We present a reliable methodology to estimate the energy associated with the subaerial diffuse degassing of volcanic‐hydrothermal fluids. The fumaroles of 15 diffuse degassing structures (DDSs) located in eight volcanic systems in the world were sampled and analyzed. Furthermore, each area was measured for soil temperature gradients and for soil CO₂ fluxes. The results show that each hydrothermal or volcanic system is characterized by a typical source fluid which feeds both the fumaroles and diffuse degassing through the soil. Experimental data and the results of physical numerical modeling of the process demonstrate that the heat released by condensation of steam at depth is almost totally transferred by conduction in the uppermost part of the soil. A linear relationship is observed between the log of the steam/gas ratio measured in the fumaroles and the log of the ratio between soil thermal gradient and soil‐gas flux. The main parameter controlling this relation is the thermal conductivity of the soil (K_c). For each area, we computed the values of K_c which range from 0.4 to 2.3 W m⁻¹ °C⁻¹. Using the CO₂ soil fluxes as a tracer of the deep fluids, we estimated that the total heat released by steam condensation in the systems considered varies from 1 to 100 MW.

For the first time a physical model, constrained by monitoring data, is used to derive a quantitative estimate of the evolution in time of magmatic gases that enter a hydrothermal system of an active volcano. The site is Campi Flegrei (west of Naples, in Italy), a caldera that had a large ground inflation in 1982–1984 followed by 20 yr of subsidence. More recently the behavior of the system has changed: the magmatic component of fumaroles has increased, swarms of earthquakes are more frequent, and the ground has started a general uplifting trend, indicating that the hydrothermal system undergoes repeated injections of magmatic fluid. Physical simulations of the process show that total injected fluid masses are the same order of magnitude as those emitted during small to medium size volcanic eruptions, and their cumulative curve highlights a current period of increasing activity. Gas emission studies coupled with physical modeling can be extremely effective in predicting magmatic evolution and eruptive activity at volcanoes.