Hydrological processes are the major processes within the system of the hydrological cycle. In the Water Cycle, we learn that the global hydrological cycle is a closed system, however, a local hydrological cycle has hydrological processes that operate within areas drained by rivers and their tributaries. These are known as drainage basins which are open systems.
Open systems mean that their inputs are not governed by outputs and more water can be lost than received.
Water is lost by:
- Evaporation and evapotranspiration to the atmosphere.
- Surface runoff to the sea.
- Percolation into groundwater stores.
Drainage basins are also known as catchment areas as they catch the precipitation falling within the watershed.
Hydrological processes in a drainage basin
When precipitation occurs, the water can follow three pathways:
Reaching the land surface and then infiltrating the topsoil.
Running off the surface as overland flow, also known as surface runoff.
Evaporating back into the atmosphere.
Any pathway taken by the water can be delayed by:
Interception by plants and buildings.
The percolation of surface water through the rocks underneath to become groundwater, stored in aquifers.
Surface runoff can flow over impermeable and saturated surfaces until it eventually reaches river channels and becomes streamflow.
Main hydrological processes in a drainage basin
| Hydrological process | Definition |
| Inputs | Precipitation | Moisture in any form. |
| Storage | Interception | Temporary storage of water before reaching soil. It is water captured by plants, buildings and hard surfaces. |
| Vegetation storage | Moisture that is taken up by vegetation. |
| Surface storage | Any water in surface water such as lakes, ponds and puddles. |
| Soil moisture | Water in soil. |
| Groundwater storage | Water held in rocks (also known as aquifer). |
| Channel storage | Water held in streams and rivers. |
| Flows and Processes | Infiltration | Water that enters topsoil. Common when there is slow or steady rainfall. |
| Throughflow | Water that seeps laterally through soil below the surface but above the water table. |
| Percolation | The vertical flow of water between soil and rock layers. |
| Base flow | Slow-moving water that flows into the river channel. |
| Channel flow | The water flowing in the river channel, also known as discharge and runoff. |
| Surface runoff | Flow over the surface when the ground is frozen, saturated or impermeable clay or during an intense storm. Also known as overland flow. |
| Outputs | Evaporation | Water into vapour |
| Transpiration | Water that has been taken up by plants and transpired onto the leaf surface. |
| Evapotranspiration | Both the effect of evaporation and transpiration. |
| River discharge | A volume of water passing a certain point in the channel over a certain amount of time. |
The impact of drainage basin factors on hydrological processes
Drainage basin factors like shape, relief, geology, vegetation, climate and land use determine what happens to the precipitation when it falls. Steeper slopes promote faster movement and shorter storage times compared to gentler slopes. Higher vegetation coverage will increase interception and evapotranspiration and decrease surface runoff.
The geology of the basin is also important. Some rocks are impermeable, preventing infiltration and causing surface saturation.
Cold climates such as snow-capped peaks can hold water until it thaws which can lead to delayed flow. Rural areas permit more natural processes compared to urban areas. Grasslands have higher infiltration, percolation, throughflow and evaporation than arable land. Urban areas have impermeability and increase rapid surface runoff, evaporation and interception.
Precipitation input and hydrological processes
When the warm air rises and cools to become condensation, this leads to clouds which then eventually form rain and snow. Looking at the UK as a case study, we can see three different types of rainfall on the west and the east.
In the west, there are drainage basins which are exposed to the air masses from the Atlantic. They are particularly prone to orographic and frontal rainfall. it is much drier on the east yet the summer months can experience heavy bursts of rain from conventional air instability. As the ground warms up, evaporation means that the air above heats and rises. The heavy rain from summer thunderstorms can lead to flash flooding as the dry soil surface becomes waterlogged quickly, causing rapid surface runoff.
Do human impacts affect hydrological processes?
The inputs, outputs, flows and stores of water of a hydrological cycle can also be affected by human impact.
Over abstraction
Over abstraction is defined as when too much water is abstracted from groundwater reserves. This can lead to rivers drying up in times of low rainfall.
Deforestation
The Tropics have a fragile natural environment with complex biodiversity. Their forests flourish on relatively thin soils. Removing trees can lead to a reduction of interception. Rainfall strikes the soil directly and compacts it, and the runoff of the rainfall that stays on the surface moves quickly to rivers causing floods.
Urbanisation
The physical character of urban areas can affect the local hydrological cycle. One example is the increase in impermeable surfaces due to built-up areas, which alter the natural flow of water.
Reservoirs
Man-made reservoirs can delay the flow through the drainage basin and increase the amounts lost through evaporation.
Impacts of man-made reservoirs include:
- Dams reduce the river flow below them, leading to vegetation loss.
- The mats of floating plants on the reservoir's surface in the Tropics lead to higher evapotranspiration rates than in open water.
- Salinity levels in the reservoir can rise as water evaporates.
- Reservoir water takes water from the drainage basin.
Hydrological Processes - Key takeaways
- Local hydrological cycles have hydrological processes that operate within areas drained by rivers and its tributaries, known as drainage basins.
- Water follows three pathways, reaching the land surface and then infiltrating the topsoil, running off the surface as overland flow, and evaporating back into the atmosphere.
- These pathways can be delayed.
- There are basin-wide factors such as the shape, relief, geology, vegetation, climate and land use of each basin which determine what happens to the precipitation when it falls.
- There are many factors that affect the hydrological processes in the drainage basins, such as the variety of precipitation and human impact.
Explanations 
Exams 
Magazine 
