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.

AcknowledgementsWe 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.

IntroductionThe 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:

1. analyses the water and energy balances at the three mature forest and Fen sites through and following the drought;

2. 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 E vis-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.

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).

Summary and Conclusions1. 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.

2. 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.

3. 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.

Measurements Methods and InstrumentsSensible and latent heat fluxes H and E

Eddy-covariance (closed-path LICOR LI6262 IRGA at forest sites and open-path LI7500 at Fen; Gill R2/R3 sonic anemometers at SOA and SOBS and CSI CSAT3 sonic anemometers at SOJP and Fen). The surface fluxes were calculated as the sum of the eddy fluxes, measured at twice the canopy height, and the rate of change of storage in the air layer below the EC measurement.

Net radiation flux RnPaired Kipp and Zonen CM11 pyranometers and Eppley PIR pyrgeometers; Kipp and Zonen CNR1 net radiometer at Fen.

Surface energy storage flux Q

Sum of the soil heat flux (measured using Middleton soil heat flux plates, corrected for storage in the soil layer above the plates), biomass heat storage flux (tree bole thermocouples) and photosynthetic energy flux (computed from eddy-covariance CO2 flux).

Precipitation P Belfort 3000 accumulating gauge.

Soil water ESI Moisture Point TDR probes at SOA (0 to 1.2 m depth); Campbell Scientific CS615 probes at SOBS (0 to 0.6 m) and SOJP (0 to 1.5 m).

Water table depth Druck piezometers. At SOA only, the measurement was 1 km from the flux tower.

Fig. 4 The energy closure-fraction as a function of the evaporative fraction (14-d means).

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.

Fig. 3. The seasonal cycle of the surface energy balance (14-d means).

Site R = P - E - S(mm y-1)

R = P - E* - S(mm y-1)

Streamflow(mm y-1)

SOA 58 19 (low)

SOBS 137 84 78

(White Gull)SOJP 134 91

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. 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%).

SOBS

SOJPH02

H94

H75

Fen

Aspen Black Spruce Jack Pine

Fig. 5. Annual vertical water balance estimates, integrated over the Sept-Aug hydrologic year.

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.

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 high P 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. 6. Cumulative P-E* at the BERMS mature forest sites.

Aspen Black Spruce Jack Pine

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.

Black Spruce Jack Pine Streamflow

Black Spruce Fen

Sites and MeasurementsTable 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.

Fig. 1. BERMS study area

Table 1. Measurements and instrumentation.