SWAT-MODFLOW INTEGRATED MODELLING APPROACH FOR SURFACE WATER-GROUNDWATER INTERACTION -A CASE STUDY

Abstract

The interaction between surface water (SW) and groundwater (GW) is an important water exchange in the hydrologic cycle and a common process that happens in saturated sections of river channels and floodplains. Analysing such interactions in delicate watersheds provides valuable insights into their impact on different water sources. The main aim of this research is to study the dynamics of SW-GW interactions of Thuthapuzha sub-basin (Kerala ) on a spatio-temporal basis using integrated SWAT-MODFLOW model, considering the impact of groundwater use and future climate change scenario. Thuthapuzha River is one of the major tributaries of the Bharathapuzha River and has a basin area of 915 km2. The average annual rainfall observed in the study area is 2534 mm and the depth to groundwater level in the study area varies from 3 to 9 m below ground level (mbgl). The SW and GW processes were simulated separately using the land surface model Soil and Water Assessment Tool (SWAT) and groundwater flow Modular Finite Difference Flow (MODFLOW) models for the simulation period of 2000- 2019. The elevation data for the models were obtained using Shuttle Radar Topographic Mission (SRTM) Digital Elevation Model (DEM) of 30 m resolution. The SWAT model calibration using observed river discharge were found satisfactory with values of 0.85, 0.85 and 0.39 for objective functions of NSE, R2 and RSR respectively. Long-term water balance of the study area obtained from SWAT model showed that runoff, evapotranspiration and shallow aquifer groundwater recharge account for 49 %, 21% and 10 % of the average annual precipitation respectively. The average annual groundwater recharge simulated by the SWAT model showed a magnitude of 265.78 mm/year. Steady state and transient simulations were made for GW flow modelling using MODFLOW-NWT flow engine in Visual MODFLOW flex 7.0 for the period 2000-2019. Model calibration showed good agreement between simulated and observed groundwater heads with RMSE (Root Mean Square Error) value <5%. The hydraulic conductivity and specific storage obtained after calibration and

validation were 0.0026 m/s and 0.0008 /m respectively. Annual GW draft in the study area was obtained as 206.92 MCM using the GW balance of the study area. To study the SW-GW interaction, SWAT and MODFLOW models were integrated using the Graphical User Interface (GUI) QSWATMOD, a Q-GIS based plugin. The integration was made possible using disaggregated Hydrological Response Units (dHRUs) and MODFLOW grid cells in the QGIS platform. The spatial and temporal variation of GW-SW exchange rate was quantified. The average annual river recharge rate to the aquifer system and GW discharge rate were obtained as 6.80 l/d per m2 and 33.65 l/d per m2 area of MODFLOW grid cell. The average river recharge to the aquifer and discharge from the aquifer for pre- monsoon, monsoon and post-monsoon seasons were quantified as 4.77, 14.56 and 7.29 l/d per m2 area and 38.34, 56.78 and 46.55 l/d per m2 area respectively. The integrated model assessed the average annual water yield over the study area 1647.30 mm (1507 MCM) and the annual average depth of available GW as 425.96 mm (389.75 MCM). Model results obtained incorporating the observed pumping data indicate a massive 23.03% decrease in rate of GW disharge from aquifer to the river. The future climate projections of temperature and rainfall simulated for RCPs (Representative Concentration Pathways) 4.5 and 8.5 using the regional climate model GFDL-RCA4 indicate a reduction of 36.35% in the rate of recharge from the river to the aquifer and a reduction of 18.39% in the rate of aquifer discharge to the river may in the future period 2056-79 with worst emission scenario (RCP 8.5). The current study was also used in identifying potential active recharge and discharge zones of Thuthapuzha river. The spatial extent of these zones and their temporal flux variation can be further used in effective planning of water conservation structures measures to help keep the river perennial and to avoid GW contamination arising from different anthropogenic sources.