8. Potential and Actual Evapotranspiration in India

PET ranges from 1400 to 1800 mm/year in arid regions (like Rajasthan) and 900 to 1200 mm/year in humid eastern India.

Potential Evapotranspiration (PET) refers to the maximum amount of water that could evaporate and transpire from a specific area under ideal conditions, which means that there is sufficient moisture available. This value varies significantly across India due to different climatic conditions. In arid regions such as Rajasthan, where there is less moisture, the PET is considerably higher (1400 to 1800 mm/year) as plants can lose a lot of water due to high temperatures. In contrast, regions with more humidity, like the eastern parts of India, see a lower range of PET (900 to 1200 mm/year) because the atmosphere is already saturated with moisture.

Think of a sponge placed in a sunny area versus one in a shaded, humid environment. The sponge in the sun will 'dry out' more quickly, similar to how plants in arid regions lose water. Conversely, the sponge in the shaded area retains moisture longer, akin to plants in humid regions where there’s less demand for evaporation.

AET is typically lower than PET; it is determined by soil moisture and vegetation cover.

Actual Evapotranspiration (AET) represents the real amount of water that is actually evapo-transpirated from the soil and vegetation, which is usually less than PET. This discrepancy is caused by several factors, primarily the availability of soil moisture and the type of vegetation present in the area. When the soil is dry and the vegetation is stressed, plants cannot transpire as much water as under ideal conditions. Therefore, AET can fluctuate based on seasonal changes and vegetation health.

Imagine a garden during a dry spell. While the garden has the potential to grow and lose moisture like it did when it was well-watered, actual water loss may be minimal due to dry soil conditions. This situation represents AET being lower than PET because, like the garden, plants won't 'drink' or 'breathe' as much when there’s no water available.

AET varies seasonally and spatially; it is higher in summer months.

AET is not constant throughout the year; it changes with seasons and across different regions. During the summer months, temperatures rise, leading to increased evaporation and transpiration rates, which ultimately increases AET. Regions with abundant rainfall during summer will also see a spike in AET as the plants have enough soil moisture to transpire effectively. However, during rainy seasons, if the soil becomes waterlogged, AET might actually decrease because of anaerobic conditions which can limit root functioning.

Consider a bicycle rider who can only ride efficiently when the weather is nice and sunny. If it rains too much or if it's too dry, the rider may struggle to keep pedaling consistently, similar to plants which thrive best under specific weather conditions. Thus, AET peaks during optimal conditions – usually in summer.

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