Ground water resources modelling of a watershed using modflow

Abstract

A study on ‘Ground water resources flow modelling and mapping of a watershed using MODFLOW’, was carried out for Kadalundi river basin of Malappuram District in Kerala. The objectives of the research were 1. to study the aquifer characteristics of the study area, 2. to study the spatial and temporal ground water variations in the study area, 3. to identify the potential ground water zones within the study area using earth resistivity studies and 4. to develop a ground water flow model for the river basin using Visual MODFLOW. Kadalundi river basin has a drainage area of 1122 km 2 with a main stream length of 130 km, originating from the Western Ghats at the western boundary of Silent Valley. River flows through the district of Malappuram and downstream reach of the river falls into Arabian Sea. In order to evaluate the aquifer characteristics of the study area, spatial and temporal variations of ground water level of dug wells and bores wells were analyzed separately. Monthly water table data from 16 bore wells and 14 dug wells during the period from 2008 to 2013 were used for the study. From the p re and post monsoon water table and piezometric water level variations and from the water table and piezometric water level hydrograph studies, it was observed that the water table and piezometric level fluctuation in low land, mid land and high land area were correlated with rainfall except at some locations like Kottakkal, Marakkara and Tanur. Hydraulic continuity studies were carried out with the help of statistical analysis and it indicated that the hydraulic continuity of ground water and surface water exists between aquifers of dug wells and bore wells in major part of study area. Areas like Thirurangadi, Kottakkal and Some part of Perinthalmanna, variation of piezometric water level was poorly correlated with rainfall, which indicated that hydraulic continuity does not exist in that areas and these areas are suitable for deep bore wells.Vertical Electrical Sounding (VES) method was carried out using Signal Stacking Resistivity Meter (MODEL-SSR-MP-ATS) at 22 locations. The apparent resistivity values obtained from the resistivity meter were interpreted with the help of ‘IPI2WIN’ software to obtain the layer parameters. The sounding curves are of A,H,K,Q,HK and KH types reulting in 3 to 4 layer sequence. The curves are prominently of H, Q, K and A type indicating the presence of three layer and combination of curves like HK and KH indicating the four sub surface layers. Most parts of the study area are dominated by H type curve which indicates the presence of good quantity groundwater. Visual MODFLOW software version 2.8.1 developed by Waterloo Hydrogeologic Inc. was used for the flow modelling of the study area. The conceptual model for the study area was developed based on the base map of Kadalundi river basin, topo sheet of the area, well logs at 30 sites and the data obtained from the geophysical studies conducted in the study area. Discretization was done by dividing it into 62 rows and 136 columns with a grid spacing of 500 m x 500 m throughout the area and the cells outside the boundary of the study area were made as inactive. Monthly pumping rate from 35 pumping wells and monthly water level data from thirty head observation wells of Ground Water Department (GWD), Government of Kerala were used as well inputs of Visual MODFLOW. Hydrogeological parameters such as hydraulic conductivity, specific storage, specific yield, porosity and initial heads and boundary conditions of the model domain including constant head, rivers, drains, recharge and evapotranspiration were used as input of Visual MODFLOW. After uploading input parameters, the model was run for steady state and transient state subsequently. Model was developed and calibrated using four years data from 2008 to 2011 and a reasonably good agreement was obtained between computed and observed water levels. After calibration, the model was validated for t w o years data of 2012 and 2013. The root mean square error (RMSE) for almost all the wells during validation were reasonably low and within acceptable limits except a few wellsvery close to the river which are most likely to be influenced by the interflow of river. After the model development and validation, it was used to predict the flow head for the next 15 years assuming five per cent yearly decrease in recharge and also to predict the ground water condition by increasing the pumping rate by 10, 25 and 50 per cent of pumping rate of the validation period (2013). From the modelling studies, it can be concluded that the Kadalundi river basin will remain safe for next five years from the point of future ground water development and subsequently the water table may reach the bed rock. This necessitates artificial ground water recharge techniques to supplement the recharge of rainfall to the ground water.