Stores and Flows in Drainage Basins

A drainage basin is an open system within the wider water cycle: water enters from outside it and can leave it. Its main input is precipitation — rain, snow, sleet or hail. Within the basin, water is held in stores and moves between them by transfers, also called flows. Water leaves as an output, either through the river’s outlet or as water vapour returning to the atmosphere.

These categories describe what water is doing. Water held in a pond is a store; water moving across the ground towards that pond is a flow.

Where water is stored

Water does not all reach a river immediately after rain falls. It can be held temporarily in several places:

• Vegetation store: leaves and branches catch precipitation, while plants also store water absorbed by their roots. Catching precipitation before it reaches the ground is called interception.
• Surface store: water collects in puddles, ponds and lakes. Snow lying on the ground is also a surface store; melting releases water from it.
• Soil moisture store: water is held in the pore spaces between soil particles. When the available spaces are full of water, the soil is saturated.
• Groundwater store: water is held in pores and cracks in permeable rock beneath the soil. Permeable rock allows water to pass through it. The upper surface of this store is the water table.
• Channel store: water is held within the river channel, even while it is moving downstream.

Stores delay the movement of water through a basin. They are not permanent containers: water can enter and leave them as conditions change.

Routes from the land surface to the river

When water reaches the ground, it can follow different routes. Infiltration is its downward movement from the surface into the soil. This adds water to the soil moisture store.

Once in the soil, water may move sideways downslope towards a river. This is throughflow. Alternatively, it may move down into the pores and cracks of the rock beneath the soil by percolation, adding to the groundwater store. Groundwater flow then carries water slowly through permeable rock towards the river.

Water can also travel across the ground surface as overland flow, or surface runoff. When soil is saturated, there is no available space for more water, so additional rain is more likely to flow over the surface into a channel. Intense rainfall can also supply water faster than it can infiltrate.

These routes are alternatives, not a sequence that every drop must follow. Some water travels over the surface, some moves through soil, and some reaches the deeper groundwater store.

How interception, infiltration and runoff are connected

Consider rain falling on a wooded hillside in a UK drainage basin. Some rain is intercepted by leaves and branches, so it does not reach the soil immediately. Some of this water evaporates; other water later drips or runs down onto the ground. Interception therefore delays and can reduce the amount of water reaching the surface.

If the soil has available pore space, water reaching it can infiltrate. This increases the soil moisture store, from which water can move by throughflow or percolation. Vegetation can also encourage infiltration, allowing more water to enter the soil rather than flow across its surface.

During prolonged rainfall, the soil moisture store may fill. As saturation develops, less additional water can infiltrate and more travels by overland flow. The balance between stores and flows has changed: a filled soil store encourages a surface route to the river.

Overland flow generally delivers water to channels more quickly than the slow route through groundwater. The route taken therefore affects both how much water reaches the river and when it arrives.

How water leaves the basin

Water reaching a river moves downstream by channel flow and eventually leaves the drainage basin through its outlet. River discharge describes the volume of water passing a point in the river per unit of time.

Water also returns to the atmosphere. Evaporation changes liquid water from surfaces such as lakes, wet soil and leaves into water vapour. Plants absorb water through their roots and release water vapour through transpiration. Together, these losses are called evapotranspiration.

A basin therefore links several routes: precipitation replenishes stores, transfers redistribute water, and outputs remove it. Changing one part of the system, such as how full the soil store is, changes the routes available elsewhere.