Irrigation scheduling is defined as the frequency with which water is to be applied based on needs of the crop and nature of the soil. Irrigation scheduling is nothing but number of irrigations and their frequency required to meet the crop water requirement. Irrigation scheduling may be defined as scientific management technique of allocating irrigation water based on the individual crop water requirement (ETc) under different soil and climatic condition, with an aim to achieve maximum crop production per unit of water applied over an unit area in unit time. Based on the above definition, the concept made is:
“If we provide irrigation facility, the agricultural production and productivity will go up automatically”.
Irrigation scheduling is a decision-making process repeated many times in each year involving when to irrigate and how much of water to apply? Both criteria influence the quantity and quality of the crop. It indicates how much of irrigation water to be used and how often, it has to be given.
Excess irrigation is harmful because:
• it wastes water below root zone
• it results in loss of fertilizers nutrients
• it causes water stagnation and salinity
• it causes poor aeration
• ultimately it damages the crops
However, irrigation scheduling has its own meaning and importance according to the nature of the work.
For irrigation engineers – Irrigation scheduling is important to cover more area with available quantity of water or to satisfy the whole command from head to tail reach in the canal or river system.
For soil scientists – It is important that the field should not be over irrigated or under irrigated as both will spoil the chemical and physical equilibrium of the soil.
For Agronomists – It is very much important to get higher yield per unit quantity of water in normal situations and to protect the crop to get as much as possible yield under drought situation by means of supplying water in optimum ratio and minimizing all field losses.
How much and how often water has to be given depends on the irrigation requirement of the crop. Irrigation requirement (IR) = Crop water requirement (CWR)-Effective rainfall (ERF)
i.e., IR = WR – ER in mm/day or mm/month.
If the crop water requirement of a particular crop is 6 mm per day, it means every day we have to give 6 mm of water to the crop. Practically it is not possible since it is time consuming and laborious. Hence, it is necessary to schedule the water supply by means of some time intervals and quantity. For example the water requirement of 6 mm/day can be scheduled as 24 mm for every 4 days or 30 mm for every 5 days or 36 mm for every 6 days depending upon the soil type and climatic conditions prevailing in that particular place. While doing so, we must be very cautious that the interval should not allow the crop to suffer for want of water.
Before scheduling irrigation in a farm or field or a command, the following criteria should be taken care for efficient scheduling.
• Sensitiveness to water shortage
• Critical stages of the crop
• Rooting depth
• Economic value of the crop
Water delivery system
• Canal irrigation or tank irrigation (It is a public distribution system where scheduling is arranged based on the decision made by public based on the resource availability).
• Well irrigation (Farmer’s decision is final).
Type of soil
• Sandy-needs short frequency of irrigation and less quantity of water
• Clay-needs long frequency of irrigation and more quantity of water
Salinity hazard – To maintain favourable salt balance, excess water application may be required rather than ET requirement of the crop to leach the excess salt through deep percolation.
Irrigation methods – Basin method allows more infiltration through more wetting surface which in turn needs more water and long interval in irrigation frequency. Furrow method allows less infiltration due to less wetting surface which needs less water and short interval in irrigation frequency. Sprinkler method needs less water and more frequency. Drip method needs less water and more frequency.
Irrigation interval – The extension of irrigation interval does not always save water. The interval has to be optimized based on the agro climatic situation.
Minimum spreadable depth – We cannot reduce the depth based on the water requirement of the crop alone. The depth should be fixed based on the soil type, rooting nature of the crop and irrigation method followed. The minimum depth should be so as to achieve uniformity of application and to get uniform distribution over the entire field.
I. Direct approach
• Depth interval and yield approach
• Soil moisture deficit and optimum moisture regime approach
• Sensitive crop approach
• Plant observation method
• Indicator plant technique
• Micro plot technique
II. Indirect or predictive approach
• Critical stage or phenological stage approach
• Meteorological or climatological approach
III. Mathematical approach
• Estimation method approach
• Simple calculation method
• Simulation approach-computing and modelling
• Empirical approach
IV. System as a whole approach
• Rotational water supply schedule
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