Bengaluru’s altering rainfall patterns: skilled factors to local weather change


residents removing water from a flooded house
Many homes in addition to the Kempegowda Worldwide Airport flooded on Monday night time. Representational picture: BBMP

Final October, round 700 homes in Bengaluru had been broken after heavy rains and flooding. The saga continues this yr – 70 homes had been flooded on June 4, and extra on July 5. For a whole bunch of low-income households in Bengaluru, heavy rainfall in the previous couple of years have meant intense injury and even shedding their residence. Flooding can even result in the influx of sewage into slums, which additional will increase the chance of water-borne and vector-borne illnesses.

Flooded roads and underpasses don’t simply choke site visitors for hours, however in addition they enhance the probability of accidents. Flooding has additionally been main to wreck to public infrastructure like stormwater drains.

Do extra of those incidents imply Bengaluru is seeing extra intense rainfalls? Or is flooding solely because of excessive ranges of concretisation within the metropolis, clogged drains, and so forth?

The research ‘Local weather change state of affairs in Karnataka: An in depth parametric evaluation’ by the Karnataka State Pure Catastrophe Monitoring Centre (KSNDMC) in June 2020 reveals that Bengaluru’s rainfall patterns have certainly been altering and that it’s an indicator of local weather change.

Elevated annual rainfall, various patterns

The research checked out rainfall knowledge from two intervals – 1960-1990 (named P1), and 1991-2017 (P2). It was discovered that the typical annual rainfall in Bengaluru City district elevated by 13% (or 107 mm – from 836 mm to 943 mm) within the interval from P1 to P2.

Map of rainfall in Karnataka: Rainfall in Bengaluru Urban district increased by 13% from 1991 to 2017 compared to 1960 to 1990
In Bengaluru City district (backside proper), annual common rainfall elevated by 13% in P2. Picture credit score: KSNDMC

Throughout the state, the variety of ‘mild rainfall’ and ‘very mild rainfall’ occasions additionally elevated within the P2 interval. The variety of ‘heavy-rainfall days’ didn’t considerably enhance. In Bengaluru, solely round one heavy-rainfall day had elevated over P2, in comparison with P1. However what was of concern was that, throughout the state, the amount of rain pouring down throughout today was a lot larger in comparison with earlier. And this was resulting in devastating results like floods.

The depth of the rainfall has elevated in Bengaluru too. With reducing open house and ineffective stormwater drains, there’s little recharge – a lot of the water is misplaced as runoff. So flooding has grow to be fairly frequent.

The research additionally discovered that Bengaluru City was one of many districts that confirmed excessive variability within the quantity of rainfall throughout years. An evaluation of annual rainfall between 1989 and 2018 confirmed a ‘co-efficient of variability’ of 25%. That’s, in 25% of those years, the quantity of rainfall diversified; it was both above or beneath regular. The very best variability was within the month of July – in 62% of the years, the rainfall in July had diversified from regular.

Map showing that Bengaluru Urban district had annual variability in rainfall across 25% of the years between 1989 and 2018
Between 1989 and 2018, the annual rainfall in Bengaluru confirmed variability throughout 25% of the years. Picture Credit score: KSNDMC

The research additionally confirmed that the variety of years with dry spells elevated in the course of the P2 interval. Dry spells are three or extra consecutive weeks with lower than 50% of regular rainfall. In massive elements of Bengaluru City, in P2, there have been 3-4 extra years with dry spells throughout southwest monsoon. Dry spells have an effect on elements like soil moisture and groundwater ranges.

Map showing that Bengaluru Urban district had more dry spells during Period 1991 to 2017
A lot of Bengaluru City district had 3-4 extra years of dry spells in P2 in comparison with P1. Picture Courtesy: KSNDMC

In an interview with this reporter, Dr G S Srinivasa Reddy, former KSNDMC Director who additionally headed the research, explains the implications for Bengaluru. Dr Srinivasa, a hydrologist, was with KSNDMC for about 30 years. After his retirement final yr, he continues to work as a Senior Advisor on the Centre.

Dr G S Srinivasa Reddy, former Director at KSNDMC
Dr G S Srinivasa Reddy, former Director at KSNDMC. Pic Courtesy: Dr G S Srinivasa Reddy

Edited excerpts from the interview:

The research reveals rising quantities of rainfall, in addition to annual variability of rainfall in Bengaluru.

As per the report, not solely Bengaluru, many elements of South Inside Karnataka is displaying an rising pattern in rainfall. On the identical time, in North Inside Karnataka, the vast majority of taluks are displaying a reducing pattern.

Second factor is, distribution of rainfall has additionally modified; the frequency of maximum occasions has been rising step by step. And there’s better variability in rainfall in Bengaluru over time.

With urbanisation, the radiation of extra warmth within the metropolis contributes to the event of clouds. Pollution additionally act as nucleus for the event of droplets of rainfall.

Is there a distinction in depth of rainfall within the metropolis?

Earlier, knowledge from the IMD (India Meteorological Division) observatory on Palace Highway, was taken as Bengaluru’s rainfall. Within the final three years, we have now established about 100 rain gauges throughout Bengaluru. This community reveals that even inside the metropolis, rainfall patterns range; it’s not uniformly distributed throughout the town. We’ve been monitoring knowledge since 2013 – in case you see these eight years’ knowledge, you will note variability.

Learn extra: Did Bengaluru actually get extra rains in 2020?

Particularly the info we have now picked up over the past 3-4 years reveals that the complete quantity of ‘heavy-rainfall days’ is much less, however the quantity of rainfall that happens isn’t just ‘heavy’, it’s ‘very heavy’. For instance, in a single station in south Bengaluru, in all probability in August 2016, 184 mm rainfall was recorded inside 24 hours. However the identical day, Yelahanka in north Bengaluru obtained solely 8 mm rainfall. A lot variability is occurring inside the metropolis. We additionally noticed that the western a part of Bengaluru, particularly Kengeri, typically will get extra rain.

Is it regular for one space to have a lot rainfall and a close-by space to have so little of it? Is that a sign of local weather change?

Variability is frequent, however a variation of 184 mm and 8mm inside a brief distance of 20 km is large. These are all gradual impacts of local weather change. Even temperatures within the metropolis have been rising.

We’re seeing this not solely in Bengaluru, however in your complete state. From 2018 onwards, we have now been observing high-intensity rainfall repeatedly.

In the event you take into account rainfall in Malnad this yr, the overall quantity of rainfall just isn’t a lot, however there was heavy rainfall inside a number of days. There have been 1-2 heavy rainfall occasions every in June and July. Suppose 1000 mm of rain is unfold over a month, we might not get a lot runoff. But when it comes inside 4-5 days, it creates a variety of runoff.

In Bengaluru additionally, we have now noticed that the runoff coefficient has elevated. It is determined by the land use sample. Earlier than the Eighties, solely 30-40% of the rainfall used to go as runoff. There have been extra open areas the place water infiltrated into the bottom and recharged groundwater. However now it’s 85-90% due to urbanisation, as open areas obtained transformed into concrete jungles, and the carrying capability of stormwater drains diminished because of encroachments/strong waste disposal. So flooding is kind of frequent in city areas as a result of excessive runoff coefficient, and in addition the elevated depth of rainfall.

As per the research, Bengaluru doesn’t present a lot of enhance within the variety of heavy-rainfall days although.

Sure, within the research we solely appeared on the variety of heavy-rainfall days. That has not elevated. However the quantity of rain that falls throughout today is far larger now. The depth could be larger in some elements of the town.

Additionally for Bengaluru City district, the research reveals extra years with dry spells. Is {that a} concern?

Sure, there are extra dry spells, and it’s not good. Excessive occasions with both extra rainfall or much less, are the results of local weather change.

Learn extra: Bengaluru’s local weather change historical past and classes to study

Bengaluru primarily will get water from Cauvery river. Even the State’s current water coverage signifies that consuming water will get precedence. Dam water in Kabini or KRS will certainly be reserved for consuming water in Bengaluru, at the same time as these areas face some drought. In addition to, the principle catchment for Cauvery water is Kodagu and a part of Wayanad (in Kerala), and rainfall has been reducing in each these districts. Sooner or later, this will have an effect on the influx coming to Cauvery.

Local weather change impacts not solely Cauvery, but additionally groundwater in lots of elements of Bengaluru which is the consuming water supply for a lot of. Already we see that in summer time, many borewells are going dry or yielding little or no.

What ought to ideally be the coverage choices from this report? What’s the way in which ahead?

No matter excessive occasions are coming, we have now to make use of it as a useful resource, particularly in city eventualities. For that, we have to implement rainwater harvesting extensively and really systematically, not only for namesake. We get 55-60 days of rain in Bengaluru, unfold throughout 4-5 months. We’ve to have a look at how surfaces like pavements may also be used for groundwater recharge. This could assist seize floodwater and in addition assist take care of water shortage throughout dry spells.

We’ve to desilt lakes and ensure sewage doesn’t enter them, after which use lake water for functions apart from consuming. As an alternative of secondary therapy of wastewater, there ought to be tertiary therapy, and this handled water may also be used for functions apart from consuming. There’s undoubtedly scope to make use of handled water for home functions, by supplying it by twin piping methods.

Just lately BBMP began giving AMC (Annual Upkeep Contract) for stormwater drains, which is an effective measure to stop flooding. Below the fifteenth Finance Fee, the Centre has allotted Rs 50 cr for Bengaluru, amongst 6-7 cities, for flood mitigation. The Centre is now bringing out the ultimate tips on easy methods to use the funds.

In fact, there ought to be bigger plans to take care of local weather change as properly.

Does this yr’s rain present any particular sample?

It follows the identical sample that we have now mentioned thus far. Rainfall was concentrated in a single or two occasions, with heavy rain and flooding from these occasions. In any other case, there has not been a lot rainfall this yr.

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