Irrigation - Foreword

• This document is intended for presenting basic facts about irrigation practices on different crops.
• Basic irrigation parameters are treated.
• Surface irrigation (furrows) and submersion irrigation are not treated.
• It is widely recognized that good water management at the farm level is of crucial importance to crop production in irrigated agriculture, to increase the effectiveness of fertilizers, plant protection measures and related inputs.

Evapotranspiration (ET)

ET is the measurement of partial water losses in the soil. Water is lost basically through evaporation from soil and transpiration through vegetal tissues (leaves, roots, stalks and the such). The sum of these losses is ET. Thus, ET is also a measurement of quantity of water needed by crops, and it depends basically on soil texture, crop and climatic conditions. ET is measured in millimeters (mm) or cube meters per hectare (mc/ha).

Crop coefficients (Kc)

Crop coefficient for estimating evapotranspiration in different crops at different stages

Four stages of crop development for field crops and vegetables are described as reference for the entirely growing season as below presented:

1. initial stage
germination and early growth when the soil is not or is hardly covered by crop (groundcover < 10%)
2. crop development stage
from end of initial stage to attainment of effective full groundcover (groundcover = 70-80%)
3. mid-season stage
time of start of maturing as indicated by discolour of leaves (beans) or leaves falling (cotton). For some crops this may extend to very near harvest (e.g. sugar beet) unless irrigation is not applied at late season and reduction in ETcrop is induced to increase yield and/or quality (sugarcane, cotton, some grains); normally well past the flowering stage of annual crops.
4. late-season stage
from end of mid-season stage until full maturity or harvest.

Field capacity

An estimation of useful water in the soil

• When in a saturated soil, gravity water has been removed, the remaining durable moisture content is called field capacity. Field capacity can be measured by determining moisture content of soil after an irrigation, which is sufficiently heavy to insure through wetting of the soil to be tested.
• In practice, field capacity is usually determined two days after an irrigation. Therefore, field capacity defines a specific point on the moisture-content time curve. Precise determinations of field capacity are generally not necessary for field applications. However, soils must be well drained before reliable field determinations can be made in this manner.
E.g., for most agricultural, clay-loam or clay soils, a soil moisture tension of 1/3 atmospheres (pF=2.54) corresponds closely to the generally accepted values of field capacity determined by moisture content. Indicative average values for fine textured soils varies between 23 and 28 per cent by dry weight or about 3,250 to 4,000m3/ha, for one meter soil depth.
Knowledge of soil-water relationship is valuable to all who have the opportunity to improve irrigation practices, including farmers who desire to obtain best use of water available for their farms. The following chapter is devoted to a consideration of the relationship of soil and water, with special reference to their influence on irrigation and drainage practices.

Irrigation planning

When water is needed

Criteria concerning irrigation planning varies depending on situations:
• Where water is scarce, irrigation must be programmed for maximizing the output (yield) from each cube meter of used water
• Where fertile soil is more scarce than water, irrigation must be programmed for maximizing the out put from each hectare.
• When to irrigate: in order to avoid any possible crop damage due to drought, the generally accepted irrigation practices recommend irrigating the crop before the soil reaches the wilting point, i.e., before complete depletion of the total available soil moisture.

Sprinkler irrigation

• Sprinkler irrigation, also called overhead irrigation, is the application and distribution of water over the field in the form of a spray, or a jet which breaks into drops or droplets, created by expelling water under pressure from an orifice (or nozzle). In contrast to surface irrigation, sprinkler systems are designed to deliver water to the field without depending on the soil surface for water conveyance or distribution.
• Sprinkler intensity rate is important in order to prevent pond and surface runoff. Sprinklers are designed and arranged in a pattern (square, rectangular or triangular) to apply water at an intensity (mm/hour) that does not exceed the soil infiltration rate.

Trickle irrigation (Drip irrigation)

With trickle irrigation water is delivered to the plants, drop by drop, via a set of plastic lateral tubes laid on the ground or buried at a depth of 15-30cm, which are supplied from a head unit through a field main pipeline. The laterals are commonly 15-25mm in diameter, and are either provided with emitters designed to drip water onto the soil. By drip irrigation daily to twiceweekly, soil water in the root zone is kept about field capacity (soil moisture tension of 0.3 to 0.5bar). Under these conditions, the emitter rate approaches closely the actual crop evapotranspiration (ETcrop). The rate of emitters is generally in the range of 1 to 10 liter/hour, (industrial crop about 1 to 3, orchards 3 to 10) and it do not exceed the basic infiltration rate. The operation pressure is usually in the of 1 to 3 atmospheres. Under trickle irrigation, the wetted portion of the soil is reduced, i.e., the active rooting volume is usually confined to a fraction (often less than 50 percent) of what would be the normal root zone of an uniformly wetted soil. Consequently the water need for irrigation is reduced by 50 percent or more compared to the above described irrigation methods. With trickle irrigation, it is possible to use salty water (concentration about 1,000mg/liter salt).

Under optimal management conditions, yield increases from 20 to 50% are realistic, as well as high product value of 40 per cent or more per unit volume of water.

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