Watershed Delineation Processes and Techniques

Watershed Concept

• A watershed is an area of land where all precipitation drains into a single outlet, such as a river, lake, or ocean. It's also known as a drainage basin.

• Watersheds collect surface runoff, rainwater, and groundwater, channeling it into a main water body.

Importance of Watersheds

• Watersheds are key units for studying hydrology and water resources.

• Most water in streams originates as precipitation within the watershed.

• Watershed characteristics control water movement.

• Watersheds are critical components of the hydrologic cycle.

Factors Influencing Watersheds

• Geology determines underground flow paths.

• Soil type affects infiltration and runoff rates.

• Topography controls surface flow direction and speed.

• Land use impacts water quality and timing of runoff.

Watersheds in Water Resource Management

• Watersheds define natural water flow within a region.

• They're frameworks for sustainable water distribution in agriculture, and industrial use.

• Watersheds help manage water distribution.

Watershed Delineation

• Watershed delineation is the process of identifying the boundaries of a watershed.

• It starts by selecting a watershed outlet.

• Outlets define the area contributing water to a specific location (e.g., a stream gauging station, reservoir or flood-prone area).

Watershed Outlet Selection

• Outlet location depends on the analysis purpose.

• For streamflow analysis, outlets are at gauging stations for water budget calculations.

• Geomorphic studies use outlets at stream junctions.

• Water resource management uses outlets at reservoirs, hydropower plants, or waste discharge sites.

• Flood management uses outlets in flood-prone areas to assess damage risk.

Importance of Manual Delineation

• Manual delineation provides valuable insight into the watershed concept.

• Manual verification is essential because digital methods can contain errors.

• Tools for manual delineation include topographic maps and stereoscopically viewed aerial photographs.

The Process of Manual Delineation

• Step 1: Begin at the watershed outlet (lowest point).

• Step 2: Draw a line perpendicular to contour lines, away from the stream bank.

• Step 3: Mark topographic high points around the stream and inspect contour patterns for accuracy.

• Step 4: Trace the divide until it encloses the headwaters and connects back.

Digital Delineation

• Digital delineation uses Digital Elevation Models (DEMs).

• DEMs provide elevation data from satellite radar reflections at grid points.

Digital Delineation Process

• Step 1: Input a DEM.

• Step 2: Fill Depressions (fill sinks).

• Step 3: Determine Flow Direction.

• Step 4: Calculate Flow Accumulation.

• Step 5: Identify Stream Network.

• Step 6: Connect Stream Links.

Water Balance

• Water balance describes the equilibrium between input, storage, and output of water.

• It's important for understanding water availability, irrigation planning, hydrological studies, and climate change impacts.

Water Balance Formula

• P = E + T + R + ΔS (Precipitation = Evaporation + Transpiration + Runoff + Change in Storage)

Time Series

• A time series is a sequence of discrete values of a variable over time with a constant interval.

Why Time Series is Important in Hydrology

• Time series data is used to identify trends and patterns in hydrological variables.

• It helps predict future behavior of hydrological events.

• Time series data facilitates managing water resources.

Specific Characteristics of Hydrologic Variables

• Key Assumptions: Sample elements represent the population, equal chances of selection, and larger sample sizes increase confidence

• Why Assumptions Fail: Hydrologic variables often show spatial and temporal variability.

Spatial Variability

• Refers to differences in hydrological processes across different locations in a watershed or region.

• Influenced by topography, soil type, vegetation, and land use.

Factors Influencing Spatial Variability

• Topography (steep slopes, flat areas)

• Vegetation (forested areas, urban areas)

• Soil types (clayey, sandy)

• Land use (agriculture, urbanization)

Temporal Variability

• Refers to changes in hydrological processes over time.

• Important for managing resources, predicting floods/droughts

• Includes precipitation, evaporation, runoff, and snowmelt

Key Hydrological Processes

• Precipitation

• Evaporation

• Runoff

• Snowmelt

Flow-Duration Curves (FDCs)

• Commonly used to show temporal variability of streamflow.

• Shows fraction of time a streamflow rate is exceeded.

Pressure-Temperature-Density Hydrostatic Relation

• Describes how atmospheric pressure changes with altitude.

• Pressure is the weight of the air column above a point.

Ideal Gas Law

• Provides a fundamental relationship between pressure, temperature, and density.

Moist vs. Dry Air

• Dry air primarily consists of nitrogen and oxygen (average molecular weight ~28.97 g/mol)

• Water vapor (H₂O) has a lower molecular weight (~18.02 g/mol).

Vapor Pressure and Saturation Vapor Pressure

• Vapor pressure is the partial pressure exerted by water vapor in the atmosphere.

• Saturation vapor pressure is the maximum vapor pressure at a given temperature

Partial Pressure and Adiabatic Processes

• Dalton's Law: Total pressure is the sum of partial pressures (air + water vapor)

• Adiabatic processes involve vertical motion of air parcels without heat exchange.

• Rising air cools (adiabatic cooling) and descending air warms (adiabatic warming).

Comparing Moist vs. Dry Air Lapse Rates

• Dry adiabatic lapse rate (Γda): Rate of temperature change from altitude change in air parcels without condensation.

• Observed gradients (e.g., near-surface lapse rates) are less steep than Γda due to latent heat release from condensation.

Latent Heat

• Energy required to change state of water without changing temperature.

• Example: Evaporation absorbs energy to break hydrogen bonds, cooling the surface.

• Change in phase(solid-liquid-gas) exchanges heat.

Key Formula for Latent Heat

• Latent Heat Transfer Formula is: λΕ = λv * ρw * E (λν: Latent heat of vaporization, ρw: Density of water, E: Evaporation rate).

Latent Heat and Sublimation

• Sublimation is the direct transition between snow/ice to vapor without becoming liquid.

• Latent Heat of Sublimation: λЕ = (λv +λf)· ρw · E (where λf is heat of fusion, and λν is heat of vaporization).

Temperature Dependence of Latent Heat (for evaporation)

• Latent heat of vaporization (Lv) slightly decreases with increasing temperature.

Practical Importance of Latent Heat

• Has a significant role in weather systems and energy exchanges in nature.

• Important in water cycle modeling, estimation of water resources.

Measures of Humidity

• Absolute Humidity: Mass of water vapor per unit volume of air.

• Specific Humidity: Mass of water vapor per unit mass of dry air.

• Relative Humidity: Ratio of actual vapor pressure to saturation vapor pressure (expressed as a percentage, commonly used).

Explore the essential concepts and techniques related to watershed delineation in this quiz. Covering both manual and digital methods, you'll learn about key factors that influence water movement and quality management. Test your knowledge on watershed definitions, calculations, and the significance of vegetation.