This study was conducted to estimate the susceptibility of landslides on a test site in Malaysia (Hulu Kelang area). A Geographic Information system (GIS)-based physical model named YS-Slope, which integrates a mechanistic infinite slope stability method and the geo-hydrological model was applied to calculate the safety factor of the test site. Input data, slopes, soil-depth, elevations, soil properties and plant covers were constructed as GIS datasets. The factor of safety of shallow landslides along the wetting front and deep-seated landslides at the bottom of the groundwater were estimated to compare with the analysis results of the existing model and actual landslides in 2008. According to the results of the study, shallow landslides mainly occurred in the central area which has many historical landslides, while deep-seated landslides were predominant in the east side of the study area. A ROC analysis was conducted and it is shown that the prediction result at the end of the northeast monsoon for shallow landslides showed relatively high accuracy compared with other predictions.

Penyusupan air hujan amat dipengaruhi oleh keadaan sempadan permukaan tanah dan sifat–sifat hidraulik tanah. Kertas kerja ini mengetengahkan kajian mengenai kesan beberapa keadaan sempadan permukaan tanah terhadap penyusupan air hujan dengan menggunakan satu program komputer VADOSE/W. Satu kajian parametrik dijalankan untuk mengkaji kesan suhu persekitaran, kelembapan relatif udara, dan kelajuan angin terhadap penyejatan dan kadar penyusupan air hujan. Kesan konduktiviti hidraulik tanah terhadap mekanisma penyusupan air hujan juga dikaji dengan menggunakan dua model tiang tanah. Keputusan menunjukkan penyusupan air hujan adalah sensitif terhadap kelembapan relatif udara dan suhu persekitaran yang lebih rendah daripada suhu tanah, tetapi tidak sensitif terhadap kelajuan angin dan suhu persekitaran yang lebih tinggi daripada suhu tanah. Konduktiviti hidraulik tanah dan keamatan hujan mempunyai pengaruh yang besar terhadap mekanisme penyusupan dan pembentukan air larian permukaan. Maka, profil ketepuan tanah yang meyakinkan hanya boleh diperolehi jika keadaan sempadan permukaan tanah yang tepat dipertimbangkan dalam analisis. Kata kunci: Penyusupan air hujan, penyejatan, keadaan sempadan, konduktiviti hidraulik, model tiang tanah Rainfall infiltration is greatly influenced by the surface boundary condition as well as hydraulic properties of soil. This paper highlights the study on the effect of several surface boundary conditions on the rainfall infiltration using a computer program VADOSE/W. A parametric study was carried out to study the effect of ambient temperature, relative humidity and wind speed on the evaporation and rainfall infiltration rate. The effect of soil hydraulic conductivity on the mechanism of rainfall infiltration was also studied by using two soil column models. Results show that rainfall infiltration is sensitive to relative humidity and ambient temperatures lower than soil temperature, but insensitive to wind speed and ambient temperatures higher than soil temperature. The hydraulic conductivity of soil and rainfall intensity have great influence on the infiltration mechanism and the development of surface runoff. Thus, reliable saturation profile can only be achieved if accurate surface boundary condition was considered for analysis. Key words: Rainfall infiltration, evaporation, boundary condition, hydraulic conductivity, soil column model