The Drainage-Basin Hydrological System and Pathways

A drainage basin (or catchment) is the area of land drained by a river and its tributaries, and the drainage-basin hydrological system describes how water enters the basin as precipitation, is stored in different places, moves along various pathways and leaves as evaporation, transpiration or river flow to the sea. In brief: the main inputs are rain, snow and other precipitation; the stores include vegetation, soil, groundwater and surface water; the pathways include infiltration, throughflow, overland flow and groundwater flow; and the outputs are evapotranspiration and river discharge. The water balance is precipitation = runoff + evapotranspiration ± change in storage.

The system explains how rivers respond to rain, why floods happen in some basins and why other basins have steady flows. This guide explains the features of a drainage basin, the stores and flows, the water balance, compares the pathways in a table, works through a water balance calculation, outlines the history of hydrology and corrects common mistakes.

What is a drainage basin?

The boundary of a drainage basin is the watershed, a line of high ground, such as a ridge, that separates it from neighboring basins. Rain falling inside the watershed drains toward the same river. The river begins at its source, is fed by tributaries at confluences and ends at its mouth, where it enters the sea, a lake or another river. Basins range from a few square kilometers to millions: the Amazon basin covers about 6 million km², which is about 25 times the area of the United Kingdom.

Basins are open systems because water and energy cross the boundary. In the global hydrological cycle, the oceans hold about 97.5% of the world’s water. Of the 2.5% that is fresh, about 69% is in ice caps and glaciers, about 30% is groundwater and less than 1% is in lakes, rivers, soil moisture and the atmosphere.

What are the inputs, stores, flows and outputs?

Inputs

Precipitation includes rain, snow, sleet, hail and dew. Its amount, intensity and form affect what happens next.

Stores

• Interception store: water held on leaves, stems and buildings, from which it evaporates or drips down.
• Surface storage: puddles, ponds, lakes and wetlands.
• Soil moisture: water held in the pores of the soil, available to plants.
• Groundwater: water filling the pores and cracks of rocks below the water table, which can be stored for years to thousands of years.
• Channel storage: water in rivers, and snow and ice stores in cold regions.

Outputs

Evaporation is the change of water to vapor from surfaces, transpiration is the release of vapor from plants, and together they form evapotranspiration. The other output is river discharge, the volume of water passing a point each second, usually in cubic meters per second (m³/s).

What pathways does water take?

• Interception, throughfall and stemflow: rain is caught by leaves, then falls through gaps (throughfall) or runs down trunks (stemflow). Dense forest can intercept 10 to 30% of rain.
• Infiltration: water enters the soil surface. Its rate depends on soil texture, vegetation and moisture, and it is high in sand and low in compacted clay and paved areas.
• Overland flow (surface runoff): water flows over the surface when rain exceeds the infiltration rate or the soil is saturated. It is the fastest pathway and brings rapid rises in rivers.
• Throughflow: lateral movement of water through the soil toward streams. It is slower than overland flow.
• Percolation and groundwater flow: water moves down through the rock to the water table and then flows slowly toward rivers as baseflow, which keeps rivers flowing in dry weather.
• Channel flow: movement in the river itself.

What is the water balance?

The water balance equation is P = Q + E ± ΔS, where P is precipitation, Q is runoff (river discharge), E is evapotranspiration and ΔS is the change in storage. Over a long time, storage change averages near zero, so P = Q + E. If precipitation exceeds evapotranspiration, water is in surplus and the soil becomes saturated, with runoff, as in humid climates. If evapotranspiration exceeds precipitation, there is a deficit and soil moisture is used up. A water balance graph shows these seasonal changes: a wet winter recharges the soil, and a dry summer uses it up.

Pathways compared

Worked example: water balance and runoff

The values are hypothetical and illustrate typical calculations.

1. Water balance: a basin receives 900 mm of precipitation in a year and loses 540 mm by evapotranspiration. Assuming no change in storage, runoff = 900 − 540 = 360 mm.
2. Runoff ratio: the share of precipitation that becomes runoff is 360 ÷ 900 = 0.40, or 40%.
3. Volume of discharge: if the basin has an area of 2,500 km² (2,500,000,000 m²), 360 mm (0.36 m) of runoff gives a volume of 0.36 × 2,500,000,000 = 900,000,000 m³ per year.
4. Mean discharge: a year has 31,536,000 seconds, so the mean discharge is 900,000,000 ÷ 31,536,000 = 28.5 m³/s.
5. Interception: a storm delivers 20 mm of rain. If the forest canopy intercepts 4 mm, the ground receives 16 mm, and the interception loss is 4 ÷ 20 = 20%.
6. Change in storage: in a dry year with P = 650 mm, E = 560 mm and Q = 130 mm, ΔS = P − Q − E = 650 − 130 − 560 = −40 mm, which means that stores (soil moisture and groundwater) fell by 40 mm.

How did the study develop?

Ancient thinkers believed that rivers were fed by water from the sea or from underground caverns. In the 17th century, Pierre Perrault and Edme Mariotte measured rainfall and river flow in the Seine basin and showed that rain was enough to explain the flow of the river. Robert Horton, in the 1930s and 1940s, developed quantitative theories of infiltration and runoff in drainage basins. The concept of the drainage basin as a system became common in geography in the 1960s. Today, rain gauges, weather radar, satellites and computer models are used to predict flows, manage floods and plan water supply.

Which mistakes are common?

Mistake 1: Confusing a store with a flow

A store, such as groundwater or soil moisture, holds water. A flow, such as infiltration or throughflow, moves it.

Mistake 2: Forgetting evapotranspiration

In many basins, more than half of the rainfall returns to the atmosphere, so runoff is only a part of precipitation.

Mistake 3: Assuming that all rain reaches the river quickly

Much water is stored and flows slowly through soil and rock, which keeps rivers flowing in dry weather.

Mistake 4: Mixing up watershed and drainage basin

In geography, the watershed is the boundary between basins. In North American usage, “watershed” often refers to the whole basin.