Forests, water fountains of the skies

Forests world over are under threat. About 35 million hectares (Mha) of forests globally have been destroyed due to urbanisation, while urban land has increased from 33.2 to 71.3 Mha between 1992 and 2015. India is witnessing the highest rise in deforestation since the last 30 years, with a stark surge recorded between 2015 and 2020, and has been ranked second after Brazil, with an average deforestation of 668,400 hectares.

Forests cover about 24.62 percent of India and are some of the most biodiverse in the world. They provide a range of important ecosystem services, such as protecting against soil erosion, regulating the water cycle, and serving as a home for a wide variety of plant and animal species. However, they are facing the increasing threat from a range of human induced activities such as illegal logging, mining and land conversion for agriculture and urban development.

Almost 45-64 percent of India’s forest cover will fall under climate hotspots by 2030 and the entire forest by 2050. It is predicted that by 2085, 20 percent of India’s forest cover may experience “catastrophic changes due to adverse impact of climate change.

Why are forests important?

Evapotranspiration (ET), is the sum of all processes by which water moves from the land surface to the atmosphere via evaporation of water from the soil surface, from the capillary fringes of the groundwater table, from water bodies on land and transpiration through plant canopy when plants take up liquid water from the soil and release water vapour into the air from their leaves. 

Forests bring rains

Every tree in the forest performs a unique function - of sucking water out of the ground through its roots and releasing water vapor into the atmosphere through pores in its leaves. Trees in large numbers thus generate huge collections or reservoirs of water in the air that form clouds that travel and bring rains in faraway places. But destruction of more and more trees is increasing the risk of drying up of these aerial rivers and the lands that depend on them for rain.

Forests generate a cooling effect

Forests balance the local climate by producing a cooling effect  partly by shading the land, but also by releasing moisture from their leaves through transpiration, which requires energy and is extracted from the surrounding air, thus cooling it. A single tree can transpire hundreds of litres of water in a day. 

Deforestation since the 1850s has been found to contribute to global warming by 40 percent, and it is feared that tropical deforestation could add 1.5 degrees Celsius (2.7°Fahrenheit) to global temperatures by 2100. Carbon dioxide emissions can add 10 percent or more to global warming due to deforestation by reducing the quantity of carbon dioxide that the forests normally pull from the atmosphere. 

What happens during dry spells?

India receives more than 70 percent of its annual rainfall in the summer monsoon from June to September, while the rainfall is scanty and scattered over the rest of the year. How does the forest system and rainfed agriculture sustain during the rest of the year when there are no rains?

The paper discusses the findings of a study that analyses the role of soil moisture and vegetation in supporting forest systems and rainfed agriculture in dry periods in India.

This study finds that:

The role of vegetation in sustaining the moisture supply to the atmosphere is pivotal

For example, the rainfall reaches its peak during the first half of August and starts withdrawing in September in India. The surface soil moisture follows the same pattern as rainfall while ET continues to increase during the monsoon withdrawal phase. However, soil evaporation immediately starts dropping after the peak of the monsoon and follows the same pattern of rainfall and surface soil moisture. Transpiration from vegetation drives the increased ET during retreat of the monsoon.

Vegetation also plays an important role in sustaining the land- atmosphere interactions during drier non rainy periods.

There are three phases after the peak of monsoon rainfall that affect evapotranspiration:

During the

first phase

This period thus has a moisture surplus because rainfall in this period is higher than ET.  Therefore the increase in ET in this period is because of the high radiation. The rainfall during this period accounts for almost 28 percent of the total annual rainfall, while the ET is approximately 20 percent of the total annual ET. The daily average incoming radiation is 97 percent of its annual daily average. This period lasts for approximately 50 days. 

The

second phase

However, the decline in ET is slower than that of rainfall. The total ET during the pre-capacitor period accounts for approximately 17 percent of total annual ET while the rainfall is 12 percent of total annual rainfall. 

The

third phase

How do ET and the associated plant processes sustain in a situation where water is limited due to less rainfall?

The radiation starts increasing after the monsoon withdrawal, the winds also change direction with the transition of the season. The ET processes during this period are sustained by the soil-vegetation continuum. The soil and vegetation receive moisture during the first and second phases due to the monsoon. This stored moisture is released to the atmosphere during the relatively dry period and supports plant activities.

Larger depletions of root zone soil moisture are observed in the Northern and Central zones, while the Western Ghats and Northeast zones, which receive rains during pre-monsoon season experience less depletion of root zone soil moisture. The Western zone with large areas under desert or scarce vegetation also show a lower rate of depletion of root zone soil moisture.

Forests and soil moisture act as buffers to store water during dry periods

The study thus concludes that the water stored by the soil–plant continuum influences the post-monsoon terrestrial water cycle over India. This stored water helps in meeting the crop and vegetation needs for the rest of the year during periods of reduced precipitation. The capacitance of soil moisture –plant continuum maintains ET, productivity, and land–atmosphere feedback by exploiting this water storage capacity. 

While climate change induced global warming threatens to generate heat waves, bring about monsoonal changes and increase frequency of droughts in India, the need to preserve forests and prevent deforestation is even more urgent in India.  Huge destruction of primary forests in the name of infrastructural projects in the country needs to stop. 

This is an open access paper under the Creative Commons Attribution 4. International License