In this article we will discuss about the need for variation in water requirement of crops, with the help of a suitable diagram.
The total water need for various plants is known as delta. However, in planning the supply of irrigation water to a field crop, it is essential to estimate the water requirement of each plot of land growing a crop or crops at any point of time. This may be done by studying the dynamic interaction between a crop and the prevalent climate and the consequent water requirement. The demand would, naturally be also dependent on the type of crop and its stage of growth.
Plant roots extract water from the soil. Most of this water doesn’t remain in the plant, but escapes to the atmosphere as vapour through the plants leaves and stems, a process which is called transpiration and occurs mostly during daytime. The water on the soil surface as well as the water attaching to the leaves and stem of a plant during a rainfall also is lost to the atmosphere by evaporation. Hence, the water need of a crop consists of transpiration plus evaporation, together called evapotranspiration.
The effect of the major climatic factors on crop water needs may be summarized as follows:
i. Sunshine.
ii. Temperature.
iii. Humidity.
iv. Wind speed.
The variation of evapotranspiration upon these factors is illustrated in Figure 2.45.
Fig. 2.45: Dependence of Evapotranspiration upon Different Climatological Factors
Since the same crop grown in different climatic variations have different water needs, it has been accepted to evaluate the evapotranspiration rate for a standard or reference crop and find out that of all other crops in terms of this reference. Grass has been chosen as standard reference for this purpose.
The evapotranspiration rate of this standard grass is, therefore, called the reference crop evapotranspiration and is denoted as ETo, which is of course, the function of the climatic variables.
Table showing the daily variation of water needs of standard grass (in mm) under different climatic patterns (ETo).
In this article, we will discuss about how to find crop water need, if ETo is known.
Agricultural scientists have evaluated a factor called crop factor and denoted it by Kc, to evaluate specific crop water needs. Naturally, Kc would be different for different crops and would not be the same throughout the growth season of one type of crop.
Thus, the crop evapotranspiration, denoted by ETc is to be evaluated as under:
ETo = Kc *ETc
Both ETo and ETC should be in the same units and generally, mm/day is used as a standard all over the world.
In order to simplify the calculations, the factor Kc has been evaluated for 4 stages of a crop growth usually denoted as:
1. Initial stage.
2. Crop development stage.
3. Mid-season stage.
4. Late season stage.
The FAO Training Manual 3 gives the growth stage periods and the corresponding Kc values for some typical crops. In the table below, that for rice is presented.
It may be mentioned that any crop doesn’t have a fixed total growth period, which is the summation of growth stage periods given above. There is usually a range, depending upon the variety of the crop and the condition in which it is cultivated.
The values of Kc also depend, upon the climate and particularly on humidity and wind speed, as shown for rice in the above table. In general, the values of Kc should be reduced by 0.05 if the relative humidity is high (> 80%) and the wind %speed is low (< 2 m/s). Likewise, the values should be increased by 0.05 if the relative humidity is low (< 50%) and the wind speed is high (> 5 m/s).
For full details, the FAO training manual 3 may be consulted as Kc values for other crops are evaluated in different manners.
For some of the crops, the following table provides information:
