Irrigation Techniques

Definition of Irrigation Irrigation is the science of artificial application of water to the land, in accordance with the crop requirements throughout the crop period for fullfledged nourishment of the crops

Necessity of Irrigation in Bangladesh

Advantages of Irrigation • • • • • • • • •

Increase in Food Production Optimum Benefits Elimination of Mixed Cropping General Prosperity Generation of Hydro-electric Power Domestic Water Supply Facilities of Communications Inland Navigation Afforestation

Disadvantages of Irrigation • • • • •

Colder and Dimmer Climate Water Logging Expensive Outbreak of diseases Environmental degradation

Types of Irrigation • Surface Irrigation • Sub-surface Irrigation

Surface Irrigation • Flow Irrigation- Water supplied from higher level to lower level with gravity force. • Lift Irrigation- Water lifted by mechanical means

Flow Irrigation • Perennial Irrigation- Constant and continuous water supply, water directed by canal by construction of weir or barrage across the river. • Flood Irrigation- Soil is kept submerged and thoroughly flooded with water.

Sub-surface Irrigation • Natural Sub-surface Irrigation – Due to leakage water from canals

• Artificial Sub-surface Irrigation- Open joined drains artificially laid below the soil

Techniques of Water Distribution in the fields • • • • • • •

Free Flooding Border Flooding Check Flooding Basin Flooding Furrow Irrigation Method Sprinkler Irrigation Method Drip Irrigation Method

Free Flooding In this method ditches are excavated in the field. Water from this ditches flow across the field. • Initial land preparation cost is low • Efficiency is low

Border Flooding • Land is divided into number of strips • Separated by low levels called Border • Each Strip is 10 to 20 m width and 100 to 400 m length

Time required to cover the given area

If we consider small are dA of the border strip area A and assume time dt to advance water over this area; if depth of water is y then the volume would be y.dA . Volume of water that percolates into soil at the same time over the same area would be f.A.dt Total water supplied to the strip during dt would be Q.dt which is equal to y.dA + f.A.dt

Check Flooding Similar to free flooding except water is controlled by surrounding check area with low and flat levels

Basin Flooding

Furrow Irrigation Method • Only one-fifth to one-half of the land surface is wetted by water • Less evaporation loss less puddling of soil

Sprinkler Irrigation Method •Water is applied to the soil in the form of spray though network pipes • It is like artificial rain so gives very good results • Most suitable for those areas where surface and sub-surface irrigation is difficult

Conditions Favoring to Sprinkler Irrigation

Advantages of Sprinkler Irrigation

Limitations of Sprinkler Irrigation

Drip Irrigation Method • It is also called Trickle Irrigation • Adopted where scarcity of water or salt problem • Water is directly applied to the root zone of the plants

Quality of Irrigation Water Impurities in Irrigation Water: • Sediment concentration • Soluble solids • Proportion of sodium ions to other cations • Bicarbonates as calcium and magnesium • Bacterial concentration

Salt Concentration • Salt concentration measured by electrical conductivity of water • Value 250 µ -mohs/cm – low conductivity (C1) • Value 250-750 µ -mohs/cm – medium conductivity (C2) • Value 750-2250 µ -mohs/cm – high conductivity (C3) • Value above 2250- µ -mohs/cm – very high conductivity (C4)

Proportion of Sodium Ions to Other Cations • Most of the soil contains Ca++ and Mg ++ with small quantities of Na+ • % of Na + is generally less than 5% of total exchangeable ions • If % increases 10% or above aggregation of soil-grain breaks down and soil become less permeable • High sodium soils are plastic, sticky when wet • The proportion of sodium ions present in the soil is generally measured by a factor called Sodium Absorption Ration, SAR

When SAR is between 0-10 – Called low sodium water (S1) When SAR is between 10-18 – Called medium sodium water (S2) When SAR is between 18-26 – Called high sodium water (S3) When SAR is above 26 – Called very high sodium water (S4)

Medium sodium water (S2) may create following problems: * Soil becomes less permeable * It starts crusting when dry * It becomes plastic and sticky when wet * Its pH increases towards alkaline soil Addition of gypsum(CaSO4) either to soil or water is suggested to overcome the sodium hazards.