2.2.2. Water Resources Model

RIBASIM [31,32] simulates the use and distribution of water using river discharges from Wflow as input. It uses a schematization of links and nodes (Figure 5) to describe the flow of water in the rivers, storage in reservoirs, diversions into canals, and consumptive use and return flows across the basin. Water can be used from precipitation, rivers, canals, or from groundwater. Conjunctive use of surface and groundwater is included. Return flows can be divided between rivers, canals and groundwater. This is important for the Ganga plains, where extensive leakage from irrigation canals recharges groundwater aquifers. RIBASIM was linked to the groundwater model by simulation of extraction and infiltration rates and by the flux between the river and the groundwater, as simulated by the groundwater model.

**Figure 5.** Schematization of the Ganga River Basin in RIBASIM.

Data for water infrastructure such as barrages, dams and canals were mainly been derived from the Ganga Basin Report [33] and the India-WRIS (Water Resources Information System [34]). The schematization was adapted with input from the first round of state and basin-wide workshops, including on the location of existing and planned reservoirs and canals, the compartmentalization of irrigated areas in command areas, and the main abstractions of surface and groundwater.

Information on irrigated crop areas was derived from the Land Use Statistics Information System of the Ministry of Agriculture and Farmers Welfare [35]. Data for 246 districts was aggregated to data for 41 irrigation nodes. The cropping calendar, describing when which crop is planted, was derived from information provided by the Crop Science Division of the Indian Council of Agricultural Research [36]. Estimates for irrigation efficiencies and return flow fractions from [37] and monthly average reference evapotranspiration data per state from [38] were used. Monthly crop transpiration coefficients for most crops are India specific values from [39], but for maize and rapeseed coefficients from [40] were used. For sugarcane, tobacco and fodder crops no information specific for India could be found, and values from FAO [41] were used.

District-level population data from the 2011 census data [2] were used to assess domestic water demands and extrapolated to 2015 based on projections for 2001 to 2026 [42] and on urbanization rates for the period 2001–2011. District data were aggregated to correspond with the 55 public water supply nodes for domestic demand. Data on water sources, leakage and return flow for major cities from [43] and data on industrial water demand from [44] were also used.

The water demand of public water supply and irrigation nodes can be fulfilled by water from surface and groundwater resources to simulate conjunctive use. The capacity of the surface water supply for irrigation is determined by the canal capacity, mostly obtained from India-WRIS, and for public water supply from [43]. The capacity of groundwater supply for both has been tuned to yield results that are comparable to the estimates presented in [45].

During periods of water shortage, RIBASIM allocates water based on priorities. The following ranking of priorities was used in the Ganga Basin Model:


The concepts used by RIBASIM to simulate water demand and allocation, and the operation of infrastructure, are described in [31,32]. RIBASIM results include monthly flows in rivers and canals, groundwater abstraction rates, and the water supplied to fulfil each water demand. The simulated groundwater abstraction rates are used as input to groundwater model. Simulated river flows are used in the groundwater model to assess the water level that determines the exchange between the river and the groundwater. In the workflow (Figure 4) the water resources model is run twice: once before the groundwater simulation with zero exchange between rivers and groundwater, and once after the completion of the groundwater simulation with the exchanges as calculated by the groundwater model. The simulated flows in the rivers are used as input to the water-quality model, the ecological assessment module, and the dashboard.
