A.1 Drainage basin hydrology and geomorphology

A drainage basin is the area of land drained by a river and its tributaries, bounded by an imaginary line called the watershed (or drainage divide), which separates one basin from its neighbours along the highest points of surrounding land. Geographers model the drainage basin as an open system: a set of interrelated components through which water and matter continuously move, exchanging energy and matter across the system's boundary with the surrounding environment. This contrasts with a closed system, which exchanges energy but not matter with its surroundings.

Every open system can be described using four categories: inputs, flows (transfers), stores, and outputs. In the drainage basin, water constantly cycles between these categories, so the system is best understood dynamically rather than as a fixed snapshot.

Flows describe water in motion between stores. Infiltration is the downward movement of water into the soil surface; its rate depends on soil type, vegetation cover, and antecedent moisture. Once in the soil, water may travel laterally as throughflow, moving slowly through soil pores towards the river channel. If the ground is saturated or impermeable, water instead moves across the surface as overland flow (surface runoff), which reaches the channel far more rapidly than throughflow. Water already stored underground can also seep into the river as base flow, sustaining discharge even during dry periods.

Stores hold water for varying lengths of time, from hours (interception by vegetation) to millennia (deep aquifers or glacial ice in the cryosphere). Soil moisture is a short- to medium-term store, while aquifers -- permeable rock layers that hold groundwater -- can retain water for very long periods. The balance between stores and flows at any moment determines how much water is available to become river discharge, which links this systems view directly to the study of river discharge and streamflow later in this subtopic.

A drainage basin functions as an open system, meaning it receives inputs and energy from outside its boundaries and releases outputs beyond them. The dominant input to this system is precipitation -- water delivered from the atmosphere to the land surface. Without precipitation, none of the basin's stores (soil moisture, aquifers, the cryosphere) or flows (infiltration, throughflow, overland flow, base flow) would be replenished. Understanding precipitation as an input is therefore the starting point for analysing everything else that happens within the basin.

Precipitation is not a single uniform input; it varies in type and intensity, and both factors control how quickly -- and how much -- water actually becomes available to the basin.

Type of precipitation determines the timing of water availability:

• Rain enters the system as liquid water immediately, becoming available for infiltration or overland flow as soon as it reaches the surface.
• Snow is stored in the cryosphere and only becomes available to the basin once temperatures rise and melting occurs, which can delay water input by weeks or months (a process central to snowmelt-fed river regimes).
• Hail, though solid like snow, tends to melt rapidly after landing because it usually falls during warmer, high-energy storm conditions, so it behaves more like a delayed pulse of intense rain than a long-term store.

Intensity (the rate of precipitation, e.g. mm per hour) determines whether the input can be absorbed by the ground or whether it overwhelms it:

• Low-intensity, prolonged rainfall allows time for infiltration, feeding soil moisture stores and groundwater.
• High-intensity rainfall, especially onto already saturated or impermeable ground, exceeds the infiltration capacity of the soil, forcing water to move as overland flow (surface runoff) instead. This is a major control on flood risk and is why short, intense storms often produce sharper river discharge peaks than longer, gentler rainfall events of the same total volume.

Because precipitation is the input that ultimately drives every downstream process -- infiltration, throughflow, overland flow, base flow, and river discharge itself -- variations in its type and intensity help explain why drainage basins in different climates behave so differently. A basin dominated by snowmelt (e.g. in a high-latitude or high-altitude setting) will show a distinct seasonal discharge pattern, while a basin subject to intense monsoonal or convective rainfall will show rapid, flashy responses to individual storm events.

In the systems approach to the drainage basin, water constantly moves between inputs, stores, flows, and outputs. Evaporation is one of the two major outputs of the drainage basin system (the other being transpiration), representing water that leaves the basin permanently by escaping into the atmosphere rather than remaining within the basin as a store or continuing to move through it as a flow.

Evaporation occurs when liquid water molecules at an exposed surface -- such as the surface of a river, lake, reservoir, ocean, or moist soil -- gain enough kinetic energy from solar radiation (insolation) and heat to change state from liquid to water vapour. This vapour then rises into the atmosphere, removing that water entirely from the drainage basin's terrestrial system. Because evaporation depends directly on the input of energy, it is highly sensitive to climatic conditions, and its rate varies enormously across different environments and seasons.