Duty and Delta | Civil Engineering Optional Notes for UPSC PDF Download

Introduction

• Definition of Duty: Duty refers to the area of land that can be irrigated with a unit volume of irrigation water. It is typically measured in terms of hectares per unit volume of water, such as cumecs. For instance, a duty of 1700 hectares per cumec signifies the extent of land that can be irrigated by one cubic meter of water per second.
• Definition of Delta (Δ): Delta represents the total depth of water required by a crop throughout its entire growth period in the field. This measurement is usually expressed in meters or centimeters. It encapsulates the cumulative water needs of the crop from sowing to harvesting.
• Inverse Relationship between Duty and Delta: There exists a reciprocal relationship between Duty and Delta. Higher Delta values correspond to lower Duty, and vice versa. In other words, crops with greater water requirements (high Delta) necessitate larger volumes of water per unit area, leading to a lower Duty.

Ways of Reckoning Duty

• By the number of hectares that one cumec of water can irrigate during the base period (e.g., 1700 hectares/cumec).
• By the total depth of water (Delta) required for the crop.
• By the number of hectares that can be irrigated by a million cubic meters of stored water (for tank irrigation).
• By the number of hectare-meters expended per hectare irrigated (for tank irrigation).

Calculation of Duty

• Duty calculation based on crop water requirements, irrigation efficiency, and water losses.
• Empirical formulas and field data for estimating Duty for different crops and irrigation methods.
• Use of software and models for accurate Duty calculation.

Calculation of Delta

• Delta calculation based on crop water requirements, irrigation intervals, and water depth per irrigation.
• Influence of irrigation efficiency, soil moisture conditions, and leaching requirements on Delta.
• Operational factors affecting Delta, such as irrigation scheduling and water distribution practices.

Duty and Delta Concepts

• Crop Period: This denotes the duration in days from the sowing of a crop to its harvesting. It delineates the temporal span over which the crop's water needs must be met.
• Base Period: The base period encompasses the entire cultivation cycle from the initial irrigation for land preparation to the final watering preceding harvesting. It serves as the reference time frame for assessing irrigation requirements.
• Significance of Duty and Delta: These metrics serve as fundamental parameters guiding irrigation strategies and infrastructure development. Duty informs the extent of land coverage per unit water volume, while Delta elucidates the total water demand of crops, facilitating efficient water allocation and management.

Relation between Duty and Delta

• Let D = Duty in hectares/cumec

• Let Δ = Total depth of water supplied in meters

• Let B = Base period in days

i) For a field of area D hectares, water supplied corresponding to depth Δ meters will be:

= Δ x D hectare-meters

= D x Δ x 10^4 cubic meters ...(1)

ii) For the same field of D hectares, one cumec flow during the entire base period B:

Water supplied = 1 cumec x B days x 24 hours x 3600 seconds

= B x 24 x 60 x 60 cubic meters ...(2)

Equating (1) and (2):

D x Δ x 10⁴ = B x 24 x 60 x 60

Δ = (8.64 B) / D meters

Therefore, Δ (Delta) = (8.64 x Base Period) / Duty

• 1 hectare = 10⁴ sq meters

• 1 cumec-day = 8.64 hectare-meters

Command Area Terminology

1. Commanded Area (CA): The area that can be irrigated by a canal system.

2. Gross Command Area (GCA): Total area that can theoretically be served by the irrigation system, including uncultivable areas.

3. Culturable Command Area (CCA): The actually irrigated area within the GCA.
   GCA = CCA + Unculturable area

4. Culturable Cultivated Area: Part of CCA put under cultivation during a crop season.

5. Culturable Uncultivated Area: Part of CCA not cultivated during a crop season, due to:
   (a) Lack of water, fertilizers, etc
   (b) Kept fallow to increase soil fertility
   (c) High water table, so no need for irrigation

Intensity of Irrigation

• Definition: Intensity of irrigation refers to the percentage of the proposed irrigation that is actually carried out annually. It is typically calculated by expressing the areas irrigated during each crop season (such as Rabi and Kharif) as a percentage of the Culturable Command Area (CCA). By summing the intensities of irrigation for all crop seasons, the yearly intensity of irrigation is obtained.

• Calculation Example: Suppose a field has a Culturable Command Area (CCA) of 120 hectares. During the Kharif season, 90 hectares of land are cultivated, and during the Rabi season, 60 hectares are cultivated. The intensity of irrigation during Kharif season would be (90/120) * 100 = 75%, and during Rabi season would be (60/120) * 100 = 50%. The yearly intensity of irrigation is the sum of these, i.e., 75 + 50 = 125%. This illustrates that the yearly intensity of irrigation can exceed 100%.

Factors Affecting Duty

• Methods and System of Irrigation: Different irrigation systems exhibit varying levels of efficiency and water usage. Perennial irrigation systems, which maintain continuous soil moisture, require less water for initial saturation and experience fewer deep percolation losses compared to inundation irrigation systems, which often entail wasteful water use. Flow irrigation systems may have lower duty due to conveyance losses in canal networks, while lift irrigation systems typically exhibit higher duty because wells serving the command area are in close proximity. Tank irrigation systems often offer high duty owing to more rigid water control mechanisms.

Method of Improving Duty
Improving the duty of water in irrigation systems involves addressing factors that contribute to reduced efficiency. Here are several methods to enhance duty:

• Proper Land Preparation
  Adequate ploughing, leveling, and providing good tilth before crop sowing are essential for optimizing water infiltration and minimizing surface runoff. This ensures that water is effectively utilized by the crops rather than being lost through runoff.

• Canal Lining
  Lining canals helps to reduce percolation and seepage losses, which are significant sources of water wastage in irrigation systems. By preventing water from seeping into the surrounding soil, more water can be delivered to the intended crop fields, improving overall water use efficiency.

• Crop Rotation:
  Implementing crop rotation practices helps maintain soil fertility and moisture balance. Different crops have varying water requirements and root depths, so rotating crops can help prevent excessive depletion of soil moisture in specific areas, contributing to more efficient water utilization.

• Farmer Training:
  Educating farmers on water-efficient practices is crucial for ensuring that they apply the correct quantity of water at the right time. By providing farmers with knowledge and skills in irrigation management, they can make informed decisions regarding irrigation scheduling and water application, leading to improved water use efficiency.

• Efficient Canal Administration:
  Ensuring efficiency, responsibility, and honesty among canal administrative staff is vital for equitable water distribution. Efficient canal operations guarantee that water reaches farmers promptly and in the required quantities, minimizing wastage and optimizing water use across the irrigation network.