- The drainage basin is the area of land drained by the main river and its tributaries. This includes water found on surface, in soil and in near-surface geology.
- The drainage basin is also called the catchment area. Essentially, it is the area which supplies a river with its water.
- Different drainage basins are separated from each other by an imaginary line called the watershed.
- Processes within the drainage basin in the water cycle:
- Precipitation – lands on bare surface or vegetation cover
- Interception storage is provided by vegetation, occurs on leaves/branches with density and type of vegetation affecting how good a store it is, some tropical rainforests intercept around 58% of precipitation.
- Evaporation from vegetation surfaces.
- Throughfall is when water drips/falls to the ground or between interception stores
- Stemflow is when water flows down stems of grass, or down tree trunks in big storms
- Infiltration is the downward movement of water from the surface into the soil movement is controlled by gravity, capillary action and soil porosity, which is the most important factor as coarse soils have larger pores than fine grained soils, so allow more infiltration.
- Soil storage is the amount of water stored in the soil as pore spaces in soil can be filled with air or water in clay soil, pores account for 40-60% of the volume in fine sand, pores account for 20-45% of the volume.
- Vegetation storage is water stored in structure of plants to survive, water is removed from soil by plants.
- Transpiration is the loss of water from vegetation through stomata on the surfaces.
- Surface storage is often in form of puddles which forms due to soil being saturated with water as it reaches infiltration capacity.
- Evaporation of surface storage.
- Evapotranspiration is the total output of water from drainage basin directly back into the atmosphere which is often referred to, as it is hard to separate output from evaporation and output from transpiration.
- Overland flow/surface runoff is when water flows horizontally across land when rainfall exceeds infiltration capacity of the soil and surface stores are full as tends to begin on slopes.
- Throughflow the movement of water down-slope through subsoil under influence of gravity which is particularly effective when underlying permeable rock prevents further downward movement. This reaches nearest channel eventually which is slower than overland flow.
- Percolation follows infiltration which is the downward movement of water within rock under soil surface with percolation rates vary depending on the nature of the rock.
- Groundwater storage.
- Groundwater flow is the slow movement of water through underlying rocks.
- Channel flow is where all the water that is in the river being transported with the amount of water leaving the basin like this called run-off.
- The water in a drainage basin is distributed between different flows in different proportions, all units are -1000km3 per year:
- Ocean precipitation – 373.
- Ocean evaporation – 413.
- Ocean to land water vapour transport – 40.
- Total land evapotranspiration – 73.
- Land precipitation – 113.
- Water returned to ocean by surface/sub-surface flows – 40
- There is a difference of 40 units between the ocean precipitation and ocean evaporation. The 40 extra from the evaporation are transported through the atmosphere and added to the total land evapotranspiration to produce the 113 units of land precipitation.
- 40 of these units then remain in the water to return to the ocean, whilst the other 73 evaporate/transpirate.
Factors Affecting Hillslope Hydrology
- Urbanisation/settlement.
- Soil type and properties.
- Rock type and properties.
- Vegetation cover.
- Temperature/humidity.
- Water extraction.
- Seasonal changes/weather.
- Land use.
- Farming practices.
Soil Water Budget
- The water budget is used to describe the balance between inputs and outputs.
- Rivers can only exist if there is precipitation/overland flow to contribute to the river or if other stores are able to release water.
- River discharge varies throughout the year, this is the river’s regime. It can also vary with short term changes, like following heavy rain.
Diagram shows the soil water budget for Eastern England.
- Recharge is when the soil water store begins to fill with water again – usually due to precipitation being greater than potential evapotranspiration.
- Utilisation is when the plants start using the water store, due to warmer weather which increases potential evapotranspiration, making it greater than precipitation.
- Deficit is when there is a lack of soil water. Plants will either wilt or some will have adaptations which aid their survival in the dry conditions.
- Surplus is when all storage in the soil is completely full. It happens because of precipitation being greater than potential evapotranspiration, so the water fills the soil. Groundwater becomes recharged and overland flow may occur.