2A - The Hydrological Cycle
The hydrological cycle is a system of linked processes: inputs (precipitation patterns and types: orographic, frontal, convectional); flows (interception, infiltration, direct runoff, saturated overland flow, throughflow, percolation, groundwater flow) and outputs (evaporation, transpiration and channel flow).
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Inputs:
The main input is precipitation, which can vary in a number of different ways. All these characteristics can have a significant effect on the drainage cycle.
Flows
There are at least seven different flows that are important in transferring the precipitation that has fallen on the land into the drainage network.
Outputs
Inputs:
The main input is precipitation, which can vary in a number of different ways. All these characteristics can have a significant effect on the drainage cycle.
- Form: rain, snow or hail. Clearly, with snow, entry of water into the drainage system will be delayed.
- Amount: this will affect the amount of water in the drainage basin and the fluxes within it.
- Intensity: the greater the intensity, the greater the likelihood of flooding.
- Seasonality: this is likely to result in the drainage basin system operating at different flow levels at different times of the year.
- Distribution: this is significant in very large drainage basins, such as the Nile and the Ganges, where tributaries start in different climate zones.
Flows
There are at least seven different flows that are important in transferring the precipitation that has fallen on the land into the drainage network.
- Interception: the retention of water by plants and soils which is subsequently evaporated or absorbed by the vegetation.
- Infiltration: the process by which water soaks into, or is absorbed by, the soil.
- Percolation: similar to infiltration, but a deeper transfer of water into permeable rocks.
- Throughflow: the lateral transfer of water downslope through the soil
- Groundwater flow: the very slow transfer of percolated water through pervious (permeable) or porous rocks.
- Surface runoff: the movement of water that is unconfined by a channel across the surface of the ground. A.k.a. overland flow.
- River or channel flow: takes over as soon as the water enters a river or stream; the flow is confined within a channel.
Outputs
- Evaporation: the process by which moisture is lost directly into the atmosphere from water surfaces, soil and rock.
- Transpiration: the biological process by which water is lost from plants through minute pores and transferred to the atmosphere.
- Discharge (channel flow): into another, larger drainage basin, a lake or the sea.