Hydrology & Elements of Drainage

Hydrology & Elements of Drainage

1. Precipitation

• Definition: Water in any form (rain, snow, hail) that falls from the atmosphere to Earth when air becomes saturated.

Key Points:

• Intensity vs duration: Heavier rain = shorter duration.

• Can fall directly into rivers (channel precipitation).

• After reaching Earth, precipitation can:

• • Be absorbed by plants.

  • Infiltrate soil → groundwater.

  • Run off into rivers/oceans.

  • Evaporate back into atmosphere.

Factors Affecting Precipitation:

• Condensation Nuclei: Particles that allow droplets to form. Influenced by natural (volcanic ash, sea salt) and human activities (construction, quarrying).

• Latitude: Lower latitudes (Caribbean) → more evaporation → more rainfall than higher latitudes.

• Distance from Sea (Continentality): Coastal areas receive more rainfall.

• Ocean & Wind Currents: Warm currents increase precipitation (e.g., Caribbean Sea), cold currents reduce it.

• Vegetation Cover: More vegetation → higher evapotranspiration → influences rainfall.

• Orographic Barriers: Mountains force air to rise → rainfall on windward side, dry on leeward side (rain shadow, e.g., Blue Mountains, Jamaica).

• Water Source Proximity: More evaporation near oceans/rivers.

• Land Surface Conditions: Texture, color, moisture affect evaporation and condensation.

• Human Activities: Deforestation, urbanization reduce moisture → less rainfall; cloud seeding can increase rainfall.

2. Evaporation

• Definition: Process by which liquid water changes into vapor, requiring heat energy.

Main Factors:

• Water Source Availability: Oceans, rivers, lakes.

• Temperature: Warmer areas (Caribbean) → more evaporation.

• Wind: Speeds up evaporation by removing moist air.

• Surface Area: Larger surfaces (oceans) → more evaporation.

• Humidity: High humidity slows evaporation.

• Air Pressure: Lower pressure → faster evaporation.

• Human Activities: Dark surfaces and deforestation can alter evaporation rates.

3. Transpiration

• Definition: Plants release water vapor from leaves into the atmosphere. About 10% of atmospheric moisture comes from transpiration.

Influencing Factors:

• Temperature: Higher → more transpiration.

• Humidity: Higher → less transpiration.

• Wind: Moves moist air away → increases transpiration.

• Soil Moisture: Low moisture → less transpiration.

• Vegetation Type: Desert plants (cactus) transpire less than forest trees.

4. Evapotranspiration (ET)

• Definition: Combined process of evaporation + transpiration. Importance: Returns large amounts of water to the atmosphere, reducing surface water availability.

Factors Affecting ET:

• Temperature & Climate: Hot, dry areas → high ET.

• Humidity: Higher humidity → lower ET.

• Wind: Increases ET by moving moist air away.

• Soil Moisture: Less soil water → less ET.

• Plant Type: Different plants transpire at different rates.

5. Interception

• Definition: Rainwater trapped by vegetation or buildings before reaching the ground.

• Interception Loss: Water evaporates from leaves back to the atmosphere.

Factors:

• Rainfall intensity and duration.

• Vegetation type/density.

• Wind speed (more wind → less interception).

• Human activities (urbanization changes interception patterns).

6. Infiltration

• Definition: Water soaks into soil from the surface.

• Infiltration Capacity: Maximum rate water enters soil (varies by soil type).

• Sandy soils → high infiltration; clay → low infiltration.

Factors:

• Precipitation amount & intensity.

• Soil type & saturation.

• Vegetation cover.

• Land use (impervious surfaces reduce infiltration).

• Slope (steeper slopes → less infiltration).

7. Percolation

• Definition: Downward movement of water through soil to reach impermeable rock → creates water table and aquifers.

• Zones: Saturation (phreatic), unsaturated (vadose), intermittent saturation.

8. Channel Flow

• Definition: Movement of water in rivers to the sea. Water enters channel by:

• Surface runoff.

• Throughflow.

• Groundwater flow (baseflow).

• Factors: Rainfall intensity, vegetation, soil type, slope, human activity.

9. Stemflow

• Definition: Rainwater runs down plant stems/trunks to soil. Influencing Factors:

• Rainfall characteristics (intensity, angle).

• Tree species (bark texture, crown shape).

• Forest structure.

• Human activity (deforestation).

10. Throughfall

• Definition: Water drips from leaves to the ground.

• Influenced by rainfall type, leaf shape, vegetation density, wind speed.

11. Surface Runoff (Overland Flow)

• Definition: Water flows over land into rivers. Factors:

• Soil type, slope, vegetation cover, rainfall intensity, human activities (urbanization, quarrying).

12. Throughflow

• Definition: Horizontal movement of water through soil to rivers after saturation.

Affected by soil type, rainfall, vegetation, and land use.