A plot of land growing a certain crop or a combination of crops has to be supplied with water from time to time. Primarily, the plot or field is expected to receive water from rain falling on the land surface. But, as we know, the distribution of rain is rather uncertain both in time and space. Also some of the rain as in a light shower does not reach the ground as it may be intercepted by the leaves of the plant during a heavy downpour; much of the water might flow away as surface runoff. Hence, only a certain amount of falling rain may be effective in raising the soil moisture that is actually useful for plant growth. Hence, for proper crop growth, the effective rain has to be supplemented by artificially applying water to the field by irrigation.
If the area of the field is small, water may be supplied from the local ground water source. If the field is large, supplemented irrigation water may be obtained from a local surface water source, like a river, if one is available nearby. The work of a water resources engineer therefore would be to design a suitable source for irrigation after knowing the demand of water from field data. In this lesson, we proceed on to find out the methods by which estimation may be made for irrigation water demand.
Crop water requirement
It is essential to know the water requirement of a crop which is the total quantity of water required from its sowing time up to harvest. Naturally different crops may have different water requirements at different places of the same country, depending upon the climate, type of soil, method of cultivation, effective rain etc.
The total water required for crop growth is not uniformly distributed over its entire life span which is also called crop period. Actually, the watering stops same time before harvest and the time duration from the first irrigation during sowing up to the last before harvest is called base period. Though crop period is slightly more than the base period, they do not differ from practical purposes. Figure 1
indicates the relative usage of water for a typical crop during its entire growth period.
FIGURE 1. Variation in the requirement of water for paddy with stage of growth (Image courtesy: Food and Agriculture Organisation, FAO)
Sometimes, in the initial stages before the crop is sown, the land is very dry. In such cases, the soil is moistened with water as t helps in sowing the crops. This is known as paleo irrigation. A term kor watering is used to describe the watering given to a crop when the plants are still young. It is usually the maximum single watering required, and other waterings are done at usual intervals.
The total depth of water required to raise a crop over a unit area of land is usually called delta. Some typical values of delta for common crops in some regions of India are as follows:
Duty of water
The term duty means the area of land that can be irrigated with unit volume of irrigation water. Quantitatively, duty is defined as the area of land expressed in hectares that can be irrigated with unit discharge, that is, 1 cumec flowing throughout the base period, expressed in days.
Imagine a field growing a single crop having a base period B days and a Delta ∆ mm which is being supplied by a source located at the head (uppermost point) of the field, as shown in Figures 2 and 3.
FIGURE 2. Border irrigation method of applying water at the head of a field (Image courtesy: Food and Agriculture Organisation, FAO)
FIGURE 3. Furow irrigation method of applying water to a field (Image courtesy: Food and Agriculture Organisation, FAO)
The water being supplied may be through the diversion of river water through a canal, or it could be using ground water by pumping (Figure 4).
If the water supplied is just enough to raise the crop within D hectares of the field, then a relationship may be found out amongst all the variables as:
Volume of water supplied = B*60*60*24 m3
Area of crop irrigated = D*104 m2
Volume of water supplied per unit area =86400/10000D = 8.64B/D
Hence, knowing two of the three variables B, D and ∆ the third party may be found out.
The duty of irrigation water depends upon a number of factors; some of the important ones are as follows:
Crop growing seasons in India
Each crop has its own sowing and harvesting seasons and it is important to have a knowledge of this which may help to decide the total water demand in a field having mixed crops.
In India, the northern and north eastern regions have two distinct cropping seasons. The first coinciding mostly with the South western monsoon is called kharif , which spans mostly from July to October. The other, called rabi, spans generally over October to March. The summer season crops are planted sometime between April and June. In southern part of India, there is no such distinct season, but each region has its own classification of seasons.
Generally, the kharif is characterized by a gradual fall in temperature, more numerous cloudy days, low intensity, high relative humidity and cyclonic weather. During Rabi, there is a gradual rise in temperature, bright sunshine, near absence of cloud days, and a lower relative humidity.
The following table indicates some the regional cropping calendars in India.
Serva or Abi
July - December
Dalwa or Tabi
December - April
In limited areas
March/April - June
Nov - May
March - July/Aug
June - Nov/Dec
Dec - June
Jammu & Kashmir
Jammu: June-Nov Kashmir: Last week of April -October
June - Dec
Feb - May
Beali (short Duration)
April/May -Sept (Only in uplands)
May - Nov
April - July
Kar or Kuruvai
June - August
Thaladi or Pishanam
June - Oct
End Nov-Mid June