Moisture losses from river basins due to human activities like extensive irrigation and groundwater pumping is essential for assessing water availability — not accounting for them can result in more than 50 per cent overestimation in India’s river basins, a study reveals.
Evapotranspiration, or the process by which water evaporates and is used up by plants, is important for water availability.
Researchers Vimal Mishra and Anuj Prakash Kushwaha from the Indian Institute of Technology (IIT) Gandhinagar said that in India, human activities such as irrigation and groundwater pumping significantly increase evapotranspiration, but most models overlook the effects.
Human-induced evapotranspiration changes water availability by intensifying losses due to evapotranspiration, reducing groundwater storage and altering surface water dynamics, they said.
The study, published in the Journal of Geophysical Research: Atmospheres, estimated human-induced evapotranspiration across 12 river basins, including the Ganga, Brahmaputra and Indus, during 2003-2020.
Observations from the Gravity Recovery and Climate Experiment (GRACE) satellites and results from five hydrological models were analysed.
“Quantifying human-induced evapotranspiration (H-ET) is essential for evaluating water availability in regions with extensive irrigation and groundwater pumping," the authors wrote.
Human-induced evapotranspiration was found to be more prominent during the dry season, contributing 30-50 per cent of total evapotranspiration in irrigation-intensive basins.
Overall, the highest human-induced evapotranspiration occurred during the summer in Brahmani (seasonal river in Odisha), Pennar (Andhra Pradesh), Krishna, Mahanadi and Narmada, among others, the authors found.
Evapotranspiration due to human activities was also found to remain consistently high across most river basins studied, revealing the persistent human-induced pressure on water resources throughout the year.
Further, “Non-accounting for H-ET leads to considerable (more than 50 per cent) overestimation in total available water in the Indus, Mahi, and Pennar river basins, while 30-40 per cent overestimation (was calculated) in the Brahmani, Ganga, Godavari, Krishna, Tapi, Narmada, and Mahanadi basins," the team said.
Annual water availability was found to be the most substantially reduced in basins with a high irrigation intensity — having more than 50 per cent of agricultural land under irrigation — such as Krishna (64.7 per cent), Mahi (63.6 per cent), Godavari (57.6 per cent), and Pennar (74.6 per cent), due to intensive groundwater and surface water based irrigation.
The authors said the results highlighted the need to integrate irrigation intensity, groundwater dynamics, and human interventions into hydrological models to improve estimates of available water.