Controls on evapotranspiration in a west Siberian bog

Abstract

This study analyzed controls on evapotranspiration ( E ) from a western Siberian bog, from a perspective that surface constraints limit the E available for atmospheric evapotranspiration demands. Ratios of E to potential evapotranspiration ( E P ) ranged from 0.2 to 0.9, and were clearly related to changes in surface constraints represented by the bulk transfer coefficient for latent heat C E (= β C H where β is the surface moisture availability and C H is the bulk transfer coefficient for sensible heat). Both E P and equilibrium evaporation ( E EQ ) showed similar seasonal trends, suggesting the importance of radiation to the seasonal variation of E P . The atmospheric drying power ( E a ) was a minor factor in E P and showed less seasonal change during most of the growing season. However, the presence of a dry air mass caused by synoptic scale advection (most frequently observed in May 1999) significantly enhanced E a ; consequently, the seasonal maximum of E P occurred earlier than the seasonal maximum of E EQ . Values for C H (0.004–0.011) increased with leaf area index except Sphagnum moss (LAI g ), indicating that vegetation growth contributes to changes in bog roughness through canopy height changes. The β value gradually decreased with decreases in the water table position ( z wt ); the open water surface area and water content of Sphagnum moss depended on z wt . Furthermore, the absence of a significant relationship between β and phenology implies that changes in evaporation contribute to variations in E more than changes in transpiration. Hence roughness change created by vegetation growth and surface wetness limit evapotranspiration to less than the potential evapotranspiration.