Percolation-evaporation balance Artificial drainage Percolation-evaporation balance

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

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

Ground water diagramatically

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 100-1000m

…..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

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

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

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

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

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

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