Introduction

Infiltration is the process of entering water from soil surface into ground. As the precipitation reaches the ground surface firstly it wets down the soil to reach its moisture level later it starts to penetrate into the soil surface in the form of infiltration after meeting the surface soil moisture conditions. The excess water penetrates downwards under the action of gravity into the soil subsurface, this process is known as percolation. The rate at which the infiltrated water is observed by the soil is known as the infiltration rate. The infiltration rate is dependent on various factors: land cover (vegetation), soil type, hydraulic conductivity, texture, structure. Different soils have different infiltration rates hence infiltration plays an important role in generation of runoff. Runoff is the overflow of accumulated water on the surface exceeding the infiltration rate. Whenever the infiltration rate of particular soil is less than the intensity of rainfall at the given time, it results in a higher rate of surface runoff. In addition the different soil conditions have different rate of infiltration, compacted soil due to agricultural activity, human movement have lower rate of infiltration, whereas natural vegetated land has higher infiltration rate. The infiltration is usually higher at the beginning and slows down exponentially with time and reaches a constant rate. During rainy days the infiltration rate will be higher on the first day of precipitation due to dry soil condition, later it decreases day by day due to soil saturation.

The infiltration rate is measured as depth of water infiltrated in measured time, usually expressed in mm/min, cm/hour. For a given soil the maximum rate at which the water can infiltrate into soil under given conditions is called the infiltration capacity of the soil. And the total depth of water infiltrated in a given period of time under given conditions is called as accumulated infiltration and is expressed in cm units. Usually the infiltration rate is measured in the field using a cylindrical infiltrometer. There are two types of infiltrometers: single ring and double ring infiltrometers. The double ring infiltrometer has metallic inner and outer rings of diameter of 30 and 60 cm respectively with a minimum height of 25 cm. These rings are driven into the soil up to 10 cm depth of soil. Then both the rings are filled with water upto marked levels. The purpose of using two rings instead of one is that the water which flows from the outer ring acts to prevent the lateral flow of the water from the inner ring. Consequently, water infiltration is measured from the inner ring to the vertical (downward) flow only. Once infiltration starts the change in water levels are measured and noted down at particular time intervals. Once the infiltration rate reaches the constant rate experiment is stopped.

Horton’s equation is a commonly used empirical equation to calculate the infiltration capacity of soil.

Where,
fp= the infiltration capacity at some time t(depth/time)
k = a Horton’s decay coefficient constant representing the rate of decrease in f capacity.
fc = a final steady state infiltration capacity t=tc
fo = the initial infiltration capacity at t=0