Drainage Basin Hydrology

The drainage basin (also called a catchment or watershed) is the fundamental unit of study in hydrology. It is defined as the area of land drained by a river and its tributaries, bounded by a ridge of high land called the watershed. Unlike the global hydrological cycle, the drainage basin is an open system — it receives inputs from the atmosphere and loses outputs through river discharge and evapotranspiration. Understanding drainage basin hydrology is essential for predicting flood risk, managing water resources, and evaluating the impact of land-use change.

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The Drainage Basin as a System

graph TD
    P["INPUTS
Precipitation
(rain, snow, hail)"] --> INT["Interception
(by vegetation canopy)"]
    P --> TF["Throughfall &
Stemflow"]
    INT -->|"Evaporation"| ATM["ATMOSPHERE"]
    TF --> INF["Infiltration
(into soil)"]
    TF --> OF["Overland Flow
(surface runoff)"]
    INF --> SM["Soil Moisture Store"]
    SM --> TH["Throughflow
(lateral movement in soil)"]
    SM --> PERC["Percolation
(to groundwater)"]
    SM -->|"Evapotranspiration"| ATM
    PERC --> GW["Groundwater Store"]
    GW --> BF["Baseflow
(to river channel)"]
    TH --> CH["River Channel"]
    OF --> CH
    BF --> CH
    CH --> Q["OUTPUT
River Discharge"]

Inputs