Introduction to Hydrology

What is Hydrology?

Hydrology is the science that encompasses the study of water on the Earth's surface and beneath the surface of the Earth, the occurrence and movement of water, the physical and chemical properties of water, and its relationship with the living and material environment of the Earth.

The Hydrologic Cycle

The Hydrologic Cycle (or Water Cycle) is the continuous process by which water is transported from the oceans to the atmosphere, to the land, and back to the oceans. It is a closed system on a global scale, meaning the total amount of water remains constant, though its distribution changes over time and space.

Key Components:

1. Evaporation: Conversion of liquid water into water vapor from oceans, lakes, and rivers.
2. Transpiration: Release of water vapor from plants.
3. Precipitation: Condensed water vapor falling to Earth as rain, snow, hail, etc.
4. Infiltration: Movement of water into the soil profile.
5. Runoff: Water flowing over the land surface to streams and rivers.
6. Groundwater Flow: Movement of water through aquifers.

The Water Balance Equation

The water balance equation is based on the Law of Conservation of Mass. For any hydrologic system (e.g., a catchment area or reservoir) over a time interval $\Delta t$:

Where:

• $P$ = Precipitation
• $R$ = Surface Runoff
• $G$ = Net Groundwater Flow (outflow - inflow)
• $E$ = Evaporation
• $T$ = Transpiration
• $\Delta S$ = Change in Storage

For a long period (e.g., annual), $\Delta S$ can be assumed to be zero, simplifying the equation to:

Where $E+T$ is often combined as Evapotranspiration (ET).

Meteorological Data

Hydrology relies heavily on meteorological data. Key parameters include:

• Temperature: Affects evaporation and snowmelt.
• Humidity: The amount of water vapor in the air. Relative Humidity (RH) is the ratio of actual vapor pressure to saturation vapor pressure.
• Wind Speed: Influences evaporation rates.
• Solar Radiation: The primary energy source driving the hydrologic cycle.