71 Journal of the Civil Engineering Forum, January 2021, 7(1): 71-84 DOI 10.22146/jcef.58350 Available Online at http: https://jurnal.ugm.ac.id/jcef/issue/archive The Effect of Slope on the Infiltration Capacity and Erosion of Mount Merapi Slope Materials Anselma Diksita Prajna Duhita 1 , Adam Pamudji Rahardjo 1,* , Ani Hairani 2 1 Department of Civil and Environmental Engineering, Universitas Gadjah Mada, Yogyakarta, INDONESIA Jalan Grafika No 2 Yogyakarta 2 Department of Civil Engineering, Universitas Muhammadiyah Yogyakarta, Yogyakarta, INDONESIA Jl. Brawijaya, Geblagan, Tamantirto, Kasihan, Bantul, Yogyakarta * Corresponding authors: rahardjo.adam@ugm.ac.id SUBMITTED 3 August 2020 REVISED 19 August 2020 ACCEPTED 18 September 2020 ABSTRACT The infiltration on slopes has a specific behavior capable of being parameterized and one of the reasons is due to the ability of the slope to generate less ponding on the sloping soil surface. This, therefore, affects infiltration rate and surface runoff proportion of water from any kind of rainfall distribution and the tendency of the surface runoff to be higher usually leads to a higher erosion rate on the slope. Moreover, slope steepness is the most important parameter of a slope, and its effect at 36%, 47%, and 58% was tested on the infiltration capacity and erosion rate of Mt. Merapi bare slope material in a laboratory using a rainfall simulator. The rainfall intensity was set constant at a rate of 116.31 mm/hour while the infiltration rate was measured by the volumetric balance principle and the erosion rates by collecting the eroded grains at the downstream end flume. Furthermore, the infiltration capacity was evaluated using the Horton method by fitting the equation to the recorded infiltration rate data while the average erosion was through the eroded grain data for each test. The results obtained represent the relationship between slope steepness, the affected infiltration capacity, and erosion for each test, and the infiltration capacity was found to be decreasing in lower slope < 47% and increasing in a higher slope while the erosion rate was increasing between 7% and 15% for each 1% increase in the slope steepness. In addition, polynomial and linear equations were developed to express the relationship between these three indicates at the Mt. Merapi bare slope material. KEYWORDS Slope Steepness; Infiltration Capacity; Erosion Rate; Rainfall Simulator; Mount Merapi Slope Materials. © The Author(s) 2021. This article is distributed under a Creative Commons Attribution-ShareAlike 4.0 International license. 1 INTRODUCTION 1.1 Background Infiltration is defined as the movement of water into the soil due to gravity and capillary forces (Bedient and Huber, 1992). It is one of the processes in the hydrological cycle which determines the amount of rainwater entering the soil and those forming surface runoff. This means infiltrated water either becomes the interflow or accumulates underneath the soil layer with the infiltration of more water reported to usually leads to lesser surface runoff. The process is mostly affected by several factors such as soil conditions and properties, surface cover and condition as well as the intensity and duration of the rain (Harto, 2000) which interact with each other to make infiltration a complex process. Moreover, in the situation rainfall intensity is lower than the ability of the soil to infiltrate water, all the rainwater is infiltrated while a pond is formed in a case where soil ability was unable to infiltrate water, thereby, causing runoff on the soil surface (Liu et al., 2011; Lei et al., 2006). There are different kinds of ground surface morphology with several surface plains found to have varying elevations connected by slopes (Schor and Gray, 2007). Meanwhile, the slope has a steepness, expressed in percent or degree, which represents the ratio of the vertical plane to the horizontal plane. The occurrence of infiltration on slopes has a specific behavior due to the generation of less pond on the soil surface (Della Sala, 2014; Lei et al., 2006). This further affects the proportion of rainwater volume infiltrating and those forming surface runoff. Another process observed to be occurring on a slope is erosion and it is defined as the removal of the soil surface layer by agencies such as wind, water, or ice (Schor and