Spatiotemporal Assessment of Evapotranspiration and Its Components in the Ahmedabad District, India: A Two-decadal Comparison Using Satellite Remote Sensing (2003-2023)

Abstract

Ahmedabad District, situated in the middle reaches of the semi-arid Sabarmati Basin, has undergone rapid urbanisation, groundwater over-abstraction, and changes in cropping intensity over the past two decades. Quantifying how these transformations have altered evapotranspiration (ET) and its partitioning among soil evaporation (E), interception (I), and canopy transpiration (T) is critical for district-scale water-resource planning. This study uses the operational SSEBop V6.1 monthly actual evapotranspiration product (1 km resolution), together with a sequential remote-sensing partitioning approach in which transpiration and interception are estimated from satellite-derived vegetation and rainfall inputs, while soil evaporation is derived as a residual. Spatial and temporal patterns of ET, E, I, and T are compared for Ahmedabad District between the water years 2003 and 2023. The results indicate that annual total ET remained nearly unchanged (729.99 mm in 2003 versus 721.88 mm in 2023, a decrease of approximately 1.1%); however, the internal composition of ET changed markedly. Soil evaporation increased by 5.8% (534.0 mm to 564.9 mm), interception increased by 32.8% (11.3 mm to 14.9 mm), and canopy transpiration decreased by 23.1% (184.8 mm to 142.0 mm). The E/ET ratio rose from 73.1% to 78.3%, while the T/ET ratio declined from 25.3% to 19.7%, indicating a shift towards direct, non-productive water loss. The monthly pattern also became less pronounced, with substantially higher winter values and lower monsoon peaks in 2023 than in 2003. These findings suggest a transition towards a more evaporation-dominated hydrological regime, although part of the transpiration signal may reflect known biases in the absolute ETₐ magnitudes reported for SSEBop and therefore warrants further validation. The study demonstrates the value of ET partitioning for diagnosing changes in water productivity in rapidly urbanising semi-arid districts.