Hydroclimatic controls and interaction-driven sensitivity of reference evapotranspiration across Indian climate regimes

Reference evapotranspiration (ET₀) represents atmospheric evaporative demand and is a key indicator of land–atmosphere interactions under climate variability and warming. However, how the sensitivity of ET₀ to its climatic drivers evolves across hydroclimatic gradients remains insufficiently quantified. Here, we examine regime-dependent sensitivity and interaction effects controlling ET₀ across five contrasting hydroclimatic regions of India (per-humid to arid) using long-term observations within the FAO-56 Penman–Monteith framework. A combined sensitivity approach integrating local perturbation analysis with global methods (Morris screening and Sobol’ variance decomposition) was employed to quantify both independent and interaction-driven effects of key meteorological variables. Results reveal a systematic transition in the dominant controls of ET₀ across the hydroclimatic gradient. Solar radiation explains more than 80% of ET₀ variability in humid and per-humid regions, whereas its contribution declines to 35–40% in arid environments, where temperature, humidity, and wind exert stronger influence. Concurrently, nonlinear interaction effects intensify with climatic dryness, with interaction strength increasing from < 5% in humid regions to 20% in semi-arid and arid regimes, reflecting strong coupling among thermodynamic and aerodynamic processes. A three-stage transition in ET₀ control emerges: radiation-dominated regimes in humid climates, vapor-pressure-deficit-driven regimes in semi-arid regions, and coupled radiative–aerodynamic regimes in arid environments.