Rain Effect Frequency of Infiltration Rate and Infiltration Capacity in Common Soil: Laboratory Test with Rainfall Simulator

Analyzing the Influence of Rain Frequency Infiltration Rate and Infiltration Capacity in Common Soil Type (Laboratory Testing Study With Rainfall Simulator). Infiltration is the flow of water into the ground through the soil surface. This process is a very important part of the hydrological cycle and in the process of transferring rain into the flow of water in the soil before reaching the river. Infiltration (infiltration rate and capacity) is influenced by various variables, including soil type, slope inclination, density and type of vegetation, soil moisture content, and rainfall intensity. This study aims to determine the effect of rainfall frequency on the infiltration rate and infiltration capacity on common soil types. This research is a type of laboratory experimental research, using rainfall simulator tool. The soil used in this study is common soil type. Furthermore, artificial rain was provided with intensity I5, I15, and I25 and performed infiltration rate reading on the Drain Rainfall Simulator. The rate and capacity of infiltration in common soils increase proportionally to the increased intensity of rainfall, the higher the intensity of rainfall the higher the infiltration occurring at the same level of rain frequency. The rate and capacity of infiltration in common soils decrease proportionally to the increasing frequency of rain, the more the frequency of rain the smaller the infiltration occurring at the same level of rainfall intensity.


Introduction
Much of the water absorbed by the soil is largely determined by the infiltration speed, intensity and duration of rain and into the layers of soil that can deviate from the water (Ritsema & Dekker, 2000;Horton, 1933). Infiltration speed is greatly affected by the condition of water saturation that existed before the rain and permeability of the soil profile above groundwater level (groundwater level). If the soil is saturated with water before the rain, the infiltration speed is very slow and close to zero so that most of the rain will run off into surface runoff. The infiltration rate of the soil, soil moisture, intensity of rainfall, amount of rainfall and length of rain are usually the most important determinants in determining infiltration capacity and runoff water volume (Seeger et. al., 2004;Ziadat & Taimeh, 2013;Kleinman et. al., 2006). The purpose of this study was to determine the effect of rain frequency on the rate and capacity of infiltration in common soil types.

Rainfall Intensity
In this study, Mononobe formula is used to calculate the intensity of rainfall.

Infiltration
Infiltration is the flow of water into the soil through the soil surface. In infiltration, there are two terms, infiltration capacity and infiltration rate, expressed in mm/hour. Infiltration capacity is the maximum infiltration rate for a particular type of soil, while the infiltration rate is the rate of infiltration of water entering the surface per unit of time, expressed in mm per hour or centimeter per hour.
Each type of soil has different infiltration rate characteristics, which vary from very high to very low. Sandy soil types generally tend to have high infiltration rates, but clays, on the other hand, tend to have low infiltration rates. This is due to differences in pore numbers in these soil types. (Triatmodjo, 2010).

Infiltration Measurement
The method commonly used to determine the infiltration rate and infiltration capacity is measurement with an infiltrometer and hydrograph analysis. Infiltrometer can be divided into inundation infiltrometer and rain simulator (rainfall simulator) (Triatmodjo, 2010).
Rainfall Simulator (Rainfall Simulator) is to apply artificial rain that is desired for research, among others: Erosion, infiltration, and interception. Rainfall simulators can control rain as desired (Harto Br, 1993). Rainfall simulator is a tool that can be used to study hydrological parameters such as infiltration and runoff under controlled rain use (Harto Br, 1993). In this study the measurement of the infiltration rate using a rain simulator.

Figure 1. Rainfall Simulator
If the rainfall intensity (I) is greater than the infiltration capacity (f) there will be inundation on the ground surface. At one time a pool of water will overflow and the overflow of water is collected in a container.
By knowing the intensity of rain (I), the volume of reservoirs in containers and inundation height, the infiltration capacity (f) will be calculated (Triatmodjo, 2010).
Based on the Standards for rainfall intensity in an artificial rain simulator determined by the United States Department of Defense (DOD) MIL-STD 810F. Method 506.4 standard), then the rainfall intensity standard for a simulator has been determined for several rainy conditions.
In this study using these standards, the following is the standard rainfall intensity displayed on the Rainfall Simulator tool used in this study. Source: Obus (2016) Infiltration Rate Formulation When rainwater collects above ground level, the water will be infiltrated through the surface and enter the soil at an initial infiltration rate (f0) whose value depends on the groundwater content at that time (Triatmodjo, 2010).
When the rain continues, the rate of infiltration will decrease because the soil will become wetter. According to Hadisusanto (2011), a simple approach but can describe the process of inflation is the Horton model. Hadisusanto (2011) and Triatmodjo (2010) describe soil infiltration with an empirical approach which is a function of time, that is:

Stages of media settings
Before the procedure of testing the rainfall simulator model is carried out it is necessary to stage the media settings in the test tank in layers with the thickness of the coating according to the planned sample height, while the planned sample height is 30 cm and every 10 cm in each layer per layer.

Staging Ranning test
1 st running test. First frequency infiltration measurement I5. Each interval of 5 minutes runoff and infiltration that is accommodated in the drain, measured the volume of water and record it in the observation table. Until the observation as long as the soil is saturated and infiltration is constant, artificial rain is stopped. For the second running test until the third running test the same as the first running test stage by using five rain frequencies and three rainfall intentions when returning

Data Processing
Existing data will then be processed using the infiltration rate formula and infiltration capacity using the horton method.

Rain Intensity Analysis
Rain intensity analysis uses the Mononobe formula because the rainfall data obtained are daily rainfall data. Mononobe formula with rainfall data for the 5, 15 and 25 year return period plan obtained from successive calculations: 246,841 mm, 307,489 mm and 344,900 mm. Examples of calculations for t = 5 minutes can be seen in the following description.

Source: Calculation result Infiltration Rate
The following table is the value of a constant infiltration rate (fc) at rainfall intensity I5, I10, I25 and the relationship between ft and fc is observed every 5 minutes until the infiltration is considered constant using the Horton Method. As well as a comparison chart Comparison of the value of the infiltration rate between the calculation results and the Horton Method.

Comparison of Infiltration Capacity between Rainfall Intentions Variables and Rainfall Frequency Variations
Data presentation and infiltration capacity analysis can be carried out in succession on three variations of rainfall intensity plans I5, I15 and I25 and five variations of rainfall frequency. A description of the comparative results of the process is presented as follows: Source: Calculation result Figure 7. Comparison of infiltration capacity variations between rainfall intentions and rainfall frequency variations

Conclusion and Suggestion
The rate and capacity of infiltration in common soils increases directly proportional to the increase in rainfall intentions, the higher the rainfall intentions, the higher the infiltration that occurs at the same frequency of rainfall. The rate and capacity of infiltration in common soils decrease proportionally with increasing frequency of rainfall, the more frequency of rain the smaller the infiltration that occurs at the same level of rain intention. In this study, the intensity of rainfall in the Makassar area was used, with periods of repeated rainfall intensity I5, I15, and I25, using five times the frequency of rain, and using common soil types, it is recommended in subsequent studies to use rainfall intensity for different regions, and types different soils of different vegetation, as well as different types of soil.