Hydrology Lecture Notes PDF

CE082 HYDROLOGY LECTURE BY: ENGR. JEZREEL S. BENLIRO  Understanding the mechanisms, types, Precipitation and measurements of precipitation in hydrological systems Introduction to Precipitation Precipitation is any form of water, li...

CE082 HYDROLOGY LECTURE BY: ENGR. JEZREEL S. BENLIRO  Understanding the mechanisms, types, Precipitation and measurements of precipitation in hydrological systems Introduction to Precipitation Precipitation is any form of water, liquid or solid, that falls from the atmosphere to the Earth’s surface It is a critical component of the hydrological cycle, influencing water availability, weather patterns, and climate HYDROLOGIC CYCLE  Formation of Precipitation Mechanisms of Formation Precipitation Occurs: Condensation: Water Cloud Formation: Droplet Growth: As When droplets are vapor in the air cools These condensed droplets collide and large and heavy and condenses into tiny droplets form clouds, coalesce, they become enough, they overcome droplets around which grow as more larger air resistance and fall condensation nuclei water vapor condenses as precipitation Types of Precipitation Convective Precipitation Orographic Precipitation Cyclonic Precipitation Types of Precipitation Convective Precipitation Convective precipitation is caused by the natural rising of warmer, lighter air in colder, denser surroundings. The difference in temperature may result from unequal heating at the surface, unequal cooling at the top of the air layer, or mechanical lifting when the air is forced to pass over a denser, colder air mass or over a mountain barrier. Convective precipitation is spotty, and its intensity may range from light showers to cloudbursts. Types of Precipitation Orographic Precipitation Orographic precipitation results from mechanical lifting over mountain barriers. In rugged terrain the orographic influence is so marked that storm precipitation patterns tend to resemble that of mean annual precipitation. Also known as relief rainfall. Types of Precipitation Cyclonic Precipitation Cyclonic precipitation results from the lifting of air converging into a low-pressure area, or cyclone. Most general storms in plains regions are of this type. Formation of Precipitation Factors Influencing Formation Atmospheric Instability: Temperature: Humidity: Higher Rising air can cool Determines whether humidity levels provide rapidly, promoting cloud precipitation will be more water vapor for formation and liquid or solid condensation precipitation  Forms of Precipitation Drizzle consists of waterdrops under 0.02-in. diameter, and its intensity is usually less than 0.04 in./hr. Rain consists of drop usually greater than 0.02-in. diameter. Drops greater than 0.25-in. diameter tend to break up as they fall through the air, so that 0.25 in. may be accepted as a practical upper limit of raindrop size. Glaze is the ice coating formed when drizzle or rain freezes as it comes in contact with cold objects at the ground. Sleet is frozen raindrops cooled to the ice stage while falling through air at subfreezing temperatures. Snow is precipitation in the form of ice crystals resulting from sublimation, i.e., from water vapor directly to ice. A snowflake is made up of a number of ice crystals fused together. Hail is precipitation in the form of balls or lumps of ice over 0.2-in. diameter formed by alternate freezing and melting as they are carried up and down in highly turbulent air currents. Single hailstones weighing over a pound have been observed. Forms of Precipitation Rain: Liquid droplets Snow: Ice crystals Hail: Frozen that fall when that form when the precipitation formed temperatures are air temperature is by strong updrafts above freezing below freezing in thunderstorms Sleet: Small ice Drizzle: Light rain Freezing Rain: Rain pellets that form with smaller that freezes upon when rain falls droplets than contact with cold through a layer of normal rain surfaces freezing air Rainfall Characteristics Depth: The total amount of rain that falls over an area, measured in millimeters or inches Duration: The time period over which rainfall occurs Intensity: The rate at which rainfall occurs, usually expressed in mm/hour Hyetograph Definition: A graph that shows the variation of rainfall intensity over time Application: Used in hydrological studies to analyze storm events and design drainage systems Point Rainfall Measurements Definition: Measurement of rainfall at a specific geographic location Importance: Provides localized data for hydrological studies, flood forecasting, and water resource management Methods Rain Gauges: Instruments that collect and measure the amount of precipitation. This is used to measure rainfall over an area in a predefined period of time. Automatic Weather Stations: Measure precipitation along with other atmospheric conditions This Photo by Unknown Author is licensed under CC BY Different Types of Rain Gauges Non-Recording Gauges Standard Rain Gauge: Simple device with a funnel that directs rainfall into a graduated cylinder for manual measurement. This measures the depth of rainfall in mm or inch.  Different Types of Rain Gauges Recording Gauges Tipping Bucket Rain Gauge: Funnels rain into small buckets that tip and record each time a specific volume is collected  Different Types of Rain Gauges Recording Gauges Weighing Rain Gauge: Measures rainfall by weighing the collected water  Different Types of Rain Gauges Recording Gauges Optical Rain Gauge: Uses lasers or infrared beams to detect and measure raindrops as they pass through a sensor Different Types of Rain Gauges Recording Gauges Automated Rain Gage  Different Types of Rain Gauges Ground-based Weather Radar - Ground weather radars are surveillance sensors that are used to discover, assess and track hazardous weather (mostly CB clouds and associated phenomena such as thunderstorms and hail). Different Types of Rain Gauges Earth-observing Satellites The Tropical Rainfall Measuring Mission (TRMM, pronounced “trim” ) satellite is the most accurate rainfall observing satellite to orbit the Earth. It carries a suite of five instruments that, when combined, allow scientists to gather a very detailed three- dimensional view of rainfall patterns. Different Types of Rain Gauges Earth-observing Satellites The Global Precipitation Measurement (GPM) mission is an international network of satellites that provide next- generation global observations of rain and snow. HYETOGRAPH  PLATE NO. 2 Characteristics of Rainfall Item No. 1: Item No. 2: