Vimal Mishra, who is Associate Professor at IIT Gandhinagar, Gujarat, focuses on understanding impacts of climate variability and climate change on water resources, energy, infrastructure, and food security. He integrates observed and remotely sensed datasets with large-scale hydrologic modelling to understand water-climate interactions. Dr. Mishra, who has developed a real-time drought monitoring system for South Asia, recently led a study titled ‘The Kerala Flood of 2018: Combined Impact of Extreme Rainfall and Reservoir Storage’ which was published in the journal Hydrology and Earth System Sciencesfor peer review.“In India, the frequency of extreme rain events has increased and the number of low-intensity rainy days has significantly declined between 1951 and 2017,” says Dr. Mishra in an interview with Pallikkutam.Your findings in the Discussion Paper identify a combination of four factors that led to the extreme flooding across Kerala in 2018, namely, above normal seasonal (May-August) rainfall; extreme rainfall across the State during the season; over 90% reservoir storage ahead of the extreme rainfall events; and lastly, the unparalleled extreme rainfall in the catchment area of major reservoirs in the State. Do you think there were deeper climatic reasons to the series of unparalleled events?
The causes of extreme rainfall event(s) in Kerala that led to widespread flooding are yet to be diagnosed. Extreme rainfall events and associated flooding have increased under the warming climate as shown by the observations and climate model simulations. However, if the 2018 extreme rainfall in Kerala was caused by climate change or not is yet to be understood. The two notable extreme rainfall events occurred in 1924 and 1961 were not caused by climate change. Therefore, such extreme rainfall events that caused flood in Kerala may or may not be caused by climate change.
The Kerala flood, as your study indicates, was caused by “multi-day extreme rainfall and partly due to high reservoir storage”. Most of these events had return periods ranging from 100 to 200 years. On the other hand, in terms of reservoir fullness, the Idukki, Kakki, and Periyar reservoirs experienced “extreme rainfall of more than 500 years return period”. In view of aggregate changes in climate either locally or globally, say over the past 50 years, do you think the return periods could shrink dramatically?
Since the frequency of extreme rainfall events has increased during the last few decades, the possibility of reduction in the return period of such events cannot be ruled out. In fact, research has shown that under the warming climate the return periods of both floods and rainfall events are reduced. In India, the frequency of extreme rain events has increased and the number of low-intensity rainy days has significantly declined between 1951 and 2017.
Reservoir storage was a critical aspect of the floods. You observe in your paper that, “reservoir operations can be more effective by incorporating the extended range extreme rainfall forecasts”. From the perspective of global best practices, could we possibly illustrate it with empirical examples?
Reservoir operations can be very critical if the frequency of extreme rainfall events has increased. The reservoirs before the 2018 flood in Kerala were almost full due to above normal seasonal rainfall, which is important for water management. Therefore, water storage in reservoirs is essential for agriculture and drinking water supply. However, what we need is a linkage between reservoir operations and meteorological forecast. Here it is important to note that meteorological forecast scan be relied upon only to a certain extent. For instance, in case of a false alarm of extreme rainfall if a significant portion of storage is released then it could be costly for irrigation during the dry season. Heavy-rainfall forecast of a longer lead time of 4-7 days will be required to properly operate major reservoirs if they are full.
Finally, and paradoxically, near-drought conditions are being reported in Kerala’s Idukki and Wayanad districts, two of the state’s district among the worst hit by the recent rains. Based on the tools you have developed that can forecast/measure drought conditions up to 5 km proximity, can we now equip our administrations to take better-informed decisions in water management?
The topography of Kerala is heterogeneous and most of the reservoirs are located in upstream areas. Therefore, if a significant portion of reservoir storage was released during the flooding, then reservoirs can experience below normal storage during the post-monsoon season. The other possibility is that most of the flood waters drained off to the sea and did not contribute to groundwater recharge. Groundwater recharge and information available on drought monitoring and forecast can probably be the two most important aspect of our current and future water management.