Hydrology of the Upper Ganga river – A report by the International Water Management Institute

To provide the background hydrological information for the assessment of environmental flow requirements at four selected ‘Environmental Flow’ sites, a hydrological model was set up to simulate the catchment in the present state (with water regulation  infrastructure) and to generate the natural flows (without water regulation infrastructure).

It further summarizes the hydrological information at these sites using a series of graphs which illustrate annual runoff variability, seasonal flow distribution, one-day flow duration curves and daily flow hydrographs for a wet and a dry year. The document also contains a table, which lists some typical flow characteristics at Environmental Flow sites on a month-by-month basis: range of expected baseflow discharges, number, magnitude and duration of flood events.

The study uses Soil and Water Assessment Tool (SWAT), a process-based continuous hydrological model that predicts the impact of land management practices on water, sediment and agricultural chemical yields in complex basins with varying soils, land use and management conditions. The main components of the model include: climate, hydrology, erosion, soil temperature, plant growth, nutrients, pesticides, land management, channel and reservoir routing.

Conceptually SWAT divides a basin into sub-basins. Each sub-basin is connected through a stream channel and further divided in to Hydrologic Response Unit (HRU). HRU is a unique combination of a soil and a vegetation type in a sub watershed, and SWAT simulates hydrology, vegetation growth, and management practices at the HRU level. The report describes the model functionality with respect to individual component of the hydrological cycle.

Since the model maintains a continuous water balance, the subdivision of the basin enables the model to reflect differences in evapotranspiration for various crops and soils. Thus runoff is predicted separately for each sub-basin and routed to obtain the total runoff for the basin. This increases the accuracy and gives a much better physical description of the water balance.

SWAT requires time series of observed climate data i.e. rainfall, minimum and maximum temperature, sunshine duration, wind speed and relative humidity. In average, the results of both evaluations, performance statics and water flow volume balance, show that the model was performed better in validation periods than in calibration in all flow sites. 

Download the report here -