Important Definitions in Irrigation Engineering are listed below
Contents:
GCA (Gross Command Area)
Gross command area is the total area lying between the drainage boundaries of an irrigation project that can be economically irrigated without considering the limitations of the quantity of available water. Gross command area consists of both cultivable and un-cultivable areas. Un-cultivable areas of GCA can be roads, ponds, residential areas, forests, unfertile land, etc.
CCA (Culturable or Cultivable Command Area)
Cultivable Command area is the area within GCA where cultivation is possible.
Culturable command area is further divided into two categories:
- Culturable Cultivated Area
- Culturable Un-cultivated Area
Culturable Cultivated Area
It is the area which is already cultivated at present i.e. this is the cultivated portion of CCA. Crops can be grown or maybe crops are growing at present in that area.
Culturable Un-cultivated Area
It is the area where cultivation is possible but not done yet at present i.e. this portion of CCA is cultivable but not yet cultivated.
Relation between GCA and CCA
GCA = CCA + Un-culturable Area
In absence of detailed data, CCA may be assumed 80% of GCA.
Intensity of Irrigation (Seasonal and Annual)
Usually, the entire portion of the culturable cultivated area is not irrigated in one season to avoid over-irrigation of a particular area as over-irrigation may cause water logging, water-borne diseases, etc. Also, sometimes due to water shortage it is not possible to irrigate the entire portion of Culturable Cultivated Area and hence, some portion of CCA is left unirrigated and is either allowed to take rest for one season or is sown with crops that don't require irrigation. Fields left un-irrigated in one season are irrigated in the next season. Thus, it can be said that in one season only about 40-60% of CCA (Culturable Cultivated Area) is irrigated.
Seasonal intensity of Irrigation
Seasonal intensity of irrigation is the percentage of CCA on which irrigation is done after sowing in a particular season.
Or
Seasonal intensity of irrigation is the percentage of CCA proposed to be irrigated in a given season.
For Example: Let GCA be 120 Ha out of which CCA be 100 Ha. Now during Kharif season 54% of CCA is irrigated and during Rabi season 62% of CCA is irrigated, then the intensity of irrigation for Kharif season will be 54% and the intensity of irrigation for Rabi season will be 62%.
Annual intensity of Irrigation
Annual intensity of irrigation may be defined as the percentage of Culturable Command Area (CCA) irrigated annually.
For a given year the Annual intensity of irrigation is obtained by adding all the seasonal intensities of irrigation in that year.
Annual intensity of irrigation = Sum of all seasonal intensities of irrigation in a year
Or
Annual intensity of irrigation = (Gross irrigated area/CCA) ✖ 100
For Example: The annual intensity of irrigation of the example stated above will be Seasonal intensity of irrigation for the Kharif season + Seasonal intensity of irrigation for the Rabi season which will be 54+62=116%.
Net and Gross Sown Areas
Usually, the area in which the crop is sown in a season is allowed to take rest for the next season but sometimes the area is not allowed to take rest and the next seasonal crop is sown in that area just after harvesting of the previous season's crop. Hence, there is an area within CCA which is sown more than once during a given year.
Net Sown Area
Net Sown Area or Net Cropped Area is the total area of land which is sown during a year, but the area is counted only once.
Gross Sown Area
Gross Sown Area or Gross Cropped Area is the sum of Area sown more than once and Net Sown Area.
Gross Cropped Area (during a year) = Net Cropped Area + Area sown more than once during the same year
Net and Gross Irrigated Areas
Net Irrigated Area
Like Net Sown Area, Net Irrigated Area is the total land which is irrigated during a year, but the area is counted only once.
Gross Irrigated Area
Gross Irrigated Area is the sum of area irrigated more than once and Net Irrigated Area.
Gross Irrigated Area (during a year) = Net Irrigated Area + Area irrigated more than once during the same year.
Area to be irrigated
The area proposed to be irrigated in any crop season or in a given year, is called Area to be irrigated. It is obtained by multiplying CCA (Culturable Cultivated Area) by the seasonal or annual intensity of irrigation. Usually, areas to be irrigated are worked out separately for different crop seasons as the water requirements of crops of varies in different seasons.
Time Factor
Time factor is defined as the ratio of the actual operating period (in days) of the canal to the crop period.
For Example: Let for a canal system the time factor for Kharif and Rabi seasons are fixed at 0.89 and 0.72 respectively which means that the canal will operate for 0.89✖180=160 days, and 0.72✖180=129 days, respectively in each crop season of 180 days.
Capacity Factor
Capacity factor for a canal is the ratio between the Mean supply discharge of the canal for a certain period and the designed full capacity of a canal.
\(\begin{array}{c} {\rm{Capacity}}\,{\rm{Factor}}\,{\rm{ = }}\,\frac{\begin{array}{c} {\rm{Mean}}\,{\rm{Supply}}\,{\rm{discharge}}\,{\rm{of}}\,{\rm{canal}}\\ {\rm{for}}\,{\rm{certain}}\,{\rm{duration}}\, \end{array}}{\begin{array}{c} {\rm{Maximum}}\,{\rm{discharge}}\,{\rm{carrying}}\\ {\rm{capacity}}\,{\rm{of}}\,{\rm{canal}} \end{array}}\\ {\rm{Capacity}}\,{\rm{Factor = }}\frac{{{{\rm{Q}}_{{\rm{required}}}}}}{{{{\rm{Q}}_{{\rm{designed}}}}}} \end{array}\)
Usually, during Kharif (summer) season the canals have to run almost to their full designed capacities and hence the value of the capacity factor varies from 0.9 to 0.95 for Kharif seasons. However, during the Rabi season the water requirement of crops reduces to about (2/3)rd times the full supply and hence the value of the capacity factor varies from 0.60 to 0.70 during Rabi season. To improve the capacity factor during Rabi season the cropped area in Rabi season is increased.
[Note]
The Rabi (Winter) crops usually mature on about 52% of the supply required by crops in the Kharif (Summer) season, but because of lower water discharges in the canal during winters, the percentage of losses enroute becomes higher during winters and hence the actual water supply at the head of the canal is kept 66% of full supply.
Full Supply Coefficient
It is the ratio of estimated area to be irrigated during the base period to the designed full discharge capacity at the head of the canal.
Full supply coefficient is the designed duty at the head of the canal and hence full supply coefficient may also be defined as the number of hectares irrigable per cumec of canal capacity at its head.
\({\rm{FSC}}\,{\rm{ = }}\,\frac{{{\rm{Area}}\,{\rm{to}}\,{\rm{be}}\,{\rm{irrigated}}}}{\begin{array}{c} {\rm{Designed}}\,{\rm{discharge}}\,{\rm{capacity}}\,{\rm{at}}\\ {\rm{canal}}\,{\rm{head}} \end{array}}\)
It is also called Duty on Capacity
Nominal Duty
It is the ratio of Area irrigated to the Mean supply discharge from the outlet of the distributary over the crop period.
\({\rm{Nominal}}\,{\rm{Duty}}\,{\rm{ = }}\,\frac{{{\rm{Area}}\,{\rm{irrigated}}}}{\begin{array}{c} {\rm{Mean}}\,{\rm{Supply}}\,{\rm{discharge}}\,{\rm{over}}\,{\rm{the}}\\ {\rm{crop}}\,{\rm{period}} \end{array}}\)
For Example: Let "x" cumec of water is released daily from the outlet of distributary for 120 days (assumed) in a total crop period of 180 days (assumed), Then the Mean supply discharge over the crop period will be = \(\frac{{x \times {\rm{120}}}}{{{\rm{180}}}} = {\rm{ }}\left( {\frac{2}{3}x} \right)cumecs\)
If "A" area is irrigated with this discharge, then the nominal duty will be \(\frac{{\rm{A}}}{{\frac{2}{3}x}} = {\rm{ }}\left( {1.5\frac{{\rm{A}}}{x}} \right)Ha/cumecs\)
⇒Nominal Duty = 1.5(A/x)
