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Evapotranspiration is the combination of two separate processes named evaporation and transpiration. Evaporation accounts for the movement of water to the air from sources such as the waterbodies and soil. Transpiration accounts for the movement of water within a plant and the subsequent loss of water as vapour through stomata in its leaves. So evapotranspiration is a bio-physical process.

Evaporation

The change of state of water from liquid to the vapour and its diffusion into the atmosphere is referred to as evaporation. During evaporation, water molecules absorb energy which gives them the motion required to escape from the surface of liquid and become a gas. Since a certain amount of energy is consumed in the process of evaporation, the remaining liquid is cooled by an equivalent amount. Thus, the process of evaporation is always accompanied by a cooling effect.

As evaporation proceeds, the surrounding air becomes gradually saturated with water vapour and the process will slow down and might stop if the wet air is not transferred to the atmosphere. The replacement of the saturated air with drier air depends greatly on wind speed. Hence, solar radiation, air temperature, air humidity and wind speed are meteorological parameters to be considered when assessing the evaporation process.

In agrometeorology, evaporation is defined as the maximum possible loss of moisture from a wet, horizontal, flat surface exposed to weather parameters, which exist in the vicinity of plants. Evaporation is an essential part of the water cycle. It is the primary pathway through which water moves from the liquid state back into the water cycle as atmospheric water vapour. The oceans, seas, lakes and rivers provide nearly 90 per cent of the moisture in the atmosphere via evaporation, with the remaining 10 per cent being contributed by plant transpiration. A very small amount of water vapour enters the atmosphere through sublimation, the process by which water changes from a solid (ice or snow) state to a gaseous one, bypassing the liquid phase.

The word ‘monsoon’ is derived from the Arabic word mawsim meaning season. The Arabs during their travel towards India recognised that by June strong southwesterly winds commence over the Arabian Sea and bring a lot of rains to the subcontinent. The term was first used in English in British India and neighbouring countries to refer to the big seasonal winds blowing from the Bay of Bengal and Arabian Sea in the southwest bringing heavy rainfall to the area. In the Indian subcontinent, monsoon is one of the oldest weather observations, an economically important weather pattern over June through September every year, and the most anticipated weather event and unique weather phenomenon. Due to its effects on agriculture, flora and fauna, and other economic, social, and environmental effects, a monsoon is one of the most anticipated, followed and studied weather phenomena of the Indian subcontinent. It has significant impact on the overall well-being of subcontinent residents.

The term ‘monsoon’ is customarily used in India to refer to the rainy season during the northern hemisphere summer from June to September and is traditionally defined as a seasonal reversing wind accompanied by corresponding changes in precipitation, but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with the asymmetric heating of land and sea. Now the term monsoon has been broadened to include almost all of the phenomena associated with the annual weather cycle within the tropical and subtropical land regions of the earth.

Precipitation is water in liquid or solid forms, falling on to the earth. In other words, precipitation is the deposition of atmospheric moisture on earth. Formation of clouds does not mean precipitation. Precipitation involves condensation, growth of hygroscopic nuclei, cooling of nuclei at different stages and their fall on to the earth’s surface. There may be several clouds but no precipitation. If the cloud particles are too small and remain suspended in the sky, they may not fall on the earth. Under certain conditions, water particles formed tend to fall but do not reach the surface since they get evaporated on the way. Only when the cloud droplets or ice crystals grow to certain size and can no more remain floating due to buoyancy of air, do they fall as precipitation.

Precipitation is the major source of water to all the living organisms on the earth. All the water sources on the earth depend on precipitation only.

Forms of precipitation

Precipitation form is the state that the moisture is in, i.e., liquid, freezing, or frozen (solid). Different forms of precipitation may occur together, such as mixed rain and snow; but such an occurrence is simply a mixture of forms, not a separate form of precipitation. Because atmospheric conditions vary greatly both geographically and seasonally, several different forms of precipitation are possible. All forms of precipitation are collectively termed hydrometeors. Hydrometeors have been classified into 50 specific types. Out of these precipitation forms, only rain (liquid form) and snow (solid form) make significant contribution to total precipitation. In many parts of the world, the term rainfall is used interchangeably with precipitation.

Clouds are visible mass of little drops of water or frozen crystals suspended in the atmosphere above the surface of the earth. The World Meteorological Organisation defined cloud as a visible aggregate of minute particles of water or ice or both, in the free air. This aggregate may include larger particles of water or ice and particles such as those present in fumes, smoke or dust.

Clouds are the most important form of suspended water droplets caused by condensation. The typical size of these water droplets and ice crystals range from less than 1 μm to 50 μm. When surrounded by billions of other droplets or crystals, they become visible as clouds. In the atmosphere sufficient moisture is required to allow condensation to take place. Then, moisture needs a suitable surface upon which it can condense. In free air, condensation begins on hygroscopic nuclei such as aerosols. These are microscopic particles and include dust, smoke, salts and chemical compounds. Nuclei range in size from 0.001 μm radius to over 10 μm. On an average, oceanic air contains one million condensation nuclei per litre and land air holds some 5 or 6 millions. On hygroscopic particles such as sea salts, condensation can begin before the air is saturated. Supersaturation in clouds rarely exceeds one per cent. On an average, clouds contain only four per cent of the total water in the atmosphere at any one time. Almost all clouds exist in the troposphere, although the tops of some severe thunderstorms occasionally extend to the stratosphere. Clouds move along with the wind and remain stationary in calm winds. They grow vertically and horizontally, depending on the temperature of the cloud and its ambient temperature. They may dissipate or precipitate. Under favourable atmospheric conditions, they burst and fall as rain.

Water vapour changes to the liquid water through condensation process. Condensation is the direct cause of the various forms of precipitation. It occurs as a result of changes in air volume, temperature, pressure or humidity. Water vapour can also be converted directly into the solid form through a process called sublimation. The latent heat expended in evaporation is liberated in the process of condensation. Four mechanisms may lead to condensation.

Water is the only substance that exists in the atmosphere in all the three phases, i.e., solid, liquid and gas. The moisture on the earth’s surface is in a constant phase change. The moisture content of the atmosphere is about 0.035% of all fresh water. The atmosphere receives its supply of moisture from the earth’s surface through evaporation from oceans, lakes, rivers, damp soil, etc. and through transpiration from vegetation. Water vapour is the gaseous state of water and is invisible.

The amount of water vapour present in the atmosphere is called atmospheric moisture or humidity. It is a measure of the amount of water vapour dissolved in the air, not including any liquid water or ice falling through the air. While humidity itself is a climate variable, it also interacts strongly with other climate variables. The humidity is affected by winds and by rainfall. Moisture content is determined by the local rate of evaporation, air temperature and the horizontal transport of moisture. Humidity indicates the likelihood of precipitation, dew or fog. It also plays a major role in the heat budget and the weather phenomenon of the atmosphere.

A big layer of air called the atmosphere surrounds the earth. The air within this layer moves from place to place when it warms up or cools down. This moving air is known as wind. In other words, air in motion is called wind. Winds move moisture and heat around the world and also produce much of our weather.

The strength of the wind can vary enormously. Sometimes air moves slowly and the wind is barely noticeable. When the weather is clear there may be a gentle breeze. When the wind is still very light, but it can be felt and there may be rustlings by leaves. At other times, the air can move very quickly and become a gale or hurricane, blowing down trees and damaging electric poles and buildings.

Winds are part of a global air circulation system that acts to balance temperature and pressure around the world. Different parts of the world receive different amounts of heat from the sun. This differential heating in turn results in differences in temperature and air pressure around the world, which drives the world’s winds.

As equatorial areas are heated most, the air above them warms and rises as it becomes lighter than the surrounding air, causing an area of low pressure. In cooler areas, the air sinks because it is heavier and results in an area of high pressure. Winds will blow as air is squashed out by the sinking cold air and drawn in under the rising warm air. Any difference in temperature like this will always cause a difference in air pressure, and therefore winds will blow. These movements result in a global wind pattern, with air moving between different areas around the world and also at different heights in the atmosphere. Colder air from the poles tends to sink and move towards the equator closer to the surface of the earth. In contrast, warm air from the equator rises and moves towards the poles high in the atmosphere because it is lighter.

Atmospheric pressure is the force per unit area exerted on a surface by the weight of air above that surface in the atmosphere of earth. The gas molecules of the atmosphere are pulled towards the earth by the force of gravity and hence exert a force on earth surface. In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point. Atmospheric pressure influences other weather elements in a significant way. Small changes in air pressure may not be felt, but they are important for bringing a change in weather. Atmospheric pressure can be calculated as the summed weight of all gas molecules between a horizontal plane and top of the atmosphere and divided by the area of the plane. Low pressure areas have less atmospheric mass above their location, whereas high pressure areas have more atmospheric mass above their location.

Measurement of atmospheric pressure

The standard unit of pressure in the SI system is the Pascal which is the name given to one Newton per square metre (N/m²). For meteorological purposes, standard sea-level pressure is 101325 pascal (Pa). To simplify this large number, the millibar unit (mb) has been adopted, which is equal to 100 pascals. Units like kiloPascal (kPa) or hectoPascal (hPa) are also used to measure pressure. 1 kiloPascal is equal to 10 millibars and 1 hPa is 1 millibar. The standard sea-level pressure is represented as 1013.2 mb or hPa.

Soil temperature is an important environmental parameter for plant growth and development. It has direct effects on microbial growth and development, organic matter decay, seed germination, root development, and water and nutrient absorption by roots. In general, the higher the temperature the faster these processes occur. Variation in soil temperature is much more pronounced because of varying characteristics and composition of soil.

The soil gains heat by absorption of solar radiation and longwave counter radiation from the atmosphere, downward transfer of sensible heat and horizontal transfer of heat from the surroundings. On the other hand, soil loses heat by longwave radiation to the atmosphere, upward transfer of sensible heat to air when the soil surface is hotter than air and with evaporation and horizontal transfer of heat from the column to surroundings.

Temperature is the physical property of an object that quantitatively expresses the hotness or coldness. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot. When a heat transfer path between them is open, heat spontaneously flows from bodies of a higher temperature to bodies of lower temperature. If no net heat flow occurs between two objects, the objects have the same temperature which are then said to be in ‘thermal equilibrium’. Temperature plays an important role in all fields of natural and applied sciences. Most of the weather parameters are dependent on it, directly or indirectly. It has important role to play in the development and growth of all living organisms in all stages of their growth.

Atmosphere receives the heat energy from the sun and its temperature increases. Due to different amount of heat energy receipt at different places, the air temperatures at different places also vary. The variation in air temperature basically results into air motion, so as to equalise the energy content of the different regions of the earth. Thus, atmospheric temperature is one of the basic elements of weather and climate.

Sun is the prime source of energy injected into the earth’s atmosphere. The sun is the nearest star to the earth. It is a hot ball of glowing gases at the heart of the solar system. Its radiant energy is practically the only energy source to the earth. Very small and insignificant quantities of energy are available from other sources such as the interior of the earth, the moon, and other stars. Without the sun’s intense energy and heat, there would be no life on earth.

The mean distance of the sun from the earth, also known as one astronomical unit (1 AU), is approximately 1.496 × 10⁸ km, though the distance varies as the earth revolves round the sun in an elliptical orbit. At this average distance, light travels from the sun to earth in about 8 minutes and 19 seconds. The energy of the sunlight supports almost all life on earth by photosynthesis, and drives earth’s climate and weather. The minimum sun-earth distance is about 0.983 AU and the maximum is approximately 1.017 AU. The earth is at its closest point to the sun (perihelion) on around January 3 and at its farthest point (aphelion) on around July 4.

The sun makes up about 99.86% of the mass of the entire solar system. The visible disk or photosphere has a radius of 6.599 × 10⁵ km, and the solar mass is 1.989 × 10³⁰ kg. The sun is a completely gaseous body. The chemical composition (by mass) of the outer layers of sun is 71 per cent hydrogen, 26.5 per cent helium, and 2.5 per cent heavier elements like oxygen, carbon, neon, and iron among others. Since the sun is not a solid body, different parts of the sun rotate at different rates. At the equator, the sun spins once about every 25 earth days, but at its poles the sun rotates once on its axis every 36 days.

The gaseous envelope surrounding the earth is called atmosphere. Thus, atmosphere can be defined as the dynamic layer surrounding the earth above its surface containing various gases, moisture, aerosols, etc. The atmosphere is retained by earth’s gravity.

The atmosphere protects life on earth by absorbing ultraviolet solar radiation, warming the earth’s surface through heat retention and reducing temperature extremes between day and night. The common name given to the atmospheric gases used in breathing and photosynthesis is air. The atmosphere has a mass of about 5.15 × 10¹⁸ kg, three quarters of which is within about 11 km from the surface. The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and outer space. The imaginary ‘Karman line’ lies at an altitude of 100 km above the mean sea level, and commonly represents the boundary between the earth’s atmosphere and outer space. The study of earth’s atmosphere and its processes is called atmospheric science or ‘aerology’. The atmosphere serves the life-system on the earth by the following activities.

Meteorology is derived from the Greek word meteoro which means above the earth’s surface; i.e., the atmosphere. The state of the atmosphere experienced at a given time, in any location on the surface of the earth is known as weather. Thus meteorology is the scientific study of the atmosphere or the weather. It is a branch of physics dealing with atmosphere and it is often quoted as the ‘physics of the lower atmosphere’. The main focus of meteorology is to understand the weather processes and forecast the weather. Solar radiation, the air mass and moisture are the three key ingredients that make up weather. The exchange of heat, moisture and momentum between the earth’s surface and the atmosphere determines the characteristic of weather. The observable weather events are known as meteorological phenomena and are defined by a set of variables called weather elements. Atmospheric temperature, atmospheric pressure, humidity, precipitation, cloudiness, wind and visibility, etc. are examples of such weather elements. The weather elements are not separate entities. They are closely related with each other and are highly variable in nature. Meteorology explains and analyses the changes of these weather elements.

Meteorology

The major subdivisions of meteorology are dynamic meteorology (or atmospheric dynamics) and physical meteorology (or atmospheric physics). The dynamic meteorology is concerned with the analysis and interpretation of the three-dimensional, time-varying, macro scale motions and energy transformations in the atmosphere. The science of dynamic meteorology is meant to develop models based on the hydrodynamic and thermodynamic equations. The physical meteorology is concerned with the processes that alter the physical properties and the chemical composition of air parcels as they move through the atmosphere.

ABSTRACT

The over extraction of water from the coastal environment, results in reduction of groundwater resources which has reflected in lowering of water level. In general, the depletion of water level enhances the landward migration of saltwater wedge. Pondicherry is one such region with recent alluvium as the major litho unit. A study has been attempted to calculate the extraction of water and to estimate the amount of recharge into this alluvial aquifer. The monthly water level fluctuation was observed for the study period in eighteen locations (20002002). The highest water level observed during 2000 was taken as the initial water level for the study and the subsequent decline in water level up to 2001 was monitored monthly until the rising trend was noted. This was taken as the fall in water level due to extraction. Keeping this as the initial value increasing trend was studied until there was a notice of decline in water level. This was taken as the rise in water level due to recharge. The spatial representation of these data in the above said eighteen locations were carried out by using GIS and the area occupied by different water level contours were calculated and the amount of water withdrawn was estimated. The maximum recharge was noted in the central and the northern part of the study area when compared to the other regions. Similarly, the maximum discharge was noted in the northern and the southern part of the study area.

Keywords: Rainfall, Recharge, Groundwater Budget, GIS.

ABSTRACT

Groundwater geochemistry is largely a function of mineral composition of aquifer through which water flows thus, differences in aquifer composition are reflected as differences in groundwater chemistry. To understand water quality and flow pattern multivariate statistical analysis are used due to its relative importance in combining chemical variables and organize voluminous data and group into variables of similar characters. R-mode factor analysis is used in the field of hydrogeochemistry to interpret complex hydrogeological process. A total of 216 samples representing complete lithology like peninsular gneiss, Charnockite and calc gneiss for four different seasons was collected and analyzed for major ions like Ca, Mg, Na, K, HCO₃, Cl, SO₄, NO₃, PO₄, F and H₄SiO₄. Correlation analysis reveals Mg, Cl and Na are spinal and others seasonal. Factor analysis indicates ascendancy of anthropogenic basis followed by lithological weight. Factor score superimpose indicate governing hydrogeochemical regimes spread in western and eastern part. In southern part the same has superior influence than northern part.

ABSTRACT

The study was aimed to identify and delineate the groundwater potable zone in and around Madurai region. The study area is composed of Charnockites and Khondalites of Archaean age. The information of lithology, geomorphology and land use/land cover was generated using the Resourcesat (IRS P6 LISS IV data) and Survey of India (SoI) toposheets of scale 1:50,000 and integrated them with GIS to identify the groundwater potable zones of the study area. On the basis of hydrogeomorphology, groundwater potential zones was identified and delineated. From the overlay analyses landuse and EC, it is inferred that, poor category groundwater is found in the scrub forest, current fallow, canal and scrub with land areas.

ABSTRACT

Topsoil erosion, loss of fertile soil and subsequent deposition in the reservoir as well as in the low-lying region are one of the major problems in the tropical mountainous region. Knowledge of sediment yield from the river basin is a prerequisite to address such issues. Rivers from the Western Ghat region of Uttara Kannada district bring lot of sediments causing siltation in their mouth. In the present study, sediment yield from a Venkatapur river basin in Bhatkal Taluk of Uttara Kannada district is estimated by Universal Soil Loss equation (USLE). Morphometric characteristics of the river basins that reflect sediment yield potential of the river basin are used to constrain the data obtained. Various parameters for USLE is derived from the IRS 1D LISS III image and data are handled in GIS environment. Sediment yields in the basin 101 tons/yr. Morphometric characters of the basin such as drainage density, bifurcation ratio, basin circularity (0.6) and form factor (0.43) suggest its low potential of sediment yield.

ABSTRACT

The present study reports shoreline changes at Gopalpur along east coast of India for 30 years during 1973-2003. In order to understand the trend of sediment transport, shoreline changes at the vicinity areas viz. Rushikulya and Bahuda estuaries are also discussed. Geomorphological investigation is also carried out to understand the processes acting on this system. One Landsat image (1973) and two IRS 1C/1D images (1993 and 2003) were used for the study. The data were subjected to digital analysis using ERDAS Imagine and management of information layers using ARC/INFO GIS software. The results of the study show that the changes occurred differently in three distinct spatial features namely, i) sand spit ii) beach and, (iii) channel zone. Changes documented in these imageries reveal that the lengths of NE spit and SW spit are increased and decreased respectively in thirty years. Areas of NE and SW spits are also showing the similar pattern. Beach is almost stabilised and a slight decrease of 0.28 km in length is observed. However, area has been increased in the southern beach. Towards north, the beach area is also increased over a period of thirty years. The change of width of the beach is not found to be significant. The mouth near channel zone is reduced to 70 meter from 135 meter during the period of study.

ABSTRACT

Remote sensing, especially high resolution satellite data has a major role to play in locating (localizing) mineral targets. The present study is carried out on exploration and discrimination of magnetite ore deposits of Tirthamalai region of part of Dharmapuri district, Tamil Nadu, India, using IRS P6 LISS IV high resolution satellite data. The applicable image processing techniques and associated put forth of field knowledge is used in the present study. The image processing classifications namely band rationing, principal component analysis and contrast enhancement are carried out in visualizing images using ERDAS Imagine 8.7 software. The visual interpretation of FCC of digital data shows that the raw pixel values of iron ore bands in the blue ranges 95 to 120, green ranges 85 to 125 and red ranges 55 to 75 and provided better delineation for iron formations.

The interpretation of satellite data shows that the iron ore band exhibits in dark color, linear shape, fine texture and sharp contact with associated rock types. To rectify and reduce misinterpretation all above image processing techniques are undertaken along with the detailed filed-check in the selected test sites using GPS 178 and delineated the in situ iron formations. The interpretations show that the occurrence of few more iron bands and weathered formations along with massive formations in the region traced on the imagery since the absorptions themselves are different. The availability spectral absorption of iron formation is inferred highly in visible and near infrared region (0.9-1 Ìm). Remote sensing high resolution satellite data proved that the satellite parameters are important criteria and play a vital role in mapping of low-grade iron ore deposits of the Tirthamalai region of Dharmapuri district of Tamil Nadu state.

ABSTRACT

Soil is the medium in which the plant growth and development occurs. Soil type might change with genesis of the parent material and with prevailing weather conditions. Dominant soils of Tamil Nadu are Black soil, Red soil, Laterite soil and Hill soils. Each soil type has an inherent potential to retain and release nutrients to plants. The availability of nutrients varies from place to place and hence spatial distribution of nutrients, quantity of nutrients along with plant available water determines phenological development of crops. Nutrients are broadly classified as Macro and Micro nutrients that are required for crops in large and small quantities respectively. Availability of both macro and micro nutrients and its spatial distribution has not been attempted so far at a regional level. Such regional level approach of nutrients would suffice the researchers and policy planners to employ these data as inputs to Crop Simulation Models (CSM) in forecasting productivity of crops.

Thiruvannamalai district in Tamil Nadu is divided into 6 taluks, 18 blocks covering an area of 6,31,205 haof land. The district lies between 12⁰ 00’ to 12⁰ 55’ N latitude and 78⁰ 37'to 79⁰ 45’ E longitude. Red loamy soil is predominantly prevalent followed by black loamy soils. Generally paddy, groundnut, sugarcane, millets and pulses are cultivated during fasli season. This district was chosen as our study area.

A study has been attempted to map micro nutrients of Thiruvannamalai district. Geo-referenced surface soil samples from eighteen blocks of Thiruvananmalai district covering maximum soil units of soil map were collected randomly as per procedure adopted by soil survey manual. Global Positioning System (GPS) were used to collect corresponding geographic coordinates from each soil sampling point covering the entire Thiruvananmalai district by using Rikaline Bluetooth GPS 6031- X7 model. Approximately one thousand five hundred surface soil samples were collected, air dried, powered and sieved as per standard conditions. Soil samples were analyzed for its physical, chemical and fertility properties. Fertility maps for Micro nutrients such as Zinc, Iron, Manganese, Copper and Boron were plotted in ARCGIS Desktop. Micronutrients maps form a part of base map in analyzing and preparing an aggregated fertility map of Thiruvannamalai district.