Understanding link between extreme rainfall events and floods

It has been reported that extreme rainfall events are increasing over India and widespread floods have increased threefold over the last several decades from an average of two events per year to six events per year. But the link between extreme rainfall events and rising incidence of floods remains tenuous.

There is scientific evidence that global warming is leading to more moisture loading in the atmosphere, which, in turn, is causingmore extreme precipitation events. However, there is no clear evidence that this is leading to any increase in floods.

A new study by Australian and American researchers reported in journal Water Resources Researchhas confirmed that increased extreme rainfall events do not necessarily lead to increased floods. Many hydraulic factors such as physiography, drainage, catchment size and vegetation cover are critical for peak flood conditions. There are also several unexpectedfactors that affect flood response to extreme precipitation.

For example, in the United States, on an average only 36 per cent of the time extreme rain events led to extreme river discharge. Much depended on how wet or dry was the river catchment area before the extreme rainfall. If the catchment area was wet prior to the extreme rainfall event, extreme discharge occurred much more frequently than the average, at 62 per cent of the time. The percentage dropped sharply to a mere 13 per cent if the catchment was dry. Clearly, soil moisture heavily influences flood response to extreme rainfall.

One of the most importantfactors for extreme rainfall is the rate at which atmospheric moisture is increasing with global warming. According to Clausius-Clapeyron equation, the capacity of air to hold moisture increases by seven per cent for each degree of warming. The increase in extreme rainfall is due to a combination of this increase in moisture as well as changes in atmospheric circulation.

Projecting extreme rainfall is a challenge for climate models. All evidence points to greater than seven per cent increase in extreme events, even though specific conclusions vary depending on how the term 'extreme events' is defined.

It is often assumed that the temperature-extreme rainfall relationship also applies to precipitation-streamflow link. However, it's not just the rainfall intensity that matters for streamflows but also duration and spread of storms. This is in addition to prior wetness and other hydraulic factors.

Historic data shows decreasing trend in streamflow. The presence or absence of dams or forest covers does not appear to matter. But increasing trends in annual maximum streamflow are seen over smaller catchment areas.Data also shows that small increase in precipitation with warming is much higher than relative increase in streamflow.

Specifically for India, the impact of extreme rain on rivers and streamflow is complicated by dams, reservoirs, urbanisation and other land use changes as well as increased evaporative losses due to global warming. Seven rivers that serve hydropower production are reported to have experienced a decrease in rainfall and streamflow, while climate projections indicate a wetter monsoon in a warmer world and a potential increase in hydropower production. The caveat is that climate models may not fully capturepast monsoon trends and may be unreliable for future projections.

This should be placed in the context of the fact that Himalayan-born rivers in India carry some of the highest sediment loads in the world. Sediment loads have a nonlinear relation with streamflow which means these loads can increase more rapidly beyond a certain threshold of streamflow. In addition, extreme precipitation events have increased over parts of the Himalayas and more of the precipitation may arrive as rain instead of snow in a warming world. This would again change the seasonality of peak streamflows.

These multitude of natural-human system interactions over India point to the urgent need for a better understanding of the complexity of the relation between extreme precipitation, streamflows and floods especially where human action may escalate flood risks. (India Science Wire)

The writer is a professor of Atmospheric and Oceanic Science and Earth System Science at the University of Maryland. He is currently a visiting professor at IIT Bombay.

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