The role of ground heat flux in estimating evapotranspiration by the Penman-Monteith method on mountain meadow

Ground heat flux (G) is often an overlooked component of the surface energy balance, and its accurate determination remains challenging. In the present study, the accuracy of various G estimation methods is examined using long-term measurements from the Central European mountain meadow. The impact of different G approximation on calculated evapotranspiration by the Penman-Monteith method (ET) is analysed. Soil heat flux measurements and surface temperature data were used to determine G, while net radiation was used to approximate G. Neglecting G led to an overestimation of ET in the daily timestep. On the contrary, the FAO-recommended hourly approximation overestimated G, underestimating ET. Site-specific calibrations of G prediction models improved their accuracy. For daily average G, as well as for hourly average G, simple constant parameter models (i.e., models including a single parameter specifying the fraction of net radiation directly) provide satisfactory accuracy of ET evaluation. However, in an hourly timestep, net radiation fails as a predictor of G shortly after sunrise and before sunset. The findings emphasise the importance of considering G in ET calculations and the need for site-specific calibrations of G estimation models.