storage in ice and snow
moisture over land condensation
precipitation on land surface runoff (overland flow)
precipitation on ocean
evaporation from land evapotranspiration
freshwater storage
soil layer permeable rock layer
percolation
evaporation
lake thro
ugh
flow
impermeable rock layer
evaporation from ocean lake surface outflow
groundwater outflow ocean water table zone of saturation figure 1.5 The hydrological cycle
Inputs Drainage basins principally have one main input – precipitation (ppt), which includes all forms of rainfall, snow, frost, hail and dew. Water is then stored or transferred in the system for an indeterminate amount of time before its eventual output in the form of evaporation (EVP), evapotranspiration (EVT) and runoff. Precipitation refers to the conversion and transfer of moisture from the atmosphere to the land. Precipitation can be very variable and several factors may impact the hydrology of an area: amount and extent of precipitation, intensity, type, duration and geographical distribution. Storage Storage refers to the parts of the system that hold or retain water for periods of time. They can be open stores on the surface of the land, within vegetation or hidden deep within the rock structure. The amount of time that water is stored for is dependent on the processes acting on it. Interception refers to water that is caught and stored by vegetation. It is affected largely by the size and coverage of plants, with large broadleaved trees catching the most water (in summer). Intercepted water may still transfer through the system using three main mechanisms: • • •
interception loss – water retained by plants and later lost as evaporation throughfall and leaf drip – water that is slowed by running off and dropping from leaves, twigs and stems stemflow – water that runs down branches and trunk to the ground.
Urban areas and areas that have been cleared for cultivation have much lower rates of interception.
12 Hydrology and fluvial geomorphology