Drainage Basin Characteristics

A drainage basin (also called a catchment area) is the land area from which all precipitation drains into a single river system or a group of streams flowing into a common outlet. The elevated land that separates two drainage basins is called a watershed or drainage divide.

Drainage basins vary in size, shape, and other characteristics due to physical, climatic, and biotic (including human) factors. These factors influence water flows such as surface runoff, infiltration, and throughflow, which in turn affect the shape of the storm hydrograph.

It is important to note that these factors do not act in isolation. For example, the slope of the land influences surface runoff, but vegetation cover also plays a role. For clarity, these factors will be examined separately.

Physical Factors Affecting Drainage Basin Characteristics and Flows

1. Relief (Slope and Elevation)
2. • Steep slopes encourage faster surface runoff, reducing infiltration. This produces a short lag time and a steep hydrograph.
   • Gentle slopes allow more infiltration and throughflow, creating a gentle hydrograph with a longer lag time.
3. Geology (Rock and Soil Type)
4. • Impermeable rocks such as granite and soils like clay restrict infiltration, causing high surface runoff and steeper hydrographs. These areas often have dense drainage networks (e.g., 5.8 km per km²), which increases flood risk.
   • Permeable rocks such as limestone and soils like sand allow rapid infiltration, reducing runoff and producing gentler hydrographs with longer lag times. An example is Cockpit Country in Jamaica, which has a drainage density of 1.2 km per km².
   • The orientation of rock layers, the presence of joints and cracks, and faults influence water flow direction and infiltration rates.
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5. Drainage Density
6. • Areas with high drainage density indicate efficient drainage of water as surface runoff, meaning there is a lower rate of infiltration. This results in a steep storm hydrograph with a short lag time.
   • Areas with low drainage density have greater infiltration, which creates a gentle hydrograph with a longer lag time.
7. Drainage Basin Size
8. • Large drainage basins usually contain many more tributaries and distributaries. Because of the larger area, water takes a longer time to reach the main river. This typically increases the lag time and reduces the steepness of the hydrograph.
   • Smaller drainage basins generally have shorter lag times and steeper hydrographs.
9. Drainage Basin Shape
10. • A circular basin is more likely to have a shorter lag time and a higher peak flow than an elongated drainage basin.

Climatic Factors Influencing Basin Flows

1. Precipitation (Intensity and Duration)
2. • Heavy rain events result in rapid saturation of the upper soil layers, and excess water reaches streams quickly as surface runoff. This creates a short lag time and a steep hydrograph.
   • If the same amount of rainfall occurs as a light drizzle over a longer period, more water will infiltrate, and the river takes longer to respond. This results in a longer lag time and a gentler hydrograph.
3. Temperature and Season
4. • Extreme temperatures, whether very cold or very hot, reduce infiltration because the soil either becomes frozen or too dry and compacted. These conditions increase surface runoff from snowmelt or heavy rainfall in arid regions. This leads to a reduced lag time and a steeper storm hydrograph.

Biotic Factors (Vegetation)

• Vegetated areas help reduce flood risk by increasing the time it takes for water to reach a river through interception and root absorption.
• Tropical rainforest canopies intercept up to 80 percent of rainfall, and about 30 percent of the water is lost through evaporation due to interception.
• Dense vegetation reduces overland flow and creates a gently sloping storm hydrograph.
• The type of vegetation matters; broadleaf trees intercept more rainwater than small shrubs, particularly during light drizzles.

Human Factors (Land Use and Impact)

1. Activities Increasing Surface Runoff and Creating Steeper Hydrographs
2. • Deforestation, mining, quarrying, urbanization, road building, pastoral farming, and dam construction (which also increases evaporation).
3. Activities Promoting Infiltration and Creating Gentle Hydrographs
4. • Reafforestation, afforestation, and agriculture involving groundwater extraction.

Areas cleared by deforestation respond quickly to rainfall due to reduced interception.

Morphometry of Drainage Basins

Morphometry refers to the measurement of the shape and physical characteristics of drainage basins. R.E. Horton and A.E. Strahler introduced morphometric studies in the 1940s and 1950s to quantify stream properties.

Drainage Density

• Drainage density provides a numerical measure of landscape dissection and runoff potential.
• Areas with dense drainage networks are more susceptible to flooding and indicate impermeable rocks or thin soils.
• Low drainage density suggests permeable soils, dense vegetation, and gentle slopes.

Factors affecting drainage density:

• Rock and soil type
• Relief or gradient of the land
• Rainfall and temperature
• Vegetation cover
• Human activity
• Time: Newly formed landscapes (e.g., volcanic areas) may have parallel streams, while older basins develop dendritic patterns.