Saturday, June 1, 2019

Ultimate guide and definition of irrigation system

Irrigation is the artificial application of water to soil in order to bring soil moisture content to field
capacity level and to supplement water available from rainfall for the purpose of crop production. Irrigation may involve conservation of water during the rainy season by construction of dam (barrier)
across a stream for water storage or using the existing river as a source of water and pumping of water to
field through pipe, hose or canal for distribution of water to crop on the farm
Objectives or purpose of irrigation
1. To add water to soil that is essential for plant growth
2. To cool the soil and atmosphere in order to provide a favourable condition for plant growth
3. To wash out or dilute harmful salt in the soil by leaching. 4. To soften the tillage pan (hardpan)
5. To reduce the hazard of frost in temperate region like USA and Canada. 6. To provide crop insurance against short dry draught. Hard pan: is a hard soil layer formed in the profile (below the soil surface) due to compaction of the soil
when machine (tractor) is frequently used in the soil. The hardpan reduces the permeability of the soil to
water thereby causing water-logging of the area and hinder the root penetration of the crop through the
soil. Frost: Freezing of soil water in the temperate region and accumulation of frozen vapour on plant can
injure the plant. Problems of irrigation
Irrigation is beneficial to mankind because it allows crop production all year round. On the other hand, irrigation have some disadvantage particularly in the area where irrigation project is sited. It has some
environmental impact like; Soil degredation, flooding and causing certain diseases to where the project is
sited if not properly planned. Developed countries like USA, England, Germany, Holland, Australia
normally pay more attention to soil degredation and they normally carry out environmental impact
assessment (EIA) of an irrigation project. Soil salinity: excessive irrigation over a long period of time can increase the accumulation of salt in the
soil profile and this render the soil unproductive for crop production
Destruction of soil structure and removal of top soil that is rich in plant nutrients by run off during surface irrigation.

Flooding: it can occur during a heavy rainfall causing overflow of water from irrigation dam or failure of
dam can destroy lives and properties. Water in the dam, canal channel are breeding areas for mosquito tsetsefly, water snail and blackfly that
can cause certain diseases to man
Forms of soil moisture or soil water
1. Gravitational water: is the water that moves freely into through and out of the soil under the
influence of gravity. It is the excess water above the field capacity. It is in excess of hygroscopic
water and capillary water. 2. Capillary water: is the principal source of water for plant growth. It is in excess of hygroscopic
water which is held by molecular attraction. It dissolves the mineral nutrients in the soil for plant
growth at a reasonable pressure. 3. Hygroscopic water: this is a thin film of water held tightly to the soil particles and it is not
normally available for plant growth. This water cannot be removed by plant. The suction pressure
is greater than 31 bars. 1 atm of water= 1036cm or 76.39cm of mercury. Soil water constant
 Saturation capacity: this occurs when all the micro and macropores are filled with water. The soil
is said to be under saturation capacity or maximum water holding capacity. The tension of water
at saturation capacity is almost zero and it is equal to free water surface. Field capacity and
permanent wilting point were formerly considered soil water constant but recently found
imprecise soil water constant but they are quantitatively useful terms.  Field capacity: it is the moisture content after the drainage of gravitational water has becomes
very small and the moisture content is relatively stable which can store water temporarily for
plant use. It occurs 2-3 days after the soil has been properly wetted by rain or irrigation. If water
content is determined at this period, it is said to be at it’s field capacity. Field capacity depends on
soil texture, organic matter in the soil, layers of impediment in the soil profile, evapotranspiration
and depth of wetting in the soil profile. For instance, a coarse than fine textured soil. In fine
textured soil, the rate of internal drainage is low and it continues for many days after irrigation. Therefore, soil texture influences the field capacity. Soil layer that can impede or restrict flow of
water in the soil profile will increase the field capacity. Organic matter when added to the soil increases the water holding capacity of soil and this will modify the field capacity. N.B: Percentage of organic matter in the soil vary from place to place and from time to time.
 When evapotranspiration is high, more water is removed from the soil after irrigation which will reduce the field capacity.

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