is negligible. In addition, since the analyzed base-flow trends are high, when executing model analysis, the depths of infiltration estimated by stable-base-flow analysis is used to obtain more reasonable groundwater recharge value. The coefficients of groundwater recharge by the precipitation in the Ching-Shui watershed estimated from the established soil moisture budget model and the base-flow model were 12.40% and 9.92%, respectively. Comparison show the result of both models to be close.

Rainfall characteristics for anisotropic conductivity of unsaturated soil slopes

The purpose of this study was to investigate the effects of the anisotropic ratio on the stability of slopes using the reliability index approach. A numerical analysis of the relationship between the three rainfall patterns, advanced, normal and delayed, and the anisotropic ratios was designed. This study also considered three different soil properties (sand, silt, and clay) to simulate rain infiltration. In this study, probability analysis was used to evaluate the stability of unsaturated soil slopes. The finite element computer program Geo-Studio was used to simulate the process of rainwater infiltrating a slope. The pore-water pressure results evaluated from seepage analysis (SEEP/W) were imported into the slope stability program (SLOPE/W). Results for the anisotropic ratio of hydraulic conductivity indicate that when the anisotropic ratios become higher, the reduction in the reliability index is insignificant. In addition, the simulation results indicated that when saturated hydraulic conductivity (ks) was less than rainfall intensities (I), the percentage probability of the occurrence of a landslide was larger than when ks was greater than I. Finally, in the cases of anisotropic ks, stability of the high ratio soil slopes was not found to be sensitive to the reliability index variation during the simulation period. Moreover, when ks was greater than I, slope stability decreased earlier than was the case in the opposite situation.

An approach to evaluate groundwater recharge from streamflow and groundwater records

To assess groundwater recharge, this study provided a composite method combining the recession-curve-displacement method and water-table fluctuation method. First, the initial recharge reference value was determined using the water-table fluctuation method. The corresponding groundwater discharge was then determined from the recharge reference value using the recession-curve-displacement method. Furthermore, the recession segment of the match between groundwater discharge and streamflow was computed. The recharge reference value was repeatedly adjusted to achieve a good fit with the recession segment for groundwater discharge and streamflow, thereby attaining the final groundwater recharge using the proposed method. Finally, the groundwater recharge of the Lanyang Creek basin in Taiwan was estimated as a case study. A comparison of recession-curve-displacement method and proposed composite method are presented. Estimation results show that the number of recharge events, recharge timing of these events, groundwater recharge events that satisfy recession theory, and range of transmissivity can be obtained using the proposed composite method. Comparison results demonstrate that the number of groundwater recharge events obtained with the composite method was greater than that acquired with the recession-curvedisplacement method. However, the annual recharge and seasonal recharge obtained with the recession-curve-displacement method and composite method were close.

Spatiotemporal Analysis of Groundwater Recharge Trends and Variability in Northern Taiwan

In this study, the base flow estimation method was used to assess long‐term changes of groundwater recharge in Northern Taiwan. The Mann‐Kendall test was used to examine the characteristics of the trends. This was followed by trend slope calculation and change‐point analysis. The annual groundwater recharge was found to exhibit a significant upward trend for the Fushan and Hengxi stations (Tamsui river basin). On the other hand, the Ximen Bridge station (Lanyang river basin) recorded a significant downward trend. Calculations showed that the rate of change for the Fengshan and Touqian river basins was small (less than 10%). However, that for the following stations was greater than 30%: Fushan, Hengxi, Ximen Bridge, and Niudou (also in the Lanyang river basin). The results of the change‐point analysis further indicated a significant change‐point for the annual recharge at Fushan, Hengxi, and Ximen Bridge stations in 1999, 1983, and 2001, respectively. The findings can be used for regional hydrological studies and as reference for water resource planning.