*3.2. Calibration and Validation of the Ganga River Basin Model*

The model components for hydrology, geohydrology and water resources management have been jointly calibrated and validated, as river flows are influenced by water use and water infrastructure operations. Flows were calibrated using 1995–2009 data and validated against 1985–1994 data. Flow calibration and validation focused on the Ganga mainstream and its main tributaries. Calibration and validation data for iMOD were specified by location not time period. The entire calibration process had six steps:


A complete description of calibration and validation results as well as sensitivity analysis results are in [5]. Figures 7 and 8 show calibration and validation for monthly flows at two locations on the Ganga River. Observed flow data are from CWC. Flow values are omitted in compliance with the Government of India Water Data Policy for classified data. Figure 7 shows results for Rishikesh, where the Ganga descends from the Himalayas onto the plains. Simulations generally agree well with the measurements but underestimate peak monsoon flows. These peak flows are less important from a water supply perspective, as during the monsoon demands (including to fill storage) are far lower than supply. Figure 8 shows results for Varanasi, the most downstream location on the Ganga for which data were available. Again, simulations match measurements well, but with an overestimation of dry season flows.

**Figure 7.** Validation (1985–1994, **top**) and calibration (1995–2009, **bottom**) results for the Ganga River at Rishikesh; monthly discharges (**left**), mean monthly discharges (**right bottom**) and location of the station (red dot on map **right top**).

**Figure 8.** Validation (1985–1994, **top**) and calibration (1995–2009, **bottom**) results for the Ganga River at Varanasi; monthly discharges (**left**), mean monthly discharges (**right bottom**) and location of the station (red dot on map **right top**).

It is difficult to compare the results of model calibration and validation with those of previous studies, since model results of previous studies are only available in the form of reports and since different data were made available to prior studies. Limited availability of measured discharge data within India for the studies reported in [12,13] made these studies focus on stations in Nepal. Ref. [13] reports results for the station Hardinge Bridge in Bangladesh for the period 1998 to 2006. These results show Nash–Sutcliffe efficiency (NSE) coefficients of 0.85 to 0.89, which is comparable to the values presented here for Varanasi (Figure 8), the most downstream station for which data were available in this study. Ref. [14] presents NSE and volume bias for simulation results for 1990 to 2004 for a number of stations within India. For Rishikesh an NSE of 0.60 and a volume bias of +30% is reported, which compares unfavorably with the results presented here where NSE of 0.73 to 0.77 and a bias of −6% to −7% were achieved (Figure 7). There are two more stations both on the Ganga River for which both this study and [14] reports results: Ankinghat and Kanpur. Both studies show comparable values for the NSE, but the volume bias reported in [14] is +30%, while our results vary between −18% to −32%.

Overall, the calibration and validation of this study benefited from better data availability than previous studies. As far as results can be compared, the hydrological results of the Ganga River Basin Model appear to be comparable to the results of previous studies and sometimes represent a slight improvement.

## *3.3. Assessment of the Impact of Scenarios and Strategies*

Herein, the term scenario describes developments that impact water resources, but that are outside the direct influence of water managers (e.g., population growth or climate change); and the term strategy describes a combination of interventions designed to address current or future management issues. The effectiveness of strategies can be assessed for different scenarios.

Except for the present scenario, all scenarios are based on assumptions or projections and are, therefore, uncertain. The 'pristine' scenario describes the basin without water resource development. Other scenarios describe possible futures for around the year 2040. All include increases in domestic, industrial, and agricultural water demand. Three climate change futures are considered: no climate change, climate described by the Intergovernmental Panel on Climate Change (IPCC) Representative Concentration Pathway (RCP) 4.5 scenario and climate described by the IPCC RCP8.5 scenario.

Based on stakeholder inputs, strategies were developed that could be implemented in combination or separately:


Most strategies can be scaled to increase their impact. Figure 9 shows basin-wide indicator values for the modelled scenarios. Impacts are most visible in the hydrological indicators: areas with critical groundwater use increase significantly, and the lowest dry-year river discharge diminishes significantly. The e-flow indicators differ significantly from the pristine condition, however, there are only small differences in e-flow indicators between future scenarios. Scenario assessments (without management interventions) indicate a significant decrease in future water availability, water quality and ecological status. Changes are mainly caused by socio-economic factors, not climate change.


E-flow: Socio-Economic status (%) E-socio 66 66 67 68 66 67 69 75

**Table 6.** Basin wide indicator scores for eight strategies under the IPCC 2040\_RCP4.5 scenario.

**Figure 9.** Basin-wide indicators for five scenarios. Values are scaled from 0 to 100, with the pristine scenario as 100). Indicator codes are explained in Table 6 and described in Table 2.

Given the significant potential degradation by 2040, it is informative to evaluate the effectiveness of strategies proposed by stakeholders. Table 6 and Figure 10 show indicator scores for individual strategies under the IPCC 2040\_RCP4.5 scenario. If multiple strategies were combined, a greater response would be expected. Details of the assessment are available in [7].

**Figure 10.** Percent change in basin-wide indicator values for intervention strategies. Values relative to the business as usual scenario. Increases indicate improvement and reduction indicate deterioration.

#### **4. Discussion**

The scenario assessments indicate significant degradation from the pristine condition. During part of the year a large fraction of the flow is diverted to canals, sometimes reducing flow in the river to almost zero. Groundwater levels have changed significantly. Flows in the shallow aquifers in the Ganga plains now entirely reflect anthropogenic influences. Water quality has been severely degraded by liquid and solid waste discharges into the river and its tributaries. It is the first time that these findings can be based on comparison of model results for the actual situation with a pristine scenario.

Model results suggest significant additional degradation in water availability, water quality and ecological status will occur in coming decades in the absence of strong management intervention. A new finding from the scenario analysis is that degradation will be mainly caused by socio-economic factors, not climate change. The projected significant increases in water demand by 2040 will mainly affect groundwater, because most available surface river water is already used. Scenario results show that drinking water and irrigation water deficits will increase, and water quality will further deteriorate. Despite considerable data uncertainty, climate change is expected to affect water demand more than water availability.

Results of the analysis show that there is no single simple intervention to address the multiple pressures on the Ganga. A combination of interventions is required. However, the suite of currently considered interventions, which would require huge investment and face significant technical challenges and stakeholder opposition, will not adequately address the future challenges of water availability, water quality and ecology. Indeed, they will not even address the current severe pressures on the river system. This is an important new finding of this study.

The intervention with the greatest potential benefits is further improvement of municipal wastewater treatment. Whether centralized or decentralized, high- or low-technology, greater reduction in pollution improves downstream water quality, improves ecosystem services, and reduces water-related illnesses and deaths. The next most important intervention is an increase in water-use efficiency, especially in irrigated agriculture. Increased efficiency will not immediately increase water availability; however, irrigation deficits may be reduced. This means greater agricultural production for the same level of irrigation withdrawals.

Model results show that water availability in the basin will be insufficient to meet projected future demands and that there are no easy technical solutions. Many interventions that are beneficial for one sector or outcome show negative effects for others.

The results of the scenario analysis show that ambitious strategies are needed to reduce demands across all sectors and that trade-offs need to be made between sectors. The agricultural sector will need to adapt to lower water availability in terms of crop choices, planting seasons and irrigation efficiency. Farmers will need to develop flexible approaches, choosing irrigated or non-irrigated crops depending on monsoon rainfall. This will affect agricultural production and sector employment.

Without coordination and careful balancing of interests, expensive interventions may fail, wasting scarce financial resources. The absence of a functioning water-resources management governance structure in the basin aggravates the challenges the basin is facing. Although not as a result of the presented study, the authors recommend from global experience that a basin management organization with a legal mandate to work across state boundaries is needed to plan the strategy and implement it.

The consequences of the conclusions presented above are far-reaching and will involve many departments and ministries beyond just water resources. Non-technical interventions, including incentives to change cropping patterns and to reduce water use, are required. Fundamentally, the focus will need to shift from more "crop per drop" to more "jobs per drop". Service and industrial sectors consume far less water per employment generated, supporting greater growth.

The participatory approach to prepare and apply an integrated river basin model as presented in this paper has potential to support improved strategic planning for the Ganga Basin as shown by the results for the scenario and strategies presented. A similar approach can also have added value to support strategic planning for other large river basins in South Asia and the rest of the world. The components of the integrated model should then be modified to reflect the river basin, the issues and the possible interventions.

**Author Contributions:** Conceptualization, M.v.d.V., K.C.A.B. and W.Y.; Methodology, M.v.d.V., P.B., K.C.A.B., M.H., G.H., M.v.O., B.O., F.R. and A.W.; Project administration, K.C.A.B.; Software, M.v.d.V., P.B., M.H., G.H. and F.R.; Supervision, M.v.d.V., K.C.A.B. and W.Y.; Validation, M.v.d.V., P.B., M.H., M.v.O., F.R., R.N.S. and S.K.S.; Visualization, G.H.; Writing—original draft, M.v.d.V., P.B., K.C.A.B., M.H., G.H., M.v.O., B.O., F.R. and A.W.; Writing—review and editing, R.N.S, S.K.S. and W.Y.

**Funding:** This research was funded by South Asia Water Initiative under World Bank contract number 8005347. The funding support from the governments of Australia, Norway and the United Kingdom are gratefully acknowledged.

**Acknowledgments:** The authors thank the Central Water Commission, the Central Ground Water Board, the Central Pollution Control Board and the Indian Meteorological Department for making available their data. Furthermore, the authors are grateful to all institutions and individuals that have participated in the stakeholder process at national and state level. The Ministry of Water Resources, River Development and Ganga Rejuvenation is thanked for providing guidance to the implementation of the project.

**Conflicts of Interest:** The authors declare no conflicts of interest.
