Unit - 1

Introduction

• The word hydrology has been derived from two Greek words ‘Hydro’ i.e., water and ‘logos’ i.e., science so it is a science of water.
• It is defined as “A science which deals with the properties, distribution and circulation of water on the surface of the land, in the soil, in the underlying rocks and also in the atmosphere mainly in the form of moisture or water atmosphere mainly in the form of vapor and in the form of water droplets during rainfall or in the solid form of snow particles during snows fall.”
• C.O. Sister and E.F barter have defined the science of hydrology as “A science which deals with the process of governing the depletion and replacement of water resources of the land areas of the earth.
• Hydrology is an applied science used by many persons working in different fields for the development of the human beings.

• Due to solar radiation called as insulation i.e., in coming solar radiation.
• The oceanic and the water on the land surface get heated and get converted into vapor.

• The vapor being lighter is lifted up in the atmosphere so the air has this vapor called as moisture starts cooling down and so its start controlling.
• It reduces the carrying capacity of air to hold the moisture and it becomes moisture saturated i.e., no more moisture it can hold. This level is called as level of condensation.

• Due to the pushing of air from the earth surface, this saturated air is still pushed in the upward direction to make it still cooler and still more compact to reduce its capacity lesser than the actual available moisture

• This extra moisture is converted into small rain drops. Which attract the dust particles to make it visible is called as clouds

• The moisture carrying clouds are carried by the winds towards the land surface.
• If it is a hilly region the clouds are still lifted up to increase the size of the raindrops and they start falling down called as rainfall.

• The rainwater received on the surface starts Moving towards the slopes, to form the running water or streams, which finally meet the oceans
• The rainwater received on the surface same times percolates (it depends upon the type of rock, if it is porous the water percolates) to form the ground water. It also comes back on the surface in the form of springs to form a surface stream to reach the ocean.
• The rain water received on the surface, which is plain and have non porous rock: forms lakes or tanks.
• The cycle is completed once the ocean or surface water comes back can the earth surface to start the first stage of Hydrology ice. It starts getting converted from the liquid state to the gaseous state. When the water is received in the form of solid state i.e., snow, it also undergoes the same cycle.
• When the process is fast the water vapour directly gets converted into solid snow particles. It is called as sublimation. This happened when the temperature of the moisture saturated air is less than O Cie. Less than the freezing point temperature. In the temperature and in the arctic zones on the earth it causes rainfall.

• There is no readily available source of information about the history of hydrology and hydrologists. This site is one of a number of initiatives (see the links below) to try and remedy that situation.
• All hydrologists are welcome to contribute information about the development of the subject (and its overlaps with hydraulics, water resource engineering, geomorphology, ecohydrology, socio hydrology, hydrometeorology, water quality, etc)
• The site is intended to provide information on hydrologists who are no longer active but who have made a valuable contribution to the history of hydrology.
• Hydrology has been a subject of investigation and engineering for millennia. For example, about 4000 BC the Nile was dammed to improve agricultural productivity of previously barren lands.
• Mesopotamian towns were protected from flooding with high earthen walls. Aqueducts were built by the Greeks and Ancient Romans, while the history of China shows they built irrigation and flood control works.
• The ancient Sinhalese used hydrology to build complex irrigation works in Sri Lanka, also known for invention of the Valve Pit which allowed construction of large reservoirs, anicuts and canals which still function.
• Marcus Vitruvius, in the first century BC, described a philosophical theory of the hydrologic cycle, in which precipitation falling in the mountains infiltrated the Earth's surface and led to streams and springs in the lowlands.
• With the adoption of a more scientific approach, Leonardo da Vinci and Bernard Palsy independently reached an accurate representation of the hydrologic cycle.
• It was not until the 17th century that hydrologic variables began to be quantified.
• Point of the modern science of hydrology includes Pierre Perrault, Edme Mariotte and Edmund Halley.
• By measuring rainfall, runoff, and drainage area, Perrault showed that rainfall was sufficient to account for the flow of the Seine. Mariotte combined velocity and river cross-section measurements to obtain a discharge, again in the Seine.
• Halley showed that the evaporation from the Mediterranean Sea was sufficient to account for the outflow of rivers flowing into the sea.
• Advances in the 18th century included the Bernoulli piezometer and Bernoulli's equation, by Daniel Bernoulli, and the Pitot tube, by Henri Pitot.
• The 19th century saw development in groundwater hydrology, including Darcy's law, the Dupuit-Thiem well formula, and Hagen-Poiseuille's capillary flow equation.
• Rational analyses began to replace empiricism in the 20th century, while governmental agencies began their own hydrological research programs. Of particular importance were Leroy Sherman's unit hydrograph, the infiltration theory of Robert E. Horton, and C.V. Theis' aquifer test/equation describing well hydraulics.
• Since the 1950s, hydrology has been approached with a more theoretical basis than in the past, facilitated by advances in the physical understanding of hydrological processes and by the advent of computers and especially geographic information systems (GIS). (See also GIS and hydrology)

• Water balance is the ratio between water inflow and outflow estimated for different space and time scales, i.e., for the Earth as a whole, for oceans, continents, countries, natural-economic regions, and river basins, for a long-term period or for particular years and seasons.
• Water balance is the most important integral physiographic characteristic of any territory, determining its specific climate features, typical landscapes, possible water management and land use.
• Analysis of water balance components for individual territories and time intervals is of great importance for studies of the hydrological cycle or water circulation in the atmosphere-hydrosphere-lithosphere system, as well as the underlying processes influenced by natural factors and human activities.
• Precipitation, evaporation, river runoff and ground water outflow not drained by river systems are basic components determining water balance.
• Besides these components, there are minor components, too, e.g., moisture due to atmospheric water vapor condensation, deep artesian water outflow, or, conversely, recharge of deep aquifers,
• Water losses for animal survival, etc. According to investigations, however, these components are very small if related to large river basins, regions and the globe—they are of no importance for water balance computation, so they can be ignored.
• It should be noted that much fresh water is used in many regions for different human needs.
• Some of this is returned to water bodies as surface and subsurface runoff, but some water is lost, particularly to evaporation (from irrigated lands, reservoirs, etc.), thus increasing evapotranspiration in the region.
• This must be taken into account in the appropriate water balance components.
• Thus, the assessments of water balances of large regions with sufficient accuracy is reduced to reliable determination of the main water balance components, i.e., precipitation, evaporation and runoff (surface and subsurface).
• Quantitative characteristics of these components for different regions of the Earth presented in this chapter are mainly based on the results of the global hydrological cycle studies carried out in Russia at the State Hydrological Institute (St Petersburg) and at the Institute of Water Problems (Moscow).
• More detailed information about individual water balance components of the Earth is given in Atmospheric Precipitation of the Earth, Evaporation from the Surface of the Globe, River Runoff to Oceans and Lakes and Groundwater Discharge to the Oceans.

• Water balance equation for any land area and any time interval (without taking account of the above minor components) is as follows:

• E is evapotranspiration;
• Rs and Run indicate surface and subsurface runoff from some land area and subsurface water inflow to the land area, respectively;
• S is water storage change in the area.
• All terms in equation (1) are in mm of water layer, which is a water volume for time unit divided by the area of the land.
• If the water balance equation is considered for a long-term period it is simplified, because S = 0.
• If the water balance is considered at the global scale, it should be noted that there are regions on each continent which differ greatly in their water balance structure.
• Most territories on the continents are the zones of so-called external runoff—river runoff from these zones discharges to the World Ocean directly.
• There are also rather large areas on the continents (probably except Antarctica) which have internal runoff. These endorheic areas are not connected to the World Ocean.
• River runoff formed in such regions is completely lost to evaporation.
• For oceanic slopes and large river basins related to areas of external runoff from the continents (when it is possible to neglect surface and subsurface inflow from adjacent areas) the water balance equation for a long-term period is as follows:

• In equation (2) Pext is a precipitation;
• Next is river runoff (from an oceanic slope) to the ocean (sea, lake);
• Run is subsurface runoff not drained by river systems and directly discharging to the ocean (sea, lake);
• Exert is evapotranspiration including additional evaporation due to human activities.
• In the areas of internal runoff (endothecia regions) the whole quantity of precipitation is ultimately lost to evaporation, so the water balance equation for a long-term period for such regions is as follows: Pint = Eint                    ......................(3)

• Eint is evapotranspiration from endorheic areas, including runoff formed within these areas and water losses for different human needs.
• For a continent with available zones of external runoff and endorheic areas, the water balance equation would probably consist of equation (2) and equation (3) joint together:

• For the World Ocean, as well as for individual oceans (and seas) the freshwater balance equation for the long-term period (without taking account of water exchange between the oceans) will be as follows:

• Where: Eoc and Poc are evaporation from the ocean and precipitation onto the ocean surface, respectively;
• R ext and Run are river water inflow and subsurface water inflow to the ocean.
• For the whole Earth for a long-term period and a steady climate situation it is evident that the total precipitation should be equal to evaporation from the water surface plus evapotranspiration from land, i.e. for the world water balance the water balance equation similar to that for endorheic areas is valid:

• Where: Pgl and Egl are global values of precipitation and evaporation from the Earth as a whole.
• It should be noted that equation (6), just like equations (1) to (5), is valid if we assume that water coming from outer space is balanced by the amount of water vapor lost to space and deep-water inflow (or juvenile water) is equivalent to water used for hydration of minerals in the lithosphere.

• In hydrology we apply scientific knowledge and mathematical principles to solve water-related problems in society: problems of quantity, quality and availability.
• Mathematical models of all Hydrological phenomena are made. They may be concerned with finding water supplies for cities or irrigated farms, or controlling river flooding or soil erosion. Or, they may work in environmental protection:
• Preventing or cleaning up pollution or locating sites for safe disposal of hazardous wastes.
• Hydrology is used to find out maximum probable flood at proposed sites e.g. Dams.
• The variation of water production from catchments can be calculated and described by hydrology.
• Engineering hydrology enables us to find out the relationship between a catchments’ surface water and groundwater resources
• The expected flood flows over a spillway, at a highway Culvert, or in an urban storm drainage system can be known by this very subject. It helps us to know the required reservoir capacity to assure adequate water for irrigation or municipal water supply in droughts condition.
• It tells us what hydrologic hardware (e.g., rain gauges, stream gauges etc) and software (computer models) are needed for real-time flood forecasting Used in connection with design and operations of hydraulic structure Used in prediction of flood over a spillway, at highway culvert or in urban storm drainage Used to assess the reservoir capacity required to assure adequate water for irrigation or municipal water supply during drought. Hydrology is an indispensable tool in planning and building hydraulic structures.
• Hydrology is used for city water supply design which is based on catchments area, amount of rainfall, dry period, storage capacity, runoff evaporation and transpiration. Dam construction, reservoir capacity, spillway capacity, sizes of water supply pipelines and affect of afforest on water supply schemes, all are designed on basis of hydrological equations.
• Hydrology provides guidance for undergoing proper planning and management of water resources. Calculates rainfall, surface runoff, and precipitation.
• It determines the water balance for a particular region.
• It mitigates and predicts flood, landslide and drought risk in the region.
• It estimates the water resource potential of the river basins
• Enables real-time flood forecasting and flood warning.
• Hydrology analyses the variations observed in the catchments by bringing a relationship between the surface water and groundwater resources of the catchment.
• Hydrology studies the required reservoir capacity that is necessary for irrigation and municipal water supply purpose during drought conditions.
• It is used in the design and operation of hydraulic structures
• It is used for hydropower generation.
• Brings measures to control erosion and sediments.

• Precipitation is the fall of water in various forms on the earth from the clouds. The usual forms area is, snow, sleet, glaze, hail, dew etc.
• Before studying the phenomenon of precipitation let us consider water vapour air in atmosphere can easily absorb moisture in the form water vapours. The amount of water vapours absorbed by air depend upon the temperature of air, them or is the temperature the more water vapour sit can absorb.
• The water vapour exerts a partial pressure on the water surface called vapour pressure.  The amount of water vapour present in air is in directly expressed in terms of vapour pressure.
• If the evaporation continues, a state of equilibrium is reached when the air is fully saturated with vapour and therefore it cannot absorb more vapours. The vapours then exert a pressure which is known as saturation vapour pressure (es). es increase with increase in temperature.
• Let us consider a parcel of air as temperature T and a vapour pressure (ea) indicated by pt. A. The saturation vapour pressure at that temperature is indicated by pt. B. The intercept BA= (e s-ea) is called saturation deficit.
• If vapours are added to the parcel of air, the pt. A will move to pt. B when air is fully saturated.
• If the parcel of air is cooled at constant pressure but without the addition of more vapours, the pt. Moves horizontally towards pt. D and the air would be saturated when pt. D is reached. At that stage, the air would have a temperature called dew point temperature (T). Cooling of air beyond this pt. Would result in condensation or formation of mint.
• If neither the temperature not the pressure remains constant, the water evaporates freely and the pt. Moves to pt. C.  In this case, water vapour is but temperature falls. The temperature at pt C. Is called wet temperature (T). The saturation vapour pressure is indicated by e w.
• Air in atmosphere can be cooled by many processes. However, which occurs by a reduction of pressure through lifting of air masses is the main natural process.

• The non recording gauge extensively used in India is the Symons rain gauge. It is installed in an open area on a concrete foundation. The distance of the rain gauge from the nearest object should be at least twice the height of the object. The gauge may defence with a gate to prevent animal and unauthorized person from entering the premises.
• Measurements are to be. Made at a fixed time. Every day normally at 08:30 hrs which is considered as the daily rainfall? In case of heavy areas, measurements are made a soft as possible. However, the last reading must be taken at 8:30AM. So that last 24 hrs data may be added up to get the rainfall of that day.

• The catch from the funnel falls on top of a pair of small buckets. These buckets are so balanced that when 0.25 mm of rainfall collects in one bucket, its sand brings the-other one in position. The tipping electrically driven record one lock work-driven chart. There cord from tipping bucket gives data on the intensity of rainfall. The main advantage of this type of instrument is that it gives an electronic pulse output that can be recorded data distance from the rain gauge.

• The catch from the funnel empties in to a bucket mounted on a weighing scale. The weight of the bucket and its contents are recorded on a clockwork-driven chart.
• That instrument plate of the accumulated rainfall against the time, i.e., the mass curve of rainfall (accumulated precipitation against time).

• This type of recording rain-gauge is also known as float type gauge here the rainfall collected by a funnel shaped collector is leading to a float chamber causing a float of rise as the float rises, a pen attached to the float through a lever system record the elevation of the float on a rotating drum driven by a dock work mechanism. A siphon arrangement empties the float chamber when the flat heads reached a pre- set maximum level which resets the pent zero, level. This type of rain gauge is adapted as the standard recording type rain gauge in India.

• A storm is any disturbed state of an environment or in an astronomical body's atmosphere especially affecting its surface, and strongly implying severe weather.
• It may be marked by significant disruptions to normal conditions such as strong wind, tornadoes, hail, thunder and lightning (a thunderstorm), heavy precipitation (snowstorm, rainstorm), heavy freezing rain (ice storm), strong winds (tropical cyclone, windstorm), or wind transporting some substance through the atmosphere as in a dust storm, blizzard, sandstorm, etc.
• Storms have the potential to harm lives and property via storm surge, heavy rain or snow causing flooding or road impassibility, lightning, wildfires, and vertical wind shear.
• Systems with significant rainfall and duration help alleviate drought in places they move through.
• Heavy snowfall can allow special recreational activities to take place which would not be possible otherwise, such as skiing and snowmobiling.
• Storms are created when a center of low pressure develops with the system of high pressure surrounding it.
• This combination of opposing forces can create winds and result in the formation of storm clouds such as cumulonimbus. Small localized areas of low pressure can form from hot air rising off hot ground, resulting in smaller disturbances such as dust devils and whirlwinds.
• A strict meteorological definition of a terrestrial storm is a wind measuring 10 or higher on the Beaufort scale, meaning a wind speed of 24.5 m/s (89 km/h, 55 mph) or more; however, popular usage is not so restrictive. Storms can last anywhere from 12 to 200 hours, depending on season and geography.
• In North America, the east and northeast storms are noted for the most frequent repeatability and duration, especially during the cold period. Big terrestrial storms alter the oceanographic conditions that in turn may affect food abundance and distribution: strong currents, strong tides, increased siltation, change in water temperatures, overturn in the water column, etc.

• Class A Evaporation pan (US Weather Bureau)
• IS Standard Pan (Used in India)
• Colorado Sunken Pan (Sunk below ground such that water level in Panisat ground level)
• US Geological Survey Floating Pan (Simulates the characterization of large water body. The evaporimeter is kept floating in lake).

• Evaporation pans are not exact models of large reservoirs.
• In the above, the evaporation observed from a pan has to be corrected to obtain the value of evaporation from a lake under similar climatic and exposure conditions. Thus, a coefficient (C) is introduced as shown below.
• Lake evaporation=Cp ×pan evaporation.