Hydrology and the Water Cycle

Hydrology

• Hydrology is the science of water, its occurrence, circulation, and distribution on Earth and in the atmosphere.
• Hydrologists study water resources, pollution, irrigation, flooding, and erosion.
• Their work is crucial for civil engineers in designing and managing water systems.

Hydrological Cycle

• The water cycle is a continuous process.
• Key stages include evaporation, condensation, precipitation, and infiltration.
• Evaporation: Liquid water turns into vapor.
• Condensation: Vapor cools and forms clouds, fog, or mist.
• Precipitation: Water falls to Earth as rain, snow, or hail.
• Infiltration: Water seeps into the ground.
• Runoff: Water flows over the Earth's surface.
• Water also moves through plants via uptake and transpiration.

Precipitation

• Precipitation is the release of water from the atmosphere.
• Orographic precipitation occurs when air masses are forced to rise over a mountain range.
• Aggregation is the process of ice particles growing by colliding with each other.
• Dry air contains no water vapor; cold clouds are predominantly composed of ice particles; warm clouds are predominantly composed of water droplets.
• Other forms include hail and dew.

Evaporation

• Evaporation is the process of water changing from a liquid to a gaseous state (vapor).
• Key factors affecting evaporation include summer weather patterns impacting water availability and vapor pressure deficit, which is the difference in saturation and actual vapor pressure.
• Evaporation pans are commonly used for measuring evaporation, with different models available.
• Thornthwaite and other methods estimate evaporation rates across various timeframes.
• Stomatal resistance (or canopy resistance) measures the rate of water vapor loss from plants (transpiration).
• Actual evaporation can be equal to potential if water availability is sufficient.

Interception and Surface Storage

• Interception is when precipitation is temporarily caught by plant surfaces.
• Throughfall is water that falls from plants to the ground.
• Stemflow is water running down the stems of plants.
• Direct throughfall and LAI (leaf area index) help to understand the proportion of water reaching the ground.
• Rainfall characteristics including amount and intensity affect interception and surface storage.
• Runoff and depression storage are key processes to understand. Precipitation can be stored in low-lying areas.

Groundwater

• Groundwater is water stored in the spaces between soil and rock particles.
• Pressure head and elevation head influence groundwater movement.
• Components of Bernoulli's equation—pressure, velocity, potential energy—are critical factors in groundwater flow.
• Darcy's law describes the relationship between hydraulic gradient and groundwater flow rate.
• Aquifers are geological formations that store significant amounts of water.
• Aquifers can be confined or unconfined.

Water Properties

• Effective Porosity: The sum of specific yield and specific retention describes the part of the total pore space that can yield water.
• Capillary Forces: Forces that act within pores and resist water flow.
• Transmissivity: A measure of groundwater's ability to transmit water through an aquifer, dependent on hydraulic conductivity.
• Storativity: The volume of water an aquifer releases or takes in per unit decline or rise in water level.

Measurement Techniques

• Microwave data provides improved detection methods for snow cover.
• Various methods can estimate precipitation interception.
• Measuring evaporation rates utilizes multiple approaches.
• Satellite data, weather, and other instruments measure aspects of the water cycle.
• Scientists calculate evaporation rate, assess water stress, and assess plant water dynamics.