Note for Advanced Transportation Engineering - ATE by akhil mahadik

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Instructional Objectives After completion of this Module, the student shall know about 1. Hydrologic cycle and its components 2. Distribution of earth’s water resources 3. Distribution of fresh water on earth 4. Rainfall distribution in India 5. Land and water resources of India; water development potential 6. Need for development of water resources 7. Forms and Types of precipitation 8. Methods of Measurement of Precipitation 9. Presentation and interpretation of rainfall data 10. Abstractions from Precipitation INTRODUCTION Water on the surface of earth is available in the atmosphere, the oceans, on land and within the soil and fractured rock of the earth’s crust. Water molecules from one location to another are driven due to the solar energy transmitted to the surface of the earth from Sun. Moisture circulates from the earth into the atmosphere through evaporation and then back into the earth as precipitation. Hydrology: It is the study of physical geographic which deals with the origin, distribution and properties of water present in earth surface. Hydrological Cycle:Precipitation ↑ Condensation ↑ Formation of Clouds ↑ Evaporation ↑ Ocean

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Water Budget Equation:Mass inflow – Mass outflow = Change in storage P – R – G – E – T = ∆S Where, P = Precipitation T = Transpiration G = Ground water Resources R = Runoff E = Evaporation ∆S = Chang in Storage Catchment Area:River Streams

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World Water Balance:Total quantity of water in the world is estimated to be about 1386 million cubic kilometer (M Km3). About 96.5% of this water is contained in the oceans as saline water. Some of the water on the land amounting to about 1% of the total water is also saline. Thus, only about 35.0 M Km 3 of fresh water is available. Out of this about 10.6 M Km3 is both liquid and fresh and the remaining 24.4 M Km 3 is contained in frozen state as ice in the polar regions and on mountain tops and glaciers. PRECIPITATION INTRODUCTION: The term “precipitation” denotes all forms of water that reach the earth from the atmosphere. The usual forms are rainfall, snowfall, hail, frost and dew. The magnitude of precipitation varies with time and space. For precipitation to form: (i) the atmosphere must have moisture, (ii) there must be sufficient nuclei present to aid condensation , (iii) weather conditions must be good for condensation of water vapour to take place, and (iv) the products of condensation must reach the earth. FORMS OF PRECIPITATION: Some of the common forms of precipitation are rain, snow, drizzle, glaze, sleet and hail. 1. Rain

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It is the principal form of precipitation in India. The term rainfall is used to describe precipitation in the form of water drops of sizes larger than 0.5mm. The maximum size of a raindrop is 6mm. Any drop larger in size than this tends to break up into drops of smaller sizes during its fall from the clouds. On the basis of its intensity rainfall is classified as follows: Light rain: trace to 2.5 mm/hr Moderate rain: 2.5mm/hr to 7.5mm/hr Heavy rain: > 7.5mm/hr 2. Snow Snow is another important form of precipitation. Snow consists of ice crystals which usually combine to form flakes. When fresh, snow has an initial density varying from 0.06 to 0.15g/cm3 and it is usual to assume an average density of 0.1 g /cm3. In India, snow occurs only in the Himalayan regions. 3. Drizzle A fine sprinkle of numerous water droplets of size less than 0.5mm and intensity less than 1mm/hr is known as drizzle. In this, the drops are so small that they appear to float in the air. 4. Glaze When rain or drizzle comes in contact with cold ground at 0 0C, the water drops freeze to form an ice coating called glaze or freezing rain. 5. Sleet It is frozen raindrops of transparent grains which form when rain falls through air at sub freezing temperature. In Britain, sleet denotes precipitation of snow and rain simultaneously. 6. Hail It is a showery precipitation in the forms of irregular pellets of lump of ice of size more than 8mm. Hails occur in violent thunderstorms in which vertical currents are very strong. WEATHER SYSTEMS FOR PRECIPITATION: For the formation of clouds and subsequent precipitation, it is necessary that the moist air masses cool to form condensation. This is normally accomplished by adiabatic cooling of moist air through a process of being lifted to higher altitude. Some of the terms and processes connected with weather systems associated with precipitation are given below. 1. Front A front is the interface between two distinct air masses. Under certain favorable conditions when a warm air mass and cold air mass meet, the warmer air mass is lifted over the colder one with the formation of front. The ascending warmer air cools adiabatically with the consequent formation of clouds and precipitation. 2. cyclone