Lec.25. Agricultural drainage - problems of drainage -

concept and factors affecting drainage.

Drainage is the artificial removal of water from the cropped fields within the tolerance limit of the crops grown in the area under consideration

Drainage problem may be caused by excess rain fall or by over irrigation of upper fields

The excess rain/irrigation water will normally drain in to the lower fields by the action of gravity. When the lower fields does not have proper drainage facility water logging condition occurs.

or

by the raising ground water table. Presence of ground water table in the root zone affects the growth of roots.

Drainage is also required for special requirements like leaching and reclamation of problem soils.

The saline soils are reclaimed by incorporation of gypsum in the field with standing water and draining the field after three days.

The tolerance limit of the crops varies with their physiological characters.

Paddy is likely to tolerate submergence for a week.

Many field crops have a tolerance of about one to three days only.

BENEFITS OF PROPER DRAINAGE:1. When drainage is proper, the length of the growing

season is more. This results in selection of better and high yielding long duration crop variety with higher net returns. The cultivation operations can be commenced timely. At times this will ensure more then one crop in a season.

2. The proper drainage prevents slippage of wheels of the machines used for land preparation. Properly drained and dried field is easier to plough. This results in saving of fuel, energy and the cost of land preparation.

3. When the soil moisture is optimum, the seeds germinate better. This ensures proper germination and coverage of crops.

4. The applied fertilizers will be useful to the crops, only if drainage problem does not occur. An increased supply of nitrogen can be obtained from the soil when drainage lowers the water table in the root zone. De-nitrification occurs in soils with poor drainage.

5. The roots also need oxygen for its growth and development. Under water logged condition, oxygen travels about 10000 times slower towards roots when compared to well aerated condition. Hence proper drainage helps in better root development and transport of applied fertilizers to the crop.

6. The inter cultural operations like weeding, application of fertilizers and pesticides will be easier only under well drained condition.

7. Heavy winds will uproot the plants and grassing animals also eat the plants with roots under wet soil condition. Better drainage ensures proper hold for the plant roots.

8. Weed problem is minimised and its control is easier with good drainage since shallow rooted weeds and undesirable grasses often thrive in wet soil, crowding about the planted crop.

9. Mechanical harvesting is difficult under slippery conditions. Also the harvester should have better ground support during operation. This is possible only under well drained conditions.

10. Reclamation of saline and alkaline soils is possible only with proper drainage facility.

11. Plant roots require certain temperature conditions for better microbial activity which results in better nutrient uptake. Seed germination is also influenced by the soil temperature.

12. Good drainage reduces diseases that thrive on wet land. These include foot rot and liver fluke that affect live stock, and diseases carried by mosquitoes to both live stock and human beings. The plant diseases such as root rot, stem rot, etc., are also reasonably controlled.

13. Soil erosion is controlled under subsurface drainage. The sediments are filtered out.

Lec.26. Drainage coefficient; principles of flow through soils - Darcy’s law - infiltration theory

DRAINAGE CO-EFFICIENT It is defined as the depth of water to be removed within 24 hours over the area under consideration.

This parameter helps in designing the rate of flow and the size of the drainage channel or pipe.

Water relations of soilWhen water is added to a dry soil either by

rain or irrigation • It is distributed around the soil particles • It is held by adhesive and cohesive

forces• It displaces air in the pore spaces and

eventually fills the pores.

When all the pores, large and small, are filled, the soil is said to be saturated and is at its maximum retention capacity.

INFILTRATIONThe movement of water from surface

into the soil is called infiltration.

Infiltration rate is the soil characteristic determining the maximum rate at which water can enter the soil under specific conditions, including the presence of excess water. It has the dimensions of velocity.

• The actual rate at which water is entering the soil at any given time is termed the infiltration rate.

• The infiltration rate decreases during irrigation.

• The rate of decrease is rapid initially and the infiltration rate tends to approach a constant rate that develops after some time has elapsed from the start of irrigation is called the basic infiltration rate.

• Accumulated infiltration, also called cumulative infiltration, is the total quantity of water that enters the soil in a given time.

FACTORS AFFECTING INFILTRATION RATE

• Initial moisture content• Condition of the soil surface• Hydraulic conductivity of the soil profile• Texture • Porosity • Degree of swelling of soil colloids • Organic matter • Vegetative cover• Duration of irrigation or rainfall • Viscosity of water

• The antecedent soil moisture content has considerable influence on the initial rate and total amount of infiltration, both decreasing as the soil moisture content rises.

• The infiltration rate of any soil is limited by any restraint to the flow of water into and through the soil profile.

• The soil layer with the lowest permeability, either at the surface or below it, usually determines the infiltration rate.

• Infiltration rates are also affected by the porosity of the soil, which is changed by cultivation or compaction.

• Cultivation affects the infiltration rate by increasing the porosity of the surface soil and breaking up the surface seals.

• The effect of tillage on infiltration usually lasts only until the soil settles back to its former condition of bulk density because of subsequent irrigations.

• Infiltration rates are generally lower in soils of heavy texture than the soils of light texture.

Darcy’s law states that the velocity of flow in a porous medium is proportional to hydraulic gradient.

i.e.: V α i

Where i = ∆h/∆L

Introducing a constant of proportionality, K,

V = Ki (or) Q = AV = KiA

Darcy’s law assumes

1.The soil is homogeneous over a large area

2.The flow is laminar through the soil

3.The soil temperature is same over space

Lec.27. Surface drainage systems - various methods

SURFACE DRAINAGE SYSTEMSurface drainage uses the potential energy

that exists due to land elevation to provide a hydraulic gradient for the movement of water.

The system can considered in three functional parts, viz.,

(1) Collection system,(2) Conveyance or disposal system, and (3) Outlet.

Lec.28. Design of open drains - problems