Hydrology and Soils
This section covers Hydrology and Soils: the water cycle, infiltration and runoff, groundwater and aquifers, watershed behaviour, and soil formation, profile, classification and conservation. Assam examples include Brahmaputra floods, char land and acidic tea soils.
Core rules
- Water balance: precipitation = evapotranspiration + runoff ± storage change.
- Infiltration is entry of water into soil; percolation is downward movement to groundwater. Porosity is pore space; permeability is the ability to transmit water.
- An unconfined aquifer has a water table; a confined aquifer is bounded by impermeable layers and may yield artesian wells.
- In a watershed, deforestation and urbanisation reduce infiltration and lag time, raising flood peaks.
- Soil forms from parent material, climate, relief, organisms and time. A typical profile is O–A–B–C–R. Assam's Brahmaputra valley has fertile alluvial soils; hill areas have laterite and forest soils; tea grows best on well-drained, acidic soils.
Worked example
A catchment receives 150 mm of rain. Infiltration is 50 mm, evapotranspiration 30 mm, and storage is unchanged. Runoff = 150 − 50 − 30 = 70 mm. If urbanisation cuts infiltration to 25 mm while evapotranspiration stays at 30 mm, runoff becomes 95 mm. The extra 25 mm travels quickly over impervious surfaces, so the flood peak rises and arrives sooner.
Traps
- Using infiltration and percolation as synonyms.
- Treating porosity as permeability; clay is porous but poorly permeable.
- Calling alluvial soil infertile; it is usually fertile but may be sandy or waterlogged.
- Confusing the water table with bedrock.