Abstract
Total forest evaporation (λE), understorey evaporation, and environmental variables were measured on nine summer days under different weather conditions in a 130-year-old stand of Larix gmelinii (Rupr.) Rupr. trees located 160 km south of Yakutsk in eastern Siberia, Russia (61°N, 128°E, 300m above sea-level (a.s.l.)). Tree and broad-leaved understorey vegetation one-sided leaf area indices were 1.5 and 1.0, respectively. Agreement of λE and sensible heat flux (H), both measured by eddy covariance, and the available energy (Ra) was generally good: (H + λE) = 0.83 Ra + 9 W m−2 with r2 = 0.92 for 364 half-hour periods and the mean ± 95% confidence limit was 129 ± 17 for (H + λE) and 144 ± 19 for Ra. Daily E was 1.6–2.2 min, less than half of the potential evaporation rate and accounting for 31–50% of Ra, with the lowest percentage on clear days. A perusal of the sparse literature revealed that average daily E of boreal coniferous forest during the tree growing season (1.9 mm day−1 for this study) is relatively conservative, suggesting that low evaporation rates are a feature of this biome's energy balance. Using the Penman-Monteith equation, the maximum bulk-surface conductance (Gsmax) was 10 mm s−1. E and Gs were regulated by irradiance, air saturation deficit, and surface soil water content during a week-long dry period following 20 mm rainfall. From lysimeter measurements, 50% of E emanated from the understorey at a rate proportional to Ra. Based on the measurements and published climatological data, including average annual precipitation equal to 213 mm, water balance calculations indicated growing season forest E equal to 169 mm, the occurrence of a late summer-autumn soil water deficit, and annual runoff of 44 mm by snowmelt.
