2. 2. 2. Tube Well Irrigation Design

Tube Well Irrigation Design

Tube wells are a modern and efficient method for extracting groundwater for irrigation purposes. Unlike dug wells, tube wells are narrow and deep, reaching aquifers located deeper underground. A tube well typically consists of a long, narrow pipe (casing) that is drilled into the ground to reach the water-bearing strata. A pump is used to lift the water to the surface for irrigation.

Components of a Tube Well

1. Casing Pipe:

   • A strong, corrosion-resistant pipe that lines the borehole to prevent it from collapsing and to keep out sand and other impurities. The diameter usually ranges from 100 mm to 300 mm.

2. Screen or Perforated Section:

   • A section of the casing pipe with slots or perforations that allow water to enter the well while keeping out sand and other particles.

3. Gravel Pack:

   • Placed around the screen, it filters out sand and fine particles, improving water quality and well efficiency.

4. Pump:

   • Usually a submersible pump is used, which is placed inside the casing pipe below the water table.

5. Headworks:

   • The surface structure, including the pump head, discharge pipe, and control valves.

Numerical Real-Life Example

Scenario: Designing a tube well for irrigation in an agricultural area with the following requirements and conditions:

• Daily water requirement for crops: 20,000 liters (20 m³)
• Depth to the water table: 50 meters
• Expected yield: 2 liters/second (lps)
• Casing diameter: 150 mm (6 inches)

Step 1: Calculate the Well Yield

The yield of the well is the amount of water it can supply per unit time. It is calculated using the formula:

$Q=\frac{\mathrm{V/}}{T}$

Where:

• $\mathrm{Q\; =\; Yield\; (lps)}$
• $\mathrm{V =\; Volume\; of\; water\; (liters)}$
• $\mathrm{T =\; Time\; (seconds)}$

Given:

• Yield ($Q$) = 2 lps

To check if the well can meet the daily requirement of 20,000 liters:

Daily Water Yield:

$Q\times T=V\mathrm{<br>}\mathrm{<br>}$

$\mathrm{There\; are\; 86,400\; seconds\; in\; a\; day\; (24\; hours\; \times \; 60\; minutes\; \times \; 60\; seconds).}$

$2\text{ lps}\times 86,400\text{ seconds}=172,800\text{ liters/day}$

$2 \text{ lps} \times 86,400 \text{ seconds} = 172,800 \text{ liters/day}$

The well can provide 172,800 liters/day, which far exceeds the daily requirement of 20,000 liters. Thus, the well can adequately supply water for irrigation.

Step 2: Design Parameters

1. Casing and Screen:

   • The casing diameter is chosen to be 150 mm (6 inches), providing a sufficient cross-sectional area for water flow. The screen length and slot size are designed based on the aquifer's properties to optimize water entry while minimizing sand intrusion.

2. Pump Selection:

   • A submersible pump is selected based on the well's depth and yield requirement. For a well depth of 50 meters and a required flow rate of 2 lps, a suitable pump with a power rating of approximately 1.5 to 2 horsepower (HP) is chosen.

3. Gravel Pack:

   • A gravel pack is placed around the screen section to filter out fine sediments and prevent clogging. The size of the gravel is selected based on the aquifer's particle size distribution.

4. Headworks:

   • Includes the installation of a pump head, discharge pipe, and control valves. The discharge pipe leads the water to storage or directly to the irrigation system.

Step 3: Cost Estimation

1. Drilling and Installation:

   • Drilling costs depend on the depth and soil conditions. For a 50-meter well with a casing diameter of 150 mm, the cost might be around  ₹ 20 per meter, totalling approximately   ₹ 1,000.

2. Casing and Screen:

   • The cost of the casing and screen material can vary, but for PVC or steel, it could be around  ₹ 300 to  ₹ 500.

3. Pump and Installation:

   • A suitable submersible pump and installation might cost between  ₹ 500 and  ₹ 1,000.

4. Gravel Pack and Headworks:

   • Additional costs for gravel pack and headworks could be around   ₹ 200 to  ₹ 300.

Conclusion

Tube wells are highly efficient for extracting groundwater, especially from deeper aquifers. They provide a reliable water supply for irrigation, even in areas with significant depth to the water table. The example demonstrates that with proper design, a tube well can meet the irrigation needs of a crop field. The design process ensures the well's efficiency, water quality, and sustainability while optimizing costs.