Are flood events really increasing? A case study of Krishna River Basin, India

Abstract

There is a heightened sensitivity about the risks posed by the prospect of climate change as deduced from the results of the global-regional climate models. Notwithstanding the moderating, albeit feebly articulated, acknowledgement of variability as being an intrinsic attribute of climate, it is often claimed that intensification of large hydrological extremes such as floods is indeed emerging as a new looming reality. This has aptly given rise to fears of prospective exacerbated socio-economic vulnerability. In an attempt to investigate the veracity of whether the hydrological flood events are really intensifying across the Krishna River Basin, the present study has examined the historical floods in Krishna River Basin (KRB) using available streamflow and precipitation records along with published reports and featured news articles. The approach followed is based on quantitative and qualitative analysis of floods in KRB at the subsystem scale (K1 to K12). The quantitative analysis involved i) Development of unregulated flow series through hydrological modelling, ii) Frequency analysis of unregulated flows and precipitation, iii) Threshold selection for defining the small, medium, and large floods, iv) Identification of flood events in observed streamflow series and v) Identification of causal rainfall and its relationship with flood peak. The qualitative analysis focused on published reports and news articles to attempt a multivariate characterization of flood flow attributes and the accompanying losses. The study concludes that the hypothesis that flood events are intensifying is untenable for most subsystems of KRB except K7 (Lower Krishna Basin) being the sole exception where, in sharp contrast, flood events show signs of moderation. Interestingly, cyclic patterns analogous to Noah and Joseph Effect are seen in the case of small floods for all the subsystems except K7. Non-recurrent, standalone extremes that bear the classical signature of Erratic Noah and Joseph elements have also been observed in medium and large floods in all the subsystems except the K12 subbasin (Munneru Basin). The study confirms that qualitative analysis alone cannot lead to an incontrovertible determination of trend like features in flood records as every event in the historical record bears a unique multi-dimensional footprint arising from a mix flood characteristics, associated losses and perceived short and long-term impacts. This study can provide a guideline to identify changes in flood typology especially in basins with altered hydrologic regimes and serve as an aid in planning and formulating policies for ameliorative flood management strategies as well as in policy restructuring when deemed to be necessary.