Relation between duty of Water and delta for crops

Delta for certain crops

The average values of deltas for certain crops are shown in table. These values represent the total water requirement of the crops. The actual requirement of irrigation water may be less, depending upon the useful rainfall. Moreover, these values represent the values on field, i.e.

‘delta             on  field’  which   includeslosses.  the  evaporation

Table: Average Approximate Values of Î' for Certain Important Crops in India

S.No         Crop Delta on field

1 Sugarcane        120    cm  (4

2 Rice        120    cm  (4

3 Tobacco 75  cm  (3

4 Garden fruits   60      cm  (2

5 Cotton   50      cm  (2

6 Vegetables       45 cm (18?)

7 Wheat    40      cm  (1

8 Barley    30      cm  (1

9 Maize     25      cm  (1

10    Fodder        22.5  cm

11    Peas  15  cm  (6

Duty of Water:

The ‘duty’ of water is the relationship betw it matures. This volume of water is generally expressed by: a unit discharge flowing for a time

equal to the base period of the crop, called Base of a duty.

If water flowing at a rate of one cubic meter per second, runs continuously for B days, and matures 200 hectares, then the duty of water for that particular crop will be defined as 200 hectares per cumec to the base of B days. Hence, duty is defined as the area irrigated per cumec of discharge running for base period B. The duty is generally represented by the letter D.

Relation between duty and delta:

Let there be a crop of base period B days. Let one cumec of water be applied to this crop on the field for B days.

Now, the volume of water applied to this crop during B days.

Volume of water applied to crop = V = (1 x60 x60 x24 xB) m3. = 86400 B (cubic metre)

By definition of duty (D), one cubic metre supplied for B days matures D hectares of land.

 This quantity of water (V) matures D hectares of land or 104 D sq.m of area.

Total depth of water applied on this land

= Volume/ Area = 86,400 B/ 104 D . 8.64B/D metres

By definition, this total depth of water is called delta (Î').

Î'=8.64B/D (metres)

Where

Î'is in cm, B is in days; and

D is duty in hectares/cumec.

During the passage of water from these irrigation channels, water is lost due to evaporation and  percolation. These losses are called Transit losses or Transmission or Conveyance losses in channels. 

Figure: Layout of a canal system

Duty of water for a crop is the number of hectares of land which the water can irrigate. Therefore, if the water requirement of the crop is more, less number of hectares of land it will irrigate. Hence, if water consumed is more, duty will be less. It, therefore, becomes clear that the

duty of water at the head of the watercourse will be less than the because when water flows from the head of the watercourse and reaches the field, some water is

lost as transit losses. Applying the same reasoning, it can be established that duty of water at the head of a minor will be less than that at the head of the watercourse; duty at the head of a distributary will be less than that at the head of a minor, duty at the head of a branch canal will be less than that at the head of a minor, duty at the head of a main canal will be less that the duty at the head of a branch canal. Duty of water, therefore, varies from one place to another, and increases as we move downstream from the head of the main canal towards the head of the branches or watercourses. The duty at the head of watercourse (i.e. at the outlet point is generally the end point of Irrigation Department. The control of Irrigation Department finishes at the outlet point, and the water is carried into the fields through watercourses by the cultivators themselves.