Engineering student Hydrology program for bsc
- 1. Weather and Climate The difference between weather and climate is a measure of time. Weather is what conditions of the atmosphere are over a short period of time, and climate is how the atmosphere "behaves" over relatively long periods of time. Weather can change from minute-to-minute, hour-to-hour, day-to-day, and season-to-season. Climate, however, is the average of weather over time and space. In short, climate is the description of the long-term pattern of weather in a particular area. Some scientists define climate as the average weather for a particular region and time period, usually taken over 30-years. It's really an average pattern of weather for a particular region. When scientists talk about climate, they are looking at averages of precipitation, temperature, humidity, sunshine, wind velocity, phenomena such as fog, frost, and hail storms, and other measures of the weather that occur over a long period in a particular place.
- 2. Precipitation Precipitation Precipitation: the single strongest variable driving hydrologic processes. The term precipitation denotes all forms of water that reaches the earth’s surface from the atmosphere (about 15 km). The usual forms of precipitation are rainfall, snowfall, hail and dew. Formation of Precipitation Moisture present in the atmosphere condenses into droplets in presence of nuclei. The nuclei are small particles of various substances (0.1 to 10 µm in diameter). After nucleation the droplets grow in size through diffusion of water vapour to it. Wind speed facilitates the movement of the cloud while its turbulence retains the water droplets in suspension. For the precipitation to occur, the cloud elements must increase in size until their falling velocity exceeds the ascensional velocity of the air. Collision and coalescence of cloud are the important factors leading to precipitation. As the particles collide they usually coalesce to form larger particles and drop to the earth when they exceed the terminal velocity. For droplet diameter of 2 mm, the terminal velocity > 20 km/hr
- 4. Droplets become heavy enough to fall (0.1 mm) Cloud Cloud Further Droplet breaks Condensation down to smaller size returns to atmosphere Rain drops (0.1- 3 mm) Droplet forms by condensation On particles of aerosols (0.1 - 10μm) Formation of Rain
- 6. Global Precipitation Precipitation averages just about 1 meter per year over Earth 30.3 45 8.7 9.8 20.7 17.8 0 20 40 60 Africa Asia Australia Europe N.America S.America Area(M km^2) 686 726 736 734 670 1648 0 500 1000 1500 2000 Africa Asia Australia Europe N.America S.America Precipitation (mm/yr) The global average annual precipitation is about 1m
- 7. Precipitation Forms of Precipitation Drizzle – also called mist consists of water particles between 0.1-0.5 mm in diameter and falling at an intensity of less than 1 mm/hour. Rain – consists of water particles larger than 0.5 mm to about 6 mm. The rainfall is: light: rates < 2.5 mm/hr moderate: rates between 2.5 – 7.5 mm/hr heavy: > 7.5 mm/hr Snow: is composed of ice crystals which usually combine to form flakes resulting from de-sublimation. The average density of snow is 0.1 g/cc and 125 to 500 mm of snow is generally required to equal 25 mm of water. Hail: balls or irregular lumps of ice 5-125 mm in diameter. Dew: moisture condensed from the atmosphere in small drops upon cool surfaces. Fog: a thin cloud of varying size formed at the surface of the earth by condensation of the atmospheric vapour. Visibility > 1 km - Mist Sleet: frozen raindrops falling through subfreezing air near the earth’s surface.
- 10. Precipitation Types of Precipitation When the moist air is lifted to higher altitudes it cools and condenses – the process is called adiabatic cooling. The rate of change of temperature with height is called lapse rate. The mean lapse rate is a decrease of 0.70C per 100 m increase in height. Convective: caused by rising of warmer and lighter air due to heating during the day into colder and denser surroundings. This is common in the tropics during the evenings. Orographic : moist air being lifted to higher altitudes due to presence of natural barriers (mountains) causing cooling, condensation and rain in the windward side (no or little rain in the leeward side). Cyclonic/Frontal: lifting of air converging into low pressure areas. Cyclonic rainfall may be frontal (warm front moving into colder air or cold front moving into warmer air). Artificial: providing the clouds with nuclei (solid carbon dioxide also called dry ice or silver iodide) by airplane to help formation of droplets.
- 12. Artificial Rain/Cloud Seeding Limitations: 1. Expensive: $1/m3 2. Toxicity 3. Regional conflict 4. Chances of failure
- 13. Rainfall Characteristics Depth: amount of rainfall at a point or over an area (usually in mm) Duration: the period of time during which rain fell (hours) Intensity: depth of rainfall per unit time i.e. depth/duration (mm/hr) Time distribution of rainfall or hyetographs: are plots of rainfall intensity (mm/hr) or amount/depth (mm) as a function of time. Hyetographs are usually plotted as a bar chart of gross rainfall. Rainfall Mass Curve: cumulative rainfall hyetographs are called mass curve. The slope of the mass curve (at any point) is the rainfall intensity. Thus, zero slope means zero rainfall and the maximum slope is the maximum intensity. Isohyets: are contours of constant rainfall and can be drawn to develop isohyetal maps of rainfall depth. Temporal and Spatial Variation of Rainfall: Rainfall varies greatly both in time and space – With respect to time – Temporal variation – With space – Spatial variation
- 14. Mass Curves & Rainfall Hyetographs 0 10 20 30 40 50 60 0 20 40 60 80 100 120 accumulated precipitation, mm Tim e, hour Mass curve of rainfall 1st storm 16 mm 0 0.5 1 1.5 2 2.5 0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 2nd storm 34 mm Rainfall Intensity (mm/hr) Time (hrs) Hyetograph
- 15. Measurement of Rainfall: There are two types of rainfall measurement: 1. Point measurement – Recording and non-recording gauge (accurate but small area coverage) 2. Non-point measurement or areal measurement – (less accurate but large area coverage; accuracy is acceptable for hydrologic analysis) - Radar - Satellite
- 16. Rainfall Point Measurement Precipitation is measured on the basis of vertical depth of water that would accumulate on a level surface if the precipitation remained where it fell. Precipitation is measured in mm and tenths. One millimeter of precipitation represents the quantity of water needed to cover the land with a 1 mm layer of water, taking into account that nothing is lost through drainage, evaporation or absorption. Precipitation is measured by gauges which can be either non-recording (no record but collect the rain) or recording (or automatic gauge). There are three types of recording gauges: tipping bucket gauge, the weighing gauge and float gauge. The rain gauge should be located such that: 1. The ground is level with horizontal catch surface. 2. The gauge must be near the ground to reduce wind effect. 3. Surrounded by an open fenced area of 5.5 x 5.5 m. 4. No object should be nearer to the gauge than 30 m or twice the height of the obstruction.
- 19. Areal Rainfall Estimates: RADAR Radar (Radio Detection and Ranging) is the “art of detecting by means of radio echoes (electromagnetic waves) the presence of objects, determining their direction and range, recognizing their characteristics and employing the data thus obtained”. The meteorological radar is the powerful instrument for measuring the area extent, location and movement of rainstorm. Unlike gages (which measure point rainfall), the rainfall over large area can be determined through the radar with a good degree of accuracy. The radar emits a regular succession of pulse of electromagnetic radiation in a narrow beam so that when the raindrops intercept a radar beam, it is partially reflected back to the radar. The reflectivity mainly depends upon the drop-size distribution and the number of drops per unit volume. In general, the more intense the precipitation, the higher the reflectivity. The major problem with radar measurement of rainfall is that the radar measures the rainfall in the atmosphere and gages measure the rainfall in ground (effect of wind, obstruction and evaporation). Moreover, hydrologists are more interested in rainfall amounts during a period (hours or day) rather than the instantaneous rates available from the radar data. But, cumulative rainfall can be obtained from radar data. The accuracy of radar measurement varies between 90-98%.
- 22. Areal Rainfall Estimation by Satellites Non-homogeneity of temporal and spatial distribution of rainfall combined with the lack or the sparse presence of ground measurement makes it one of the most difficult parameters to quantify over a large area for hydrological analysis. The maximum range of radars vary from 100 to 300 km and also, radars are prohibitively expensive. So, satellite-based measurements offer much cheaper and wider coverage over larger basins. Meteorological satellite provide a unique opportunity for monitoring the precipitation for regions where ground measurement is limited and the accuracy of estimation is acceptable to the hydrologists. Satellites carry instruments designed to observe specific atmospheric characteristics such as cloud temperatures and precipitation particles. The higher the cloud albedo (reflected solar energy), the more droplets and/or ice crystals it contains and the deeper it tends to be, so the more likely rainfall is on the ground. And the lower the top infrared brightness temperature, the higher the cloud top, and the more likely the rainfall. TRMM (Tropical Rainfall Measuring Mission) satellite is the most widely used satellite for measurement of rainfall.
- 23. Satellite images used for rainfall predictions
- 24. Advantages: 1. Better estimation of spatial and temporal distribution of observed precipitation for large areas 2. Data on precipitation in areas of poor coverage of radars and rain gauges 3. Availability of data with adequate lead time for input to river and flash flood models, analysis of floods and droughts, water resources assessments 4. Availability of data in absence of data sharing across transboundary catchments. Rainfall Estimation by Satellites
- 25. Scatter plot of the average monthly accumulated rainfall for gauge data versus Tropical Rainfall Measuring Mission (TRMM) 3B42V7 data.
- 26. Average monthly rainfall derived from Tropical Rainfall Measuring Mission (TRMM) 3B42V7 data and gauge data for the period 1998–2010.
- 27. Rainfall Accuracy of Rainfall Measurement The accuracy of rainfall measurement is mainly affected by wind, by the height of the gauge and exposure. Wind and exposure errors can be very large, even more than 50 percent. 1. Human Error: faulty scale reading and water displacement (spilling of water). 2. Instrumental Defect: water to moisten the gauge; speed at which mechanical devices work (such as tipping bucket gauges); and leakage. 3. Improper Siting: height above the ground of the gauge orifice and exposure angle. 4. Wind Error: error due to systematic wind field deformation above the gauge orifice: typically 2 to 10 per cent. Wind deformation is largest for (1) and smallest for (6).
- 28. P = kPc P is the corrected rainfall and Pc is the catch. Effect of wind on rainfall
- 29. Rainfall Rain Gauge Network A gauge network should be planned such that a representative picture of the areal distribution of rainfall is obtained. The gauge density depends upon the purpose, accuracy, topography, accessibility and cost involved in measurement. The following are some of the criteria set by WMO: 1. For flat regions: 600-900 km2 per station. 2. For mountainous regions: 100-250 km2 per station. 3. For arid regions: 1500-10,000 km2 per station. In Bangladesh, rain gauge networks are maintained by Bangladesh Water Development Board (BWDB) and Bangladesh Meteorological Department (BMD). BWDB has 290 rainfall stations spread all over Bangladesh and BMD maintains 33 rainfall stations (mainly at district headquarters).
- 30. Rain gauge network of Bangladesh Water Development Board (BWDB) Considering the area of Bangladesh the rain gauge network can be considered as good; (147,500 / 290 = 508 km2 gauge).