These lecture notes provide a comprehensive overview of Surface Irrigation, covering its principles, methods, advantages, and design considerations.

Lecture Notes: Surface Irrigation

1. Introduction to Surface Irrigation

Definition: Surface irrigation is the oldest and most common method of applying water to crops. In this system, water is distributed over the soil surface by gravity. The soil acts as both the medium for water conveyance and the storage reservoir for the root zone.

Key Characteristic: Unlike sprinkler or drip irrigation, surface irrigation does not require high-pressure pumps or extensive piping networks within the field.

2. Factors Influencing the Choice of Surface Irrigation

Before selecting a specific surface irrigation method, the following factors must be considered: 1. Soil Type: Infiltration rates are crucial. Sandy soils (high infiltration) may lead to excessive water loss, while clay soils (low infiltration) may cause waterlogging. 2. Topography: The land must be relatively flat or capable of being leveled. Steep slopes lead to erosion and uneven distribution. 3. Water Availability: Surface irrigation usually requires a large discharge (flow rate) of water to cover the land quickly. 4. Crop Type: Some crops (like rice) prefer flooding, while others (like potatoes) require furrows to keep the foliage dry.

3. Classification of Surface Irrigation Methods

Surface irrigation is generally divided into four main categories:

A. Uncontrolled (Wild) Flooding

  • Description: Water is diverted from a canal or stream and allowed to flow over the land without any artificial barriers or levees.
  • Suitability: Low-value crops, pastures, or hilly areas where leveling is impossible.
  • Limitations: Very low efficiency; uneven water distribution.

B. Border Irrigation

  • Description: The field is divided into long, narrow strips (borders) separated by low earth ridges (levees). Water is applied at the top end and flows as a thin sheet toward the bottom.
  • Types:
    • Directional: Follows the slope.
    • Contour: Follows the land’s natural contours.
  • Suitability: Close-growing crops like wheat, barley, and alfalfa.

C. Check Basin Irrigation

  • Description: The field is divided into small, level plots (basins) surrounded by dikes. Water is filled to a desired depth and allowed to infiltrate.
  • Suitability: This is the most popular method for rice (paddy) and orchards (Ring Basin method).
  • Advantage: High control over water application; excellent for leaching salts from the soil.

D. Furrow Irrigation

  • Description: Small, parallel channels (furrows) are used to carry water between crop rows. Only a portion of the surface is wetted (usually 1/5 to 1/2), reducing evaporation.
  • Suitability: Row crops like corn, cotton, potatoes, sugarcane, and vegetables.
  • Types:
    • Corrugations: Small furrows used for grain crops.
    • Surge Irrigation: Applying water in “on-off” cycles to improve distribution.

4. Phases of Surface Irrigation

A typical surface irrigation event consists of four distinct phases: 1. Advance Phase: From the time water is introduced at the inlet until it reaches the end of the field. 2. Storage (Ponding) Phase: The time between water reaching the end and the moment the inflow is shut off. 3. Depletion Phase: The time between stopping the inflow and the moment water begins to disappear from the top end. 4. Recession Phase: The time from the end of depletion until the water has completely receded from the soil surface.

5. Advantages and Disadvantages

Advantages:

  • Low Cost: Minimal capital investment; no expensive equipment or power required (gravity-fed).
  • Simplicity: Easy to operate and maintain by farmers.
  • Salinity Management: Effective at leaching salts down below the root zone.
  • Versatility: Can utilize variable water qualities, including water with some sediment.

Disadvantages:

  • Low Efficiency: High losses due to deep percolation (at the top end) and runoff (at the bottom end).
  • Land Leveling: Requires precise land leveling, which can be expensive and may remove fertile topsoil.
  • Labor Intensive: Requires constant monitoring to move water from one basin/furrow to the next.
  • Waterlogging: Risk of rising water tables if drainage is poor.

6. Performance Indicators (Efficiencies)

To evaluate how well a surface system is working, we use:

  1. Water Application Efficiency (\(E_a\)): \[E_a = \frac{\text{Water stored in root zone}}{\text{Water delivered to the field}} \times 100\]
  2. Water Storage Efficiency (\(E_s\)): Measures how much of the crop’s water requirement was actually met.
  3. Water Distribution Efficiency (\(E_d\)): Measures how uniformly water was distributed along the length of the run.

7. Improving Surface Irrigation

To modernize surface irrigation, engineers use: * Precision Land Leveling (Laser Leveling): Using laser-guided buckets to create perfectly flat surfaces. * Gated Pipes: Replacing open ditches with pipes that have small gates to control flow into furrows. * Tailwater Recovery: Collecting runoff at the end of the field and pumping it back to the top to be reused.

Summary for Students

  • Surface Irrigation = Gravity.
  • Basins are for rice/trees.
  • Borders are for grains/alfalfa.
  • Furrows are for row crops.
  • Efficiency depends on how well the “Advance” matches the “Recession” time.