Using the Complementary Relationship Between Actual and Potential Evaporation to Diagnose the Onset of Heatwaves

Prolonged periods of extremely high temperatures with lack of precipitation mark heatwaves that pose health risks and damage ecosystems via rapid soil water depletion and reduced evaporative cooling. Identifying the conditions for the onset of heatwaves and their effects on land-atmosphere energy and mass fluxes remains a challenge. We propose using the generalized complementary relationship (CR) to estimate actual evaporation from heterogeneous landscapes overlain by different vegetation types (i.e., grasslands and forests) and quantify responses to radiation and air temperature anomalies. A strong correlation between air temperature and sensible heat flux anomalies deduced from FLUXNET data suggests that abrupt exceedances of sensible heat flux above climatological means are indicators for the onset of heatwaves. We capitalize on the coupling between latent and sensible heat fluxes and their links to soil moisture availability within the CR framework to predict anomalous increases in regional sensible heat flux associated with soil water depletion (low precipitation) and extreme evaporative demand (hot air and high radiation). The systematic and energy-constrained framework based on the CR concept provides insights into the triggering and feedbacks associated with heatwaves and hydro-climatic extremes such as regional droughts.