Extreme rainfall in mountainous regions poses flood threat to Delhi
NEW DELHI : Although high intensity rainfall in Delhi is decreasing, the threat of flooding persists in Delhi because of an increase in high-intensity and extreme rainfall this decade in Uttarakhand and Himachal Pradesh, states through which the Yamuna passes.
The Yamuna river has been flowing above the danger level of 205.3 metres in Delhi since 19 August. Residents of low-lying areas along the river have been asked to move to safer locations as the water level is expected to reach the 207-metre mark by August 21, as per the Central Water Commission’s forecast.
If it happens, this will be only the fourth instance of the water level of the Yamuna crossing the 207-metre mark in Delhi since 1978. It is to be noted that while Delhi’s total monsoon rainfall was 966.7 millimetres (mm) in September 1978, the fifth highest since 1950, Delhi has received 274.1mm of cumulative rainfall until August 19 this year, which is only 68% of the long-period average (LPA) rainfall the capital receives until this date.
An analysis of the India Meteorological Department (IMD) rainfall data for Delhi since 1950 shows that high-intensity rainfall during monsoon has been decreasing in Delhi over the last two decades. It is a rise in low-intensity rainfall this decade that has ensured that the total rainfall has not decreased drastically.
HIGH-INTENSITY, LOWINTENSITY RAINFALL
IMD gives grid-wise rainfall data for India, where each grid is a box covering 0.25-degree latitude and longitude. Delhi consists of two such grids. High-intensity rainfall for Delhi is calculated by adding daily rainfall, if it is greater than 35.5mm, in either of the grids over the total monsoon period (June 1 to September 30) and dividing it by two.
Low-intensity rainfall can be calculated by a similar calculation for all days with less than 35.5mm rainfall. Extreme rainfall is a subset of high-intensity rainfall where the daily rainfall threshold is 150mm.
Until 2010, both low-intensity and high-intensity rainfall in Delhi were recorded in tandem. They would increase if the total monsoon rainfall increased and decrease when the rainfall was less. This has changed in this decade, as the high-intensity and low-intensity rainfall seem to be diverging.
As a result of this, the average share of high-intensity rainfall in Delhi’s total rainfall is the lowest (19%) in this decade, since the 1950s. Values from 2010-18 have been used to calculate this decade’s shares. Locations around Delhi — Gurugram, Faridabad, Gautam Budh Nagar, Ghaziabad, Baghpat, Jhajjar and Sonepat — also show the same rainfall pattern. Before reaching Delhi, the Yamuna crosses Uttarakhand, Himachal Pradesh, Haryana and Uttar Pradesh. Both Uttarakhand and Himachal Pradesh have experienced an increase in highintensity and extreme rainfall this decade.
Explaining the current rising level of Yamuna in Delhi, an irrigation department official told ANI that water had to be released from the Hathni Kund barrage in Haryana because of heavy rainfall in Uttarakhand. According to IMD, Uttarakhand received 100.6mm rainfall from August 15 to 19. Himachal Pradesh received 147mm of rainfall in the same period. Uttarakhand went from being 29% deficient from LPA on August 14 to being 23% deficient on August 19. Himachal Pradesh, which was 23% deficient on August 14, became 2% deficient on August 19.If these trends continue, Delhi might have to reconcile itself with being vulnerable to flooding without receiving much rainfall.