Module-3 Precipitation PDF

CE 118 HYDROLOGY Chapter 3 PRECIPITATION Objectives: By the end of this lesson, students will be able to: 1. Understand the process of precipitation and the conditions required for it to occur. 2. Identify and differentiate between the main types of pr...

CE 118 HYDROLOGY Chapter 3 PRECIPITATION Objectives: By the end of this lesson, students will be able to: 1. Understand the process of precipitation and the conditions required for it to occur. 2. Identify and differentiate between the main types of precipitation, including cyclonic, convective, and orographic precipitation. 3. Explain the factors influencing different forms of precipitation such as rain, snow, sleet, and hail, and how these are related to atmospheric conditions. 4. Analyze the methods used to measure precipitation, including the use of rain gauges and radar systems, and understand the potential errors involved in measurement. 5. Understand the significance of precipitation in the hydrologic cycle and its impact on water resources, ecosystems, and human activities. DEFINITION Precipitation is a critical component of the water cycle, as it represents the process where water in various forms falls from the atmosphere to the Earth’s surface. It can occur as rain, snow, sleet, hail, or freezing rain, depending on atmospheric conditions. 1  CE 118 HYDROLOGY Conditions for Precipitation: For precipitation to occur, three key conditions must be met: 1. Cooling of air: Air must be cooled to its dew point, the temperature at which it becomes saturated with moisture and condensation occurs. Cooling usually happens as air rises. 2. Condensation nuclei: Tiny particles like dust or salt present in the atmosphere act as a surface for water vapor to condense into droplets. 3. Sufficient moisture: There must be enough moisture in the air, typically provided by evaporation or transpiration processes, to generate significant precipitation. Types of Precipitation: Precipitation is classified based on how the air is lifted and cooled. The main types are: 1. Cyclonic (Frontal) Precipitation: o Occurs when two air masses with different temperatures and densities meet. The colder, denser air acts as a barrier, forcing the warmer air to rise. As the warm air rises, it cools and condenses, leading to precipitation. This type of precipitation is common in areas with cold and warm fronts. 2. Convective Precipitation: o This occurs due to local heating of the Earth's surface. Warm air, being lighter, rises rapidly into the atmosphere where it cools and condenses, forming precipitation. Convective precipitation is often short-lived but can be intense, leading to heavy downpours and thunderstorms. 2 CE 118 HYDROLOGY 3. Orographic Precipitation: o When moist air is forced to ascend over a mountain or high terrain, it cools as it rises, leading to orographic precipitation. This is commonly observed on the windward side of mountain ranges, while the leeward side often experiences a rain shadow effect, receiving less precipitation. 3  CE 118 HYDROLOGY Measuring Precipitation: Precipitation is typically measured using rain gauges. These can be divided into: 1. Non-recording gauges: These simply collect and measure the total amount of rainfall over a given period. 2. Recording gauges (Pluviographs): These instruments provide continuous records of rainfall intensity over time. Types of recording gauges include weighing gauges, tipping-bucket gauges, and float-type gauges. More advanced techniques like radar can also be used to measure rainfall intensity by detecting the size of raindrops. Common Errors in Precipitation Measurement: 1. Wind Error: High wind speeds can cause underreporting of rainfall because rain may be deflected away from the gauge. 2. Obstacle Error: Nearby buildings, trees, or other obstacles can block precipitation, leading to lower recorded values. Proper placement of rain gauges is essential to avoid such errors. Analysis of Precipitation Records Once precipitation data is collected, it is crucial to analyze the records to understand rainfall patterns, intensities, and distributions over time. This helps in water resource management, flood control, and agricultural planning. Two important tools for analyzing precipitation data are the mass curve and the hyetograph. 1. Mass Curve of Precipitation: The mass curve is a graphical representation showing the cumulative depth of precipitation over time. It provides a visual way to track how precipitation accumulates during a storm or over a specific period.  It is helpful in identifying the total rainfall received over a time period and in determining how evenly the rainfall was distributed. For example, steep slopes on the curve indicate periods of intense rainfall, while flatter portions show intervals of lower intensity or no rain. 4  CE 118 HYDROLOGY 2. Hyetograph of Precipitation: A hyetograph illustrates the intensity of rainfall over time, typically represented as bars on a graph. The height of each bar shows the amount of precipitation during a specific time interval.  Hyetographs are particularly useful in stormwater management and flood analysis. By observing how quickly precipitation falls, hydrologists can predict potential runoff and flooding risks. Computation of Average Rainfall Depth Over a Basin When managing large areas like watersheds or basins, it is important to determine the average rainfall depth over the region. This helps in designing dams, irrigation systems, and other water management infrastructure. There are several methods for calculating average rainfall depth when multiple rain gauge stations are spread across the area. 1. Arithmetic Average Method: The simplest method for computing the average rainfall depth over a basin is the Arithmetic Average Method. This is calculated by taking the sum of the rainfall recorded at all stations and dividing by the number of stations: 5  CE 118 HYDROLOGY Where:  P1, P2... Pn are the recorded rainfall values at different stations.  n is the number of rain gauges.  This method works best in areas where rainfall is uniformly distributed. However, for large or topographically diverse regions, more sophisticated methods may be required. This method can be used in regions smaller than 500 km2. Sol’n: Average rainfall = (5 + 8 + 12 + 15)/4 = 10.0 mm. 2. Thiessen Polygon Method: The Thiessen Polygon Method accounts for the uneven distribution of rain gauges and the varying areas they represent. This method assigns more weight to gauges that cover larger areas. Procedure: a. Joining the rain gauge station locations by straight lines to form triangles. b. Bisecting the edges of the triangles to form the so-called “Thiessen polygons”. c. Calculate the area enclosed around each rain gauge station bounded by the polygon edges (and the catchment boundary, wherever appropriate) to find the 6  CE 118 HYDROLOGY area of influence corresponding to the rain gauge. Where A1, A2,... An are the areas of the Thiessen polygons and P1, P2...Pn are the rainfall values recorded at each station.  This method is more accurate than the arithmetic average, especially in larger or irregular basins where rain is not evenly distributed. Thiessen polygon does not change in time and is drawn only once. The method can be used in regions 500-5000 km2 size. It considers the non-uniformity of the areal distribution of gauges. Example: Calculate the average precipitation depth for the area given in the figure above with four stations and rainfall depths given in table below. 7  CE 118 HYDROLOGY 3. Isohyetal Method: The Isohyetal Method is the most precise method for calculating average rainfall in regions where rainfall patterns are influenced by factors like topography. An isohyet is a line, on a rainfall map of the basin, joining places of equal rainfall readings. An isohyet map showing contours of equal rainfall represents a more accurate picture of the rainfall distribution of the basin. This method can be preferred for orographic precipitation. Procedure:  From the rainfall values recorded at various rain-gauge stations, the isohyetal map is prepared for the storm causing the rainfall over the area.  Measure the areas enclosed between successive isohyets with the help of planimeter.  Multiply each of these areas by the average rainfall between the isohyets.  The average rainfall is then computed from the expression. 8  CE 118 HYDROLOGY Example: Calculate the average rainfall depth for the area given with Isohyetal method. 9