28. Drought Characterisation and Impact Assessment on Basin Hydrology—A Geospatial Approach

Characterisation and quantification of drought are important considerations in the planning and management of water resources. Present study examines the impacts of meteorological droughts on various hydrological aspects of the Godavari River basin by characterising and quantifying the droughts using a geospatial approach. Standardised indicators such as Standardised Precipitation Index (SPI) and Standardised Stream Flow Index (SSI) were used to identify the meteorological and hydrological droughts, respectively at different accumulation periods (1 month to 24 months) for the years 1951–2018. The meteorological drought index was used to characterise the droughts in the Godavari River basin, considering their severity, spatial extent and duration at various time scales. The long-term hydrological regime of the basin (1951–2018) was simulated using the Variable Infiltration Capacity (VIC) hydrological model. The time periods (of each accumulation period) with normal rainfall were identified and segregated. The mean (average) discharge (both observed and modelled) of each accumulation period (1 month to 24 months) was estimated from discharge data of these normal meteorological periods. The percentage deviation of observed and simulated discharge values from the ‘mean’ values during each drought period for each station were obtained and their relation with the various category of meteorological drought was quantified. The hydrological drought index (SSI) was computed by applying the appropriate probability distribution (concluded from the KS test) for both observed and simulated discharge values for the Polavaram discharge station representing the whole basin. Majorly for all timescales, log-normal was observed as the optimum probability distribution for both the observed and modelled hydrological data. Hydrological drought events were then analysed in accordance with the meteorological droughts in order to examine their interrelationships. The results showed a linear relationship when we compared the meteorological droughts with percentage deviation in the modelled discharge. However, similar relation doesn’t exist in the case observed discharge values. This might be due to the fact that every basin will have its anthropogenic resilience towards droughts. The observed  discharge data from the Godavari basin does not describe the natural flow regime of the river, which is why the behaviour of hydrological droughts in the basin is anthropogenically controlled. The numerical impact of each category of meteorological drought on the hydrological regime of the Godavari basin and its sub-basins was quantified successfully for the virgin flow conditions.