Meteorology Chapter on Latent Heat and Humidity

Watershed Concepts

• A watershed is an area of land where all precipitation drains into a common outlet, such as a river, lake, or ocean.
• Also known as a drainage basin.
• It collects surface runoff, rainwater, and groundwater, channeling it into a main water body.

Importance of Watersheds

• Key unit for studying hydrology and water resources.
• Most water in streams originates as precipitation within the watershed.
• Watershed characteristics control water movement.
• Watersheds are a key part of the hydrologic cycle.

Factors Influencing Watersheds

• Geology: Determines underground flow paths.
• Soils: Affect infiltration and runoff rates.
• Topography: Controls surface flow direction and speed.
• Land Use: Impacts water quality and timing.

Watersheds and Water Resource Management

• Define the natural flow of water within a region.
• Help manage water distribution for agriculture, drinking, and industrial use.
• Act as a framework for sustainable water use.

Watersheds and Water Quality

• Activities within watersheds impact water quality.
• Pollutants from farms, factories, or urban areas flow into rivers and lakes.
• Proper management reduces pollution and protects water quality.

Role of Watersheds in Flood Control

• Predict flooding during heavy rainfall or snowmelt.
• Size, shape, and vegetation influence water flow.
• Manage runoff to reduce flood risks.

Watershed Delineation

• Process of identifying the boundaries of a watershed.
• Begins with selecting the watershed outlet.
• Outlets define the area contributing water to a specific location.

Watershed Outlets Selection

• The location depends on the purpose of the analysis.
• Streamflow Analysis: Outlets at gauging stations for water budgets.
• Geomorphic Studies: Outlets at stream junctions or where streams meet larger water bodies.
• Water Resource Management: Outlets at reservoirs, hydroelectric plants, or waste-discharge sites.
• Flood Management: Outlets in flood-prone areas to assess damage risk.

Importance of Manual Delineation

• Valuable Insights: Manual delineation provides valuable insight into the watershed concept.
• Essential for Validation: Digital methods often contain errors, requiring manual verification.

Tools Needed for Manual Delineation

• Topographic maps.
• Stereoscopically viewed aerial photographs.

The Process of Manual Delineation

• Step 1: Start at the watershed outlet (lowest point).
• Step 2: Draw a line perpendicular to contour lines, away from the stream bank.
• Step 3: Mark the location of the topographic high points around the stream, inspecting contour patterns frequently to ensure accuracy.
• Step 4: Trace the divide until it encloses the headwaters and connects back.

Digital Delineation

• Based on Digital Elevation Models (DEMs).
• DEMs provide elevation data at grid points, derived from satellite radar reflections.

Advantages of Digital Delineation

• Rapid data processing.
• Accessibility of hydrological insights (e.g., elevation, slope).
• Elimination of tedious manual efforts.

Digital Delineation Process

• Input Digital Elevation Model (DEM).
• Fill Sinks (Depressions).
• Flow Direction.
• Flow Accumulation.
• Stream Network.
• Stream Links.

Water Balance

• Equilibrium between input, storage, and output of water in a particular system.
• Essential for understanding water availability, irrigation planning, hydrological studies, and climate change impacts.

Example of Water Balance

• A polder, a low-lying area below sea level, has an area of 5 km².

Water Balance Equation

• P = E + T + R + AS
• P - (R + G + E + T) = AS

Hydrological Modeling

• To estimate available water (Reservoirs, Lakes, Groundwater).
• Application of the Water Balance Formula.

Agricultural Planning

• To manage irrigation based on precipitation and evapotranspiration.

Flood and Drought Prediction

• Extreme weather events.
• Water availability.

Time Series

• Time-ordered sequence of discrete values of a variable separated by a constant time interval (∆t).
• Precipitation, Streamflow, Groundwater Levels, Temperature, Evapotranspiration

Why is Time Series Important in Hydrology?

• Unveiling the power of time series in hydrology.
• Identifying trends and patterns.
• Predicting future behavior.
• Managing water resources.
• Understanding hydrological events.

Identifying Trends and Patterns

• Seasonal variability of Daily Minimum Temperature.

Special Characteristics of Hydrologic Variables

• Key assumptions in classical statistics.
• Why these assumptions often fail in hydrology.
• Spatial variability (spatial distribution issues).
• Temporal variability (temporal distribution issues).
• Solutions.

Spatial Variability

• Differences in hydrological processes across different locations in a watershed or region.
• Influenced by various physical and environmental factors (topography, soil types, vegetation, and land use).

Factors Influencing Spatial Variability

• Topography (steep slopes vs. flat areas).
• Vegetation (forested areas vs. urban areas).
• Soil types (clayey soils vs. sandy soils).
• Land Use (agriculture vs. urbanization).

Temporal Variability in Hydrology

• Variation in hydrological processes over time.
• Key processes: precipitation, evaporation, runoff, snowmelt.

Key Hydrological Processes

• Precipitation, Evaporation, Runoff, Snowmelt

Timescales

• Daily, Seasonal, Annual, Multi-Decadal

Importance

• Water Resource Management, Flood Prediction, Drought Response

Temporal Variability of Streamflow

• Long-term average streamflow: Indicates potential water availability.
• Affected by seasonal and interannual variations in precipitation, snowmelt, evapotranspiration.
• Key points: Streamflow in unregulated rivers varies widely, even in humid regions.
• Typical variability spans over three or more orders of magnitude.

Flow-Duration Curves (FDCs)

• Duration curves depict the temporal variability of streamflow.
• Show the fraction of time a streamflow rate is exceeded.
• Illustrate variability and limitations of using average streamflow as a metric.

Pressure-Temperature-Density

• Hydrostatic relation: How atmospheric pressure changes with altitude.
• Ideal Gas Law: Fundamental relationship between pressure, temperature, and density.

Moist Air vs. Dry Air

• Dry air primarily consists of nitrogen (N2) and oxygen (O2).
• Water vapor has a lower molecular weight than nitrogen and oxygen.

Vapor Pressure and Saturation Vapor Pressure

• Vapor Pressure: Partial pressure exerted by water vapor in the atmosphere.
• Saturation Vapor Pressure (e*): Maximum vapor pressure at a given temperature.

Partial Pressure and Adiabatic Processes

• Dalton's Law: Total pressure equals the sum of partial pressures (P = Pda + e).
• Adiabatic cooling: Rising air cools as pressure decreases.
• Adiabatic warming: Descending air warms as pressure increases.

Comparing Moist vs. Dry Air Lapse Rates

• Dry Adiabatic Lapse Rate (da): Applies to air parcels without condensation.
• Observed Temperature Gradient: Near-surface lapse rate is less steep than da due to latent heat release from condensation.
• Importance: Affects cloud formation, weather systems, and turbulence.

Latent Heat

• Energy required to change the state of water without changing its temperature.
• Example: Evaporation absorbs energy, cooling the surface.
• Condensation releases energy, warming the surroundings.

Key Formula for Latent Heat

• Latent Heat Transfer Formula: λE = λv * ρw * E

Latent Heat and Sublimation

• Sublimation: Process where snow or ice transitions directly to vapor.
• Latent Heat of Sublimation: λE = (λv + λf) * ρw * E

Temperature Dependence of Latent Heat

• Latent Heat of Vaporization (λ₁) decreases slightly with increasing temperature.

Practical Importance of Latent Heat

• Weather systems, energy exchanges in nature, water cycle modeling
• Example: Sublimation in snowpack loss and water resource estimation.

Measures of Humidity

• Absolute Humidity (pv): Mass of water vapor per unit volume of air.
• Specific Humidity (q): Mass of water vapor per unit mass of dry air.
• Relative Humidity (RH): Ratio of actual vapor pressure to saturation vapor pressure.