The rainfall in Jakarta on New Year’s Day 2020 was the highest in history. Flooding depends on not just heavy rainfall, but also geological and geographical conditions, infrastructure, management and how people react.
By
Ahmad Arif
·4 minutes read
The rainfall records at the Meteorology, Climatology and Geophysics Agency (BMKG) shows that extreme rain occurred across and around Jakarta on 31 Dec. 2019 and 1 Jan. 2020. The heaviest recorded rainfall on New Year’s Day was 377 millimeters at Halim Perdanakusuma Airport, 335 mm at Taman Mini Indonesia Indah (TMII), 260 mm in Jatiasih, 246 mm per day in Cikeas and 226 mm per day in Tomang.
The 377 mm that fell at Halim airport was the highest recorded rainfall in Greater Jakarta since the Dutch colonial government started recording the region’s rainfall in 1866. The previous highest recorded rainfall was 367 mm, which occurred in Sunter in February 2015. Flooding also hit Jakarta in 2015, but it was not as severe as this year.
BMKG climate researcher Siswanto said that the extreme rainfall in 2015 only affected North Jakarta, while this year’s extreme rainfall affected almost everywhere in the region, including midstream and upstream regions such as Depok and Bogor.
Flooding also hit Jakarta in 2015, but it was not as severe as this year.
The two highest recorded rainfall during the 154-year record for Jakarta, occurred in 2015 and 2019-2020 and showed a shift in the rain pattern, according to Siswanto’s research into long-term rainfall trends.
A 2015 study in the Royal Meteorological Society states that between 1866 and 2010, Jakarta saw a decrease in the number of rainy days, but that the rate of heavy rain increased significantly to exceed 50 mm in daily rainfall and 100 mm per day in annual rainfall. This increase in extreme rainfall is just one part of the story, as Jakarta and all Indonesian territory south of the equator have experienced a decrease in total annual rainfall over the same period.
BMKG researcher Supari wrote in the International Journal of Climatology in 2016 that rainfall in Indonesian territory north of the equator (central and northern Sumatra, eastern Kalimantan, central and northern Sulawesi) had seen increased rainfall.
Rainfall has shifted globally. A 2007 study by Lau and Wu indicated that the probability distribution function for tropical rain in several countries had shifted significantly in 1979-2003. This included the increasing prevalence of heavy rain (10 percent of the heaviest rainfall) and the decreasing prevalence of light rain (5 percent of the lightest rainfall).
A 2015 study by Kevin E. Trenberth in Nature Climate Change also linked the increasing frequency of extreme weather in various parts of the world with land and sea surface temperatures. The weather has surely changed as global temperatures have risen by 1.1 degrees Celsius since the 1850s.
Based on the undeniable meteorological data, the chance of extreme rain will increase. Accordingly, the risk of flooding due to weather will also increase for Jakarta. Similarly, the risk of a water deficit during the dry season will also increase, as has happened in all regions across Java.
Specifically for Jakarta, rainfall is not the only cause of flood risk. Geographically, Jakarta is prone to flooding, as it is located in a low-lying region at the estuary of 13 rivers. In the past, Jakarta had many swamps and lakes that were used as natural reservoirs, due to its geographical position. As time went by, these areas dried up and were filled.
Jakarta’s fate also depends on the runoff from upstream buffer regions, especially Bogor. However, many parts of these buffer regions have been transformed into residential areas. Many natural reservoirs and swamplands around Jakarta have also dried up and been turned into residential areas. Consequently, an increasing volume of runoff flows to Jakarta from these former water catchment areas.
Water pumps are vital to Jakarta and careless operation will lead to disaster.
Many rivers in and around Jakarta that serve to contain runoff during the rainy season are narrower today. Some have disappeared and been turned into housing complexes.
Geologically, the land in Jakarta has continued to subside since 1974 by a maximum 4.1 meters. At the same time, the sea has risen due to global warming. Naturally, under such conditions, it will be more difficult for rivers to flow out to sea. Therefore, water pumps are vital to Jakarta and careless operation will lead to disaster.
With such a complex problem, solving Jakarta’s floods will not be easy. Normalizing Jakarta’s rivers, which many understand to mean installing concrete barriers at riverbanks and dams, is no guarantee.
Integrated upstream and downstream spatial planning is needed. In solving the flooding in Jakarta, we also need to consider the potential of the increasing water deficit during the dry season.