Summer carbon dioxide and water vapor fluxes across a range of northern peatlands

Sherpa Romeo green journal. Permission to archive final published versionNorthern peatlands are a diverse group of ecosystems varying along a continuum of hydrological, chemical, and vegetation gradients. These ecosystems contain about one third of the global soil carbon pool, but it is uncertain how carbon and water cycling processes and response to climate change differ among peatland types. This study examines midsummer CO2 and H2O fluxes measured using the eddy covariance technique above seven northern peatlands including a low-shrub bog, two open poor fens, two wooded moderately rich fens, and two open extreme-rich fens. Gross ecosystem production and ecosystem respiration correlated positively with vegetation indices and with each other. Consequently, 24-hour net ecosystem CO2 exchange was similar among most of the sites (an average net carbon sink of 1.5 ± 0.2 g C m 2 d 1) despite large differences in water table depth, water chemistry, and plant communities. Evapotranspiration was primarily radiatively driven at all sites but a decline in surface conductance with increasing water vapor deficit indicated physiological restrictions to transpiration, particularly at the peatlands with woody vegetation and less at the peatlands with 100% Sphagnum cover. Despite these differences, midday evapotranspiration ranged only from 0.21 to 0.34 mm h 1 owing to compensation among the factors controlling evapotranspiration. Water use efficiency varied among sites primarily as a result of differences in productivity and plant functional type. Although peatland classification includes a great variety of ecosystem characteristics,peatland type may not be an effective way to predict the magnitude and characteristics of midsummer CO2 and water vapor exchanges.Ye

Evaporation from natural nonsturated surfaces

Evaporation from nonsaturated surfaces is investigated. The concept of potential evaporation is first examined; a series of definitions are developed and classified, and it is shown that the relationship between potential and actual evaporation rates depends on the controlling variables in the chosen definition for potential evaporation. Extending the work of Penman (1948) to the unsaturated case, a general equation is derived to describe evaporation from nonsaturated surfaces. Applying Bouchet's (1963) hypothesis with a consistent set of definitions also leads to the same general equation. To account for departures from the saturated condition, the equation makes use of the concept of relative evaporation, the ratio of the actual evaporation rate to the rate which would occur under the prevailing atmospheric conditions if the surface was saturated at the actual surface temperature. A relationship is established between the relative evaporation, G, and a dimensionless parameter called the relative drying power, D, the ratio of the drying power (the evaporation rate which would occur if the surface was saturated at the actual air temperature) to the sum of the drying power and the net available energy. The relationship is non-dimensional and appears to be single-valued. An experimental investigation of evaporation from bare soil and growing wheat is carried out; data from an energy balance installation show that the G-D estimates of evaporation are in close agreement with calculations obtained using the Bowen ratio approach. The data are also used to refine the relationship between relative evaporation and relative drying power. The G-D estimates of evaporation also agree closely with independent estimates obtained from the soil water balance at three sites during the growing season. The combination of this G-D relationship with the derived general evaporation equation constitutes a simple model for obtaining estimates of evaporation from nonsaturated surfaces; no prior estimate of the potential evaporation is required, and the surface conditions of temperature and humidity need not be known. A preliminary relationship is found for the vapour transfer coefficient (used in a Dalton type transfer equation) for daily time periods. Algorithms are presented for the estimation of daily net radiation and for soil heat flux. A new approach is proposed for the application of remotely-sensed surface temperature data to the estimate of regional evaporation. A relationship is derived between the surface temperature and the vapour pressure deficit in the air. The relationship allows for the use of remotely-sensed data in evaporation models such as that presented above. The method is shown to provide superior results to the simplified energy balance approach currently being applied