Understanding link between extreme rainfall events and floods

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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. However, the link between extreme rainfall events and the 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 causing more extreme precipitation events. However, there is no clear evidence that this leads to increased floods.

A new study by Australian and American researchers, published in the journal Water Resources Research, has 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. Several unexpected factors also affect flood response to extreme precipitation.

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

Another factor is the size of the thunderstorms. Storms are found to become smaller in size when temperatures are warmer since the convection intensity increases with temperature. In addition, warmer temperatures prolong droughts and dry up the soils, which affects the flood response following extreme rainfall events. Other non-obvious factors include the tree canopy’s water storage capacity and the trees’ leaf wettability. Leaf wettability was found to increase with temperature for many tree species.

There have been some increases in flooding relating to earlier snowmelt, which shifts the timing of the floods to spring. Likewise, late winter storms with warmer temperatures or rain on snow are found to cause late winter floods in parts of Europe. The timing and magnitude of floods are altered in these cases. Many of the coastal floods are also unrelated to extreme rainfall.

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

Projecting extreme rainfall is a challenge for climate models. All evidence points to a greater than seven percent 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 the precipitation-streamflow link. However, not just the rainfall intensity matters for streamflows but also the duration and spread of storms. This is in addition to prior wetness and other hydraulic factors.

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

Specifically for India, the impact of extreme rain on rivers and streamflow is complicated by dams, reservoirs, urbanization, 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. At the same time, 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 capture past 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, meaning 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 precipitation may arrive as rain instead of snow in a warming world. This would again change the seasonality of peak streamflows.

These multitudes of natural-human system interactions in India point to the urgent need for a better understanding of the complexity of the relationship between extreme precipitation, streamflows, and floods, especially where human action may escalate flood risks.

Journal Reference

  1. Sharma, A., Wasko, C., & Lettenmaier, D. P. (2018). If Precipitation Extremes Are Increasing, Why Aren’t Floods? Water Resources Research, 54(11), 8545-8551. DOI: 10.1029/2018WR023749

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