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Assessing the Water and Energy Balances at the BERMS Flux Towers, 1999 to 2005

SOBS. H02. SOJP. H94. H75. Black Spruce Fen. Fen. Aspen Black Spruce Jack Pine. Black Spruce Jack Pine Streamflow.

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Assessing the Water and Energy Balances at the BERMS Flux Towers, 1999 to 2005

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  1. SOBS H02 SOJP H94 H75 Black Spruce Fen Fen Aspen Black Spruce Jack Pine Black Spruce Jack Pine Streamflow Fig. 8. A comparison of annual runoff at the SOJP and SOBS flux towers and gauged streamflow from the White Gull basin. The estimates are for the Sept-Aug hydrologic year. Fig. 3. The seasonal cycle of the surface energy balance (14-d means). Fig. 5. Annual vertical water balance estimates, integrated over the Sept-Aug hydrologic year. Aspen Black Spruce Jack Pine Fig. 6. Cumulative P-E* at the BERMS mature forest sites. Fig. 4 The energy closure-fraction as a function of the evaporative fraction (14-d means). Assessing the Water and Energy Balances at the BERMS Flux Towers, 1999 to 2005 A. G. Barr 1, G. van der Kamp 1, T. A. Black 2, J. H. McCaughey 3, R. Granger 1, N. Hedstrom 1, K. Morgenstern 2 and Z. Nesic 2 1 Science and Technology Branch, Environment Canada; 2 Agroecology, U.B.C.; 3 Geography, Queen’s U. • Introduction • The Boreal Ecosystem Research and Monitoring Sites (BERMS) study area is located in central SK, Canada, near the southern limit of the boreal forest. The 1999-2005 period includes three extreme drought years (2001-2003) followed by two extreme wet years (2004-2005). This study: • analyses the water and energy balances at the three mature forest and Fen sites through and following the drought; • examines the relationship between energy-and water-balance closure by comparing stand-level water balances with gauged streamflow. Surface Energy Balance The surface energy balance may be written as: Rn = H + λE + Q (1) Energy imbalances occur when the left and right hand sides of (1) do not balance. We defined the energy-closure fraction CF and the energy imbalance  as: CF = (H + E) / (Rn – Q) (2)  = Rn – Q – H - E (3) Fig. 3 shows the surface energy balance at the three BERMS mature forest sites for 2000 to 2004. The most striking features of Fig. 3 are the contrasts in the seasonal cycles of H and E among sites and years. Note the general seasonal lag in Evis-à-vis H, the dominance of H at the conifer sites, and the suppression of E at SOA during the later years of the 2001-2003 severe drought. Surface Water Balance Runoff R was estimated as: R = P – E* - S (4) where P is measured precipitation, E*is the energy-closure adjusted value of E (= E / CF), and S is the rate of change of vertical water storage, inferred from soil water and water table depth. Fig. 5 plots annual estimates of the terms in (4) at the BERMS forest sites. Note the severity of the 2001-2003 drought at SOA, the highP in 2004 and 2005, the lack of P at SOA vis-à-vis SOBS and SOJP, the similarity in annual E from SOA and SOBS, and the high inter-annual variability in P - E*, S and R. Fig. 7 compares the water balance of the BERMS SOBS and Fen sites. Note the dynamic S and R terms at the Fen. The Fen acts as a water reservoir. Water from the surrounding uplands flows into the Fen in wet years, mostly via the groundwater. In dry years the flow reverses and the water storage in the Fen is depleted. Surface runoff occurs when the water table in the Fen rises above the peat surface. The SOBS and SOJP flux towers lie within the White Gull watershed (Fig. 2). Fig. 8 compares the local estimates of R at SOBS and SOJP from (4) with measured streamflow from the White Gull basin. The agreement validates the application of energy-closure adjustments to E prior to the calculation of R in (4), as shown in Table 2. Fig. 1. BERMS study area Table 2. The impact of energy-closure adjustments to E on the mean annual estimates for R at the BERMS flux towers, 1999 to 2005, in comparison with gauged streamflow. Fig. 2.White Gull watershed (629 km2) in the eastern half of the BERMS study area. The + signs show the BERMS flux towers and the yellow arrow shows the streamflow station. The watershed is dominated by black spruce (41%), wetlands (22%), aspen (17%) and jack pine (12%). Sites and Measurements Table 1 summarizes the measurements of the energy and water balances at the three BERMS mature forest (SOA Old Aspen, SOBS Old Black Spruce, SOJP Old Jack Pine) and Fen sites. • Summary and Conclusions • Boreal deciduous and coniferous forest stands had contrasting seasonal cycles of H and E. The contrast was diminished by drought, which deeply depleted soil water at the aspen site. • The ratio of runoff to precipitation was ~ 20% for the two coniferous stands and the Fen, and ~ 5% for the deciduous stand, showing a fundamental difference in water use. The wetlands acted as water reservoirs for the surrounding landscape. • Independent assessment of the water and energy balances confirmed the importance of energy-closure adjustments to H and E. Without a closure adjustment to E, runoff was seriously overestimated at all sites. Fig. 4 plots the energy-closure fraction CF as a function of the evaporative fraction EF (defined as E /(Rn-Q)). CF was less than one at all values of EF, with a weak dependency on EF at SOA but not the other sites. The mean CF values were 0.89 (SOA), 0.84 (SOBS) and 0.85 (SOJP). Table 1. Measurements and instrumentation. Fig. 6 plots cumulative P - E*. The large water deficit at SOA during the 2001-2003 drought caused a deep depletion of soil water. At SOBS and SOJP, there was a water excess in all years. Fig. 7. Annual vertical water balance, Sept to Aug, at the SOBS and Fen sites. Acknowledgements We gratefully acknowledge the work of Joe Eley, Dell Bayne, Charmaine Hrynkiw, Erin Thompson, Alison Theede and Steve Enns, who oversaw the meteorological measurements and data management; Andrew Sauter, Rick Ketler, Don Zuiker and Sheila McQueen, who provided laboratory, field and data management support for the flux measurements; and Barry Goodison and Bob Stewart, who championed the BERMS program. Financial support was provided by the Climate Research Branch of the Meteorological Service of Canada, the Canadian Forest Service, Parks Canada, the Action Plan 2000 on Climate Change, the Program of Energy Research and Development, the Climate Change Action Fund, the Natural Sciences and Engineering Research Council of Canada, the Canadian Foundation for Climate and Atmospheric Science, BioCap Canada, and the National Aeronautic and Space Agency.

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