Introduction

Infiltration is the process by which water enters the soil from the ground surface. When precipitation reaches the ground, it initially wets the soil to satisfy its moisture requirement. After this condition is met, water begins to penetrate into the soil surface in the form of infiltration. The excess water then moves downward into the subsurface soil under the action of gravity; this process is known as percolation.

The rate at which water is absorbed by the soil is termed the infiltration rate. This rate depends on several factors, including land cover (vegetation), soil type, hydraulic conductivity, texture, and structure. Since different soils have different infiltration rates, infiltration plays an important role in the generation of runoff. Runoff is the flow of accumulated water over the surface when it exceeds the infiltration rate.

Whenever the infiltration rate of a particular soil is less than the rainfall intensity at a given time, it results in a higher rate of surface runoff. In addition, soil conditions significantly influence infiltration: compacted soils, resulting from agricultural activities or human movement, have lower infiltration rates, whereas naturally vegetated land exhibits higher infiltration rates.

Generally, the infiltration rate is higher at the beginning and decreases exponentially with time until it reaches a constant value. During rainy periods, the infiltration rate is higher on the first day due to dry soil conditions and decreases gradually in subsequent days as the soil becomes saturated.

The infiltration rate is defined as the depth of water infiltrated per unit time and is usually expressed in units such as mm/min or cm/hr. For a given soil, the maximum rate at which water can infiltrate under specified conditions is termed the infiltration capacity of the soil. The total depth of water that infiltrates over a given period under specified conditions is known as cumulative (or accumulated) infiltration, and it is expressed in units of length such as cm.

Infiltration rate is commonly measured in the field using a cylindrical infiltrometer. There are two types of infiltrometers:

1. Simple or tube-type infiltrometer
2. double-ring infiltrometer

Double-Ring Infiltrometer

The double-ring infiltrometer is a widely recognized instrument used for accurately measuring soil infiltration rates. It was developed to overcome the limitations of the single-ring infiltrometer, which tends to overestimate infiltration due to lateral movement of water. In contrast, the double-ring infiltrometer minimizes lateral divergence by incorporating an outer ring that saturates the surrounding soil. This arrangement promotes predominantly vertical flow within the inner ring and reduces systematic errors associated with single-ring measurements.

Proper installation of the instrument is essential. Inadequate contact between the thin metal walls of the rings and the soil may result in leakage and inaccurate readings. A typical setup consists of an inner ring with a diameter of 15 cm or 30 cm, and an outer ring with a diameter of 45 cm or 60 cm.

Two standard methods are commonly used with the double-ring infiltrometer:

1. Constant Head Method

In this method, water is continuously supplied to maintain a constant water level in both the inner and outer rings. The volume of water required to sustain the water level in the inner ring over time is measured.

2. Falling Head Method

In this method, no additional water is supplied after the initial filling. The decrease in water level within the inner ring is measured at specified time intervals.

To measure the water depth inside the rings, a graduated steel scale or a precision measuring rod is typically used.

Applications

The double-ring infiltrometer is suitable for use in most soil types, except for clogging soils, stony soils, and soils on steep slopes. The presence of the outer ring ensures that infiltration from the inner ring occurs predominantly in the vertical direction.

Using this instrument, several soil-hydrological parameters can be determined for each soil layer, including:

• Infiltration capacity
• Near-saturated hydraulic conductivity
• Infiltration curve
• Cumulative infiltration over a specified period

The double-ring infiltrometer is widely applied in various fields, including:

• Surface irrigation and drainage projects
• Infiltration and water purification basins
• Study of seepage from watercourses, canals, basins, and wastewater lagoons
• Soil leaching at waste storage sites
• Research on the effects of cultivation practices
• Studies on drainage effects
• Investigation of poorly permeable layers in sports fields

The use of two rings serves to minimize lateral flow. Water from the outer ring reduces the horizontal movement of water from the inner ring, thereby ensuring that infiltration measured within the inner ring represents primarily vertical (downward) flow. As infiltration proceeds, the changes in water level are recorded at specified time intervals. The experiment is continued until the infiltration rate attains a constant value, at which point the test is terminated.

Horton's equation is a widely used empirical equation for estimating the infiltration capacity of soil.

Horton's equation for infiltration capacity is expressed as:

f_p = f_c + (f_o - f_c) e^-kt

Where, f_p= infiltration capacity at time t (depth per unit time),
k = Horton’s decay coefficient, representing the rate of decrease in infiltration capacity,
f_c = final steady-state infiltration capacity,
f_o = initial infiltration capacity at t=0,
t = time (often denoted as t_c in specific contexts)

Advantages

• The presence of the outer ring ensures that infiltration from the inner ring occurs predominantly in the vertical direction, thereby improving measurement accuracy.
• Minimizes lateral flow of water, leading to more reliable results.
• Provides in-situ measurements, reflecting actual field conditions.
• Simple and effective instrument for field use across a wide range of soil types (except clogging, stony soils, and steep slopes).