- Background of the Study
Infiltration is the movement of water into the soil from the surface. The water is driven into the porous soil by force of gravity and capillary attraction. First the water wets soil grains and then the extra water moves down due to resulting gravitational force (Amin, 2005). Infiltration rate is simply how fast water enters the soil and is usually measured in inches or millimeters per hour. This rate depends on soil Characteristics such as soil texture (amount of sand, silt, and clay), hydraulic conductivity, soil structure, vegetation cover etc and plays an important role in generation of runoff volume. If infiltration rate of given soil is less than intensity of rainfall then it results in either accumulation of water on soil surface or in runoff (Ghorbani et al., 2009).
Soil is a reservoir that stores water for plant growth. Soils in good condition have well developed structure and continuous pores to the surface. As a result, water from rainfall or snow melt readily enters these soils. The water in soil is replenished by infiltration (Amin, 2005) . The infiltration rate can be restricted by poor management. Under these conditions, the water does not readily enter the soil and it moves down slope as runoff or ponds on the surface, where it evaporates. Thus, less water is stored in the soil for plant growth, and plant production decreases, resulting in less organic matter in the soil and weakened soil structure that can further decrease the infiltration rate. The different soil conditions affect its rate and mass. For example the compacted soils due to movement of agricultural machines might have a low infiltration rate which in turn accelerates runoff generation and thus hazards like soil erosion.
Additionally, from irrigation and hydrologic applications point of view, the rate at which water can be applied to the soil without causing runoff use to be another important factor, and decides the effectiveness as well as overall influences of rains, sprinkler or other irrigation. Good soil structure improves infiltration. Soils with good structure have more pores for the movement of water than soils with poor structure (Rahman, 2010).
Commonly used methods for determining infiltration capacity are hydrograph analyses and infiltrometer studies. Infiltrometers are usually classified as rainfall simulators or flooding devices. In the former, artificial rainfall is simulated over a test plot and the infiltration is calculated from observations of rainfall and runoff, with consideration given to depression storage and surface detention (Thrash, 1997). Infiltrometers are usually tubes and rings inserted in the ground level of soil. Ring infiltrometers consist of a single metal cylinder that is driven partially
into the soil. The ring is filled with water, and the rate at which the water moves into the soil is measured. This rate becomes constant when the saturated infiltration rate for the particular soil has been reached. The size of the cylinder in these devices is one source of error. A 15-cm diameter ring produces measurement errors of approximately 30%, while a 50-cm diameter ring produces measurements errors of approximately 20% compared to the infiltration rate that would be measured with a ring of an infinite diameter. It has been suggested that a diameter of at least 100 cm should be used for accurate results (Singh and Bhakar, 2004). However, cylinders of this size become very difficult to use in practice on light soils, because large volumes of water are required to conduct tests on sandy soils with high infiltration rates. Single-ring infiltrometers overestimate vertical infiltration rates. This has been attributed to the fact that the flow of water beneath the cylinder is not purely vertical, and diverges laterally. This lateral divergence is due to capillary forces within the soil, and layers of reduced hydraulic conductivity below the cylinder. A number of techniques for overcoming this error have been developed (such as a correction procedure that uses an empirical equation) for 15-cm diameter rings. Double-ring infiltrometers minimize the error associated with the single-ring method because the water level in the outer ring forces vertical infiltration of water in the inner ring (Amin, 2005).
1.2 Statement of Problem
Infiltration has received great attention in the last few years, as the relative amount
of surface runoff which occurs following natural rains or irrigation is dominantly affected by the rate & volume of infiltration. And in Nigeria today, most farmers hardly prioritize the use of infiltrometer, which could be as a result of the high cost of purchasing one. This made it necessary to design an infiltrometer making use of readily available materials, thus making the device much more affordable for the farmers to acquire.
1.3 AIMS AND OBJECTIVE
The aim of this study is to design a double ring infiltrometer.
- To design and fabricate a double ring infiltrometer.
- To evaluate the performance of the double ring infiltrometer.
The infiltration rate of a soil determines the maximum rate at which irrigation should be applied. When irrigation water is applied at a higher rate it results in ponding of water and surface runoff. Infiltration plays very important role in the process of hydrological cycle. And there is always a need for agriculturist to measure the infiltration capacity of a soil before planting on it. For this to be done conveniently, an infiltrometer is required. And in this study, a double ring infiltrometer will be designed. Reason being that, double-ring infiltrometers minimize the error associated with the single-ring method because the water level in the outer ring forces vertical infiltration of water in the inner ring.
1.5 Scope of Study
The scope of this work is based on the design of a double ring infiltrometer making use of only local facilities available that will be cheaper than imported infiltrometer. Though, more emphasis will be laid on the cost of developing the infiltrometer as well as its infiltration capacity measurement.