One of the following two methods is generally used for the estimation of irrigation requirement:
1. Method Based on ‘Duty of Water’:
In this method, the designer has to first decide the duty of water for different crops in the command area of the system. Duty of water (or simply duty) for a given crop is defined as the area irrigated by the unit discharge of water flowing continuously for the duration of the base period of the crop.
The base period of a crop is the time duration between the first watering at the time of sowing and the last watering prior to the harvesting of the crop. On the other hand, crop period of a crop is defined as the total time duration between the sowing and harvesting of the crop. This means that the base period of a crop is smaller than its crop period.
The duty is measured in hectares per cubic metre per second and depends on:
(i) The crop,
(ii) Type of soil,
(iii) Irrigation and cultivation methods,
(iv) Climatic factors, and
(v) Channel conditions.
As water flows in a canal, there is loss of water due to evaporation and percolation. Therefore, duty is different at different points of a canal system. The duty at the head-works of a canal system is less than the duty at an outlet or in the tail end region of the canal. Duty is usually calculated for the head discharge of a canal. Duty calculated on the basis of the outlet discharge is called outlet discharge factor or simply outlet factor which excludes all losses in the system.
The volume of water needed for the growth of a crop during its entire crop-growing period is known as the water requirement of the crop. This requirement varies at different stages of the growth of the plant. The water requirement of a crop expressed in volume divided by the cultivated area would result in depth of water which term is usually used as a measure of water requirement of a crop.
Any irrigation system should be capable of supplying peak requirement of the crops being grown in the command area. One of the methods to decide the water requirement of a crop is on the basis of kor watering.
When a plant is only a few centimeters high, it needs to be given its first watering which is 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 the kor period, their growth is retarded and the crop yield reduces considerably.
The kor watering depth and kor period vary depending upon the crop and the climatic factors of the region. For example, in UP, the kor watering 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 hilly 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/sec), then, by definition,
1 m3/s of water flowing for b (i.e., base period) 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 of land gives the water depth, ∆.
... ∆ = 86400/(D/b) x 104 = 8.64b/D (in metres) (3.12)
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 A are replaced by the kor period and the kor watering requirement, respectively.
Example 3.2:
The culturable command area for a distributary channel is 15,000 hectares. The intensity of irrigation is 35% for wheat and 20% for rice. The kor periods for wheat and rice are 4 and 3 weeks, respectively. The kor watering depths for wheat and rice are 135 and 190 mm, respectively. Estimate the distributary discharge.
Since the water demands for wheat and rice are at different times, these are not cumulative. Therefore, the distributary channel should be designed, for higher of the two values, i.e., 3.14 m3/s. The kor period for a given crop in a region depends on the duration during which there is likelihood of the rainfall being smaller than the corresponding water requirement.
Accordingly, the kor period is least in humid regions and more in dryer regions. The kor depth requirement must be met within the kor period. As such, the channel capacity designed on the basis of kor period would be large in humid regions and small in dry regions. Obviously, this method of determining the channel capacity is, therefore, not logical and rational.
A more rational method for determining the channel capacity would be to compare evapotranspiration (or consumptive use) and the corresponding effective rainfall for, say, 10-day or 15-day or 30-day periods of the entire year and determine the water requirement for each of these periods. The channel capacity can then be determined on the basis of peak water requirement of those periods.
2. Method Based on Consumptive Use:
During the crop period of a crop, there is continuous movement of water from soil into roots, up the stem and leaves, and out of leaves to the atmosphere. This movement of water is necessary for carrying plant food from the soil to the various parts of the plant.
Only a very small portion (less than 2%) of water absorbed by the roots is retained in the plant and the rest of the absorbed water, after performing its tasks, gets evaporated to the atmosphere mainly through the leaves and stem.
This process is called transpiration. In addition, some water gets evaporated to the atmosphere directly from the adjacent soil and water surfaces and from surfaces of the plant leaves. The water needs of a crop, thus, consist of transpiration and evaporation, and is called consumptive use or evapotranspiration.
Evapotranspiration or consumptive use can be defined as the amount of water needed to meet the water loss through evaporation and transpiration. Consumptive use can be expressed in terms of volume per unit area or simply the depth of water. Consumptive use helps determine the irrigation requirement (at a farm) which should, obviously, be the difference between the consumptive use and the effective precipitation.
Consumptive use (or evapotranspiration) depends on:
(i) Crop and its stage of growth;
(ii) Climatic conditions such as temperature, humidity and wind movement, and
(iii) Physical and chemical properties of soil.
Since pan evaporation (a measure of evaporation due to atmospheric temperature) and evapotranspiration depend on climatic factors, evapotranspiration Det can be correlated to the pan evaporation Ep as
Det = KEp (3.13)
in which K is the crop factor for the period under consideration. Pan evaporation data for different parts of India during different periods are published by the meteorological department. The crop factor K depends on the crop as well as its stage of growth (Table 3.1).
In the absence of pan evaporation data, the consumptive use u can be estimated using Blaney-Criddle formula expressed as
u = 25.4 kf (3.14)
in which u = consumptive use of crop in mm,
k = empirical crop consumptive use coefficient (Table 3.2),
f = consumptive use factor,
= p/100 (1.8t + 32) (3.15)
t = mean temperature in °C for the chosen period, and
p = percentage of daylight hours of the year occurring during the period (Table 3.3).
The consumptive use is generally determined on a monthly basis and the irrigation system is designed for the maximum monthly water needs.
Example 3.3:
Using the Blanrey-Criddle formula, estimate the yearly consumptive use of water for sugarcane for the data given in the first four columns of Table 3.4.
Solution:
Using Eqs. (3.14) and (3.15),
u = 25.4k p/100 (1.8t + 32)
Values of monthly consumptive use calculated from the above formula have been tabulated in the last column of Table 3.4. Thus, yearly consumptive use
= ∑µ = 1.75m.
The growth of all plants can be divided into three stages, viz., vegetation, flowering and fruiting. Different crops are harvested during different stages of crop growth. The consumptive use increases during the vegetative stage and attains the peak value around the flowering stage and, thereafter, the consumptive use decreases.
At each rainfall (or precipitation), some water is added to the crop field. However, the entire rainfall cannot be stored within the root zone of the soil. The part of the rainfall, which contributes to the soil moisture in the root zone soil, is effective precipitation (or rainfall) and is obtained by subtracting the sum of runoff, evaporation and deep percolation from total rainfall.
If for a given period, the consumptive use (Det) exceeds the effective rainfall (Dp – Dpl), the difference (i.e., the net irrigation requirement (NIR)) has to be supplied by the irrigation system. Thus, the field irrigation requirement (FIR) is given by
in which Det = depth of evapotranspiration (or consumptive use),
Dp = depth of total rainfall (precipitation),
Dpl = depth of precipitation that goes as surface runoff and/or infiltrates into the ground and/or that intercepted by the plants, and
Ea = irrigation efficiency or application efficiency
Example 3.4:
Using the data given in the first four columns of Table 3.5 for a given crop, determine the field irrigation requirement for each month assuming irrigation efficiency to be 60%. 
Field irrigation requirement for each month of the crop growing season has been calculated using Eqs. 3.13 and 3.16 and tabulated in the last column of Table 3.5.