Crops under irrigation need water, but when water is applied incorrectly, it could lead to nutrient leaching, soil erosion, or accumulation of salt in the soil.
The knowledge of water requirements, crop irrigation requirements, and consumptive use is not enough to determine the amount of water to apply. It is necessary to know when to apply water to crops, and this process is known as irrigation scheduling.
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Definition of Irrigation Scheduling
Irrigation scheduling is the process of determining when to irrigate and how much water to apply per irrigation. Proper scheduling is essential for the efficient use of water, energy, and other production inputs such as fertilizers.
It allows irrigation to be integrated with other farming activities, including cultivation and chemical application. Among the benefits of proper irrigation scheduling are:
1. Improved crop yield and quality
2. Water and energy conservation
3. Lower production costs
Methods Used in Determining Irrigation Scheduling
Different methods are used to determine when to irrigate. They may be grouped into the following categories:
1. Plant Indicators for Irrigation Scheduling
Monitoring crops or plants is the most direct method of determining when to irrigate.
Since the primary objective of irrigation is to supply plants with the water they need when it is required, it is necessary to relate plant parameters to soil water content to determine the amount of irrigation.
i. Appearance and Growth
Visual indicators of the need for water include leaf wilting, shoot wilting, and leaf color changes. Measurement of stem diameter and height is regularly monitored to determine growth rate.
The need to irrigate is indicated by a low growth rate. Appearance and growth are not always effective parameters for scheduling full irrigation, as plants may have been short of water for too long to adversely affect production when changes in appearance or growth become noticeable.
Crop appearance must be carefully interpreted, as diseases and improper nutrient levels may produce changes similar to those associated with water stress. The primary advantage of appearance and growth as indicators of when to irrigate is simplicity.
ii. Leaf Temperature
Rises in leaf temperature, in addition to those related to increases in air temperature, are associated with reduced transpiration rates, resulting in partial or total stomatal closure. Air and leaf temperature may be remotely sensed from the ground, aircraft, or possibly satellites.
One popular method is using a handheld infrared thermometer to measure the difference between plant canopy or leaf and ambient air temperature each day (usually 1-1½ hours after noon).
The number of degrees by which canopy temperatures exceed air temperature is accumulated until a certain critical level is reached. When this level is reached, it is time to irrigate. Days when canopy temperature is less than the air temperature are neglected.
iii. Leaf Water Potential
The measurement of leaf water potential is another indicator of the plant’s need for water. Lower potentials indicate a greater need for water. Leaf water potential measurement is a destructive procedure that involves removing a leaf and placing it in a pressurized chamber.
The pressure in the chamber is slowly increased until fluid is forced from the leaf stem. The pressure required is a measure of the leaf moisture potential.
Care must be taken in carrying out this measurement, as leaf age, leaf exposure to solar radiation, and the time of day can significantly affect the result.
Usually, mature leaves are selected from a specific standardized location on the plant, and measurements are made at a particular time of the day.
Although commercial instruments for measuring leaf water potential by irrigators are not widespread, considerable time, care, and training are required to obtain reliable results.
iv. Stomatal Resistance
Stomatal resistance is an index of the need for water, as it is related to the degree of stomatal opening and the rate of transpiration. In general, high resistances indicate significant stomatal closure, reduced transpiration rates, and the need for water.
Commercially available leaf or diffusion promoters are used to measure stomatal resistance. However, the large amount of time and skill required to measure and interpret stomatal resistance limits their use to research purposes.
2. Soil Indicators for Irrigation Scheduling
Soil-based irrigation scheduling involves determining the current water content of the soil, comparing it with a predetermined minimum water content, and irrigating accordingly.
i. Appearance and Feel Method
With experience, one can judge soil water content by the appearance and feel of the soil. A soil probe (or auger) is needed to obtain soil samples from the lower portion of the root zone for examination. Tables present guides for judging how much available water has been removed from the soil.
ii. Gravimetric Sampling
This is a direct method of measuring the water content of soil samples taken from a field. Samples are weighed, dried at 105-110°C, and re-weighed after drying. Usually, 24 hours is required to dry samples to a constant weight. The equation below is used to compute the percentage water content on a dry weight basis:
%MC = (Mwet – Mdry) x 100 / Mdry
On a volume basis, it is computed using the following equation:
%Mc by volume = (Volume of water in soil x 100) / Total volume of soil and voids
Example:
Given the following data, calculate the soil water content:
Weight of wet soil 100 cm³ = 131g
Weight of dry soil 100 cm³ = 121g
Solution:
%Mc = (Mwet – Mdry) x 100 / Mdry
= (131 – 121) x 100 / 121
= 10 x 100 / 121
= 8.26%
Although the gravimetric method is simple and reliable, the sample is destructive and techniques are time-consuming. In addition, data are at least one day old when they become available for scheduling.
iii. Tensiometer
A tensiometer is a ceramic cup filled with water and connected through a wetted-filled tube to either a vacuum gauge or a mercury manometer. Water moves in and out of the cup in response to changes in soil water content.
Tensiometers provide measurements of soil water potentials that are related to soil. Tensiometers may interfere with cultivation and require considerable time for installation and maintenance. In addition, they have a relatively limited range of operation (0-80 centibars).
3. Water Budgeting Techniques for Irrigation Scheduling
Water budgeting uses pan evaporation data of reference crop evapotranspiration to compute when to irrigate. It is similar to the soil indicator method. Instead of measuring soil water content, it is computed using the following equation:
Oi = O(i-1) + 100 (ET – Pe) / Drz
Where:
Oi = soil water in percentage by volume at the end of the i and i-1, respectively
Pe = Effective precipitation
ET = Evapotranspiration
Drz = Depth of root zone
Other methods used in determining irrigation scheduling include farmers’ experience, which farmers usually rely on, neutron probes, porous blocks, and feeling the soil with hands.
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Irrigation Scheduling Strategies
Irrigation scheduling strategies are designed to either fully or partially provide the irrigation requirement. They include:
i. Full Irrigation
This involves providing the entire irrigation requirement and results in maximum production as shown in the production function in the figure below.
Exceeding full irrigation reduces crop yield by reducing soil creation and restricting gas exchange between the soil and atmosphere.
Full irrigation is economically justified when water is readily available and irrigation costs are low. However, it does not represent efficient water use.
ii. Deficit Irrigation
This strategy involves partially supporting the irrigation requirement, which is a practice of deficit irrigation.
Irrigation scheduling enables knowing when to irrigate and the quantity of water to apply to growing crops at different stages. This makes water use more efficient, reduces irrigation costs, and increases crop yields for farmers.
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