ESTIMATION OF DESIGN DISCHARGE OF A CANAL
The amount of water needed for the growth of a crop during its entire crop-growing period is known as the water requirement of the crop, and is measured in terms of depth of water spread over the irrigated area. This requirement varies at different stages of the growth of the plant. The peak requirement must be obtained for the period of the keenest demand. One of the methods to decide the water requirement is on the basis of kor watering.
When the plant is only a few centimeters high, it must be given its first watering, called the kor watering, in a limited period of time which is known as the kor period. If the plants do not receive water during kor period, their growth is retarded and the crop yield reduce considerably. The kor watering depth and the kor period vary depending upon the crop and the climatic factors of the region. In UP, the kor watering depth for depth for wheat is 13.5 cm and the kor period varies from 8 weeks in north-east UP (a relatively dry region) to 3 weeks in the hill region (which is relatively humid). For rice, the kor watering depth is 19 cm and the kor period varies from 2 to 3 weeks.
If D represents the duty (measured in hectares/m3/s) then, by definition, 1 m3/s of water flowing for b (i.e., base period in days irrigates D hectares.
\ 1 m3/s of water flowing for 1 day (i.e., 86400 m3 of water) irrigates D/b hectares
This volume (i.e., 86400 m3) of water spread over D/b hectares gives the water depth, D
D = 86400 / (D/b) x 104 =8.64 b/D (metres)
For the purpose of designing on the basis of the keenest demand (i.e., the kor period requirement) the base period b and the water depth Dare replaced by the kor period and kor water depth, respectively.
Example: The culturable command area of a distributary channel is 10,000 hectares. The intensity of irrigation is 30 per cent for wheat and 15 per cent for rice. The kor period for wheat is 4 weeks, and for rice 3 weeks. Kor watering depths for wheat and rice are 135 mm, respectively. Estimate the outlet discharge.
Area to be irrigated (hectares) : Wheat : 0.30 ´10,000 = 3000 Rice : 0.15 ´10,000 = 1500
Outer factor D = 8.64 b/D (in hectares/m3/s) : Wheat : 8.64(4 ´7) /0.135 = 1792 Rice : 8.64(3´7)/ 0.19 = 954.95
Outlet discharge (m3/s) : 3000/1792 = Wheat : 1.674 »1.7 Rice : 1500.954.95 = 1.571 »1.6
Since the water demands for wheat and rice are at different times, these are not conservative. Therefore, the distributary channel should be designed for the larger of the two charges, viz., 1.7 m3/s. The above calculations exclude channel losses and the water measurement of other major crops during their kor period.
The kor period for a given crop in a region depends on the duration which there livelihood of the rainfall being smaller than the corresponding water requirement. Increasingly, the kor period is least in humid regions and more in dryer regions. The kor depth placement must be met within the kor period. As such, the channel capacity designed on the kor period would be large in humid regions and small in dry regions. Obviously, this period of determining the channel capacity is, therefore, not rational, and is not used in ……
A more rational method to determine the channel capacity would be to compare transpiration and corresponding effective rainfall for, say, 10-day (or 15-day) periods of entire year and determine the water requirement for each of these periods. The channel capacity can then be determined on the basis of the peak water requirement of the 10-day (or periods). This method has already been explained in section.