Southwest monsoon brings 26% excess rain to Coastal A.P., improves groundwater

While the rainfall improved groundwater in Rayalaseems the flood in Budameru caused misery to the people living in many areas of Vijayawada City, explains a former Director of the IMD

The torrential rains and the floods in the Budameru caused untold misery to the people living in many areas of Vijayawada City. The heavy rains, which occurred towards the fag end of the Southwest monsoon, however, resulted in 26% excess rain in coastal A.P. and improved the groundwater levels.

Retired Director of the IMD P. V. Rama Rao, has analysed the ‘performance of the SW monsoon-2024’ using data obtained from websites of the India Meteorological Department (IMD) and the A.P. Ground Water Department as also from newspapers.

The SW monsoon advanced into Rayalaseema and coastal Andhra Pradesh on June 2, this year, it covered the entire Rayalaseema region by June 4 and coastal A.P. by June 20. The advance of moist SW monsoon current suppressed the hot spell and brought relief to the people.

Synopsis of SW monsoon

The low pressure systems which form or move over westcentral and adjoining northwest Bay of Bengal off Andhra-Odisha coasts, provide triggering mechanism and activate or even revive monsoon flow and cause heavy rainfall over Andhra Pradesh and Telangana States.

After crossing the coast, these systems generally move in a west-northwesterly direction and increase rainfall in their vicinity and the areas along the path followed by them. They cause vigorous monsoon with very low clouds and heavy spells of rain, especially in the west and south and more so in southwest sector of the system, says Mr. Rama Rao.

The system in the affected area may cause heavy rains at many places and on few occasions extremely intense spells of rain, cause extremely heavy rain falls (> 21 cm) or exceptionally heavy rainfalls (>30 cm) in 24 hours. They are followed by flash floods which may cause extensive damages to property and in extreme cases even cause loss of human lives.

In the season, normally low velocity wind systems up to deep depression stage form and no major damages occur due to winds over land areas due to frictional forces. However, on few occasions ‘Rain Storms’ occur and cause extensive damages to agriculture and infrastructure.

The performance of monsoon is evaluated based on rainfall amount received and its percentage rainfall departure from normal. The performance of southwest monsoon in the year 2024, is ‘good’ over coastal A.P. with percentage excess rainfall of 26%, with actual average rainfall of 76 cm against seasonal normal of 60 cm. It was normal over Rayalaseema with percentage rainfall of 18%, with actual average rainfall of 48 cm against the seasonal normal of 41 cm.

Groundwater levels in A.P.

Rainfall is the principal factor which contributes to increase or decrease in groundwater levels. The behaviour of groundwater table is essentially governed by rainfall, its quantity, intensity and frequency.

The other factors that impact groundwater levels are seepage and percolation processes from irrigation canals and nearby surface water sources.

Due to excess rainfall in coastal Andhra Pradesh and normal rainfall in Rayalaseema, after utilisation of groundwater for agriculture and other needs, the net rise in groundwater levels between for June 1 to September 30 was 2 m (6.5 ft.) in coastal Andhra Pradesh and 1.91 m (6.3 ft) in Rayalaseema.

By the end of September 2024, groundwater is available in coastal A.P. at an average depth of 8.99 m (29.5 ft). The lowest average depth groundwater available in Konaseema Ambedkar district at 3.11 m (10.2 ft) and the deepest depth below ground of 22.7m (74.5 ft) in Prakasam district.

In Rayalaseema, groundwater is available at an average depth of 11.78 m (38.6 ft) by end of September, 2024.

In the SW monsoon period, significant rise in groundwater levels occurred at Palanadu district of coastal Andhra Pradesh by 3.89 m (12.76 ft) and in Sri Satya Sai district of Rayalaseema by 4.93m (16.17 ft).

Vijayawada floods: Synoptic Situation

A depression formed and lay over west central and northwest Bay of Bengal off north Andhra and south Odisha coasts on August 31 morning. The depression crossed north Andhra-South Odisha coast close to Kalingapatnam during the midnight of August 31. Subsequently, after crossing the coast the system moved west-northwestwards and moved over to Vidarbha across Telangana by September 2, says Mr. Rama Rao.

A shear zone ran roughly along 180° N over north Peninsular India between 4.5 and 7.6 km above mean sea level on Sept 1 and along 200° N between 3.1 and 5.1 km on September 2. This is one of the weather factors which caused heavy rains in Andhra and Telangana States.

Under the influence of the depression torrential rains with extremely intense spells of rain occurred on August 31 and September 1, 2024 in the erstwhile Krishna and Guntur districts in coastal Andhra Pradesh.

Amaravati in Guntur district received heaviest rainfall of 26 cm in 24 hours ending at 8.30 a.m. on September 1. The remaining districts of north coastal A.P. received rainfall ranging between 5 and 10 cm and in Rayalaseema between 1 and 7 cm on those two days. Coastal Andhra Pradesh received average rainfall 11 cm. In the present districts, Guntur district received 31.5 cm, NTR district 26.6 cm, Krishna district 21.5 cm and Palanadu district 20.1 cm respectively on those two days. These are the districts worst affected due to rain havoc.

Published - October 04, 2024 04:15 pm IST