Hydrology and Water Resources

Why it matters

Hydrology and Water Resources are critical for managing water supply, flood control, and environmental conservation. Understanding these concepts helps in designing sustainable water management systems, which are essential for supporting agriculture, industry, and domestic needs.

Key ideas

• Hydrological Cycle: The continuous movement of water on, above, and below the surface of the Earth. It includes processes like evaporation, condensation, precipitation, infiltration, and runoff.
• Precipitation: Any form of water - liquid or solid - falling from the atmosphere to the Earth's surface. It is a primary input to the hydrological cycle.
• Evapotranspiration: The sum of evaporation from the land surface plus transpiration from plants. It is a significant component of the water balance.
• Runoff: The part of precipitation that flows over the land surface towards streams, rivers, and lakes.
• Infiltration: The process by which water on the ground surface enters the soil. It affects groundwater recharge and surface runoff.
• Groundwater: Water stored underground in aquifers, which can be tapped for human use.
• Water Resource Management: Strategies and practices to manage water resources sustainably, including allocation, conservation, and quality control.

Formulas

The basin balance assumes all depths refer to the same interval and area, with no other net imports/exports.

• P = E + T + R + ΔS

  • P: Precipitation (mm)
  • E: Evaporation (mm)
  • T: Transpiration (mm)
  • R: Runoff (mm)
  • ΔS: Change in storage (mm)

• Q = C·i·A / 3600000

  • Q: Rational-method peak discharge (m³/s); use only for a suitable catchment and rainfall duration/intensity, not as a complete hydrograph
  • C: Runoff coefficient (dimensionless)
  • i: Rainfall intensity (mm/hr)
  • A: Catchment area (m²)

Worked example

Given:

• Precipitation (P) = 1000 mm
• Evaporation (E) = 200 mm
• Transpiration (T) = 150 mm
• Change in storage (ΔS) = 50 mm

Find: Runoff (R)

1. Use the water balance equation: P = E + T + R + ΔS
2. Substitute the given values: 1000 = 200 + 150 + R + 50
3. Solve for R: R = 1000 - 200 - 150 - 50
4. Calculate: R = 600 mm

Final Answer: 600 mm

Common mistakes

• Confusing units, especially when converting between mm and m³.
• Ignoring the change in storage (ΔS) in the water balance equation.
• Misapplying the runoff coefficient in different catchment conditions.

For GATE CE

Questions often involve calculating components of the hydrological cycle, such as runoff or evapotranspiration. Practice problems on water balance equations and understanding the impact of different variables on water resources.

Quick check

1. What is the primary input to the hydrological cycle?
2. Define evapotranspiration.
3. What does the runoff coefficient represent?

Answers: 1. Precipitation 2. The sum of evaporation and transpiration 3. In this peak-flow model it is an empirical factor relating peak runoff to rainfall intensity and area; do not assume it is universally a storm-volume runoff fraction.