PhD

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 premonsoon, 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