Factors affecting frequency of irrigation
The frequency of irrigation and the quantity of water needed depend upon a number of factors, which are detailed below:
(1) Depth and distribution of rooting:
Deep-rooted crops generally penetrate deep into the soil. They are thus able to draw moisture from the lower layers of the soil as compared to shallow rooted crops.
So deep-rooted crops need heavy irrigations so that the water may reach the required depth. On the other hand, the interval between the two irrigations may be increased.
In contrast to this, shallow-rooted crops possess fibrous roots which spread in all directions but do not go deep into the soil. These crops, there. fore, require light irrigations at shorter intervals.
(2) Size of the top growth:
Generally speaking, the greater the size of the top growth, the larger would be the amount of irriga tion water required to cope with the increased amount of water lost by the plants by transpiration through the leaves and other soft parts.
However, a larger size of top growth means greater coverage of the soil and, to that extent. less scope for the evaporation of water from the soil surface.
Thus, in addition to other factors, it is by the interaction of such counteracting influences that the amount and frequency of irrigation water to be applied to a crop are determined.
(3) Thickness of stem:
A thin stem implies, on the one hand, less transpiration loss and, on the other, greater loss through evapo ration.
In practice, if the loss through evaporation can be somewhat controlled by means of a mulch, natural or artificial, a thin crop would need less water as the plant surface from which water is lost through transpiration in this case would be less than in the case of a thick crop.
(4) Weather conditions:
In determining the quantum of moisture losses through the process of evapo-transpiration. weather conditions exercise a great influence.
For instance, other factors remaining the same, high atmospheric temperature and windy weather result in increased loss of moisture.
One advantage of having a suitable windy break around the sensitive crops is for keeping down the moisture losses through high winds.
(5) Texture of soil:
Frequent irrigations are needed on lighter soils because their water-holding capacity is low.
A cm of water applied to the surface of the soil will replace the available moisture content to a depth of 30 to 40 cm in sand, 20 to 25 cm in loam, but only 10 cm to 20 cm in clay. Soils having 5 cm low permeability frequent light irrigations result in higher yield than less frequent heavy irrigation if the total quantity of water remains the same.
(6) Organic matter in the soil:
The physical and chemical properties of a soil are affected to a very large extent by the amount and state of decay of the organic matter present in it. It particularly influences the water-holding capacity and permeability of the soil.
Both these properties, in turn, determine the quantum and frequency of irrigation to be applied to a crop. A soil rich in organic matter retains more moisture and for a longer period than the one with a poor organic content.
Obviously, the former soil will require less water to mature a crop than the latter. That explains why organic manures are so much sought after by vegetable growers.
(7) Soil depth irrigated:
Normally wheat requires an average of 37.5 cm of water depth. The same crop can be grown even with 20 to 25 cm of irrigated water, provided the soil is irrigated to a depth of 4 cm from the surface, filling each time to its storage capacity.
(8) Type of crop grown:
Rice and sugarcane are very high water consuming crops. Cotton, wheat, jowar, groundnut and other oilseeds require light irrigation.
(9) Rainfal pattern:
If the rainfall occurs when water is needed by the crop, then the irrigation requirement is less. How ever, this is seldom the case.
(10) Method of irrigation:
Flooding methods require more irrigation water than sprinkling or dripping methods. Moreover injudicious application of water will entail considerable losses by percolation. There is a danger of leaching of plant nutrients also.
(11) Quantity and type of fertiliser used: Nitrogen fertilisers require more and frequent watering than other types.