The seasonal water and energy exchange above and within a boreal aspen forest P.D. Blanken a, * , T.A. Black b , H.H. Neumann c , G. den Hartog c , P.C. Yang b , Z. Nesic b , X. Lee d a Department of Geography and Program in Environmental Studies, University of Colorado, 260 UCB, 110 Guggenheim, Boulder, CO 80309 0260, USA b University of British Columbia, Vancouver, BC, Canada c Meteorological Service of Canada, Downsview, Ont., Canada d Yale University, New Haven, CT, USA Received 21 August 2000; revised 10 January 2001; accepted 2 February 2001 Abstract The seasonal water and energy exchange of a boreal aspen forest underlain by a hazelnut understory is described. Measure- ments of above-aspen latent and sensible heat, short-wave and net radiation, and photosynthetically active radiation are compared to those measured above the hazelnut understory. Understory radiation measurements were made using a tram system. Energy storage at each measurement height was determined, and measurements of the soil moisture, temperature, and heat ¯ux were made using an array of probes. The mean annual air temperature and total precipitation during 1994 were 1.28C and 488.4 mm, respectively, above the 1951±1980 average 20.28C and total 462.6 mm. There was a pronounced seasonal development of leaves, with the maximum leaf area index of the hazelnut 3.3 m 2 m 22 ) exceeding that of the aspen 2.3 m 2 m 22 ). Beneath-aspen radiation decreased exponentially as the aspen leaf area increased, and the calculated effective extinction coef®cients decreased as the plant area index increased. At full aspen leaf, 27, 23, and 20% of the above-aspen short-wave, net, and photosynthetically active radiation, respectively, reached the hazelnut. The diurnal energy balance at both heights showed pronounced seasonal trends. Sensible heat from the forest ¯oor dominated during the leaf-free period, whereas latent heat from the overstory dominated during the leafed period. The fraction of the annual precipitation evaporated was 82±91%, with 67±68%, 26±28%, and 4±7% originating from the aspen, hazelnut, and soil, respectively. Over the leafed period, soil water was depleted from the root zone 0±60 cm depth) and accumulated between the 61±123 cm depth, overall resulting in a de®cit of 34.7 mm between 0±123 cm depths. This soil water balance compared well with the daily integrated difference between precipitation and eddy-covariance deter- mined measurements of evaporation. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Biometeorology; Boreal forest; Deciduous forest; Transpiration; Radiation; Surface energy balance 1. Introduction The boreal forest's size, role in contemporary north- ern hemisphere climatology and global carbon cycle, and sensitivity to climatic change, were all reasons given to initiate the boreal ecosystem±atmosphere Journal of Hydrology 245 2001) 118±136 www.elsevier.com/locate/jhydrol 0022-1694/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S0022-169401)00343-2 * Corresponding author. Tel.: 11-303-492-5887; fax: 11-303- 492-7501. E-mail address: blanken@spot.colorado.edu P.D. Blanken).