Infiltration and Ground Water

Introduction

Infiltration and ground water are important concepts in engineering hydrology. Understanding these concepts is crucial for various engineering projects. Infiltration refers to the process by which water enters the soil from the surface. Ground water, on the other hand, is the water that is stored beneath the Earth's surface in the saturated zone. Both infiltration and ground water play a significant role in the water cycle and have implications for water resource management.

Infiltration

Infiltration is the process of water entering the soil from the surface. Several factors affect the rate of infiltration:

1. Soil Characteristics: The texture, structure, and compaction of the soil influence its permeability and, therefore, the rate of infiltration.
2. Vegetation Cover: Vegetation acts as a barrier to infiltration by intercepting rainfall and reducing the impact of raindrops on the soil surface.
3. Slope of the Land: Steeper slopes increase the velocity of runoff, reducing the time available for infiltration.
4. Rainfall Intensity and Duration: Higher rainfall intensity and longer duration can saturate the soil surface, reducing the rate of infiltration.

To measure infiltration, various methods are used:

1. Infiltration Equation and Indices: The infiltration equation relates the rate of infiltration to the properties of the soil and the rainfall characteristics. Infiltration indices, such as the infiltration capacity, provide a measure of the soil's ability to absorb water.
2. Methods for Measuring Infiltration: Several techniques, such as the double ring infiltrometer, single ring infiltrometer, and tension infiltrometer, are used to measure the rate of infiltration.

Ground Water Occurrence and Exploration

Ground water refers to the water stored beneath the Earth's surface in the saturated zone. It occurs in aquifers, which are underground layers of permeable rock or sediment that can transmit water. Aquifers are separated by aquitards, which are impermeable layers that restrict the flow of water.

There are different types of aquifers, including unconfined aquifers, confined aquifers, and artesian aquifers. Unconfined aquifers are not overlain by an impermeable layer, while confined aquifers are sandwiched between two aquitards. Artesian aquifers are confined aquifers in which the water is under pressure and can rise above the level of the aquifer.

Ground water flow is governed by Darcey's law, which states that the flow rate is proportional to the hydraulic gradient and the hydraulic conductivity of the aquifer. The hydraulic gradient is the change in hydraulic head per unit distance, and the hydraulic conductivity is a measure of the aquifer's ability to transmit water.

To explore ground water, various techniques are used:

1. Geophysical Methods: Electrical resistivity, seismic refraction, and electromagnetic methods are used to determine the subsurface characteristics and locate potential aquifers.
2. Drilling and Test Pumping: Wells are drilled into the ground to access the ground water. Test pumping involves pumping water from the well and measuring the response of the aquifer to determine its properties.
3. Ground Water Sampling and Analysis: Samples of ground water are collected and analyzed to determine its quality and suitability for various purposes.

Flow in Wells

Wells are structures that are used to extract ground water from aquifers. There are different types of wells, including dug wells, driven wells, and drilled wells. Wells operate based on the principle of creating a pressure difference between the well and the aquifer, causing water to flow into the well.

Steady flow in wells occurs when the rate of pumping remains constant over time. The flow rate and drawdown, which is the lowering of the water level in the well, can be calculated using equations based on assumptions about the aquifer properties and the well design.

Unsteady flow in wells occurs when the rate of pumping changes over time. Transient drawdown, which is the change in water level in the well due to pumping, can be calculated using equations that consider the time-dependent behavior of the aquifer.

Wells have advantages, such as providing a reliable source of water, and disadvantages, such as the potential for contamination and the depletion of the aquifer.

Yield of Wells

The yield of a well refers to the amount of water that can be extracted from an aquifer over a given period. Several factors affect the yield of a well:

1. Aquifer Properties: The hydraulic conductivity and storage coefficient of the aquifer influence its ability to transmit and store water.
2. Well Design and Construction: The diameter, depth, and screen placement of the well affect the flow of water into the well.

The yield of a well can be determined through test pumping, which involves pumping water from the well at a known rate and measuring the response of the aquifer. Sustainable yield, which is the long-term average yield that can be extracted without depleting the aquifer, can also be estimated.

Real-world examples of yield determination can be found in various water supply projects and groundwater management plans.

Conclusion

Infiltration and ground water are fundamental concepts in engineering hydrology. Infiltration is influenced by soil characteristics, vegetation cover, slope of the land, and rainfall intensity and duration. Ground water occurs in aquifers and can be explored using geophysical methods, drilling, and test pumping. Flow in wells can be steady or unsteady, and the yield of wells depends on aquifer properties and well design. Understanding infiltration and ground water is essential for effective water resource management and engineering projects.

Summary

Infiltration and ground water are important concepts in engineering hydrology. Infiltration refers to the process of water entering the soil from the surface, while ground water is the water stored beneath the Earth's surface in the saturated zone. Factors affecting infiltration include soil characteristics, vegetation cover, slope of the land, and rainfall intensity and duration. Infiltration can be measured using infiltration equations and indices, as well as various measurement methods. Ground water occurs in aquifers and can be explored using geophysical methods, drilling, and test pumping. Flow in wells can be steady or unsteady, and the yield of wells depends on aquifer properties and well design. Understanding infiltration and ground water is crucial for water resource management and engineering projects.

Analogy

Understanding infiltration and ground water is like understanding how water enters and is stored in a sponge. Infiltration is the process of water seeping into the sponge, while ground water is the water that is stored within the sponge. Factors such as the sponge's texture, the presence of obstacles on the surface, and the intensity and duration of water pouring onto the sponge affect the rate at which water infiltrates. Similarly, ground water occurs in different layers of the Earth's subsurface, and its flow and availability can be influenced by various factors. Just as understanding the properties of a sponge helps in managing water absorption, understanding infiltration and ground water is essential for effective water resource management and engineering projects.