Total Water Requirement in Growing a Crop | Irrigation | Agriculture

In this article we will discuss about the total water requirement in growing a crop, with the help of a suitable formula.

The water that is required to irrigate a field or plot of land growing the particular crop not only has to satisfy the evapotranspiration needs for growing the crop, but would also include the following:

i. Losses in the form of deep percolation while conveying water from the inlet of the field upto its last or tail end as the water gets distributed within the field.

ii. Water requirement for special operations like land preparation, transplanting, leaching of salts, etc.

Further, the evapotranspiration requirement of crops (ET) really doesn’t include the water required by crops for building up plant tissues, which is rather negligible compared to the evaporation needs. Hence ET_c is often equivalently taken as the consumptive irrigation requirement (CIR).

The net irrigation requirement (NIR) is defined as the amount of irrigation water required to be delivered in the field to meet the consumptive requirement of crop as well as other needs such as leaching, pre-sowing and nursery water requirement (if any).

Thus,

NIR = CIR + LR + PSR + NWR

Where,  

LR = Leaching requirement

PSR = Pre-sowing requirement

NWR = Nursery water requirement

Field Irrigation Requirement (FIR) is defined as the amount of water required to meet the net irrigation requirements plus the amount of water lost as surface runoff and through deep percolation. Considering a factor ƞ_a called the water application efficiency or the field application efficiency which accounts for the loss of irrigation water during its application over the field, we have,

FIR = NIR/ƞ_a

Now, consider an irrigated area where there is a single source of ground water pump) is supplying water to a number of fields and water is applied to each field by rotation (Fig.2.47). Naturally, some water is lost through the respective turnouts. Hence, the source must supply a larger amount of water than that required at any point of time by adding up the flows to the fields turnouts that are open at that point of time.

Thus, the capacity of the water supply source may be termed as the gross irrigation requirement (GIR), defined as:

GIR = FIR/ƞ_c

In the above equation, ƞ_c is the water conveyance efficiency. Fig. 2.48 show the factors that decide the overall irrigation efficiency.