Proper Irrigation and Drainage Guide
Irrigation is not only important for the arid regions but is becoming equally important in the humid regions. Although irrigation is very important and is becoming a basic part of well-developed agriculture throughout the world, it also has its negative sides.
The pressure for survival and the need for additional food supplies to meet the demands of increasing populations are necessitating a rapid expansion of irrigation throughout the world.
Irrigation can convert agriculturally productive land into waterlogged land and create problem of salinity. The successful cultivation of crops largely depends upon adequate drainage of the land in which they are grown.
This unit treats all aspects of irrigation and drainage i.e. methods of irrigation and drainage, the advantages and disadvantages of each method are discussed. The suitability of each method in terms of crops to be grown and topography of the land is also treated.
What is irrigation? Irrigation is generally defined as the artificial application of water to the soil for the purpose of supplying the moisture essential for plant growth. Irrigation water is supplied to supplement the water available from rainfall.
Important Terms and Definitions
1. Water requirement
The water requirement (WR) of a crop may be defined as the quantity of water, regardless of its source, required by a crop in a given period for its normal growth and development under field conditions at a specific place.
Water requirement includes the losses due to evapo-transpiration (ET) or consumptive use (CU) plus the losses during the application of irrigation water and the amount required for special operations such as land preparation, transplanting, leaching, etc. it may thus be formulated as follows: WR = ET or CU + application losses + special needs.
2. Irrigation requirement
The field irrigation requirement (IR) of a crop, therefore, refers to the water requirement of the crop, excluding effective rainfall and the contribution from soil profile, and may be formulated as: IR =WR – (ER+S).
A farm irrigation requirement depends on the irrigation needs of the individual crops, their area and losses in the farm water distribution systems, mainly by seepage.
3. Net irrigation requirement
This is the amount of irrigation water required to bring the soil moisture level in the effective root zone to field capacity. Thus it is the difference between field capacity and the soil moisture content in the root zone before starting irrigation.
4. Gross irrigation requirement
This is the total amount of water applied through irrigation. In other words, it is the net irrigation requirement plus losses in water application and other losses.
Gross irrigation requirement can be determined for a field, a farm, an outlet command area, or an irrigation project, depending on the need, by considering the appropriate losses at various stages of the crop growth.
Gross irrigation requirement in field = net irrigation requirement divided by irrigation efficiency. For example, if the net amount of irrigation is 10 cm and the irrigation efficiency is 70%, the gross amount of water to be applied to the field is 10 cm divided by 0.70 = 14.29 cm.
5. Irrigation efficiency
Irrigation efficiency is a measure of the amount of water delivered by irrigation that actually ends up as available water to the plant.
To illustrate the point, let us assume a rooting depth of 1.5 m. ideally, the soil should be wetted evenly down to a depth of at least 4 m. In practice this is rarely attained. Irrigation efficiency is a measure of how close the water delivery system comes to achieving this ideal situation.
Read Also : Irrigation and Drainage: All You Need to Know About
It gives a measure of the amount of water effectively delivered to a farm and varies from soil to soil. The ratio between water requirement and irrigation requirement is a measure of irrigation efficiency. It indicates how efficiently the available water supply is being used.
Factors Influencing Irrigation Efficiency
The principal factors influencing irrigation efficiency are the design of the irrigation system
The degree of land preparation
And the skills and care of the irrigator.
Water is lost from the distribution system in several ways, by evaporation, unwanted wetting of banks of delivery ditches, deep percolation, seepage and run-off. In sprinkler system, for example, the tendency for high evaporation reduces irrigation efficiency. Any factor which causes loss of water will also reduce irrigation efficiency.
Ways of Minimizing the Loss of Irrigation Water
– These losses can be minimized by the following;
– Adequate planning of the irrigation system
– Proper design of the irrigation method
– Proper land preparation
– Efficient operation of the system.
Irrigation Frequency
This refers to the number of days between any two subsequent irrigations during periods without rainfall. It depends on the consumptive use rate of a crop and the amount of available moisture in the crop root zone. It is a function of crop, soil and climate.
Sandy soils are irrigated more often than fine-textured soils. Moisture use rate increases as the crop grows and the days become longer and hotter. In general, irrigation should start when about 50% of the available moisture has been used from the zone in which most of the roots are concentrated.
A record of the growth stages of the crop with reference to the critical periods of growth is also kept with a view to determining the frequency of irrigation.
Irrigation Scheduling
The number and timing of irrigation vary widely for different crops. Earlier concept for scheduling were based on the soil water regime in which the water content at field capacity (the upper limit of the regime) was considered as 100% available for crop growth, and that at the permanent wilting point as 0% available.
About 50% available water was accepted as the lower limit of the regime and it was taken as a criterion for scheduling irrigation.
Later on it was realized that climatic parameters play a predominant role in governing the water needs of crops. This leads to the concept of evapo-transpiration, which has been used as the criterion for timing irrigations.
The latest approach for scheduling irrigation is the plant water status itself. This may be considered as an ideal criterion as the plant is a good integrator of soil, water and climatic factors.
Read Also : The Products Derived From Food Waste
