The design of irrigation canal system requires the knowledge of some of the important terminologies and definitions associated with the command area of irrigation, the irrigation intensity and the intensity of cropping.
Each term is explained one by one briefly in this article.
Gross Command Area (G.C.A)
The gross command area is the total area bounded within the irrigation boundary of the project that can be irrigated economically without bothering about the limitation of quantity of water available.
The gross command area includes the cultivable area and un-cultivable area. The examples of the un-cultivable area include ponds, residential areas, reserved forests and roads.
A doab is an area between two drainages. When a canal system lies on a doab, the irrigation is economical. Here, the gross command area is the area enclosed by the drainages on both the sides and hence G.C.A forms the geographical area of the doab.
Cultivable Command Area (C.C.A)
The cultivable command area is also called a culturable command area. This is defined as the cultivable part of the gross command area. This is that area where cultivation is possible. Pastures or fallow lands that can be converted into cultivable land comes under this category. C.C.A does not include un-cultivable land mentioned before.
It is not necessary that 100 percent of C.C.A is cultivated. Hence C.C.A can be classified as:
- Cultivated Portion of CCA
- Cultivable but not cultivated Portion of CCA
C.C.A is generally taken as 80 percent of GCA if adequate data is not provided.
Intensity of Irrigation
The intensity of irrigation of a particular land area can be defined based on a particular season or annually. Hence, the intensity of irrigation of a season or seasonal intensity of irrigation is defined as that percentage of cultivable command area (CCA) that is proposed to be irrigated in a given season.
The percentage of C.C.A which may be irrigated annually is called as the annual intensity of irrigation (AII). This may include the irrigation of two or more crops.
AII = Gross Irrigated Area/CCA
Gross Irrigated area = Total area irrigated once in a year + area irrigated more than once in that year
or AII = Sum of total of intensities of irrigation of all the seasons of the year
Area to be Irrigated
The area to be irrigated is the area proposed to be irrigated in a year or that season. It is obtained by multiplying the CCA with the annual intensity of irrigation or seasonal intensity of irrigation.
As the water requirement for each crop varies with season, the area to be irrigated is calculated for each crop season.
Net and Gross Sown Areas
There are situations when in a year, two or more crops are grown during a particular year in the same area in two seasons. Hence, this area is sown more than once in a year. Then,
Gross Cropped Area or Gross Sown Area (During a Year) = Net cropped area i.e. area sown once in a year + area sown more than once in a year
Net and Gross Irrigated Area
The area of land irrigated once in a year is called a net irrigated area. The area of land irrigated once in a year plus the area of land irrigated more than once in a year forms the gross irrigated area.
Time Factor and Capacity Factor
The time factor is defined as the ratio of actual operating period of a distributary to the crop period. This factor is to check the danger of over-irrigation which can result in water clogging and high salinity.
The capacity factor is defined as the ratio of mean supply discharge in a canal during a period to its design full capacity.
Full Supply Coefficient
The design duty at the head of the canal is called a full supply coefficient. It is also called as the number of hectares irrigable per cumec of the canal capacity at its head. Hence,
Full Supply Coefficient = Area Estimated to be Irrigated during the base period/ Design Full supply discharge at the head of the canal
The full supply coefficient is also called as Duty on Capacity.
The ratio of area actually irrigated by the cultivators to the mean supply discharge let out at the outlet of the distributary over the crop period is called as Nominal Duty.
It is the ratio of the land area of a crop irrigated under the Kharif season () to the land area of that crop irrigated in the Rabi season.
Ak = A. [x/ (x+1)]
AR = A/(x+1)
Where, x is the crop ratio, Ak is the Area grown in Kharif season, AR is the area grown in Ravi season and A is the total area of crop irrigated.
Intensity of Cropping
The intensity of cropping is defined as the ratio of the Gross cropped area to the net cultivated area or net cropped area. The net cultivated area can be equated to the CCA.