Seasonality of energy and water fluxes in a tropical moist forest in Vietnam

Agricultural and Forest Meteorology. 2021. Vol. 298–299. Article 108268.

Kuricheva A. O., Avilov V., Sandlersky Robert B., Dinh D., Kuznetsov A., Julia A. Kurbatova

This study presents the results of over 6 years (11/2011–12/2017) of energy and water flux measurements on the first eddy covariance site in Vietnam, in a mature tropical moist forest under monsoon climate. The dry season, lasting approximately four months, has strong interannual variations. Six-year average precipitation and evapotranspiration amounted to 2518 ± 253 mm and 1519 ± 105 mm per year, respectively, and net radiation total was 4897 ± 145 MJ m−2 per year. On average, the evapotranspiration increased from 3.64 ± 1.02 mm d−1 in a dry season to 4.65 ± 1.02 mm d−1 in a wet season, in parallel with a slight counterintuitive increase in net radiation, from 5.44 ± 1.20 mm d−1 to 5.70 ± 1.40 mm d−1 (equivalent water layer that evaporates using the relevant energy). In the driest months, evapotranspiration remained at a level of 60–80 mm per month in spite of nearly absent precipitation. Sensible heat increased in the second half of a dry season. The Bowen ratio was small in a wet season, increased to 1.0 in dry months, while it reached 2.2 in the anomalously hot conditions of April 2016. The average ratio of daily latent heat to daily net radiation in dry and wet season equalled to 68.9 ± 21.4% and 83.0 ± 8.5%, respectively. The energy balance closure in different years varied from 77.2 to 82.4% and was better in the hottest months. Canopy stomatal conductance fell sharply in hot-dry conditions and restricted evapotranspiration. Stomatal regulation of evapotranspiration occurred during three to five months depending on the severity of the dry season; however, in a wet season, evapotranspiration was fully dependent on net radiation and evapotranspiration occurred as a passive response to atmospheric demand.

Keywords: evapotranspiration, heat fluxes, Eddy covariance, тепловой поток, эвапотранспирация, метод турбулентных пульсаций