Week 2 PDF - Watershed Concepts

Understanding the Watershed Concept EXPLORING THE ROLE OF WATERSHEDS IN THE HYDROLOGIC CYCLE MINA FAGHIH 2025.01.13 A watershed is an area of land where all the water that falls as precipitation (rain, snow, etc.) drains into a common outlet, such...

Understanding the Watershed Concept EXPLORING THE ROLE OF WATERSHEDS IN THE HYDROLOGIC CYCLE MINA FAGHIH 2025.01.13 A watershed is an area of land where all the water that falls as precipitation (rain, snow, etc.) drains into a common outlet, such as a river, lake, or ocean. Also known as a drainage basin , 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. The Importance of Watersheds in Water Resource Management: 1-Watersheds and Water Resource Management Define 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. 2-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. 3-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: Understanding the Process What is 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. (The outlet is the location where water exits the watershed (e.g., a stream gauging station, reservoir, or flood-prone area)). 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 map. 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, Inspect contour patterns frequently to ensure accuracy. Step 4: Trace the divide until it encloses the headwaters and connects back.  Digital Delineation Digital watershed delineation is 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 1. Input Digital Elevation Model (DEM) 2. Fill Sinks (Depressions) 3.Flow Direction 4. Flow Accumulation 5. Stream Network 6. Stream Links Convert DEM to Flow Direction Calculate the slop from each cell to each of neighbors 𝑟𝑖𝑠𝑒 𝑒𝑙𝑣𝑓𝑟𝑜𝑚 −𝑒𝑙𝑣𝑡𝑜 Slop= = 𝑟𝑢𝑛 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 Distance is same as model path Water Balance ❑ Water balance refers to the equilibrium between the input, storage, and output of water in a particular system, such as a watershed, river basin, or a specific location. It is essential for understanding water availability, irrigation planning, hydrological studies, and climate change impacts. Example of water balance: Exercise 3.11.2c of 'Introduction to Physical Hydrology'. A polder is a low-lying area below sea level from where we have to pump out the water in order to keep our feet dry. We have a polder area of 5 km². The polder is made up of open water and a land part: 2 km² is open water, and 3 km² is land. We have a precipitation of 750 mm/year, an open-water evaporation of 600 mm/year, and an evaporation from the land part of the polder of 420 mm/year. The pumping discharge is 2 x 10^6 m³/year, and we have a storage coefficient for the land part of the polder of 0.4. The storage coefficient is defined as the ratio of added or extracted water depth by precipitation or evaporation (mm) and the accompanying change in water table level (mm); the storage coefficient is a dimensionless number. Determine the seepage (mm/day) for a year in which the water table and open water level have risen 200 mm. How do we go about solving this? Water Balance Formula P=E+T+R+ΔS P-(R+G+E+T)= ΔS (P+I)-(R+G+E+T)= ΔS Where, P = Precipitation, R = Surface runoff G = Net groundwater flows out of the catchment E = Evaporation T = Transpiration ΔS = Change in storage I = other inflow  Time Series A time series is a time-ordered sequence of discrete values of a variable separated by a constant time interval Δt. Why is Time Series Important in Hydrology? Identifying Trends and Patterns seasonal Variability of Daily Minimum Temperature for three years (1971-1973) at Central Park, New York Special Characteristics of Hydrologic Variables: Challenges in Analyzing Hydrologic Variables: Key Assumptions in Classical Statistics: 1.Sample elements equally represent the population. 2.Equal chance for all elements to be selected. 3.Larger sample size increases confidence. Why These Assumptions Often Fail in Hydrology: Spatial Variability (Spatial Distribution Issues) Temporal Variability (Temporal Distribution Issues) Solutions: Use specialized statistical techniques for spatial and temporal data. Address challenges with methods like trend analysis and cycle detection.  spatial variability Spatial variability refers to the differences in hydrological processes that occur across different locations in a watershed or region. These differences are influenced by various physical and environmental factors, such as topography, soil types, vegetation, and land use. Temporal Variability in Hydrology ❑ In hydrology, temporal variability refers to the variation in hydrological processes that occurs over time. These processes, such as precipitation, evaporation, runoff, and snowmelt, change continuously, and understanding these variations is key to managing water resources, predicting floods, and responding to droughts.. Temporal Variability of Streamflow Long-term Average Streamflow : Indicates potential water availability for human use. Affected by seasonal and interannual variations in precipitation, snowmelt, and evapotranspiration. Key Points: Streamflow in unregulated rivers varies widely, even in humid regions. Typical variability spans over three or more orders of magnitude. "Available water" is best represented by the flow rate exceeded 95% of the time. Flow-Duration Curves and Variability Management Flow-Duration Curves (FDCs): Duration curves are commonly used to depict the temporal variability of streamflow; Show the fraction of time a streamflow rate is exceeded. Illustrate variability and the limitations of using average streamflow as a metric. Example (Pemigewasset River, NH): Average flow: 39.2 m³/s. Flow exceeded 95% of the time: 5.38 m³/s (14% of average flow). Maximum to minimum daily flow ratio: 1,270. Pressure-Temperature-Density hydrostatic relation: The hydrostatic relation describes how atmospheric pressure changes with altitude. Atmospheric pressure at any given point is the weight of the air column above it. As altitude increases, there is less air overhead, which means less weight and therefore lower pressure. Ideal Gas Law The Ideal Gas Law provides a fundamental relationship between pressure, temperature, and density: P=ρRT where: T= absolute temperature P = pressure (Pa) z = altitude (m) ρ= air density (kg/m³) g = gravitational acceleration (9.81 m/s^2) R = specific gas constant (J/kg·K) Moist Air vs. Dry Air Molecular Weight: Dry air consists primarily of nitrogen (N2) and oxygen (O2), with an average molecular weight of ~28.97 g/mol. Water vapor (H2O) has a molecular weight of 18.02 g/mol, which is lighter than both 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 that can exist at a given temperature. Calculated using empirical equations: Key Points: e* increases with temperature (shown graphically in figure). At saturation, any addition of water vapor or lowering of temperature causes condensation (e.g., fog, clouds).  Partial Pressure and Adiabatic Processes Dalton’s Law of Partial Pressures: Total pressure is the sum of partial pressures: P=Pda+e Pda: Pressure of dry air, e: Vapor pressure. Adiabatic Processes: Vertical motion of air parcels with no heat exchange: Adiabatic cooling: Rising air cools due to pressure decrease. Adiabatic warming: Descending air warms due to pressure increase. Dry Adiabatic Lapse Rate (Γda): Rate of temperature change with altitude: Γda=9.75 K/km Comparing Moist vs. Dry Air Lapse Rates Dry Adiabatic Lapse Rate (Γda): Applies to air parcels without condensation. Observed Temperature Gradient: Near-surface lapse rate: ~6.5°C/km. Less steep than Γda due to latent heat release from condensation. Importance of Lapse Rates: Affect cloud formation, weather systems, and turbulence.  Latent Heat Latent Heat: Energy required to change the state of water without changing its temperature. Example: Evaporation absorbs energy to break hydrogen bonds, cooling the surface. Condensation: Bonds reform, releasing heat and warming the surroundings. Latent-Heat Exchange: Energy exchange between latent heat processes. Key Formula for Latent Heat Latent Heat Transfer Formula: λ E =λ v ⋅ρ w ⋅E Where: 𝜆v : Latent heat of vaporization (MJ/kg) 𝜌w : Density of water (kg/m³) E: Evaporation rate (depth/time) Latent Heat and Sublimation Sublimation: Process where snow or ice transitions directly to vapor. Latent Heat of Sublimation λ E =(λ v +λ f )⋅ρ w ⋅E Where: 𝜆f : Latent heat of fusion = 0.334 MJ/kg (constant) 𝜆v : Latent heat of vaporization  Temperature Dependence of Latent Heat Formula: λv=2501−0.00236⋅Ts Where Ts is in °C, and λv is in MJ/kg. Latent Heat of Vaporization (for Evaporation): For water vapor: can vary slightly with temperature. Latent Heat of Vaporization (𝜆ᵥ) decreases slightly with increasing temperature. Examples of Latent Heat Values for Water: Latent heat of vaporization (at 100°C): Lv=2.26 MJ Latent heat of fusion (at 0°C): Lf=334 kJ/kg Practical Importance of Latent Heat Importance: Weather systems Energy exchanges in nature Water cycle modeling Example: Sublimation in snowpack loss and water resource estimation.  Measures of Humidity: Absolute, Specific, and Relative Humidity Absolute Humidity (ρv): Mass of water vapor per unit volume of air. Related to vapor pressure using the Ideal Gas Law: Rv=461 J/kg Specific Humidity (q): Mass of water vapor per unit mass of dry air: p: Total air pressure Relative Humidity (RH): Ratio of actual vapor pressure to saturation vapor pressure: Key points Absolute humidity describes the actual water vapor content. Specific humidity is often used for atmospheric studies. Relative humidity is a familiar and practical measure.