GROUNDWATER Dr. DEEPAK KHARE GENERAL HYDROLOGY CYCLE FORMATIONS

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1 GROUNDWATER By Dr. DEEPAK KHARE Associate Professor Department of Water Resources Development & Management Indian Institute of Technology Roorkee, ROORKEE (Uttaranchal) , India GENERAL The term groundwater is used to denote the water which has saturated pores or interstices of the subsoil. Groundwater is derived from precipitation on earth s surface that gradually percolates to the subsoil through porous strata or openings through rock formations. Groundwater is one of the earth s most important resources and its development plays a vital role in country s economy. It becomes a usable resource when the water bearing formations are permeable enough to yield adequate quantity water for use by wells, springs or stream channels and can be replenished from recharge sources each season to permit continued exploitation. Compared to surface water, ground water is relatively free from the effect of surface pollutants, and has the advantage of being less susceptible to changes in quality, chemical composition and temperature vibration. The aquifer complex transports the ground water from areas of replenishment to areas of need. HYDROLOGY CYCLE The most important part to under stand the any water source is hydrological cycle. The Figure-1 is self explanatory vary components of the cycle. FORMATIONS There are different types of formations below the earth surface. They are very important and essential to make assessment of groundwater flow. The various formations are;

2 Fig. 1: The Water Cycle Fig. 2: Concept of Ground Water Table

3 Aquifer : Aquifers are the permeable formations having structures which permit appreciable quantity of water to move through them under ordinary field conditions. Thus these are the geologic formations in which ground water occurs (i.e. sands and gravels). Aquicludes : Aquicludes are the impermeable formations which contain water but are not capable of transmitting or supplying a significant quantity (e.g., clays). Aquifuge : any water. Aquifuge is an impermeable formation which neither contains water not transmits IMPORTANT TERMS There are few important terms related with groundwater hydrology; Porosity : Porosity is defined as the ratio of the volume of openings or pores (or voids) V v in the material to its total volume V and is expressed as percentage : V v N = x 100 V Specific Yield : The capacity of a formation to contain water is measure by porosity. However, a high porosity does not indicate that an aquifer will yield large volumes of water to a well. The only water which can be obtained from the aquifer is that which will flow by gravity. The specific yield of an aquifer is defined as the ratio expressed as a percentage, of the volume of water which after being saturated, can be drained by gravity to its own volume.

4 Specific Retention : Specific yield is always less than porosity since some water will be retained in the aquifer by molecular and surface tension forces. The specific retention (S r ) of an aquifer is the ratio, expressed as a percentage, of the volume of water it will retain after saturation against the force of gravity of its own volume. Water Table : The saturated zone is bounded at the top by either a limiting surface of saturation or overlying (confining) impermeable strata, and extends down to underlying impermeable strata. In the absence of the confining impermeable layer, the static level of water in wells penetrating the zone of saturation is called the water table. The water table is the surface of a water body which is constantly adjusting itself towards an equilibrium condition. If there were no recharge to or outflow from the ground water in a basin, the water table would eventually become horizontal. The concept of ground water table is shown in Figure-2 and the details of types of wells with formations are given in Figure-3 Fig. 3: Types of Aquifers and Wells

5 GROUNDWATER ASSESSMENT For the assessment of groundwater in any area, it is important to understand the models available. The present state of knowledge in Ground Water hydrology provides adequate know-how to study the impact of a deterministic Ground Water withdrawal and/or recharge pattern on the piezometric elevations through lumped models or distributed models. (A) Water Balance / Lumped Model The water balance equation proposed by Sokolov and Chapman (1974) can be written as follows: Inflows - Outflows - Change in Storage + Error term = 0 The inflows include the rainfall recharge, recharge from rivers, subsurface horizontal inflow, artificial recharge and inflow from other aquifers (Overlying of underlying ). The outflows include base flow to the rivers, outflow from the Ground Water into the zone of aeration for moisture recovery lost by evapotranspiration, outflow to the overlying or underlying aquifers, subsurface horizontal outflow, Ground Water outflow through springs, and Ground Water pumped from aquifers. The lumped aquifer response to known inflows or outflows can be obtained by defining the Ground Water storage fluctuations in terms of the piezometric head fluctuations and the storage coefficient. (B) Distributed Ground Water Model The distributed parameter Ground Water flow models are based on the solution of differential equations governing two -dimensional or three -dimensional transient Ground Water flows in saturated zone. Closed form or series solution of governing equations are available only for idealised boundary and recharge conditions. These are generally based upon the assumption of homogeneity and isotropy. For predicting the aquifer response to spatially and temporally varying differential equations have to be solved by appropriate numerical methods.

6 (C) Finite Difference and Finite Element Mothods For obtaining numerical solution of Ground Water flow problems two distinct types of digital computer-based methods viz., finite difference and finite element are available. Finite difference methods employ finite difference approximation to convert the partial differential equation into a determinate set of linear algebraic equations. The discretization of space, necessary for finite difference approximations, can be based upon either polygons or a more rectangular grid pattern. The former is generally known as Tyson and Weber model, whereas the latter as finite difference model. Since Tyson and Weber model has-been modified and then used in the present study, it has been discussed in detail in the present section. Tyson and Weber invested the physical aspects of a Ground Water basin using simulation. The computational procedure involves two phases : (1) development and verification of the model; and (2) use of the model in predicting behaviour of the basin under imposed situation. An electronic differential analyser, or analog computer is used for the first phase and a digital computer is used in the second phase.in order to develop the mathematical model of Ground Water system, they divided the Ground Water basin in small polygon zones. Assumptions used in deriving the model are that (i) the aquifer is unconfined, (ii) there is no vertical variation in aquifer properties, and (iii) the aquifer thickness is small in comparison to its lateral dimensions. Flow in the aquifer is defined by a single linear equation derived by time-dependant flow rate in the aquifer is the algebraic sum of several extraction and replenishment flows. For modelling on the analog computer the flow equation is transformed to an equivalent system of difference -differential equations. The system is solved simultaneously on the analog computer to give the Ground Water level at the nodes of the polygonal zones. However, the solution of a system of difference -differential equation on the analog computer is subject to inherent instability which is difficult to overcome.

7 Once the model on the analog computer is verified by comparing computed water levels with historical data, the equations are modelled on the digital computer for operational studies of the basin. Alternative schemes for operation of the basin are studied by successive iterations using different inputs for aquifer replenishment and withdrawals. The system is gradually improved by choosing the best alternatives tried on the model. Simulation of this type provides great detail concerning system operation but does not necessarily provide the optimal alternative. The model is important in detailed investigation of the Ground Water basin responses to recharge and withdrawals. The finite difference approach is a general method for calculating approximate solution of partial differential equations. This method has been programmed to solve twodimensional and three dimensional transient Ground Water flow problems. This method is widely used for the Ground Water problems The finite element method has been employed to obtain the solution of differential equations for the evaluation of aquifer response. This followed the development of variational principles for linear initial value problem. The finite element method is reported to circumvent many difficulties relating to irregular geometry of area, heterogeneity and boundary condition in addition to giving results of higher order accuracy. INVERSE AND DIRECT PROBLEMS OF GROUND WATER MODELLING Ground Water modelling is essentially a forecasting problem wherein the solution of the equations governing the flow of water in an aquifer is sought. To obtain the solution appropriate initial and boundary conditions are imposed with known aquifer parameters. The model can then be used to simulate the response of the aquifer to any excitation. This excitation may be in the form of pumpage, recharge of a change in the boundary conditions. The response of the aquifer would be in the form of either the water table fluctuations or stream-aquifer inter flows

8 One of the most difficult steps in mathematical modelling of Ground Water flow is the collection of data relating to spatial distribution of storage coefficient (S) or specific yield (S y ) and transitivity (T). The most widely used method for determining aquifer parameters S and T is known as pumping test (Kruseman and de Ridder,1970). The aquifer parameter so determined represent only that portion of the aquifer which lies within the radius of influence of the wall used for pumping tests. The hydraulic parameters are known to vary considerably within the same aquifer. Therefore a number of tests are needed for delineating various zones of having similar characteristics within the aquifer. Besides, pumping tests are expensive and the cost of parameter identification for a regional aquifer may, therefore become prohibitive. Further, the aquifer parameters determined from pumping tests cannot be used as such in a simulation model for the purpose of forecasting. An indirect method of estimation of aquifer parameters is though Ground Water modelling. The model is first calibrated against historical data. The calibration process invariably requires adjustment in the aquifer parameters. The estimation of theses parameters is, thus, an inverse process wherein these parameters are calculated from the historical excitation response and the associated initial and boundary conditions data. The system in this case is represented by the equation correlating the response to excitation GROUNDWATER MANAGEMENT THROUGH CONJUCNTIVE USE PLANNING The quantity and quality of available water resources have been recognised as limiting factors in the development of most arid and semi -arid regions. Recent experiences have shown that these limiting factors may also apply in the more humid areas previously thought to be immune to water storage problems. The optimum utilisation of existing water resources is therefore of ever increasing importance. As the population is increasing rapidly, the corresponding agricultural production need to be increased. This realisation has led to the development of high yielding varieties of crops, increased reliance on chemical fertilisers and more intensive irrigation. All these measures have increased considerably the water requirement for irrigation. To meet these

9 increased requirements, a large number of water resources projects incorporating a dam or a barrage and a network of canals have been implemented. These projects increase the Ground Water recharge which, without proper drainage measures, increases in water table. The excessive rise of water table has adverse consequences in the form of water logging and salt accumulation. The maximum permissible level of water table in such areas is governed by the requirement of maintaining a minimum depth of water table from the ground surface that primarily depends on the depth of the root zone of the crops being grown in the area. In areas where canal irrigation has not been introduced, irrigation requirement is met by the Ground Water withdrawals. However, larger Ground Water withdrawals can lower the water table/piezometeric elevations excessively. This excessively lowering of the water table can render many shallow wells dry, reduce the base flow in the hydraulically connected rivers and induce salt-water intrusion in coastal aquifers. The reduction of base flow in rivers can adversely affect the ecology of the surrounding regions, operation of the downstream Surface Water projects and quality of river water. Therefore, the base flow in these rivers should not be allowed to fall below a minimum level. Artificial recharge may have to be resorted to in such regions. It is important that all the available water resources are considered in unison to maximize the benefits that could accrue from the combined use of Surface Water and Ground Water. The advantages of both the resources have to be taken in to consideration for effective and optimum management of the available water resources. CONJUNCTIVE USE Conjunctive use is the combined and integrated management of Surface Water and Ground Water for optimal utilization of available water resources. In other words, Conjunctive use of Surface Water and Ground Water offers a great potential for enhanced and assured water supplies at minimum cost. According to Todd Future demand for water requires planning maximum utilization of all existing supplies. This can most economically be obtained by Conjunctive use of Surface Water and Ground Water

10 reservoirs". Hall and Buras stated that maximum development of water resources requires Conjunctive utilization of Surface Water and Ground Water resources. Kazman pointed out that Surface Water and Ground Water are inextricably interconnected and are not to be arbitrarily separated. Further, the Irrigation commission report in, stated the following : "We have already stressed the need for taking Ground Water resources in to account while preparing river basin plans. This is particularly desirable where the Ground Water supply is ample or where it is expected to improve with advent of canal irrigation. There are several ways of making combined use of Conjunctive use of Surface Water and Ground Water. It can take the form of full utilization of Surface Water supplies supplemented by Ground Water, or the direct use of Ground Water during periods of low canal supplies or canal closures. It can also take the form or irrigation pockets exclusively with Ground Water in a canal command especially where the terrain is uneven. Planning for combined use surface and Ground Water calls fro great ingenuity than is needed for their separate use. It has to be admitted that so far not many projects have been planned on the basis of such combined use of water. Such combined use as in now practiced was not planned but came into being out of necessity. The primary aims in any water resources project based on Conjunctive use concept is to optimize the combined utilization of available and proposed surface and Ground Water facilities and sustain the supply over a longer period. The term optimization means the achievement of the best results and may be interpreted in different ways depending on the relative importance of the specific objectives. A benefit out of a given volume of water, or of minimizing water losses through flood runoff, evaporation and seepage. Most comprehensive optimization schemes however will necessarily have to be based on some kind of economic evaluation. Additional benefits will, obviously be achieved if a water resource system is planned and operated taking into consideration the advantages offered by the Conjunctive use of Surface and Ground Water.

11 CONCLUDING REMARK For the assessment of groundwater in any region, it is necessary to have the complete information of the area regarding all possible pumping structures viz; state tube wells, private tubewells etc. These should be proper estimate of unit draft from these structures. All possible recharge structures should be considered. The assessment would basically, mean the overall resources in any region and can be obtained if these parameters are calculated properly. For knowing the groundwater levels, the distributed modelling has to be done.