K. Sridharan

Identification of parameters in unconfined aquifers

A method is presented for identification of parameters in unconfined aquifers from pumping tests, based on the optimisation of the objective function using the least squares approach. Four parameters are to be evaluated, namely: The hydraulic conductivity in the radial and the vertical directions, the storage coefficient and the specific yield. The sensitivity analysis technique is used for solving the optimisation problem. Besides eliminating the subjectivity involved in the graphical procedure, the method takes into account the field data at all time intervals without classifying them into small and large time intervals and does not use the approximation that the ratio of the storage coefficient to the specific yield tends to zero. Two illustrative examples are presented and it is found that the parameter estimates from the computational and graphical procedures differ fairly significantly.

Parameter estimation in an anisotropic leaky aquifer system

Abstract Parameter estimation is made for an anisotropic leaky aquifer system in which the direction of principal axes is unknown. There is a declining water table in the aquitard. The seven governing parameters are the direction of principal axes, the degree of anisotropy, the equivalent transmissivity and storage coefficient of the aquifer, and the leakage coefficient, specific storage and specific yield of the unconfined aquitard. A modified parameter perturbation technique which is computationally efficient is used for the determination of sensitivity coefficients. The parameter estimation procedure is applied to three test problems including one field problem. Situations where the total number of wells is only three are handled with two pump tests by locating the pump in a different well in each test.

Identification of parameters in semiconfined aquifers

Abstract A method is presented for identification of parameters from pumping tests, in semiconfined aquifers with storage release from the aquitard. Four parameters are to be evaluated, namely: the transmissivity of the aquifer; the storage coefficient of the aquifer; the leakage coefficient of the aquitard; and the storage coefficient of the aquitard. The Neuman and Witherspoon solution for the drawdown, valid for all time intervals, is used and this solution is not amenable to the graphical procedure. The optimisation problem is solved by the sensitivity analysis technique. The method has the merit of utilising the field data for all times without classifying them into small and large time intervals. A computational procedure for evaluating the drawdown integral is presented. Two illustrative examples are presented to demonstrate the application of the method.

Parameter estimation in an aquifer-water table aquitard system

Abstract The weighted least squares approach is used for the estimation of parameters in an aquifer-water table aquitard system. Six parameters are to be evaluated, namely: the equivalent transmissivity, degree of anisotropy and storage coefficient of the aquifer, and the leakage coefficient, specific storage and specificv yield of the aquitard. The coupled aquifer-aquitard equations are solved by an iterative numerical procedure and the optimisation problem is solved by the sensitivity analysis technique. This method is applied to one hypothetical problem and two field pumping tests of 7 days duration.

Analysis of an Aquifer-water table aquitard system

The problem of pumping an aquifer in an aquifer-water table aquitard system is considered, accounting for the elastic properties of both the aquifer and the aquitard, the gravity drainage in the aquitard and treating the water table as an unknown boundary. The coupled partial differential equations are nondimensionalised, yielding three principal parameters governing the problem. The numerical solution of these equations is obtained for a wide range of parameter values. Type curves are generated and their use is illustrated through a field application.

Computer model for vedavati ground water basin. Part 2. Regional model

A computer model for regional ground water resource evaluation, based on the leaky aquifer concept, is presented for the Vedavati basin which is a crystalline hard rock area of 24,200 km2 in Peninsular India. The model can handle nonhomogeneity, anisotropy, varied pumping, rainfall and river stages. The model is calibrated based on regional water level observations for one year. The calibrated model is used to determine the distribution of safe yield, overexploited regions and regions of maximum potential over the basin. Regions which are most affected by drought conditions are also identified.

Analysis of flow near a dug well in an unconfined aquifer

A numerical analysis of flow to a dug well in an unconfined aquifer is made, taking into account well storage, elastic storage release, gravity drainage, anisotropy, partial penetration, vertical flow and seepage surface at the well face, and treating the water table in the aquifer and water level in the well as unknown boundaries. The pumped discharge is maintained constant. The solution is obtained by a two-level iterative scheme. The effects of governing parameters on the drawdown, development of seepage surface and contribution from aquifer flow to the total discharge are discussed. The degree of anisotropy and partial penetration are found to be the parameters which affect the flow characteristics most significantly. The effect of anisotropy on the development of seepage surface is very pronounced.

Modified Strongly Implicit Procedure for groundwater flow analysis

Abstract A Modified Strongly Implicit Procedure (MSIP) to solve two dimensional groundwater flow problems in non-rectangular regions is presented. At present while using SIP, such problems are solved over a superscribed rectangular computational region with zero transmissivities for nodes outside the region of interest. This leads to wastage in computer storage and time. The MSIP is developed to handle non-rectangular regions directly without such dummy nodes. It is found that except for four types of boundary nodes, the same equations as in the normal SIP are applicable. The relationships for the special types of boundary nodes for row computations are presented. The method is tested on two test problems and compared with other finite difference methods, and MSIP is found to be the best. The MSIP will be particularly advantageous in regions with non-rectangular boundaries and where a number of parameters have to be stored for each node.

Analysis of transients in a canal network

A generalised formulation of the mathematical model developed for the analysis of transients in a canal network, under subcritical flow, with any realistic combination of control structures and their multiple operations, has been presented. The model accounts for a large variety of control structures such as weirs, gates, notches etc. discharging under different conditions, namely submerged and unsubmerged. A numerical scheme to compute and approximate steady state flow condition as the initial condition has also been presented. The model can handle complex situations that may arise from multiple gate operations. This has been demonstrated with a problem wherein the boundary conditions change from a gate discharge equation to an energy equation and back to a gate discharge equation. In such a situation the wave strikes a fixed gate and leads to large and rapid fluctuations in both discharge and depth.