Irrigation and Crop Water Requirement - All Basic Components and Terminologies

2.1 Irrigation

“Irrigation may be defined as the process of artificially supplying water to soil for raising crops”. It is a science of planning and designing an efficient, low cost, economic irrigation system tailored to fit natural conditions. It is the engineering of controlling and harnessing the various natural sources of water, by the construction of dams and reservoirs, canals and headwork’s and finally distributing the water to the irrigation fields. (Jain)

“Science of Artificial Application of Water to the land, in accordance to the ‘crop requirements’, throughout the ‘crop period’ for full fledge nourishment of crop” (SK GARG, 1999)

“Irrigation is the art of applying water to the land by artificial means to fulfill the water requirement of crops in the areas where rainfall is insufficient.” (Iqbal Ali, 2010)

2.2 Crop water requirement (CWR):

It is the total amount of water required by the crop in a given period of time for normal growth, under field conditions.

It includes;

1.      evapotranspiration,    

2.      water used by crops for metabolic growth,

3.      water lost during conveyance and application of water and

4.      Water required for special operations such as land preparation, tillage and salt leaching etc.

It is expressed as the surface depth of water in mm, cm or inches.

2.3 Conveyance Losses:

1.      These losses take place from diversion structure (barrage) to the field (outlet).

2.      Major loss of water in an irrigation channel is due to absorption, seepage or percolation and evaporation.

3.      In earthen channels losses due to seepage are much more than the losses due to evaporation.

2.4 Evapotranspiration:

It is defined as the water transpired by crop plants and the water evaporated from the soil in the crop field and intercepted precipitation by areal parts of plants in any specified time period

Figure 2 Mechanism of Evapotranspiration

2.5 Consumptive use:

It is the amount of water required by a crop for its vegetated growth to evapotranspiration and building of plant tissues plus evaporation from soils and intercepted precipitation.

·         It is expressed in terms of depth of water.

2.5.1 Daily consumptive use:

·         The amount of water consumptively used during 24-hours.

·         It is usually estimated to record the peak period consumptive use rates to formulate the cropping pattern and

·         To decide the water supply from sources during different periods of cropping.

2.5.2 Peak period consumptive use:

·         It is the average daily consumptive use during a few days (6 to 10 days) of highest consumptive use in a season.

·         It occurs when the vegetation is in abundance, temperature is high and the crops are in flowering stage.

·         It is used in the planning of an irrigation system

2.5.3 Seasonal consumptive use:

·         It is the amount of water consumptively used by crops during the entire cropping season/period.

·         It is used to evaluate and decide the seasonal water supply to a command area of an irrigation project.

2.6 Actual crop evapotranspiration (ETc):

It is the rate of evapotranspiration by a particular crop in a given period under prevailing soil water and atmospheric conditions. It refers to the evapotranspiration from a disease free crop growing in a large field under optimal soil conditions with adequate water and fertility and giving full potential production under the given environment.

It is usually calculated by multiplying the Crop Coefficient (Kc) with ETo, thus:

ETc = Kc x ETo

2.6.1 Crop coefficient:

It is the ratio b/w the actual crop evapotranspiration to the reference crop evapotranspiration.

Kc = ETc / ETo

It is determined experimentally for various crops. ETc is determined by Lysimeter and ETo is determined with USWB class A evaporation pan. Kc is different for different crop and for different crop growth stages. It is mainly affected by crop type, soil type and climate of the area.

2.7 Water Availability

2.7.1 Crop Period:

It is the time normally in days that a crop takes from the instance of its sowing to harvesting.

2.7.2 Base period:

It is the time between first watering of crops at the time of its sowing and the last watering of crops before harvesting

2.7.3 Delta of crops:

Total depth of water required by the crop in unit area during base period. In other words, it is the total depth of water required for maturing the crop.

Volume of water required by the crop = Delta x Area

or

Delta (ft) = Volume (acre-ft) / Area (acres)

2.7.4 Duty of irrigation water:

It is defined as the no. of hectares (or acres) of land irrigated for full growth of a given crop by supply of 1 m3/sec (1 ft3/sec) of water continuously during the entire base period.

2.7.5 Full Supply Factor or Duty:

The term duty is only used for existing or running projects, but in a proposed project it is known as full supply factor.

2.7.6 Variation of Duty with respect to location:

Duty reduces as we move from field to head of canal.

2.7.7 Irrigation Intensity:

Percentage of culturable area irrigated during a year.

2.7.8 Cropping Intensity/cultivation intensity:

It is the %age of area cropped with respect to culturable command area (CCA) in a year. If whole CCA is cropped once in Rabi, and once in kharif, the Cropping intensity is 200%.

2.7.9 Cropping Pattern:

Cropping Pattern represents the %age area of land under a crop in a season in command of a canal.

2.7.10 Command Area: 

The area which lies on downstream side of project (canal) to which water can reach by gravity action.