What's behind deficit rainfall?

The globe has registered the highest average July temperature in 2023. Copernicus Climate Change Service, a European Climate monitoring organisation, officially declared that July 2023 was the world’s hottest month on record by a wide margin.

July’s global average temperature of 16.950 degrees Celsius was a third of a degree Celsius higher than the previous record set in 2019—16.62 degrees Celsius. Normally, global temperature records are broken by a hundredth or a tenth of a degree, so this margin is unusual.

August 2023 has become India’s driest August since recordkeeping began in 1901. That is a record rainfall deficit in 122 years. The staggering rain deficit of over 33% has heightened concern about this monsoon season, culminating in a significant shortfall.

El Nino at play

United States-based National Oceanic and Atmospheric Administration (NOAA) predicted six months ago that the El Nino effect would bring a drought-like situation and increase food inflation. El Nino is an abnormal warming of surface Ocean currents in the east and central equatorial Pacific that corresponds with the changes in the wind flow affecting weather patterns across many regions of the globe. It leads to poor summer monsoon rains in India.

As opposed to this, La Nina conditions correspond to water cooling in the East and Central Pacific. Such wet weather conditions have finally dissipated after persisting for three years.

El Nino-related drought and famine have been recorded in history. There is evidence to conclude that there have been political upheavals in El Nino years, especially when the responses of various governments were inadequate. It can be tricky for dispensations in election years.

Warming: East vs West

The weather pattern is also understood by a new term found by experts in the 1990s; it is Indian Ocean Dipole (IOD), which is determined by the difference between the sea surface temperature of the western and eastern Indian Ocean.

If the temperature in the west is higher than in the east, it leads to more rainfall in India “or positive IOD” and vice versa. The early monsoon forecast of normal rainfall with a 5% variation by the Indian Metrological Department (IMD) during this year was with the hope that IOD would be positive, and that is the only way El Nino effects can be neutralised.

The unprecedented rainfall deficit indicates that India had a negative IOD this year, and the adverse impact of El Nino could not be neutralised. In other words, the eastern part of the Indian Ocean was warmer than its western part. It was a bad coincidence that accounts for unfavourable conditions and poor rainfall. 

A large part of the country, except for North West India and North East India, has experienced a deficit ranging from 10% in West Bengal to 60% in Kerala. The situation in Karnataka is as bad as it is in Kerala. The reservoirs are empty. As a direct political implication, the Cauvery water dispute between Tamil Nadu and Karnataka has reached the Supreme Court.

Forests and precipitation

Rapid changes in land use patterns, especially the loss of natural forests, have increased the runoff and reduced the water absorption potential of tree roots and soil. As a result, the reservoirs have been silted up, and the water-holding capacities have decreased.

The roots of the trees work like sponges and can hold a substantial quantity of water during rainfall. A forest is the most important tool for conserving soil and water and reducing runoff. Many perennial streams become seasonal when the catchment is devoid of tree growth.

Two physicists named A M Makarieva and V G Gorshkov from St Petersburg Nuclear Physics Institute, Russia, wrote a paper, Biotic pump of atmospheric moisture as driver of the hydrological cycle on land, which was published in Hydrology and Earth System Sciences in 2007. It observed that water from the land flows to oceans under gravity, and water loss from land is continuously compensated by atmospheric moisture transport from ocean to land.

Using data from terrestrial transects, researchers found that air fluxes can transport moisture over non-forested areas for a few hundred kilometres; precipitation decreases exponentially with distance from the ocean. In contrast, precipitation over extensive natural forests is independent of the distance from the ocean for several thousand kilometres.

This explains the existence of an active biotic pump transporting atmospheric moisture inland from the ocean. The phenomenon is explained as low-level air moving from areas with weak evaporation to areas with intense evaporation. Due to the high leaf area index, tropical evergreen forests maintain high evaporation fluxes, which support the ascending air motion over the forest and “suck in” moist air from the ocean. This is the essence of the biotic pump of atmospheric moisture, which can enhance precipitation at any distance from the ocean. The evidence shows that an intense terrestrial water cycle is unachievable without extensive forests covering the continent's width.

If the forests were to disappear, then moisture would no longer be sucked in, and given the natural fallout rate of rainfall, some 600 kilometres from evaporation to precipitation, the land would dry out and turn into desert. We must save our forests to get good rains.

(The author is a retired principal chief conservator of forests (Head of Forest Force) Karnataka)