The water balance:
Precipitation = evapotranspiration + streamflow +/- storage
• Precipitation forms when vapour in the atmosphere cools to its dew point and
condenses to form droplets which aggregate and leave clouds.
• Cumuliform clouds (high with flat bases) form when air is heated locally through
surface contact causing convection, expansion and then cooling (adiabatic
expansion). Produces convectional rainfall.
• Stratiform clouds (layered) form when air mass moves horizontally across cooler
surface (ocean). This combined with turbulence is advection. Produces frontal
rainfall.
• Cirrus clouds (wispy, tiny ice crystals) form at high altitude but don’t produce
precipitation. When air is forced to rise over hills orographic rainfall is produced.
Lapse rates describe the vertical distribution of temperature in lower atmosphere on rising
air.
• Environmental lapse rate = falls at 6.5 degrees/km in lower atmosphere
• Dry adiabatic lapse rate = falls at 10 degrees/km due to cooling by adiabatic
expansion.
• Saturated adiabatic lapse rate = falls at 7 degrees/km as condensation releases
latent heat. Condenses at dew point (8 degrees) until atmospheric stability above
4000m is reached as air is cooler than its surroundings and so cannot rise freely.
Catchment Hydrology:
Drainage Basin = an area of land drained by a river and its tributaries that forms a subsystem of the hydrological cycle.
Pore space varies in different soils – Clay has 50% volume, sand has 30%.
