Synoptic controls on summer evapotranspiration from a bog peatland in southern Canada

Abstract

The relationship between surface hydrometeorological variables and mid-tropospheric circulation was used to describe and model the synoptic controls on daily and monthly evapotranspiration (ET) and potential evapotranspiration (PET) measured above a peat bog near Ottawa, Ontario. The four most dominant modes of variability in daily 500 hPa geopotential height over eastern North America were defined by S-mode principal component analysis and subjected to K-means clustering in order to produce discrete patterns of 500 hPa height. A total of 10 clusters were retained, based on a compromise between the number of legitimate patterns defined in a separate manual classification scheme and the proportion of variance grouped between clusters (61%) by the K-means algorithm.Height patterns defined by the 10-cluster scheme exhibited differences in their control over bog ET. Pre-high anticyclonic conditions were most efficient at drying the bog, whereas conditions prior to and during the passage of cyclonic systems were the least efficient at drying the bog. The remaining seven clusters exhibited insignificant control over ET on a seasonal basis. Variability in heights directly over the study site, associated with the second principal component, exhibited the closest relationship with hydrometeorological fluxes at the bog surface. However, attempts to model daily ET based, first, on multiple linear regression using principal components and, second, on cluster analogues of ET produced unsatisfactory skill. The inability to forecast daily ET was attributed to the influence of non-linear variation in the rate at which mid-tropospheric circulation controls propagated down to the surface and also to unresolved hydrological controls at the bog surface. Days following rainfall frequently resulted in amplified ET, regardless of cluster type, indicating that the control of ET by antecedent moisture conditions was a limiting factor in downscaling ET from large-scale atmospheric predictors. On a monthly time scale, however, the circulation indices explained 64% and 45% of the variation in monthly ET and PET respectively.

Effects of Serial Dependence and Large-Scale Tropospheric Circulation on Midlatitude North American Terrestrial Carbon Dioxide Exchange

“…These modes consist of regimes, described by Stocker et al (2001, p. 435) as local maxima in the probability density function of atmospheric fields, and abrupt shifts in those regimes, such as the Pacific decadal oscillation (PDO) or El Niño-Southern Oscillation (ENSO). Such modes may directly affect regional temperatures through advection, adiabatic expansion, and compression (Wallace et al 1996), and affect regional hydrology through alteration of atmospheric moisture transport (Liu et al 2004;Dominguez and Kumar 2005), stormtrack trajectories (Rogers 1997;Favre and Gershunov 2006;Wang et al 2006), and evaporation (Hember et al 2005).…”

Estimating evapotranspiration from small watersheds using a water and energy balance approach

“…Furthermore, multiple linear regression (MLR) analysis was employed to evaluate the order of importance of the meteorological and soil water factors involved and finally to develop equations for the calculation of AET. MLR of evaporation on the relevant meteorological and environmental factors has been attempted in the past (Shih, 1984;Pook et al, 1991;Di Bella et al, 2000;Hamber et al, 2005;Cleverly et al, 2006), and different conclusions have been drawn, depending on the meteorological and environmental parameters measured and the climatic region in which the measurements were made. The development of suitable means for estimating AET in a semiarid watershed will lead to better management decisions related to water use.…”

Peatland evaporation across hemispheres: contrasting controls and sensitivity to climate warming driven by plant functional types

“…Mer Bleue bog is a 28 km 2 ombrotrophic peatland (45.411°N, −75.481°E) located near Ottawa, Canada, in a cool continental climate. Mer Bleue has many features in common with boreal peatlands but is located near the south-ern limit of the boreal climate zone (Hember et al, 2005). This peatland was initiated 8400 years ago as a fen, transitioning into a bog around 7100-6800 years ago .…”