Unit 1 Precipitations and Abstractions.pptx

Precipitation and Abstractions
Hydrological cycle - Meteorological measurements – Types and forms of precipitation - Rain gauges - Spatial analysis of rainfall data using Thiessen polygon and Iso-hyetal methods - Interception – Evaporation: Measurement, Evaporation suppression methods – Infiltration: Horton’s equation - Double ring infiltrometer - Infiltration indices.

1. AI3404 HYDROLOGY AND WATER RESOURCES ENGINEERING UNIT I PRECIPITATION AND ABSTRACTIONS

2. UNIT I PRECIPITATION AND ABSTRACTIONS Hydrological cycle - Meteorological measurements – Types and forms of precipitation - Rain gauges - Spatial analysis of rainfall data using Thiessen polygon and Iso- hyetal methods - Interception – Evaporation: Measurement, Evaporation suppression methods – Infiltration: Horton’s equation - Double ring infiltrometer - Infiltration indices.

3. Hydrological cycle Hydrological cycle , also known as water cycle , is a continuous movement of water on, above and below the surface of the Earth . During this process, water changes its state from one phase to another, but the total number of water particles remains the same.

4. Stages of Water Cycle 1. Evaporation The sun is the ultimate source of energy, and it powers most of the evaporation that occurs on earth. Evaporation generally happens when water molecules at the surface of water bodies become excited and rise into the air. These molecules with the highest kinetic energy accumulate into water vapor clouds. Evaporation usually takes place below the boiling point of water. Another process called evapotranspiration occurs when evaporation occurs through the leaves of plants. This process contributes to a large percentage of water in the atmosphere.

5. Stages of Water Cycle 2. Sublimation Sublimation occurs when snow or ice changes directly into water vapor without becoming water. It usually occurs as a result of dry winds and low humidity. Sublimation can be observed on mountain peaks, where the air pressure is quite low. The low air pressure helps to sublimate the snow into water vapor as less energy is utilized in the process. On earth, the primary source of sublimation is from the ice sheets covering the poles of the earth.

6. Stages of Water Cycle 3. Condensation The water vapor that accumulated in the atmosphere eventually cools down due to the low temperatures found at high altitudes. These vapors become tiny droplets of water and ice, eventually coming together to form clouds.

7. Stages of Water Cycle 4. Precipitation Above 0 degrees centigrade, the vapors will condense into water droplets. However, it cannot condense without dust or other impurities. Hence, water vapors attach itself on to the particle’s surface. When enough droplets merge, it falls out of the clouds and on to the ground below. This process is called precipitation (or rainfall). In particularly cold weather or extremely low air pressure, the water droplets freeze and fall as snow or hail.

8. Stages of Water Cycle 5. Infiltration Rainwater gets absorbed into the ground through the process of infiltration. The level of absorption varies based on the material the water has seeped into. For instance, rocks will retain comparatively less water than soil. Groundwater can either follows streams or rivers. But sometimes, it might just sink deeper, forming aquifers .

9. Stages of Water Cycle 6. Runoff If the water from rainfall does not form aquifers, it follows gravity, often flowing down the sides of mountains and hills; eventually forming rivers. This process is called runoff. In colder regions, icecaps form when the amount of snowfall is faster than the rate of evaporation or sublimation. The biggest icecaps on earth are found at the poles.

10. Precipitation Precipitation is any liquid or frozen water that forms in the atmosphere and falls back to the earth . It comes in many forms, like rain, sleet, and snow. precipitation occurs when a part of the atmosphere saturates itself with water vapor and when the right temperature comes it condenses and precipitates. The two processes which make the air saturated are the cooling of air molecules and the addition of water vapor.

11. Forms of Precipitation In Liquid Form precipitation occurs in: Drizzle Rain When the above comes in contact with the air mass at the subfreezing temperature it becomes Freezing Rain Freezing Drizzle The frozen forms of precipitated water include: Snow Ice Needles Hail Graupel Sleet

12. Forms of Precipitation 1. Rain Rain is a form of precipitation that is in the form of water drops of a size that is larger than 0.5mm. The maximum raindrop size is about 6mm. Drops of larger size break up into smaller drops as it falls down on the Earth’s surface. Rainfall is the predominant form of precipitation and therefore, the term precipitation is used synonymously with rainfall. The magnitude of the rainfall shows high temporal and spatial variation. This variation causes the occurrence of hydrologic extremes like floods and droughts.

13. Forms of Precipitation 2. Snow Snow consists of ice crystals in a flaky form, having an average density of 0.1g/cc. It is also an important form of precipitation that usually forms in colder climates and higher altitudes. 3. Drizzle Drizzle is a fine sprinkle of tiny water droplets that have a size less than 0.5mm and an intensity greater than 1mm/h. The tiny drops that form a drizzle appear floating in the air.

14. Forms of Precipitation 4. Glaze or Freezing Rain The glaze is formed when rain or drizzle comes in direct contact with the cold ground at around 0 degrees celsius . This water drops freeze to form an ice coating known as glaze. 5. Sleet Sleet is frozen raindrops that are formed when rainfall passes through the air in the atmosphere at subfreezing temperatures.

15. Forms of Precipitation 6. Hail Hail is a kind of showery precipitation in the form of pellets or lumps that have a size greater than 8mm. Hail occurs during violent thunderstorms. These are examples of precipitation. Let us now look at what are the different types of precipitation. 7. Sun shower A sun shower occurs when rain falls while the sun shines. In the absence of clouds, raindrops fall from the sky when rain-bearing winds blow several miles away. As a result, a sun shower occurs when a single rain cloud crosses the earth's surface, allowing the sun's rays to flow through. It is usually accompanied by a rainbow.

16. Forms of Precipitation 8. Grains of Snow Snow grains are tiny white ice grains. Snow grains are flat, with a diameter of about 1mm. They are almost as big as a drizzle.  9. Diamond Dust Diamond dust is made up of tiny ice crystals that occur at low altitudes and temperatures. The dazzling effect caused by light reflecting off ice crystals in the air gave diamond dust its name. 10. Ice Crystal This is a common occurrence in colder regions of the globe. The crystals resemble fog, with water particles freezing into ice. Ice crystals look similar to needles, plates, or columns in shape.

17. Forms of Precipitation

18. Types of Precipitation Cyclonic Precipitation. Convective Precipitation. Orographic Precipitation.

19. 1. Cyclonic precipitation Cyclonic precipitation is caused by the lifting of an air mass because of the pressure difference. If low pressure occurs at an area, air will flow horizontally from the surrounding area, causing the air at the low-pressure area to lift. The cyclonic precipitation may be divided into two-part Frontal Precipitation Non-Frontal Precipitation

20. 1. Cyclonic precipitation 1.1. Frontal Precipitation: When two air masses, because of contrasting densities and temperatures, clash with each other, precipitation and condensation, occur in the surface of contact. The surface of contact is called a ‘ Front ‘ or ‘ Frontal Surface,’ and the precipitation is called frontal precipitation. Cold Front Warm Front

21. 1. Cyclonic precipitation When a cold air mass drives out a warm air mass, it is called a ‘ Cold Front,’ and if a warm air mass replaces the retreating cold air mass, it is called a ‘Warm Front.’ On the other hand, if both air masses are drawn simultaneously towards a low-pressure area, the front developed is stationary and is called a ‘ Stationary Front .’ Cold Front causes intense precipitation in comparatively small areas, while the precipitation due to the warm front is less intense but is spread over a comparatively larger area. Cold fronts move faster than warm fronts.

22. 1. Cyclonic precipitation Stationary Front

23. 1. Cyclonic precipitation 1.2. Non-Frontal Precipitation: In the case of non-frontal precipitation, the moist warm air mass is stationary, and the moving cold air mass meets it. Thus, due to the lightness of the warm air mass, there is a passive ascent of warm air over cold air owing to this active undercutting. When this lifted warm air cools down at higher altitude, precipitation occurs.

24. The air on getting heated becomes light and rises in convection currents. As the air rises, it expands and drops the temperature and subsequently, condensation takes place and cumulus clouds are formed. Heavy rainfall with lightning and thunder takes place which does not last long. Such rain is usually in the summer or the hotter part of the day. This type of rainfall generally takes place in the equatorial regions and internal parts of the continents, predominantly in the northern hemisphere. This rainfall is usually associated with hail and graupel 2. Convective precipitation

25. When the saturated air mass comes across a mountain, it is forced to rise. The rising air expands, eventually, the temperature falls, and the moisture gets condensed. The principal characteristic of this type of rain is that the windward slopes get more rainfall. After giving rain on the windward side, when these winds reach the other slope, they drop away, and their temperature increases. Then their ability to take in moisture increases and hence, these leeward slopes remain dry and rainless. The region situated on the leeward side is known as the rain-shadow area. 3. Orographic precipitation

26. Orographic precipitation is caused by moist air masses. Which strikes some natural topographic barriers like mountains, causing, rise up, condensation, and precipitation. This greatest amount of precipitation falls on the windward side, and the leeward side often has very little precipitation. 3. Orographic precipitation

27. Meteorological measurements

28. The precipitation gauges are based on the simple idea of exposing in an open area a hollow cylindrical vessel with a rigid bottom and no top cover. Rain or other forms of precipitation fall into the vessel, and its depth (or volume or weight) is measured, snow or other forms being melted before taking the measurement. The various types of precipitation gauges used are broadly classified as: Non- recording gauges Recording gauges Types of rain gauges

29. Most common type of non-recording rain gauge extensively used in India is the Symon's gauge. It consists of a circular collecting area of 127 mm diameter connected to a funnel. The rim of the collector is set in a horizontal plane at a height of 305 mm above the ground level. The funnel discharges the rainfall catch into a receiving vessel. The funnel and receiving bottle are located in a metallic container and water measured with a graduated measuring cylinder, with accuracy up to 0.1 mm. Rainfall is collected in the bottle taken out at 8:30 A.M. and put in a graduating measuring cylinder which gives the depth of the rainfall. If the rainfall on a particular day is very heavy, than more number of measurements are taken and summed up to given the rainfall depth for the day. Non- recording gauges

30. Non- recording gauges

31. The recording type rain gauges provide a continuous record of rainfall over time and yield the important information such as duration of the storm, intensity and cumulative rainfall. The recording type of rain gauges are commonly installed along with non-recording type rain gauge for checking and calibration. In a recording type rain gauge, there is a clock-driven mechanical system by which rainfall at a given time is recorded on a graph paper by a pen. The curve of cumulative rainfall with time thus obtained is called mass curve. The slope of the curve at any point of time shows rainfall intensity. Recording gauges

32. The following are some of the commonly used recording type rain gauges. ( i ) Float, (ii) Weighing bucket (iii) Tipping bucket Recording gauges

33. This type of recording rain gauge also known as natural-syphon type gauge, rain is collected through a funnel leading into a float chamber causing a float to rise. A pen fitted on the stem of the float traces the rainfall on the graph paper fitted round a clock driven drum. The drum rotates once either in 1 day or in 7 days depending upon the design after which a new chart is fitted round the drum. The receiver also consists of a siphon chamber. When the float and the pen rise to the maximum level, the water from the float chamber is siphoned off. The pen immediately reaches the zero level of the graph paper and continues to rise if the rainfall continues. 1. Float type

34. 1. Float type

35. In this type of rain gauge, a bucket is supported by spring or a lever balance. Movement of the bucket is transmitted to a pen which traces the record of the increasing weight of the bucket and its contents on a clock driven graph paper enough for one week. The catch of the funnel empties into a bucket mounted on a weighing scale. In some instruments of this type the recording unit is so constructed that the pen reverses its direction at every preset value so that a continuous plot of storm is obtained. 2. Weighing bucket type

36. 2. Weighing bucket type

37. The tipping bucket rain gauge comprises of a pair of buckets pivoted under a funnel in such a way that when one bucket receives 0.25 mm of rainfall it tips, discharging its contents into a reservoir and bringing the other bucket under the funnel. A recording mechanism represents the time of occurrence of each tip. The tipping bucket rain gauge is suited to the measurement of rainfall intensity/or short periods. The catch from the funnel falls onto one of a pair of small buckets. These buckets are so balanced that when 0.25 mm of rainfall collects in one bucket, it tips and brings the other one in position. The water from the tipped bucket is collected in a storage can. The tipping actuates an electrically driven pen to trace a record on clockwork-driven chart. 3. Tipping-bucket type

38. The tipping bucket rain gauge comprises of a pair of buckets pivoted under a funnel in such a way that when one bucket receives 0.25 mm of rainfall it tips, discharging its contents into a reservoir and bringing the other bucket under the funnel. A recording mechanism represents the time of occurrence of each tip. The tipping bucket rain gauge is suited to the measurement of rainfall intensity/or short periods. The catch from the funnel falls onto one of a pair of small buckets. These buckets are so balanced that when 0.25 mm of rainfall collects in one bucket, it tips and brings the other one in position. The water from the tipped bucket is collected in a storage can. The tipping actuates an electrically driven pen to trace a record on clockwork-driven chart. 3. Tipping-bucket type

39. 3. Tipping-bucket type