The duty is the largest
on the field
Understanding Irrigation Duty and its Variation
The question asks about where the irrigation duty is largest. To answer this, we first need to understand what irrigation duty means and how water flows through an irrigation system.
What is Irrigation Duty?
Irrigation duty is defined as the area of land that can be irrigated by a unit volume of water flowing continuously for the entire duration of the base period of the crop. It is commonly expressed in hectares per cumec (ha/cumec).
Mathematically, duty (D) can be related to the area (A) irrigated by a flow rate (Q) over a base period (B) as:
\( D = \frac{A}{Q} \)
However, a more practical formula relating duty, delta (\(\Delta\)), and base period (B) is also used:
\( D = \frac{8.64 \times B}{\Delta} \)
where D is in ha/cumec, B is in days, and \(\Delta\) (delta) is the total depth of water required by the crop in meters over the base period.
Water Flow in Irrigation Systems and Losses
Water in an irrigation system flows from the source (like a reservoir or river) through a network of canals: main canal, branch canals, major and minor distributaries, watercourses, and finally to the field. As water travels through this network, there are losses.
These losses primarily include:
- Seepage: Water lost through the bed and sides of canals into the ground.
- Evaporation: Water lost from the surface of canals into the atmosphere.
- Operational Losses: Losses due to spills, leakages, or improper regulation.
Due to these losses, the volume of water available decreases as it moves downstream from the head of the main canal towards the field. This means:
- The volume of water entering the main canal at its head is the maximum.
- The volume of water reaching the watercourse head is less than that at the main canal head.
- The volume of water finally reaching the field is the minimum, as it has passed through the entire distribution system and incurred all intermediate losses.
Analyzing Duty at Different Points
Let's consider irrigating a fixed area, say 1 hectare, using the definition \( D = \frac{A}{Q} \). Since the area \(A\) is the same (1 hectare) regardless of where we measure the water, the duty \(D\) is inversely proportional to the flow rate \(Q\) at that point.
\( D \propto \frac{1}{Q} \)
This means where the flow rate (\(Q\), or effectively the volume of water available) is high, the duty is low. Where the flow rate (\(Q\)) is low, the duty is high.
Comparing the points mentioned in the options:
- Head of the main canal: Maximum volume of water is present here. Thus, \(Q\) is maximum, and \(D\) is minimum.
- Head of the watercourse: Volume of water is less than at the main canal head due to losses. \(Q\) is less than at the main canal head, so \(D\) is greater than at the main canal head.
- On the field: Minimum volume of water finally reaches the field after all losses. \(Q\) is minimum here, so \(D\) is maximum.
Conclusion: Where Duty is Largest
Because the volume of water decreases progressively from the head of the main canal down to the field due to various losses, the flow rate (\(Q\)) is lowest on the field. Since duty (\(D\)) is inversely proportional to \(Q\) for a given area, the duty is largest on the field.
Let's summarise the water volume and duty at different points:
| Point in the System | Volume of Water Available | Duty (\(D \propto 1/Q\)) |
|---|---|---|
| Head of main canal | Maximum | Minimum (Lowest Duty) |
| Head of watercourse | Intermediate | Intermediate Duty |
| On the field | Minimum | Maximum (Largest Duty) |
Therefore, the duty is largest on the field.
Revision Table: Irrigation Duty Concepts
| Term | Definition | Typical Units | Variation along Canal |
|---|---|---|---|
| Duty (D) | Area irrigated per unit flow rate over base period | ha/cumec | Increases as water flows downstream (largest on field) |
| Delta (\(\Delta\)) | Total depth of water required by crop | meters (m) | Remains constant for a given crop and area |
| Base Period (B) | Time between first and last watering of a crop | days | Remains constant for a given crop |
Additional Information: Factors Affecting Irrigation Duty
Several factors can influence the irrigation duty in a region:
- Type of Crop: Different crops have different water requirements (\(\Delta\)). Crops needing less water have higher duty.
- Soil Type: Permeable soils (sandy) lead to higher seepage losses and thus lower duty compared to less permeable soils (clayey).
- Climate: Hot, dry climates increase evaporation losses, reducing water availability downstream and affecting duty.
- Efficiency of the Irrigation System: Well-maintained canals with lining reduce seepage losses, leading to higher duty overall. Poor maintenance increases losses and reduces duty.
- Method of Irrigation: More efficient methods like drip or sprinkler irrigation reduce water losses on the field, potentially leading to higher duty compared to flood irrigation.
- Rainfall: Effective rainfall during the crop period reduces the amount of irrigation water needed, thus increasing the duty of the supplied water.
Important Questions from Water Requirement of Crops
Which of the following food grain crops occupies the largest part of the cropped area in India?
Which of the following statement is correct for sprinkler irrigation method?
Watering done prior to the sowing of the crops is called ________.
- An identified source of irrigation water has ion concentrations of Na+ , Ca++ and Mg++as 20, 10 and 8 mill-equivalent per liter, respectively. The SAR of this water is approximately:
If B is the base period in days, D is the duty in hectares/cumec and Δ is the delta of the crop
in m, the relationship between them is given by:
