1. Percolation-evaporation balance
Artificial drainage
Percolation-evaporation balance

2. Liquid losses of water Losses are two fold: Runoff
carries dissolved salts and sediment
Causes erosion
percolation and drainage water
Occurs when rainfall exceeds soil WHC
Depends on
rainfall amount and distribution
Soil characteristics
Evaporation losses
Vegetation type
Carries chemicals, may lead to pollution
Recharges ground water

3. Percolation evaporation balance
Actual ET is constrained by available water supply
In tropical dry season:
Large vapour pressure gradients ETo is high
Little rainfall and no percolation out of profile
Evapotranspiration exceeds percolation,
→ soil moisture is depleted
hazard of salinization
Irrigation should incorporate excess water to remove salts

4. Ground water diagramatically

5. Ground water As drainage water moves down through soil
it reaches the saturated zone- all pores are water-filled
Saturated zone usually overlies an impervious layer (rock or clay)
Ground water- water within the saturated zone
Water table:- the upper surface of the zone of saturation
 Humid regions water table- is : 1-10m below soil surface
→ Arid region: up to m

6. …..Ground water continued
Vadose- unsaturated zone above water table
Shallow ground receives percolating drainage waterwater
ground water may seep laterally through acquifers-discharge into rivers and streams
↠water table moves up and down in response to balance between amount of drainage recharge and losses through seepage to springs, steams and pumped wells

7. Ground water resources
Important source of domestic water
aquifer;-
Unconfined aquifer- shallow water bearing layer not seperated from soil by an impermeable layer
Aquiclude;- confine deeper acquifers
- impervious stata
Replenished by water draining from soils
water removed by crop uptake – may be replaced by capillary movt from shallow water table
Capillary fringe: zone of wetting by capillary rise

8. Movt of chemicals in drainage water
Carries leached organic and inorganic chemicals eg Nitrate poisoning and eutrophication
Ground water contaminants may be:
Human pathogens,
Toxic compounds from waste disposal sites
Macropores contribute to rapid water flow to depths below 1m
Contaminants are carried beyond the zone of microbial degradation
Preferential or bypass flow more harzardous where chemicals are surface placed not incorporated
Pollution hazard worsened by high intensity rainfall
Occurs through earthworm burrows, old root channels and clay shrinkage cracks

9. Bottom land soils
Some soils tend to be saturated in upper part of profile
-especially in low lying positions
-perched water table
Wetlands
- transitional ecosystems between land and water
Characterized by water logging, poor aeration and anaerobic conditions
These conditions are suitable for functionality of wetland ecosystems and the survival of wetland adapted species
These conditions are detrimental to most cultivated crops

10. Problems associated with bottomland farming
Anaerobic conditions are detrimental to the growth of most cultivated crops
Soils present engineering problems:
Low bearing strength
Uneven settlement and cracking of structures
Flooded basements
Potholes
Frost heaving
Plant production problems:
Difficult to perfome mechanized operations
Plants are starved of oxygen for respiration
Root systems are poorly developed and confined

11. LAND DRAINAGE This is the removal of gravitational water
Also the removal of excess salts in arid areas
Benefits of artificial drainage
Improved soil strength
Less damage to structures, pavements and plants
Enhanced root zone aeration
Less Mn and Fe toxicity
Reduced levels of funag diseases esp seedlings
More rapidn soil warming
Less atmc pollution theough N gases and methane
Removal of excess salts

12. Detrimental Effects of artificial drainage
Wildlife habitat is disturbed
Organic matter decomposition is accelerated
Nutrient assimilation and other biochemical processes are reduced
There is increased flooding-
Run-off losses are higher
Lower moisture retention
Greater cost of drainage due to flooding on alluvial lands developed after drainage