Hydrology PDF

Hydrology Definition: The science, which deals with the occurrence, distribution and circulation of water, is called hydrology. The study of hydrology is useful for the following: 1. For the proper design and operation of hydraulic structures. 2. For ascertaining flood flows expected at a spillw...

Hydrology Definition: The science, which deals with the occurrence, distribution and circulation of water, is called hydrology. The study of hydrology is useful for the following: 1. For the proper design and operation of hydraulic structures. 2. For ascertaining flood flows expected at a spillway or highway culvert or in a city drainage system. 3. For ascertaining reservoir capacity to ensure adequate water for irrigation or municipal water supply during draughts. 4. For ascertaining the effect of reservoirs, levees, and other control works in flood flow in a stream. Notes: (a) The science, which deals with the physical features and conditions of water on the earth surface, is called hydrography. (b) The science that deals with the measurement of water is called hydrometry. Hydrological Cycle The water of the universe always changes from one state to another in a cyclic order under the influence of the sun. The water from the surface source like lakes, rivers, ocean etc. converts to vapour by evaporation. These vapours get condense due to the sudden fall in temperature and pressure and thus clouds are formed. These clouds again causes precipitation (i.e. rainfall). Some of the vapours arc converted to ice at the peak of the mountains. The ice again melts in summer and flows as rivers to meet the sea or ocean. These processes of evaporation, precipitation and melting of ice go on continuously and thus a balance is maintain in the atmosphere. This earth’s water circulatory system (from the stage of evaporation from ocean to the final return to the ocean) is called hydrological cycle. If E is the evaporation, P is precipitation and R is run-off, then the hydrological cycle is express by the relation, P=E+R Hydrograph Definition: A graph showing variations of discharge (in cumec) with time (in hours or days), at a particular point of a stream, is known as hydrograph. The hydrograph representing one cm of run off from a rainfall of some unit duration and specific area distribution is known as unit hydrograph. L.K. Sherman first gave the concept of unit hydrograph in 1932. The graphical representation of average rainfall and rainfall excess (i.e. rainfall minus infiltration) rates over specified areas during successive unit time intervals during a storm is called hyetograph. Precipitation We have discussed that water goes on evaporation continuously from the water surfaces on earth (e.g. river, lake, sea, ocean, etc.) by the effect of sun. The water vapour goes on collecting in the atmosphere up to a certain limit. When this limit exceeds, the water vapours get condensed and there by cloud is formed. Ultimately droplets are formed and returned to earth in the form of rain, snowfall etc. This is known as precipitation. Depending upon the various atmospheric conditions, the precipitation may be of the following types: Cyclonic precipitation. It results from lifting of air masses converging into low-pressure area. Convective precipitation. It results from natural rising of warmer, lighter air in colder and denser surroundings. Orographic precipitation. It results from lifting of warm moisture-laden air masses due to topographic barriers.  Run-off and Catchment Definition: The amount of water, which flows over the surface of the earth after all the losses have taken place, is called run-off. A tract of land, which contributes the above run-off into a stream or reservoir, is called catchment and its area is known as catchment area The run-off is measured in terms of depth of water (i.e. cm) or in tents of discharge (i.e. cumecs); the following are the factors affecting the run-off from a catchment i. Intensity of rainfall ii. Soil characteristics of catchment iii. Topography of the catchment iv. Shape and size of catchment v. Geological condition of catchment and Weather condition Ground Water Flow  The Thiem equation is used to estimate the drawdown (change in water level) in a confined aquifer around a fully penetrating pumping well.  The equation assumes steady-state conditions and radial flow towards the well. It is derived from Darcy's law and is applicable when the aquifer is confined (i.e., there is a confining layer both above and below the aquifer). The Thiem equation is given by: S= Q/4𝜋T. ln⁡(r2 /r2) Where:  s is the drawdown (change in water level).  Q is the pumping rate.  T is the transmissivity of the aquifer.  R1is the distance from the well to the initial water level.  R2is the distance from the well to the observation point. 2. The Theis Equation:  The Theis equation is used to analyze the drawdown in an unconfined or confined aquifer around a pumping well during a pumping test. It assumes radial symmetry and is based on the assumption of homogeneous and isotropic aquifer conditions.  The equation is derived from the solution of the groundwater flow equation and is applicable to both confined and unconfined aquifers. The Theis equation is given by: s= Q/4πT⋅W(u) Where:  s is the drawdown (change in water level).  Q is the pumping rate.  T is the transmissivity of the aquifer.   W (u) is the well function, which is a complex mathematical function related to the dimensionless parameter u, representing time and aquifer properties. Both of these equations are commonly used in aquifer test analysis to estimate aquifer properties such as transmissivity. The analysis involves comparing observed drawdown data from pumping tests to the theoretical solutions provided by these equations, often using graphical methods or numerical methods to match observed and calculated drawdown curves.