Hydrology Midterm PDF

**Groundwater** is the water that occurs in a saturated zone of variable thickness and depth below the earth's surface. The main processes in the hydrologic cycle are: 1- Precipitation (rainfall) P 2- Runoff (surface) R 3- Transpiration (from plants) T 4- Evaporation E 5- Infiltration F 6- Gr...

**Groundwater** is the water that occurs in a saturated zone of variable thickness and depth below the earth's surface. The main processes in the hydrologic cycle are: 1- Precipitation (rainfall) P 2- Runoff (surface) R 3- Transpiration (from plants) T 4- Evaporation E 5- Infiltration F 6- Groundwater flow G **I.FORMATION OF PRECIPITATION** Conditions required to form the precipitation are: i\. [Presence of moisture in the atmosphere] ii\. [Presence of sufficient nuclei particles to help condensation] iii\. [Weather conditions must be optimum for condensation to take place] **Types of Precipitation** **Convective Precipitation** This type of precipitation happens in varying intensities. The areal extent of convective precipitation is small in the range of less than **10km in diameter.** **Cyclonic Precipitation** It occurs when [warm, moist air is drawn into a low-pressure cold front.] The warm air rises as it is drawn into the low-pressure zone and is subjected to adiabatic cooling. **Orographic Precipitatio**n [Precipitation which is caused by hills or mountain] ranges deflecting the moisture-laden air masses upward, causing them to cool and precipitate their moisture. **II.DIFFERENT FORMS OF PRECIPITATION** Precipitation occurs in the following forms: 1\. **Rain**: This term is used generally when the water droplets are of size **0.5-6 mm**. Rain can be classified on the basis of its intensity as follows : 2\. **Snowfall**: It [consists of ice crystals] which is combined form of ice flakes. Initially these flakes have density in the range of [0.06-0.15 g/cc] (avg of 0.1 g/cc). 3\. **Drizzle**: When water droplets are of **size lesser than 0.5 mm**. Its intensity should be less than 1 mm/hr. In this case, particles are of such range that they can be seen **floating into the ai**r. 4\. **Glaze:** [When rain or drizzle comes in contact with cold ground at around 0°C,] the water is converted into ice coating termed as glaze. 5\. **Sleet**: It is [frozen rain drop which is formed when rain falls through air at sub-atmospheric] temperature or sub-freezing temperature. 6\. **Hail**: [consisting of balls or lumps of ice], called hailstones, that form during thunderstorms. **size \> 8 mm** **III.RAINFALL CHARACTERISTICS** 1\. **Rainfall Depth** -[Total rainfall over a specific area] \- Rain gauges (in mm) Significance: \- Flood risk assessment \- Reservoir design \- Agricultural planning 2\. **Rainfall Duration** -[Length of time rainfall occurs] \- Start to end of rainfall event Impact: \- Affects flooding potential \- Influences drainage system design 3.**Rainfall Intensity**- rate of rainfall (mm/hr) Calculation: **Depth ÷ Duration** Significance: Stormwater management Flood prevention Classification: **HYETOGRAPH**- a [graphical representation of rainfall intensity over time] Types: Uniform, Triangular, Realistic Application: Hydrological modeling Flood control design IV\. **POINT RAINFALL MEASUREMENTS** Point rainfall measurement refers to the [quantification of precipitation at a specific] [location using instruments like rain gauges]. This method captures the amount of rain that falls over a fixed area, providing data that can be used for hydrological studies, weather forecasting, and agricultural planning **Point measurement**s are crucial for understanding localized weather patterns. While point measurements [provide precise data for specific location]s, they might not reflect variations in rainfall across a larger area. For a broader understanding, data from multiple gauges or networks of gauges are often used to analyze rainfall patterns over a region. **Example of Point Rainfall Measurement Instruments**: 1\. Standard Rain Gauge 2\. Tipping Bucket Rain Gauge 3\. Weighing Rain Gauge V. **DIFFERENT TYPES OF RAIN GAUGE** A **rain gauge** [is an instrument used to measure the amount of precipitation, typically rain,] [over a specific period.] It usually consists of a cylindrical container with a funnel at the top that directs rainwater into a graduated measuring tube. The amount of water collected in the tube is then measured, usually in millimeters or inches, to determine the total rainfall. 1.**Standard Rain Gauge** (Non-Recording Rain Gauge) Design: A simple cylindrical container with a funnel leading to a graduated measuring tube. Usage: [Measures the total rainfall over a period]. The user manually checks and records the measurement. Example: The 8-inch Standard Rain Gauge used by the U.S. National Weather Service. 2.**Tipping Bucket Rain Gauge** Design: Contains a small bucket that tips and empties when it fills with a predetermined amount of water (often 0.2 mm or 0.01 inch). Each tip is recorded electronically. Usage: [Provides continuous, real-time measurements] of rainfall, making it useful for automatic weather stations. Advantages: Records both the amount and intensity of rainfall. 3\. **Weighing Rain Gauge** Design: Collects rain in a container placed on a scale. The weight of the water is measured and converted into a depth of rainfall. Usage: [Used in environments where snow or ice might accumulate, as it can weigh precipitation] [in all forms.] Advantages: Highly accurate and can measure all types of precipitation (rain, snow, sleet). 4\. **Optical Rain Gauge** Design: U[ses laser or infrared beams to detect raindrops as they pass through a sensing area]. Usage: Commonly used in automated weather stations and aviation, where precise, real-time data is essential. Advantages: Can measure the size and velocity of raindrops, which helps in calculating rainfall intensity. 5\. **Acoustic Rain Gauge** Design: [Measures the sound of raindrops hitting a surface], typically used underwater to detect rainfall on the water\'s surface. Usage: Used in marine environments to monitor rainfall over oceans and large bodies of water. Advantages: [Can measure rain over a large area without direct contact]. 6\. **Float**-Type Rain Gauge Design: [Rainwater is collected in a large container with a float inside]. [The rise of the float] [indicates the amount of rainfall, which is then recorded.] Usage: Used in hydrological studies, particularly in areas prone to heavy rainfall. Advantages: Can handle large volumes of water, making it suitable for areas with intense rainfall. VI\. **ESTIMATION OF MISSING RAINFALL DATA** Historical Rainfall Data Sources: Obtain historical rainfall data from meteorological stations, government agencies (e.g., PAGASA in the Philippines), or online databases. **Interpolation Methods**- [When data is sparse or missing, you can use spatial interpolation] methods like: Inverse Distance Weighting (IDW): A simple method that estimates rainfall at unsampled locations based on the weighted distance to known points. **Statistical Analysis**- Use [regression models to predict future rainfall trends based on] historical data. Rainfall-Runoff Models If you are dealing with hydrological studies, you can use rainfall-runoff models like: SCS Curve Number Method: Estimates runoff based on rainfall, soil type, land use, and antecedent moisture condition VII\. **CONVERSION OF POINT RAINFALL TO AERIAL RAINFALL** **Point rainfal**l refers to the [amount of rainfall measured at a specific location, typically] [using a rain gauge.] **Methods to convert point rainfall to aerial rainfall:** 1.**Arithmetic Average** This method involves [taking the simple average of rainfall measurements from multiple] [points within the area of interest]. **2. Thiessen Polygon Method** This method [assigns weights to each rain gauge based on the area it represent] 3\. **Isohyetal Method** [This method involves drawing contour lines (isohyets) of equal rainfall on a map.] The area between these lines is then used to calculate the average rainfall. 4\. **Areal Reduction Factors (ARF)** C[onverting point rainfall to aerial rainfall is crucial for designing and managing water] [resources projects, such as dams, drainage systems, and flood control measures.] VIII\. **DOUBLE MASS ANALYSIS** Is a [graphical method used to check the consistency of precipitation data over] [time.] If the relationship is linear, the data is consistent. A break in the slope indicates a change in the relationship. Formula: Pcx = Px \* (Mc / Ma) Where: Pcx = Corrected precipitation at any time period t at station X. Px = Original recorded precipitation at time period t at station X. Mc = Corrected slope of the double-mass curve. Ma = Original slope of the double-mass curve **Infiltratio**n is the process [where water on the ground surface enters the soil]. \- It happens naturally when rain, irrigation, or any water source moves through the soil, filling gaps and spaces between particles. **Importance of Infiltration**  **Groundwater Recharge** - [Essential for sustaining water supplies and ecosystems, especially during dry periods.]  **Reduction of Surface Runoff** - [Helps prevent flooding and erosion by reducing] [the volume of water that flows over land.]  **Pollutant Filtration** - [Improves water quality by filtering contaminants as water] [moves through the soil.] **II. Factors affecting infiltration, and infiltration measurements** Factors affecting infiltration **Precipitation Level**  Among the various factors affecting infiltration, precipitation level is often referred to as the most contributing factor. **Precipitations like rain and snowmelt** infiltrate the ground surface to the water bed.  **High intensity, duration, and amount of precipitation; will lead to greater infiltration.** **Soil Characteristics**  The soil characteristics influence capillary forces and adsorption;  **The rate of infiltration on the soil surface is largely dependent on the porosity and** **permeability of the soil profile.**  **Water infiltrates faster into large pore spaces of coarse subsoils**. ** For continuity of water percolation, the pore spaces should be interconnected.** ** This prevents run-off after the initial stage of water adsorption.**  While **permeability refers to** [how connected pore spaces are to one another.]  **If the material has high permeability, then the pore spaces are connected to one** **another allowing water to completely flow through.**  **However, if there is low permeability then the pore spaces are isolated and water** **is trapped within them.** **If a soil is both permeable and porous, infiltration rate is** **increased.** **Vegetative Cover** [ Vegetative cover can either increase or decrease infiltration.]  **Vegetation coverage [protects the soil surface from the impact of raindrop]s.** ** Therefore, it will take a longer time before raindrops have direct contact with the** **soil surface.**  The infiltration rate is therefore reduced.  On the other hand, vegetation root system and organic matter crumbles soil structure and improves its permeability.  In this case, the infiltration rate is increased.  Therefore, the type of vegetative cover has a role to play in either increasing or decreasing the soils infiltration rates. **Slope of the Land**  **Infiltration is faster in areas with flat land surfaces compared to steeply-sloped** **surface where the water will run off quickly.**  **Steeply-sloped surfaces encourage surface water runoff and in critical cases leads** **to erosion.** S**oil Saturation**  **Soil becomes saturated whenever it reaches its infiltration capacity.**  **Infiltration capacity is the maximum amount of rainwater that can enter a soil at** **any given time.**  **Once this maximum level is reached, the excess water will overflow as surface** **Runoff** **Evapotranspiration Level**  Different crops have varying stomata distribution, sizes, internal resistance to water transport.  These independent properties have its effect on the transpiration of crops.  Regions with higher evapotranspiration levels will imply a faster pull of water from the soil surface by crop root through infiltration. Human Activities  Prominent in urban setting; soil surface often gets compacted due to road construction, operation of tractors and large farm machineries.  This reduces both the porosity and permeability of the soil leading to a drastic decrease in infiltration rates.  **Poor soil management reduces infiltration rate and makes it difficult for water to** **penetrate the soil.** **Infiltration Measurements** **Infiltration measurements** refer to the [assessment of how much water seeps into] [the soil from precipitation, irrigation, or other sources.] **Several Methods to Measure Infiltrations:** 1\. **Double Ring Infiltrometer** - The double ring infiltrometer is a simple instrument that is **used to determine the rate of infiltration of water into the soi**l. The rate of infiltration is determined as **the amount of water per surface area and time unit,** **that penetrates the soil.** 2\. **Single Ring Infiltrometer** - Using a single-ring infiltrometer, [a ring is driven into] [the ground and water is either continuously supplied or provided at a falling head] [condition.] 3\. **Tension Infiltrometers** - [Tension infiltrometers, which apply water over a circular] [portion of the soil surface and measure the flow rate of water drawn into the soil,] are widely used instruments for measuring the hydraulic conductivity of soil in the field. **III. Horton Model and Philip's equation** The **Horton Model** is [a significant empirical framework in hydrology, developed by] [Robert E. Horton] in the early 1930s. It **describes how the infiltration capacity of soil** decreases over time during a rainfall event. The Horton Infiltration Equation: f(t) = fc + (fo − fc)e −kt Where:  f(t) = Infiltration capacity at time t  fc= Constant minimum value of infiltration capacity (asymptotic value)  fo = Initial infiltration capacity at the start of the event  k = Constant controlling the rate of decrease in infiltration capacity  t = Time Limitations Despite its widespread use, the **Horton Infiltration Mode**l has notable deficiencies: o Parameter Measurement - the model is not directly linked to measurable physical properties of the soil o Limited Applicability - [best suited for short-duration, high-intensity rainfall] [events and may not perform well under continuous or prolonged rainfall] [condition]s The **Horton Model** is important in hydrology for several reasons: 1\. Estimation of Infiltration Rates - provides a reliable method for estimating how quickly water can infiltrate into soil 2\. Dynamic Modeling - model captures the decrease in infiltration capacity over time 3.Applications in Urban Planning - aids in designing effective stormwater management systems 4.Foundation for Further Research - Horton\'s work has laid the groundwork for subsequent advancements **Philip's Equation** The **Philip's Equation** (or Philip\'s infiltration equation) is [used to describe water] [infiltration into soils]. It was developed by J.R. Philip in 1957 and is [commonly applied to] [model unsteady infiltration of water into soil]. I(t) = S⋅t + A⋅t Where:  I(t)- is the cumulative(increasing) infiltration at time t,  S - is the sorptivity, which measures the capacity of the soil to absorb water,  A - is a constant related to the steady-state infiltration rate, and IV\. **Green-Ampt Model** The **Green-Ampt model** is an [approximate model utilizing Darcy's law]. [The model] [is developed with the assumption that water is ponded on the ground surface]. Consider a vertical column of soil of unit horizontal cross-sectional area and let a control volume be defined around the wet soil between the surface and depth L. The model was developed by [Heber Green] and [G.A. Ampt] in the early 1900s, estimates downward infiltration when water ponds on the soil surface. It builds on earlier research in soil physics, particularly Edgar Buckingham\'s work on unsaturated soil flow. Key Components:  Infiltration Rate Equation: The **Green-Ampt model** [helps explain why infiltration rates decrease over time, as] [the hydraulic gradient driving infiltration diminishes]. Despite its assumptions, it remains a valuable tool for understanding and predicting infiltration dynamics. **V. Ponding Time** Ponding time in hydrology r[efers to the period during which water accumulates] [on the surface of the ground or in depressions], such as ponds or fields, before it begins to infiltrate into the soil or runoff into nearby water bodies. It is a key factor in surface water dynamics, especially in areas with low infiltration rates or impervious surfaces, such as urban settings. Key Points about Ponding Time: Definition: **Ponding time** is the [time interval between the onset of rainfall and when water] [starts to pool or accumulate on the ground surface due to the ground's inability to absorb] [all the rainfall immediately.] Influence of Soil Properties:  Infiltration Rate: The infiltration rate of the soil (how quickly water can move into the soil) is a primary factor in determining ponding time. Soils with high infiltration rates (like sandy soils) have shorter ponding times, while clayey soils with low infiltration rates have longer ponding times.  Saturation: **Once the soil becomes s** **aturated, ponding time starts because the** **excess water can no longer infiltrate and begins to accumulate.** Role in Flooding: **Prolonged ponding times can contribute to localized flooding, especially in areas** **where the drainage systems are inefficient or where the ground is compacted or** **imperviou**s (e.g., in urban environments). Runoff Generation: Once ponding occurs, excess water typically flows as surface runoff. The duration of ponding time influences how quickly runoff begins, which is crucial in predicting peak discharge during storm events in hydrological models. Influencing Factors:  Rainfall Intensity: High-intensity rainfall can overwhelm the infiltration capacity of the soil, leading to a shorter ponding time.  Land Use: Urbanization, deforestation, and agricultural practices can reduce the ground's ability to absorb water, shortening ponding time and increasing surface runoff. Relevance in Hydrological Modeling: In hydrological models, **ponding time** [helps determine when and how much water] [will transition from infiltration to runoff, influencing predictions of peak flows, water] [storage, and flood risk.]  Ponding time is the elapsed time between when rainfall begins and when water begins to pond on the soil surface.  **VI. Fitting infiltration** models to infiltration data using excel This subtopic explores the process of fitting infiltration models to observed infiltration data. It discusses the significance of infiltration modeling, commonly used models, and the steps involved in the model fitting process. Hydrological Processes: Infiltration models help understand the interactions between precipitation, surface runoff, and groundwater recharge. Water Resources Management: Accurate infiltration estimates are vital for water resource planning, allocation, and conservation. Environmental Modeling: Infiltration models are used in studies of soil erosion, nutrient cycling, and pollutant transport. Engineering Design: Infiltration data is essential for designing drainage systems, stormwater management facilities, and irrigation systems. Commonly used Infiltration Models Several models have been developed to describe the infiltration process. Some of the most commonly used models include:  Horton\'s Equation  **Green-Ampt Model** ** Philip\'s Equation** ** Kostiakov Model** Model Fitting Process: The process of fitting an infiltration model to data involves the following steps:  Data Collection: Gather infiltration data from field experiments, lysimeters, or rain simulators.  Model Selection: Choose an appropriate model based on the characteristics of the soil, land cover, and the specific application.  Parameter Estimation: Determine the model parameters using statistical methods, such as least squares or maximum likelihood estimation.  Model Refinement: If necessary, modify the model or collect additional data to improve its accuracy.