Modeling of River-Aquifer Interactions: A Top-Down Approach

Abstract

Surface water interacts with groundwater in many types of physiographic and hydrogeological conditions. The heterogeneity in hydrological processes over a catchment affects the location, time and extent of interaction phenomenon. Exchange of water varies spatially and temporally due to the effect of natural and anthropogenic factors. It is essential to identify and quantify the surface water and groundwater (SW-GW) exchange, since the quantity and quality of these resources affect each other. The SWGW interactions are characterized at various scales of flow systems such as regional, intermediate and local scale. Driving forces influence the SW-GW exchange depending upon different flow scales but the challenge of the present research is to bring out its relevance. In the past, studies related to SW-GW interactions using integrated models are not dealt in detail for smaller scales. Small-scale processes may seem insignificant when accounted for a larger perspective of consideration. Therefore, an effort is put to analyze the effect of driving forces on a larger scale and a small scale in the present study. The objective of the present study is to investigate the surface water and groundwater interactions at two different hydrological extents of a catchment. The present study proposes a new approach based on the top-down hierarchy from regional scale to sub-catchment scale to assess the SW-GW flux. For the regional scale study, Nethravathi basin is chosen whereas for the intermediate scale, sub-catchment of Gowri-hole, a tributary of Nethravathi River is considered. In this study, river-aquifer interaction processes are simulated by using RIVER package of MODFLOW for an unconfined aquifer system. A regional groundwater flow model is built as a pre-requisite to identify the dynamic exchange occurring over the catchment area using potentiometric maps and flow budgets. In the present study, a MODFLOW-based regional groundwater model was simulated under steady-state condition for a calibration period of 2004 - 2009 and validation period of 2010 - 2011. The calibrated model was validated using the observed groundwater level data of 15 open wells measured by Department of Mines and Geology, Government of Karnataka. The simulated regional groundwater model is in good agreement with most of the wells reasonably matching the observed and computed groundwater heads. It shows that the simulation of regional groundwater model is reasonable and well suitable for the studies related to SW-GW interactions. In the present study,ii intermediate scale model for the Gowri-hole sub-catchment was calibrated for transient analysis from June 2004 - May 2010 with a daily step input. Automated Parameter Estimation analysis was carried out to get better results from the study. The calibrated model was validated from June 2010 - October 2012 for two monthly observation wells of Department of Mines and Geology and one seasonal observation well of Central Ground Water Board (CGWB). Groundwater heads gradually increase from June - August with the arrival of monsoons and decline significantly from September upto the month of May. Groundwater swelling is noticed near Well No. 3 of Bellare village in the month of October. River leakage decreases from 10 – 11 % in June to 4 – 5 % in July with the commencement of peak monsoon flows. It steadily increases from 12 – 14 % in September and continues to occur up to 41 – 42 % until the end of May. Aquifer discharge increases from 24 – 25 % in June to 34 – 35 % in July due to quick saturation during monsoon. From September, aquifer contribution into the river flow significantly decreases upto 9 – 10 % in May. The contribution of aquifer discharge into the river flow is consistent at the confluence points. Some parts of river segments are under the influence of aquifer discharge. However, majority of river segments are dominated by river leakage areas throughout the year. Consequently, Gowri hole acts as a Gaining River during monsoons due to aquifer discharge. And, it acts as Losing River due to river leakage throughout post-monsoon and summer months. SW-GW interactions are driven by aquifer parameters such as recharge rate and hydraulic conductivity. In the present study, these driving forces are calculated for the simulation of both the regional scale and sub-catchment scale groundwater flow model. During the calibration, the driving force values are adjusted until the model is simulated with a good match between computed and observed groundwater heads. The study identified that recharge rate is the driving factor influencing the SW-GW interactions at regional scale and hydraulic conductivity is the driving factor of sub-catchment scale SW-GW interactions.