Hydrology Reading Notes PDF

Reading 1.1: ============ Introduction to Physical Hydrology 1\. Overview of Physical Hydrology Definition: Study of the occurrence, movement, and physical properties of water on and below the Earth\'s surface, excluding oceanic water (studied in oceanography). Major Water Types: Atmospheric Wa...

Reading 1.1: ============ Introduction to Physical Hydrology 1\. Overview of Physical Hydrology Definition: Study of the occurrence, movement, and physical properties of water on and below the Earth\'s surface, excluding oceanic water (studied in oceanography). Major Water Types: Atmospheric Water: Found in vapor, liquid (rain), or solid (snow) forms. Surface Water: Includes rivers, lakes, and ponds. Soil Water: Stored above the water table in unsaturated zones. Groundwater: Stored in saturated zones below the water table. 2\. Water Storage on Earth Freshwater: Only 2.5% of Earth\'s water. 69% in polar ice. 30% in freshwater groundwater. 1% in surface water, soil water, and atmospheric water. Significant portion of groundwater is brackish or saline. 3\. The Hydrological Cycle Describes the continuous movement of water through evaporation, condensation, precipitation, and flow. Key Processes: Evaporation: Purifies water and leaves salts in oceans. Condensation: Formation of water droplets in clouds. Precipitation: Rain, snow, or hail falling to Earth. Runoff and Flow: Water moves via overland, soil, and groundwater flows to surface water bodies. Residence times: Oceans: Thousands of years. Atmosphere: \~10 days. Groundwater: \~20,000 years. 4\. Drainage Basins Definition: Geographical area draining into a river or reservoir. Boundaries: Often determined by the highest landscape points for surface water; groundwater boundaries may differ due to permeability variations. Processes within a drainage basin: Precipitation: Gross (all rainfall) vs. net (reaching the ground). Interception: Rain captured by vegetation. Infiltration: Water enters soil pores/cracks. Percolation: Water moves to groundwater. Overland Flow: Water flows across surfaces to rivers. 5\. Water Balance Equation where: In: Water entering the area (e.g., precipitation). Out: Water leaving (e.g., evaporation, runoff). ΔS/Δt: Change in storage over time. Applications: Long-term averages set ΔS=0. Useful for understanding effects of land-use and climate changes. 6\. Global and Regional Implications Global warming can speed up the hydrological cycle, potentially increasing extreme weather events. The melting of polar ice due to climate change could lead to significant sea-level rise, with long-term and short-term implications for water storage and flow. Key Figures and Tables Figure 1.2: The hydrological cycle (illustrates evaporation, precipitation, flow, and storage processes). Table 1.1: Global water distribution by type and percentage. Table 1.2: Example of the water balance in the Netherlands, showing seasonal variations in precipitation, evaporation, and storage. Important Concepts Greenhouse Effect: Water vapor and greenhouse gases maintain Earth\'s average surface temperature at 15°C; enhanced greenhouse effect could lead to global warming. Residence Time: Average time water resides in a specific part of the cycle (e.g., oceans, atmosphere, or groundwater). Interception Loss: Water captured by vegetation and evaporated before reaching the ground. Reading 1.2.: ============= **Chapter 2: Virtual Campus in Hydrology and Water Resources** **Watershed Characteristics** **2.1. Watershed Defined** - **Definition**: A watershed is a geographical unit for analyzing the hydrological cycle, often defined by the area contributing water to a stream at a given cross-section. - **Delineation**: - May differ between topographical and real watershed, especially in karst areas. - Artificial changes (roads, reservoirs) must be considered. - Conventional method uses topographic maps to trace divides based on contour lines. **2.2. Watershed Characteristics** **2.2.1. Physical Characteristics** 1. **Geomorphology**: - **Surface & Shape**: - Shape influences hydrograph behavior (e.g., long shapes result in slower flows). - Gravelius\'s index (KG) measures watershed shape via its perimeter and equivalent circular area. - **Orientation**: - Orientation affects snowmelt rates (e.g., North-South watersheds melt slower). 2. **Topography**: - **Hypsographical Curve**: Shows elevation distribution and is useful for hydrological comparisons. - **Altitudes**: - Max and min altitudes define watershed limits. - Average and medium altitudes provide hydro-meteorological insight. - **Average Slope**: - Indicates watershed reaction to rainfall. - Calculated using contour line data. 3. **Hydrography**: - Defined by the network of watercourses influenced by geology, climate, and relief. - Characteristics: - Strahler\'s classification categorizes stream orders. - Longitudinal slope and network development degree impact runoff and hydrological processes. 4. **Agro-Pedo-Geological Factors**: - Soil and vegetation affect water infiltration, retention, and runoff. - Vegetation reduces extreme runoff but offers limited drought/flood resilience. - Geological substrate influences runoff and groundwater flow, especially in karst regions. **2.2.2. Hydrological Characteristics** 1. **Hydrograph**: - Represents the discharge (Q) versus time post-rainfall. - Reaction intensity and speed depend on the watershed. 2. **Concentration Time**: - Defined as the maximum time for water to reach the watershed outlet. - Components: - **th**: Humidification time. - **tr**: Runoff time. - **ta**: Moving time through hydrographic networks. 3. **Isochrones**: - Contour lines joining points of equal concentration time. - Useful for analyzing hydrological behavior and constructing hydrographs. Reading 1.3. : ============== **Watershed Planning and Management** **5.1. Characteristics of Watersheds** - **Definition**: A watershed is a geographical unit of hydrological analysis that includes surface and subsurface features, climate, soils, vegetation, and land use. - **Water Flow**: Water from precipitation moves through soil, groundwater, and streams to larger water bodies. **5.2. Classification of Watersheds** 1. **Based on Size**: - **Small (\2500 km²)**: - Channel storage dominant. - Less sensitive to short-duration rainfalls. 2. **Based on Land Use**: - **Agricultural**: High infiltration and erosion due to cultivation. - **Urban**: Increased runoff, reduced infiltration, and flood vulnerability. - **Mountainous**: Dominated by surface runoff, significant snowmelt contribution. - **Forest**: High interception and subsurface flow; reduces peak discharge. - **Desert**: Low vegetation, minimal stream development, and high evaporation. - **Coastal**: Influenced by tidal actions; prone to flooding and saltwater intrusion. - **Wetland**: Minimal infiltration, significant runoff, and prolonged hydrograph peaks. - **Mixed**: Combines multiple land uses, common in India. **5.3. Geomorphological and Physical Characteristics** 1. **Basin Characteristics**: - **Area**: Key factor in hydrological analysis; determines runoff potential. - **Length**: Longest dimension affecting travel time of water. - **Slope**: Reflects runoff velocity and erosion potential. - **Shape**: Affects flow concentration; circular shapes have higher flood peaks. 2. **Channel Characteristics**: - **Order**: Hierarchical stream classification (Horton-Strahler system). - **Length**: Increases geometrically with order (Horton's law of lengths). - **Slope**: Affects flow velocity. - **Profile**: Gradual slope towards the outlet. - **Drainage Density**: Indicates closeness of streams; affects flow efficiency. **5.4. Quantitative Characteristics** - **Form Factor**: Ratio of basin area to square of axial length; measures basin compactness. - **Circularity Ratio**: Comparison of basin area to equivalent circle. - **Elongation Ratio**: Ratio of equivalent circle diameter to basin length. - **Compactness Coefficient**: Perimeter-to-circumference ratio. Reading 3.1.: ============= **Elements of the Hydrologic Cycle** **1. Overview of the Hydrologic Cycle** - **Components**: Includes precipitation, interception, infiltration, percolation, evaporation, transpiration, and runoff. - **Key Points**: - Precipitation drives the hydrologic cycle. - Water undergoes processes like infiltration and evaporation before contributing to surface runoff or discharge. **2. Precipitation** - **Mechanisms**: - **Condensation**: Forms water droplets on condensation nuclei. - **Convection**: Urban heat islands and adiabatic cooling can cause localized precipitation. - **Frontal and Orographic Lifting**: Warm air rising over cold fronts or mountains cools to form precipitation. - **Measurement**: - **Instruments**: Includes standard precipitation gauges and tipping bucket rain gauges for short-term data. - **Error Analysis**: Errors arise from gauge type, wind, and location; systematic errors identified using double mass analysis. **3. Infiltration** - **Process**: - Water infiltrates the soil and percolates into groundwater or evaporates. - Movement varies based on soil type and topography. - **Measurement**: - **Ring Infiltrometer**: Measures infiltration under ponded conditions. - **Horton Equation**: Describes infiltration capacity over time. - **Key Variables**: - Initial infiltration is high but decreases over time as soil saturates. **4. Evaporation and Transpiration** - **Evapotranspiration**: Combination of evaporation and transpiration, influenced by energy availability. - **Methods of Measurement**: - **Pan Evaporimeter**: Overestimates evaporation; corrected by a coefficient. - **Penman Equation**: Calculates potential evapotranspiration using climate data. - **Factors**: Soil type, temperature, wind, and water availability affect rates. **5. Runoff** - **Surface Runoff**: Visible water on the ground flowing to streams, lakes, or seas. - **Components**: - **Direct Runoff**: From rainfall events. - **Baseflow**: Sustained by groundwater. - **Hydrograph Analysis**: - Displays runoff over time. - Helps separate direct runoff and baseflow. - **Urban vs. Natural Runoff**: - Urban areas: Quick runoff, high peaks, and reduced infiltration. - Natural areas: Slower runoff, promoting biodiversity and pollution reduction. **6. Statistical Analysis** - **Precipitation Statistics**: - Rainfall intensity, duration, and frequency aid in infrastructure design. - Intensity-Duration-Frequency (IDF) curves help predict extreme events. - **Runoff Estimation**: - Use runoff coefficients to estimate water volume based on precipitation. **7. Applications and Practical Examples** - **Calculation Examples**: - Estimating effective rainfall and losses using methods like the Thiessen polygon. - Using infiltration and hydrograph data to calculate runoff volumes and coefficients. - Determining pollutant transport in runoff for environmental assessments. **8. Challenges in Hydrological Measurements** - **Error Margins**: - Precipitation and discharge data often contain 1-20% errors. - Observations require careful calibration and uncertainty acknowledgment. Reading 3.2.: ============= **Precipitation Overview** **Main Component of the Hydrologic Cycle**: Precipitation drives the hydrologic cycle. **Formation Mechanisms**: Cooling of air (via advection, radiative cooling, or lifting). Condensation of water vapor on nuclei like dust and salt particles. Growth of droplets/ice crystals via accretion and diffusion. Moist air supply maintains precipitation intensity. **Types of Precipitation** **Drizzle**: Droplets (0.1-0.5 mm diameter). **Rain**: Droplets (\>0.5 mm); categorized into light, moderate, or heavy. **Snow**: Ice crystals forming snowflakes, with density varying based on temperature. **Sleet and Freezing Rain**: Ice pellets and rain freezing upon contact with cold surfaces. **Hail**: Ice balls (5-50 mm), forming during convective storms. Other types include snow pellets, soft hail, and dew. **Major Weather Systems for Precipitation** **Extratropical Cyclones and Fronts**: Cold fronts: Steeper, cause strong precipitation (stratiform or convective). Warm fronts: Gradual slopes, leading to widespread clouds and moderate rainfall. **Convective Systems**: Thunderstorms with updrafts and downdrafts; often organized into mesoscale systems. Squall lines and mesoscale convective complexes are common forms. **Tropical Systems**: Monsoons and tropical cyclones (e.g., hurricanes, typhoons) bring heavy rainfall. **Orographic Effects**: Increased precipitation on windward slopes; rain shadows on leeward sides. **Precipitation Distribution** **Measurement Techniques**: Thiessen polygon, isohyetal methods, and inverse distance weighting. Objective methods like kriging improve accuracy. **Temporal Distribution**: Hyetographs (precipitation intensity vs. time) and mass curves (cumulative precipitation). Double mass analysis ensures data consistency over time. **Design Rainfall Data**: Intensity-duration-frequency relationships for infrastructure planning. **Interception** **Mechanism**: Precipitation intercepted by vegetation, some lost to evaporation before reaching the ground. **Factors Affecting Loss**: Dense vegetation and moderate rainfall intensities increase interception loss. Storage capacity and evaporation rates are key determinants. **Empirical Estimates**: Relationships established using leaf area index and storage parameters. **Reliability of Measurements** **Challenges**: Wind-induced errors: Distortion around precipitation gauges affects accuracy. Systematic errors in historical data require caution and corrections. Reading 5.1.: ============= **Hydrological Measurements Guide** **Water Levels of Rivers, Lakes, and Reservoirs** **Definition**: Water level (stage) is the elevation of a water surface relative to a datum. **Uses**: Forecasting flows, flood hazard delineation, structure design. **Measurement Instruments**: **Non-recording Gauges**: Graduated staff, inclined gauges, wire-weight gauges. **Recording Gauges**: Float-operated, pressure-actuated, ultrasonic, radar-based sensors. **Site Selection**: Hydraulic conditions, accessibility, and purpose determine gauge location. **Special Considerations**: Freeze prevention for winter operation, telemetering systems for real-time data. **Ice Observations** **Importance**: Critical for navigation, structure design, flood management, and ice jam prediction. **Key Observations**: Ice thickness, freeze and breakup dates, ice flow, and stationary ice coverage. **Measurement**: Augers for thickness, aerial or satellite surveys for large-scale observations. **Frequency**: Every 5-10 days during critical periods; aircraft observations for special cases. **Discharge Measurements** **Definition**: River discharge is the rate of water flow through a cross-section. **Methods**: **Current Meter**: Velocity-area approach; requires stable channels and accurate site selection. **Float Method**: Uses surface or rod floats; best for turbulent or hard-to-access sites. **Dilution Method**: Uses tracers to measure mixing and flow; suitable for turbulent flows. **Indirect Methods**: Slope-area, culvert, contraction, and weir flow calculations used for floods or inaccessible sites. **Key Factors**: Accurate cross-section and velocity measurements. Site selection critical for reliability. Specialized techniques for ice-covered, tidal, or unstable channels. **Non-Traditional Stream Gauging Methods** **Emerging Techniques**: **Moving Boat**: Measures flow using instruments on a boat across a cross-section. **Ultrasonic/Doppler**: Uses sound waves for velocity and discharge measurement. **Electromagnetic**: Detects water movement using magnetic fields. **Challenges and Special Conditions** **Unstable Channels**: Require artificial controls or alternative gauging sites. **Mountain Streams**: High velocity and debris require dilution or tailored methods. **Weed Growth**: Regular clearing necessary; affects velocity and discharge accuracy. **General Best Practices** Regular maintenance of gauges and recalibration of instruments. Safety precautions during extreme conditions like floods or ice-covered rivers. Proper training for observers to ensure consistent and accurate data. Reading 5.2.: ============= **Runoff, Stream Flow, Reservoir Yield, and Water Quality** **Introduction** - **Runoff**: Flow of water on the Earth\'s surface, either as overland or channel flow. - **Stream Flow**: Flow in natural streams; a part of the hydrological cycle, often expressed in volume (e.g., cubic meters or feet) or flow rate (e.g., m³/s). - **Runoff Depth**: Spatially averaged runoff obtained by dividing runoff volume by the basin area. **Runoff Processes** 1. **Surface Runoff**: - Caused by precipitation exceeding soil infiltration or saturation. - Types: Hortonian flow, saturation flow, surface crusting. 2. **Stream Flow**: - Comprises surface runoff, interflow, and groundwater flow. **Runoff Measurement Techniques** 1. **Direct Methods**: - Using **current meters** to measure velocity at specified verticals. - Common tools: Standard Price meters and Pygmy meters for shallow depths. - Portable electromagnetic sensors for additional flexibility. 2. **Moving-Boat Method**: - Used for large streams; measures discharge while a boat traverses a cross-section. - Combines stream velocity with boat velocity. 3. **Other Methods**: - **Tracer-dilution**: For small, turbulent streams. - **Volumetric or flumes/weirs**: Effective for small flows. 4. **Indirect Methods**: - **Slope-Area Method**: Uses Manning\'s formula to relate discharge to channel cross-sectional area and slope. - **Contracted-Opening Method**: Suitable for bridge sites with known contraction geometry. **Continuous Records** 1. **Conventional Gaging Stations**: - Measure stage (water level) continuously, using rating curves to determine discharge. 2. **Advanced Techniques**: - **Ultrasonic Meters**: Measure velocity via sound signals. - **Electromagnetic Meters**: Compute discharge based on near-wall velocity. **Hydrographs** - Graphs that display variations in water flow over time. - **Components**: - Rising limb, recession limb, and baseflow. - **Types**: - Event-based (e.g., storms, floods) or continuous (e.g., annual flows). **Factors Affecting Hydrographs** 1. **Watershed Characteristics**: - Shape, size, slope, and drainage patterns. 2. **Precipitation Losses**: - Initial abstraction, infiltration, and évapotranspiration. **Statistical Analysis** - Used to predict flood frequencies and return periods based on historical data. Reading 5.3.: ============= **Hydrologic Measurement** **Introduction** - Hydrologic measurements gather data on processes like precipitation, streamflow, and evaporation to aid hydrologic simulations for design, analysis, and decision-making. - Key developments include the computerization of hydrologic data and advances in electronic measurement and real-time analysis. **6.1 Hydrologic Measurement Sequence** 1. **Sensing**: Sensors measure hydrologic phenomena either directly (e.g., rain gauges) or indirectly (e.g., thermometers). 2. **Recording**: Data is preserved manually or automatically (paper charts, tapes, or digital systems). 3. **Transmission**: Remote data is transferred to central locations via manual or real-time methods (e.g., microwave, satellite). 4. **Translation**: Data converted from field-record formats to digital formats for storage. 5. **Editing**: Data checked for errors or redundancies. 6. **Storage**: Archived in systems like WATSTORE. 7. **Retrieval**: Accessed in machine-readable or print formats. **6.2 Measurement of Atmospheric Water** - **Precipitation**: - **Non-recording gauges**: Measure depth manually. - **Recording gauges**: Automatically record rain intensity. - **Radar**: Estimates rainfall rates and tracks precipitation patterns. - **Evaporation**: Measured using pans (e.g., Class A) with correction factors (pan coefficients). - **Evapotranspiration**: Measured using lysimeters (soil tanks simulating ground cover). **6.3 Measurement of Surface Water** - **Water Surface Elevation**: - Crest stage gauges capture flood peaks. - Staff gauges and bubble gauges measure continuous water levels. - **Flow Velocity**: - Measured using current meters (manual or electromagnetic). - Velocity profiles obtained by sampling at different depths. - **Streamflow**: - Computed using cross-sectional velocity and depth data. - Rating curves convert gage heights to flow rates. **6.4 Measurement of Subsurface Water** - **Soil Moisture**: Measured by drying soil samples or using neutron probes. - **Infiltration**: Measured using ring infiltrometers or tracers. - **Groundwater Levels**: Observed using wells with float devices or electronic sensors. **6.5 Hydrologic Measurement Systems** 1. **Urban Hydrology Monitoring**: - Includes rain gauges, flow sensors, and microprocessor-based data logging. - Real-time systems monitor rainfall, runoff, and water quality. 2. **Flood Early Warning Systems**: - Use remote sensors, pressure transducers, and radio communications for rapid data transmission. 3. **Large River Systems**: - Real-time data acquisition networks provide flood forecasts. **6.6 Measurement of Physiographic Characteristics** - Physiographic factors (e.g., drainage area, slope, land use) influence runoff characteristics and are mapped using topographic data and aerial photography. Reading 7.1.: ============= **Hydrologic Frequency Analysis** **Overview** - **T-year Event**: Refers to an event with a return period of T years. - Example: A 100-year flood has a 1% annual probability of occurrence. - QT denotes the estimated magnitude of such an event. - Statistical tools, not laws, are used to predict hydrologic events, emphasizing the inherent uncertainties. **Key Concepts** 1. **Return Period and Probability** - Return period (T): The average time interval between occurrences of an event of similar magnitude. - Annual probability of a T-year event, PT = 1/T. - Probability of occurrence in n years, P = 1 - (1 - PT)\^n. 2. **Risk Analysis** - Risk increases with shorter return periods for a fixed design life. - Economic optimization balances cost against acceptable risk of exceedance. **Data Analysis Approaches** - Different scenarios depending on available data: 1. Long flow records at the site. 2. Records available at nearby locations. 3. Short records at the site. 4. Data from analogous catchments. 5. No data available. **Frequency Determination** - **Annual Series**: Uses peak flow data for each year. - **Partial Duration Series**: Uses all peaks above a threshold, suitable for shorter intervals. **Probability Distributions** 1. **Normal Distribution**: - Symmetrical, unsuitable for flood data as it allows for negative values. 2. **Lognormal Distribution**: - Suitable for positively skewed data. 3. **Extreme Value Type I (EVI)**: - Often used in hydrology for maximum values. 4. **Log Pearson Type III (LP3)**: - Commonly used in U.S. flood frequency analysis, accommodates skewed data. **Statistical Parameters** - Mean, standard deviation, skewness, and coefficient of variation describe data distributions. - Skewness indicates asymmetry in data: - Zero skew = symmetrical. - Positive skew = tail extends to the right. **Curve Fitting** - Frequency factors (K) help estimate event magnitudes for various return periods based on chosen distribution. - Confidence intervals provide estimates of the range where true event magnitudes lie. **Design and Optimization** - Determining return periods involves balancing protection costs against risks of system failure. - Structures are designed for specific return periods: - E.g., rural culverts for 10-year events; urban systems for 25-year events; major structures for 100-year events. **Practical Application** - Steps to apply frequency analysis: 1. Collect and rank data. 2. Determine probabilities or return periods. 3. Select a probability distribution. 4. Compute statistical parameters. 5. Fit a curve and estimate event magnitudes. Reading 7.2.: =============