ASGEOGRAPHYREVISIONHYDROLOGYANDFLUVIALGEOMORPHOLOGY

1.1 DRAINAGEBASINSYSTEMS

KEYTERMSANDDEFINITIONS

Hydrology is the study of water as it moves on, under and through the Earth’s surface.The hydrological cycle (also called the water cycle) is the movement of water between air, land and sea. It varies from place to place and over time.Precipitation is the conversion and transfer of moisture in the atmosphere to the land. It includes all forms of rainfall, snow, frost, hail and dew.

KEYTERMSANDDEFINITIONS

Interception is the precipitation that is collected and stored by vegetation.Overland flow (or surface run-off) occurs in two cases:

1 when the intensity of precipitation that reaches the surface exceeds the infiltration capacity of the soil. This process is known as Hortonian overland flow2 when the combination of precipitation intensity and duration saturates the soil and raises the water table to the surface. This process is known as saturation overland flow.

KEYTERMSANDDEFINITIONS

Infiltration is the movement of water downwards from the surface into the soil.Evaporation refers to water from the ground or a lake that changes into a gas.Transpiration is water loss from vegetation to the atmosphere.Evapotranspiration is the combined losses of evaporation and transpiration.Drainage basin is the area of land drained by a river system (a river and its tributaries).Interception is the precipitation that is collected and stored by vegetation.

KEYTERMSANDDEFINITIONS

Throughfall is water flowing off wet leaves onto the ground.Stemflow is water flowing down the trunks of trees and stems of grasses and shrubs.Percolation is the movement of water down into the groundwater zone.Throughflow is water flowing through the soil down to the groundwater zone.Baseflow is the movement of water through rock. It is the slowest form of such water movement, and accounts for the constant flow of water in rivers during times of low rainfall.

KEYTERMSANDDEFINITIONSWater table is the upper layer of permanently saturated pore spaces in the groundwater zone.Groundwater is water stored underground in a permeable rock, e.g. chalk or sandstone. The water emerges at the surface as springs and streams. The groundwater’s upper level is called the water table. Layers of rock that are filled with groundwater are called aquifers.Recharge is the ‘topping up’ of groundwater levels following abstraction.Capillary movement is the upward movement of water in a soil. It usually occurs in climates where evaporation exceeds precipitation either annually or seasonally.Springs are where water seeps directly out of the ground where an aquifer reaches the ground surface.

TOPICSUMMARYThe hydrological cycle refers to the cycle of water between atmosphere, lithosphere and biosphere.The drainage basin system is an open system as it allows the movement of energy and matter across its boundaries.Water can be stored at a number of stages or levels within the cycle. These stores include vegetation, surface, soil moisture, groundwater and water channels.Human modifications are made at every scale.

TOPICSUMMARYPrecipitation is the main input into the drainage basin system. It is the transfer of moisture from the atmosphere to the land.Interception refers to water that is caught and stored by vegetation.Evaporation is the process by which a liquid or a solid is changed into a gas.Factors affecting evaporation include meteorological factors such as temperature, humidity and wind speed.Transpiration is the process by which water vapour escapes from plants.The combined effects of evaporation and transpiration are normally referred to as evapotranspiration (EVT).

TOPICSUMMARYInfiltration is the process by which water soaks into or is absorbed by the soil.Soil moisture refers to the subsurface water in the soil.Groundwater refers to subsurface water. Groundwater accounts for 96.5 per cent of all freshwater on the earth.Aquifers are rocks that contain significant quantities of water.

ADDITIONALWORK1. Outline the ways in which human activities have modified the

hydrological cycle.2. Describe and explain the role of vegetation in the hydrological cycle.3. Discuss the importance of groundwater.4. Describe and explain how the hydrological cycle varies between

different geographic regions.