5.7 RESULTS AND DISCUSSION
5.7.1 Groundwater Flow Modelling with SWAT Model Recharge
The monthly sub-basin scale recharge obtained using SWAT model was used in the groundwater flow model along with satellite-based evapotranspiration data, aquifer characteristics, soil characteristics, initial and boundary conditions to predict spatio- temporal variations of groundwater levels and river-aquifer exchange flux.
5.7.1.1 Model Calibration and Validation at Supaul Well Station
(a) Calibration
Figure 5.4(a) shows the groundwater hydrograph variations between observed and simulated data at Supaul well station during the calibration period (1997-2003). In this period, i.e. during pre-monsoon Rabi season (January), the simulated groundwater levels varied between the ranges of 48.72-49.29 m whereas, the observed groundwater levels are noted to be in the range of 48.38-48.94 m that point toward the over-prediction of simulated groundwater levels from that of observed groundwater levels. During the pre-monsoon period (April) the simulated groundwater levels varied in the range of 48.48-48.73 m while the observed groundwater levels varied in the range of 48.05-48.50 m, also indicates the over-prediction of simulated groundwater levels. Similar observations are drawn during monsoon (August) and post-monsoon Kharif (November) seasons. The simulated groundwater levels in the months of August and November varied in the ranges of 49.83- 51.45 m and 48.91-49.80 m whereas, observed groundwater levels varied in the range of 49.25-50.09 m and 48.68-49.28 m, respectively. It can be noted that irrespective of a season, the simulated values are observed to be over-predicted than that of the observed values. However, at a very few months (for example, in November, 2002) the simulated groundwater levels are observed to be under-predicted from that of observed groundwater levels. Figure 5.4(c) shows the scatter plot between observed and simulated groundwater levels during the calibration period. It is observed that the model can able to predict the groundwater levels during post-monsoon Kharif and pre-monsoon Rabi seasons. The coefficient of determination (R2), Nash-Sutcliffe efficiency (NSE) root mean square error (RMSE) between observed and simulated groundwater levels are noted to be 0.71, 0.33, and 0.61 m respectively during the calibration period. In case I, hydraulic conductivity of 0.0004- 40 m/day and specific yield of 0.02-0.27 are found be the most sensitive calibration parameters
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Figure 5.4: Observed and simulated groundwater level hydrograph variations at Supaul well station during (a) calibration period (b) validation period; scatter plot between observed and simulated groundwater levels during (c) calibration period (d) validation
period.
(*Note: Simulated groundwater levels were obtained using SWAT recharge; calibration period: 1997-2003;
validation period 2004-2010) 47.5
48.5 49.5 50.5 51.5
Jan-97 Apr-97 Jul-97 Oct-97 Jan-98 Apr-98 Jul-98 Oct-98 Jan-99 Apr-99 Jul-99 Oct-99 Jan-00 Apr-00 Jul-00 Oct-00 Jan-01 Apr-01 Jul-01 Oct-01 Jan-02 Apr-02 Jul-02 Oct-02 Jan-03 Apr-03 Jul-03 Oct-03
Groundwater Level (m)
Observed Ground Water Level (m) Simulated Groundwater Level (m) (a)
47.5 48.5 49.5 50.5
Jan-04 Apr-04 Jul-04 Oct-04 Jan-05 Apr-05 Jul-05 Oct-05 Jan-06 Apr-06 Jul-06 Oct-06 Jan-07 Apr-07 Jul-07 Oct-07 Jan-08 Apr-08 Jul-08 Oct-08 Jan-09 Apr-09 Jul-09 Oct-09 Jan-10 Apr-10 Jul-10 Oct-10
Groundwater Level (m)
Month
48 48.5 49 49.5 50 50.5 51 51.5 52
48 49 50
Simulated Groundwater Level (m)
Observed Groundwater Level (m)
47.8 48.3 48.8 49.3 49.8 50.3
47.8 48.3 48.8 49.3
Observed Groundwater Level (m) (b)
(c) (d)
R2– 0.71 NSE – 0.33 RMSE – 0.61
R2– 0.67 NSE – 0.28 RMSE – 0.42
(b) Validation
The groundwater hydrograph variation between observed and simulated data during the validation period (1997-2003) at Supaul well station is shown in Figure 5.4(b).
It can be noted that during pre-monsoon Rabi season (January), the simulated groundwater levels varied between the ranges of 48.19-49.24 m whereas, the observed groundwater levels varied in the range of 48.30-48.90 m. However, the simulated groundwater levels are found to be over-predicted than that of observed groundwater levels in few years. During pre-monsoon (April), the observed groundwater levels are observed to be varied in the range of 48.19-48.69 m whereas, simulated groundwater levels varied in the range of 47.85-48.55 m which indicates that the over-prediction of simulated groundwater levels than that of observed groundwater levels. The similar behavior can be observed during monsoon (August) and post-monsoon (November) seasons. The simulated groundwater levels varied between the ranges of 49.34-50.17 m and 48.71-49.15 m in August and November months whereas, observed groundwater levels remain in the range of 48.70-49.51 m and 48.60-48.90 m respectively. Similar to the calibration period, the simulated values in the validation period are found to be over-predicted than that of observed values irrespective of a season. Also, the scatter plot between observed and simulated groundwater levels (Figure 5.4(d)) indicates that the model predictions are not well captured by the model during validation period and R2, NSE and RMSE between observed and simulated groundwater levels remain to be 0.67, 0.28, and 0.42 m respectively.
5.7.1.2 Model Calibration and Validation at Jaynagar Well Station
(a) Calibration
The model was also calibrated and validated at Jaynagar well station and the comparative plots are shown in Figure 5.5. The groundwater hydrograph variations between observed and simulated data of the calibration period (1997-2003) are shown in Figure 5.5(a). During pre-monsoon Rabi season (January), the simulated groundwater levels varied between the ranges of 64.14-64.93 m while, the observed groundwater levels varied in the range of 63.57-64.93 m. This difference indicates the over-prediction of simulated results from the observed groundwater levels. However, during the pre-monsoon period (April), the simulated groundwater levels varied between 63.54-64.26 m and the observed groundwater levels varied in the range of 63.56-65.22 m. It is to be noted that the
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simulated results over-predict the observed ones during April except in few years. During the monsoon (August) and post-monsoon Kharif (November) seasons, a similar observation can be seen. It is found that in the month of August and November, the simulated groundwater levels varied in the ranges of 64.77-66.58 m and 64.26-65.39 m whereas, observed groundwater levels vary in the range of 63.87-67.04 m and 64.12-65.35 m respectively. It can be noted that similar to the observations at Supaul well station, the simulated values at Jaynagar station are found to be over-predicted than observed values irrespective of season. However, in some months (for example April month) the simulated groundwater levels are found to under-predict the observed groundwater levels. Scatter plot between observed and simulated groundwater levels during calibration period (Figure 5.5(c)) indicates better model performance in comparison with the results at Supaul well station with R2 of 0.71, NSE of 0.56, and RMSE of 0.45 m respectively.
(b) Validation
Figure 5.5(b) shows the groundwater hydrograph variation between observed and simulated data during the validation period (1997-2003) at Jaynagar well station. During pre-monsoon Rabi season (January), the simulated groundwater levels varied in the range of 63.84-65.13 m whereas, the observed groundwater levels are noted to vary in the range of 63.59-65.53 m. However, the simulated groundwater levels closely matched the observed groundwater levels except in few years. During pre-monsoon (April), the simulated groundwater levels varied in the range of 62.87-64.31 m whereas, observed groundwater levels varied in the range of 62.92-64.62 m which also indicated close match with observed groundwater levels except in few years. Whereas, during monsoon (August), the simulated groundwater levels are found to vary between the ranges of 65.17-66.81 m while the observed groundwater levels varied between 64.51-66.17 m which indicates that over-prediction of simulated results. Similar to the observations found in pre-monsoon, the simulated groundwater levels (ranging between 64.45-66.04 m) are noted to have a close match with observed groundwater levels (ranging between 64.30-65.15 m) during post- monsoon Kharif (November) except in few years. Figure 5.4(d) shows the scatter plot of observed and simulated groundwater levels. It indicates the model performance in the validation period is similar to the calibration period, having R2 of 0.69, NSE of 0.67 and RMSE of 0.51m. Overall, the simulated results of the groundwater flow model in Figure 5.3 and 5.4 indicates the simulated groundwater levels in case I are observed to be over- predicted than the observed groundwater levels except at few time periods.
Figure 5.5: Observed and simulated groundwater level hydrograph variations at Jaynagar well station during (a) calibration period (b) validation period; scatter plot between observed and simulated groundwater levels during (c) calibration period (d)
validation period.
(*Note: Simulated groundwater levels were obtained using SWAT recharge; calibration period: 1997-2003;
validation period 2004-2010)
Section 3.10 of chapter 3 revealed that the recharge obtained using SWAT model was observed to be over-predicted due to the underprediction of actual Evapotranspiration (AET) and the model was not able to capture the effects of wet and dry years during the study period. As the observed monthly groundwater recharge values were not available for
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the study area at the sub-basin scale, the validation of groundwater recharge was not included at that point of discussion. The validation of these observations is attempted by incorporating the over-predicted recharge into the groundwater flow model in case I.
At Supaul station, it is observed that the simulated groundwater levels (varied in the range of 48.72 - 49.96 m) are found to be over-predicted slightly than that of observed groundwater levels (varied in the range of 48.32 - 50.08 m) during extremely wet year (2001) (Figure 5.4(a)). Also, during the extremely dry year (2005), the simulated groundwater levels (varied in the range of 48.18 - 50.08 m) are noted be over-predicted than that of the observed groundwater levels (varied in the range of 47.85 - 49.40 m) (Figure 5.4(b)). The comparison of simulated and observed values indicates the model is not able to capture the effects of wet and dry years. Similarly at Jaynagar station, the simulated groundwater levels (varied in the range of 64.10 - 66.12 m) are found to be slightly over- predicted than that of observed groundwater levels (varied in the range of 64.21 - 66.17 m) during extremely wet year (2007) (Figure 5.5(a)) Whereas, during extremely dry year (2005), the simulated groundwater levels (varied in the range of 64.01 - 65.81 m) are also found to be over-predicted than that of the observed groundwater levels (varied in the range of 63.76 - 65.67 m) during extremely dry year (2005) (Figure 5.5(b)). The comparison of simulated and observed values at Jaynagar also indicates the model is not able to capture the effects of wet and dry years. Overall, during calibration and validation period, the performance of the model (in case I) is found to be over-predicted (Figures 5.4 and 5.5) which indirectly reconfirms that the over-prediction of recharge by the SWAT model.