Snow ablation in an open field and larch forest of the southern mountainous region of eastern Siberia

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1 Hydrological Sciences Journal ISSN: (Print) (Online) Journal homepage: Snow ablation in an open field and larch forest of the southern mountainous region of eastern Siberia KAZUYOSHI SUZUKI, JUMPEI KUBOTA, YINSHENG ZHANG, TSUTOMU KADOTA, TETSUO OHATA & VALERY VUGLINSKY To cite this article: KAZUYOSHI SUZUKI, JUMPEI KUBOTA, YINSHENG ZHANG, TSUTOMU KADOTA, TETSUO OHATA & VALERY VUGLINSKY (2006) Snow ablation in an open field and larch forest of the southern mountainous region of eastern Siberia, Hydrological Sciences Journal, 51:3, , DOI: /hysj To link to this article: Published online: 19 Jan Submit your article to this journal Article views: 239 View related articles Citing articles: 11 View citing articles Full Terms & Conditions of access and use can be found at

2 Hydrological Sciences Journal des Sciences Hydrologiques, 51(3) June Snow ablation in an open field and larch forest of the southern mountainous region of eastern Siberia KAZUYOSHI SUZUKI 1, JUMPEI KUBOTA 2, YINSHENG ZHANG 1, TSUTOMU KADOTA 1, TETSUO OHATA 1 & VALERY VUGLINSKY 3 1 Hydrological Cycle Observational Research Program, Institute of Observational Research for Global Change, JAMSTEC, 2-15 Natsushima-cho, Yokosuka, Kanagawa , Japan skazu@jamstec.go.jp 2 Research Institute for Humanity and Nature, Kyoto, Japan 3 State Hydrological Institute, St Petersburg, Russian Federation Abstract The southern mountainous taiga region of eastern Siberia is the runoff source area of the basins of the rivers Lena and Amur, where snowmelt discharge is an important hydrological process. To evaluate the effect of the sparse larch forest canopy on snow ablation and energy balance in the snowpack, meteorological conditions and snow ablation were observed in a larch forest (LF) and an open field (OP). At the beginning of snowmelt, the snow water equivalent was 54.4 and 95.5 mm at OP and LF, respectively. The snow disappeared at LF three days later than at OP. Sublimation accounted for about 8% of snow ablation at both sites from 1 April to 5 May 2002, the snowmelt period. The energy balance of the snowpack at the two sites was dominated by the net all-wave radiation onto the snow surface. The difference in snowmelt between the sites was primarily caused by a difference in the net all-wave radiation. Snow surface albedo correlated with snow surface density for densities from 150 to 350 kg m -3 at both sites. Key words net all-wave radiation; Siberia snow ablation; snow surface albedo; snowmelt; sublimation; taiga Ablation de la neige à découvert et sous mélézin dans la région montagneuse sud de la Sibérie orientale Résumé La région de taïga montagneuse méridionale de la Sibérie orientale est la zone d origine des écoulements des bassins des Fleuves Lena et Amour, zone dans laquelle le débit de fonte des neiges est un processus hydrologique important. Pour évaluer les effets du maigre couvert de mélèze sur l ablation de la neige et sur le bilan énergétique dans le couvert neigeux, les conditions météorologiques et l ablation de la neige ont été observées dans une forêt de mélèzes (FM) et hors forêt (HF). Au début de la fonte des neiges, l équivalent en eau était respectivement de 54.4 et 95.5 mm en HF et FM. La neige a disparu en FM trois jours plus tard qu en HF. Entre le 1er avril et le 5 mai 2002, période de la fonte des neiges, la sublimation a représenté environ 8% de l ablation de la neige. Le bilan énergétique du couvert neigeux a été dominé, dans les deux sites, par le rayonnement net total à la surface de la neige. La différence de fonte entre les sites a été principalement causée par une différence de rayonnement net total. L albédo de la surface de la neige a été corrélé avec la densité de la surface de la neige, pour des densités variant entre 150 et 350 kg m -3, pour les deux sites. Mots clefs rayonnement net total; Sibérie; ablation de la neige; albédo de surface de la neige; fonte des neiges; sublimation; taïga INTRODUCTION In a study of the runoff source area of the Lena River basin, Ma et al. (2000) showed that most of the river water comes from the southern mountainous taiga region of eastern Siberia and that the maximum discharge of the Lena River results from snowmelt. Hence, it is important to investigate snow ablation there. This region is covered by forest consisting mainly of larch (Larix spp.). Most studies on the effects of water and energy balance on snow ablation under a forest canopy (e.g. Hardy et al., 1997; Link & Marks, 1997; Pomeroy & Granger, 1997; Suzuki et al., 1999; Giesbrecht & Woo, 2000; Woo et al., 2000; Koivusalo & Kokkonen, 2002) have been carried out in North America, northern Europe and Open for discussion until 1 December 2006

3 466 Kazuyoshi Suzuki et al. northern Japan. Although Siberia, which is located in northern Eurasia, has the largest permafrost area on Earth and seasonal snow cover, few studies on snow ablation in the Siberian region have been reported. In the southern mountainous taiga region of eastern Siberia, in particular, Vasilenko (2004) described the properties of the water and energy balance in an open field of the Mogot experimental watershed using a profile method with manually observed meteorological data. However, most of the land surface in eastern Siberia is covered by larch forest. Thus, it is necessary to evaluate the effect of the larch forest canopy on snow ablation. Suzuki et al. (1999) showed that the most important factor leading to a difference in snowmelt between open areas and dense larch forest is the difference in sensible heat flux in the cooltemperate climate of Japan, using a snowmelt model developed by Kondo & Yamazaki (1990). However, unlike in cool-temperate Japan, the cold climate in eastern Siberia would also be expected to affect snow ablation. Net all-wave radiation is the most important energy balance component of snow ablation at high latitudes. Suzuki & Ohta (2003) developed a simple forest snowmelt model to describe the effects of snow surface albedo on net all-wave radiation and snowmelt energy. They found that when the initial snow surface albedo is low (<0.6), snowmelt energy decreases as forest density increases. However, when the initial snow surface albedo is high (>0.6), only a slight decrease in snowmelt energy occurs as forest density increases, because net all-wave radiation increases with the increase in forest density. Sokolov & Vuglinsky (1997) reported that the snow surface albedo in the southern mountainous region of eastern Siberia is high. However, the precise way in which snow surface albedo affects net all-wave radiation in larch forest and open sites in eastern Siberia is not well understood. The goals of this study were to describe the properties of the water and energy balances and their effect on snow ablation at an open and a forested site in the southern mountainous taiga of eastern Siberia. To accomplish these goals, an intensive field campaign was carried out. This project was undertaken collaboratively by the Frontier Observational Research System for Global Change (FORSGC) of Japan and the State Hydrological Institute (SHI) of Russia. Observed and simulated data on snowmelt, snow sublimation and energy balance obtained during spring 2002 are presented. METHODOLOGY Site description The study area is located in the southern mountains of eastern Siberia (55 36 N, E), approximately 60 km north of Tynda in the Amur region of Russia (Fig. 1). The study area is in the Nelka River catchment, which is about 12 km long and 2.5 km wide, with a total area of approximately 30.8 km 2. The slopes of the main valley face northeast and southwest, and the altitude of the surrounding ridges ranges from approximately 550 to 1150 m. The land surface is covered predominantly by larch forest (Larix cajanderi). The forest floor is covered by about a 10-cm thickness of true mosses (Aulacomnium turgidum, Cetraria cucullata) and lichen (Cladina arbuscula). Continuous permafrost, about m thick, is present throughout the watershed. Annual mean air temperature is around 7.7 C, and annual precipitation is from 500 to 600 mm.

4 Snow ablation in the southern mountainous region of eastern Siberia 467 Fig. 1 Location of the Intensive Observation Sites, OP and LF. Two sites in the bottom of the main valley, one in the forest (LF) and one in an open field (OP), were selected to observe meteorological conditions. Site LF is covered by a sparse larch forest, and site OP is grassland (during the snow cover period, all grass was covered by snow). The plant area index (PAI), estimated from photographs taken with a fisheye lens, was 0.4 at the LF site and 0.0 at the OP site. At the LF site, measurements were also taken of forest stand density and mean tree height, which were 4128 trees ha -1 and 4.3 m, respectively, and the crown projection area (Fig. 2). Furthermore, an 18.6-m tower was installed at the LF site to measure meteorological elements above and within the forest canopy. In 2001, larch leaves (needles) emerged at the beginning of June and fell in the middle of September, so the larch branches were free of leaves during the spring study period. In order to observe spatial variability of the snow cover at the LF site, a 50 m 50 m grid composed of 25 (10 m 10 m) grid cells was established. Measurements Precipitation was measured with a Tretyakov gauge twice a day at 08:00 and 20:00 h local time. The snow survey and snow depth measurements were performed manually at each site. Field studies were also conducted to examine the water and energy balances during the period of snow ablation, from 1 April 2002 to 11 May Meteorological parameters, including air temperature, relative humidity, wind speed and direction, incident and reflected shortwave radiation, net all-wave radiation, air

5 468 Kazuyoshi Suzuki et al. Fig. 2 Crown projection diagram showing meteorological and snow depth observation sites at the LF site. Small closed circles denote snow stick locations for determining the spatial variation in snow depth. The large grey circle denotes the meteorological observation sites at LF. Table 1 List of observed hydrometeorological elements and equipment used. The numbers in parentheses denote the observational height above or below the ground surface. Meteorological elements OP LF Net all-wave radiation REBS Q7 (1.3 m) REBS Q7 (1.4 m) Air temperature Vaisala HMP-45D (1.6 m) Vaisala HMP-45D (1.7 m) Relative humidity Vaisala HMP-45D (1.6 m) Vaisala HMP-45D (1.7 m) Shortwave radiation Silicon photodiode Silicon photodiode Hamamatsu (1.3 m) Hamamatsu (1.4 m) Soil temperature Thermocouple Thermocouple (0, 0.1, 0.2, 0.4, 0.6, 0.8, ( 0.2, 0.4 m) 1.0 m) Hakusan PT-100 ( 0.05, 0.15, 0.25, 0.35, 0.45, 0.55 m) Soil moisture None TRIME IMKO P2 sensor ( 0.1, 0.3, 0.5 m) Wind direction Makino WS-104 (1.6 m) None Wind speed Makino AC-750 (1.6 m) Makino AC-750 (1.7 m) Air pressure Vaisala PTB101B (0.5 m) None Soil heat flux EIKO MF-81 EIKO MF-81 Surface temperature OP: open field; LF: larch forest. (0 m) Everest GL (0.5 m) (0, 0.05 m) Everest GL (0.7 m)

6 is Snow ablation in the southern mountainous region of eastern Siberia 469 pressure, and surface temperature, were measured, for most variables every 10 min, by a data logger (CR-10X, Campbell and Datamark 3300, Hakusan) (Table 1). To examine the spatial distribution of snow water equivalent below the larch forest canopy at the LF site, 16 snow sticks (1.5 m long with 1-cm gradations) were installed around the tower site (Fig. 2) at the intersections of the grid lines. The snow sticks were read manually from 13 March to 13 April 2002 and from 28 April to 2 May For April 2002 and 3 May 2002 to the date of snow disappearance (5 May 2002 at OP and 8 May 2002 at LF) no data were available for snow water equivalent for either site. At site OP, spatial variations in snow water equivalent were not measured, because at that site snow water equivalent was almost uniform spatially. Snow density, snow grain size and snow temperature for each layer were measured in a snow pit at each site. Total snow density was measured by taking two snow samples at each site, using a snow sampler (50 cm 2 50 cm). The snow disappearance dates for the two sites were determined from photographs of ground surface conditions and daily surface temperature. Note that the snow measurements (snow depth and snow density) were performed only three times during the snowmelt period and then only at LF. Theory Lundberg & Halldin (2001) showed that when the closure of a forest canopy is less than 0.1, the ratio of snow interception to total precipitation above the canopy is also less than 0.1. In this study, the larch forest canopy was very sparse, and the canopy closure as determined from fisheye-lens photographs was less than 0.1. Thus, it was assumed that the precipitation loss due to snow interception was negligible. The energy balance at the snow surface is described by the formula: Q S = Q M + Q C = R N + H + LE + G (1) where Q S (W m -2 ) is the energy of the entire snowpack; Q M (W m -2 ) is the snowmelt energy; Q C (W m -2 ) is the internal energy change within the snowpack (the energy for freezing liquid water or the change of snow temperature within the snowpack); R N (W m -2 ) is the net all-wave radiation beneath the forest canopy; H (W m -2 ) the H sensible heat flux beneath the canopy; LE (W m -2 ) is the latent heat flux beneath the canopy; and G (W m -2 ) is the ground heat flux under the snowpack, which was measured by a heat flow plate. Net all-wave radiation under a forest canopy is: R N N N 4 ( 1 α) I + L + L = I + I + L - εσts = I + L = (2) where I N (W m -2 ) is the net shortwave radiation beneath the canopy; L N (W m -2 ) is the net longwave radiation beneath the canopy; α is the albedo on the snow surface, I (W I m -2 ) is the incident shortwave radiation beneath the canopy; I (W m -2 ) is the reflected shortwave radiation beneath the canopy; L (W m -2 ) is the downward longwave radiation beneath the canopy; L (W m -2 ) is the upward longwave radiation from the snow surface; ε is the emissivity of the snowpack ( 0.97), σ is the Stefan-Boltzmann constant; and T S (K) is the snow surface temperature. Here, the downward longwave radiation was not measured, but estimated using equation (2) and the measured values of I I, II and T S. Each energy component was defined as positive when its direction was towards the snowpack.

7 is 470 Kazuyoshi Suzuki et al. Sensible and latent heat fluxes beneath a larch forest canopy are described by the following equations, based on the bulk transfer method: and H p ( TZ TS ) U Z H = C c ρ (3) E ( qz qs ) U Z LE = LC ρ (4) where c p (J kg -1 K -1 ) is the specific heat of air; ρ (kg m -3 ) is the air density; C H and C E are the bulk transfer coefficients for sensible and latent heat fluxes, respectively; L (W m -2 ) the latent heat for sublimation; and T (K), U (m s -1 ), and q (kg kg -1 ) are air l temperature, wind speed and specific humidity, respectively. The subscripts Z and S denote the reference height and the snow surface, respectively. Verification of simulated snow ablation Continuous snowmelt or energy balance fluxes were not observed at the OP or LF sites. Thus, the physically-based snow process model SNTHERM (Jordan, 1991) was applied to estimate the energy balance of the snowpack and snow ablation for the entire snowmelt period. The input variables were air temperature, water vapour, wind speed, incident shortwave radiation, downward longwave radiation, snow surface albedo and ground heat flux. Downward longwave radiation was estimated by equation (2) using the measured surface temperature (T S ). Furthermore, at the LF site, daily mean values were used for the snow surface albedo because hourly snow albedo measurements sometimes exceeded 1, owing to shade from the canopy. Zhang et al. (2004) estimated the bulk transfer coefficient for latent heat flux (C E ) at this study site from the relationship between the observed sublimation from the snow surface, measured by microlysimeter, and meteorological elements. In the SNTHERM model, the values reported by Zhang et al. (2004) were used for the bulk transfer coefficients for sensible and latent heat fluxes. The outputs of the SNTHERM model are snow temperature, snow density, snow grain size in each layer, and water and energy balances within the snowpack. To verify the simulated energy balance, simulated and observed daily snow surface temperature and net all-wave radiation were compared at sites OP and LF (Table 2). The simulated results agreed well with the observed values, showing a clear linear relationship, for both parameters. Based on the RMSE of net all-wave radiation, the estimated error of downward longwave radiation was from 3 to 8 W m -2, because the mean difference in observed and estimated snow surface temperatures was less than 1.0 C, a difference that should be negligible for the estimation of upward longwave radiation. Table 2 Statistical information (square of the linear correlation coefficient, r 2, and root mean square error, RMSE) corresponding to the simulations. OP LF r 2 RMSE r 2 RMSE Surface snow temperature C C Net all-wave radiation W m W m -2

8 Snow ablation in the southern mountainous region of eastern Siberia 471 Fig. 3 Temporal variations in observed and simulated snow processes at the OP and LF sites. Straight and broken lines denote the simulated snow water equivalent at OP and LF, respectively. Open and closed circles denote the observed snow water equivalent at OP and LF, respectively. Vertical bars denote standard deviation in snow water equivalent at the LF site. Next, the temporal variations in simulated snow ablation were compared with observed snow ablation values at the OP and LF sites (Fig. 3). Here, vertical bars denote the standard deviations of the spatial variations in snow ablation among the 16 snow gauges at LF, whereas snow ablation at OP was observed at a single point. The simulated snow disappearance date agreed well with the observed date. However, it should be noted that almost all of the snow observations were done when the snow water equivalents were constant. Thus, melt computations based on the modelling had to be used, with detailed meteorological data as input. The simulated results of snow ablation were used to compare water and energy balances at the OP and LF sites because it was not possible to observe the main snowmelt events. RESULTS AND DISCUSSION Characteristics of snow ablation at the open and larch forest sites Figure 4(a) (d) describes the temporal variations in daily hydrometeorological conditions at the LF and OP sites during the intensive field campaign. The differences in air temperature and relative humidity between the sites were insignificant, but incident shortwave radiation and wind speed from 1 April to 11 May 2002, when snow cover disappeared from LF, were significantly different. Here, the hydrometeorological conditions and energy balance are described separately for the cold days (days with no snowmelt) and for the snowmelt days (days when snowmelt occurred) at the two sites from 1 April to 5 May 2002 (Table 3). On cold days, the mean air temperature was below freezing, and snow surface albedo was more than 0.7, at both sites. The mean incident shortwave radiation at LF was 68% of that at OP, and mean wind

9 472 Kazuyoshi Suzuki et al. Wind speed (m s -1 ) 3.0 OP (a) LF Air temperature ( o C) Incident short-wave radiation (W m -2 ) Water vapour pressure (hpa) /04/ /04/ /04/ /05/ /05/ /04/ /04/ /04/ /05/ /05/ /04/ /04/ /04/ /05/ /05/ (b) (c) (c) (d) /04/ /04/ /04/ /05/ /05/11 Date Fig. 4 Temporal variations in daily hydrometeorological variables at the OP and LF sites from 1 April 2002 to 11 May 2005: (a) wind speed, (b) air temperature, (c) water vapour pressure, and (d) incident shortwave radiation. Closed and open circles denote data from the OP and LF sites, respectively. speed at LF was 58% of that at OP during both periods. During the snowmelt period, air temperature, water vapour pressure and incident shortwave radiation were higher than during the cold period. In contrast, snow surface albedo decreased at both sites during the snowmelt period, and the difference between the two sites became larger, because background albedo under the shallow snow depth at OP increased the reduction in snow surface albedo at that site. Even on snowmelt days, mean air temperature at both sites was close to 0 C.

10 Snow ablation in the southern mountainous region of eastern Siberia 473 Table 3 Mean meteorological conditions during the snow ablation period from 1 April to 5 May 2002 for snowmelt days and cold days. Period Site Air temperature ( C) Water vapour pressure (hpa) Incident shortwave radiation (W m -2 ) Wind speed (m s -1 ) Snowmelt OP days LF Cold days OP LF Snow surface albedo Table 4 Comparisons of the snow ablation, snowmelt, sublimation and energy balance components of snow ablation from 1 April 2002 to 4 May 2005 for snowmelt days and cold days. Period Site Accumulated snow ablation (mm) Snowmelt Accumulated snowmelt (mm) Accumulated sublimation (mm) Energy for snowpack (W m -2 ) Net allwave radiation (W m -2 ) Sensible heat flux (W m -2 ) Latent heat flux (W m -2 ) days OP LF Cold days OP LF Ground heat flux (W m -2 ) Table 4 summarizes snow ablation, snowmelt and sublimation at the OP and LF sites from 1 April to 5 May 2002, separately for the snowmelt and cold periods. Snowmelt was defined as meltwater discharged from the bottom of the snowpack. Thus, if surface meltwater did not reach the bottom of the snowpack, no snowmelt event was recognized. Sublimation, which was overall an important component of snow ablation, became a minor factor during snowmelt periods. In general, snow ablation at OP was greater than that at LF during both periods. Figure 5 shows the temporal variations in estimated snowmelt and sublimation at the OP and LF sites. The initial snow water equivalent was 54.4 mm at OP and 95.5 mm at LF; thus, the initial snow water equivalent at OP was only 57% of that at LF. The wind may have blown snow off the OP site, causing this difference. On the other hand, the snow disappeared at the OP and LF sites at almost the same time, on 5 and 8 May 2002, respectively. Some surface snowmelt occurred on 8 April 2002 at both sites, but snowmelt water did not reach the bottom of the snowpack on that date. Snowmelt events occurred on 15 and 16 April 2002, but the main snowmelt period did not begin at either site until 25 April Snowmelt temporarily stopped on 30 April 2002, when a heavy snowfall occurred, and restarted on 2 May The maximum snowmelt (about 23 mm day -1 at both sites) occurred on 27 April 2002 at OP and on 6 May 2002 at LF. In general, the daily snowmelt value tended to be greater at OP than at LF whenever snowmelt occurred at both sites. Snowmelt events occurred on 12 days at OP and 14 days at LF. The snow disappearance date was three days later at LF than at OP; this lag was caused by the differences in the initial snow water equivalent and the snowmelt rate at the two sites. From 1 April to 8 May 2002, the maximum daily sublimation from the snowpack was greater at OP (>1 mm day -1 ) than at LF (0.6 mm day -1 ). Before 25 April 2002,

11 474 Kazuyoshi Suzuki et al. Snowmelt (mm d -1 ) (a) Sublimation (mm d -1 ) /4/1 2002/4/ /4/ /5/1 2002/5/ (b) /4/1 2002/4/ /4/ /5/1 2002/5/11 Date Fig. 5 Temporal variations in snow ablation processes at OP and LF during spring 2002: (a) snowmelt and (b) sublimation. Black and white bars denote sites OP and LF, respectively. Table 5 Comparison of sublimation in boreal and temperate larch forests under different climate regimes. Site Boreal larch forest, eastern Siberia Temperate sparse larch forest, northern Japan Sublimation (mm) Snow ablation (mm) Sublimation in relation to total snow ablation (%) Duration Reference April 4 May 2002 Present study March 26 April 1994 Suzuki et al. (1999) snow ablation was caused mainly by sublimation at both sites. The overall contribution of sublimation to snow ablation was about 8% at each site. Thus, sublimation is an important component of snow ablation in this region, especially at open sites. In contrast, the contribution of sublimation to snow ablation in a larch forest in temperate Japan (Suzuki et al., 1999) over a period of 33 days was only 0.4% (Table 5). Energy balances at two sites In the following, the components of the energy balances and their temporal variations at the two sites (Fig. 6) are studied. On cold days, ground heat fluxes at both sites were positive to the snowpack, and they varied diurnally at OP but not at LF. This result

12 Snow ablation in the southern mountainous region of eastern Siberia 475 (a) (b) Fig. 6 Temporal variations in energy balance components at (a) OP and (b) LF. agrees with the findings of a previous study by Granger & Male (1978). On the other hand, during the snowmelt days, there were large differences in ground heat fluxes between the two sites. It is assumed that the large ground heat fluxes into the soil increased the soil temperature and released the latent heat when meltwater refroze within the frozen surface soil, but no data on the refreezing of liquid snowmelt water in the frozen ground are available. Table 4 shows the mean energy balance components for the snowpacks at the OP and LF sites from 1 April to 5 May 2002, separately for the snowmelt and cold days. The main contributor to the energy balance at both sites was net all-wave radiation, especially during the snowmelt period, and temporal changes in net all-wave radiation corresponded to the temporal changes in snowmelt (Fig. 6). The difference in snowmelt between the sites was apparently caused primarily by the large difference in net all-wave radiation between the sites. During most of the snowmelt period covered in this study, sensible heat flux was small compared to net all-wave radiation, because the mean daily air temperature was close to freezing. In contrast, during the cold days, the energy of the snowpack at LF was greater than that

13 476 Kazuyoshi Suzuki et al. at OP, perhaps as a result of the high albedo and increased longwave radiation under the forest canopy, as described by Suzuki & Ohta (2003). Therefore, the dominant energy for snow ablation was latent heat flux; and the difference in energy between the sites during the cold period was caused by the higher energy at the LF site during the cold period, which reflects the higher net all-wave radiation at that site during that period. Thus, the difference in snow ablation between the OP and LF sites was caused by the differences in latent heat flux between the two sites during the cold period. Characteristics of the radiative balance at the open and larch forest sites The results obtained in this study showed that the main energy source contributing to snow ablation was net all-wave radiation and that the difference in snow ablation between the sites was caused by the difference in the net all-wave radiation. Here, the daily variations in net all-wave radiation at the OP and LF sites are discussed in more detail. The variations in net all-wave radiation between the sites were determined from observed radiation data, excluding data from days when snow fell. The incident shortwave radiation at OP was linearly related to that at LF (Fig. 7). However, the mean incident shortwave radiation at LF was 62% less than that at OP, suggesting that canopy shading was responsible for the reduction in shortwave radiation at LF, as shown by Suzuki et al. (1999). However, the mean net all-wave radiation during the cold days at LF was greater than that at OP (Table 3), indicating that emitted longwave radiation from the forest canopy caused the difference in net allwave radiation when the snow surface albedo was high. Daily net longwave radiation values were compared between the OP and LF sites during 1 27 April 2002, when snow completely covered the watershed, and were found to be linearly related (Fig. 8). Moreover, the slope of the regression line for netlongwave radiation was similar to that for downward longwave radiation between an open area and sparse larch forest with a similar PAI (Suzuki et al., 1999), suggesting that the slope can be explained by the difference in downward longwave radiation. Fig. 7 Relationship of daily incident shortwave radiation between the LF and OP sites. The long dashed line denotes the regression line.

14 Snow ablation in the southern mountainous region of eastern Siberia 477 Daily net long-wave radiation at LF site (W m -2 ) 0-20 y=0.54x-1.02 r 2 = y=x Daily net long-wave radiation at OP site (W m -2 ) Fig. 8 Comparison of daily net longwave radiation between LF and OP. When net longwave radiation at the OP site increased, such as on cloudy days, the difference between the OP and LF sites became small. Even on sunny days, an increase in net longwave radiation at the OP site caused a reduction in the difference between net all-wave radiation at the OP and LF sites. This result implies that, during the cold period, the longwave radiation emitted from the larch canopy contributed to an increase in downward longwave radiation onto the snow surface at the LF site and caused the increase in net all-wave radiation compared with that at the OP site. However, because longwave radiation was not measured, the role of emitted downward longwave radiation from the forest canopy could not be confirmed. Snow surface albedo was strongly correlated with snow surface density for densities between 150 and 350 kg m -3 at the LF and OP sites (Fig. 9); thus, snow surface density was one of the most important factors determining snow surface albedo. Note that the slopes of the regression lines for the LF and OP sites are similar, but the intercept value for OP is higher than that for LF. Melloh et al. (2001) showed that litter on the snow surface in a forest reduces snow surface albedo compared to that in an open field. It is possible that the smaller intercept value for LF resulted from the presence of litter on the snow surface at that site. Kojima (1979) described the relationship between snow surface albedo and snow surface density under various snow conditions in Sapporo, Hokkaido, Japan. Ohta (1994), using all of the data collected by Kojima (1979), calculated a linear equation representing the relationship between snow surface albedo and snow surface density (Fig. 9). The intercept of the equation reported by Ohta (1994) is lower than those of the regression lines at the two sites of this study, but the slope reported by Ohta (1994) is almost the same as those of this study. The lower intercept can be explained as an effect of soot or other contaminants within the snow, because Sapporo is in an urban zone. Warren (1982) suggested that snow surface density might depend on grain size, since density normally increases as grain size increases. Nakamura et al. (2001) showed that the snow grain size within the upper 3 cm of a snow layer is important for determining surface snow albedo. However, it is difficult to observe snow grain size

15 478 Kazuyoshi Suzuki et al. 1.0 y = r 2 = x + 1.0, Snow surface albedo y = r 2 = x + 1.0, y = x , Ohta(1994) Snow surface density (kg m -3 ) Fig. 9 Relationship between snow surface albedo and snow surface density for snow surface density from 150 to 350 kg m -3. Long-dashed, solid, and dashed-dot lines denote regression lines for OP, LF and Ohta (1994), respectively. with high resolution in the field. The liquid water content within a snow layer is also important in relation to snow density, but because liquid water content within a snowpack causes an increase in the snow grain size and snow density, it is difficult to isolate the effect of liquid water content. The liquid water content within the snow layer was not measured in this study, so its effect could not be evaluated. Further research will focus on the influence of litter coverage, snow grain size and liquid water content within the snow surface layer on snow surface albedo. Potential problems with the forcing data First, point radiation data were used to evaluate the energy balance in the snowpack below the larch canopy. However, point measurements of radiation at forested sites sometimes introduce error into water and energy balances determined using such measurements. Giesbrecht & Woo (2000) showed that downward radiation and snow ablation varied greatly in a subarctic spruce woodland. The one-point radiation measurement results of the present study would thus contain errors in the downward radiation values, but not enough data were obtained to determine the magnitude of this problem. This will be investigated in future research. Second, a heat flux plate was used under the snowpack to measure ground heat flux so that the SNTHERM model could be used to estimate snow processes. As snowmelt proceeded, the snow depth decreased significantly; thus, transmittance of solar radiation through the shallow snowpack and the surrounding patchy snow cover would affect the measurement of ground heat flux. Future studies should focus on how ground heat flux from or to the frozen ground affects snowmelt and sublimation.

16 Snow ablation in the southern mountainous region of eastern Siberia 479 CONCLUSIONS Hydrometeorological conditions were measured in the southern mountainous taiga region of eastern Siberia during spring Energy balances in the snowpack were estimated in an open field (OP) and at a forested site (LF) using a physically-based snow process model. The properties of snow ablation and energy balances within the snowpack were determined at the two sites as follows: 1. The snow water equivalent at the beginning of the study period was 54.4 mm at OP and 95.5 mm at LF. At the end of the snow ablation period, snow disappeared at OP three days earlier than at LF. 2. The contribution of sublimation to snow ablation at the OP and LF sites from 1 April to 5 May 2002 was about 8%. Therefore, sublimation is important for understanding snow ablation in this region during the snowmelt period. 3. The energy balance of snowmelt at the OP and LF sites was dominated by net allwave radiation onto the snow surface. Most of the difference in snow ablation between the two sites was caused by the difference in net all-wave radiation. 4. Snow surface albedo correlated with snow surface density for densities from 150 to 350 kg m -3 at both sites. The intercept of the regression line for the OP site was greater than that for the LF site. It was assumed that the litter coverage on snow surface at LF affected the difference in interception. Further research will focus on the parameterization of snow surface albedo and the emitted downward longwave radiation from the forest canopy. In addition, the spatial variability of incident radiation should be addressed in future studies to evaluate representative radiation values beneath a canopy. Acknowledgements This project was coordinated by the GEWEX Asian Monsoon Experiment (Prof. Tetsuzo Yasunari, Nagoya University, Japan). The authors acknowledge Prof. Lars Bengtsson (Lund University, Sweden) and an anonymous reviewer for suggesting revisions of a first draft, Dr Rachel Jordan of the Cold Regions Research Engineering Laboratory (USA), Dr Masuyoshi Matsuda of MTS Inc. (Japan), Dr Hiroyuki Hirashima of the Japanese National Research Institute for Earth Science and Disaster Prevention, Dr Takashi Kuwada of the CREST project of the Japan Science and Technology Agency (JST), Nagoya University, and the staff of the State Hydrological Institute in St Petersburg (Russian Federation) for their help in conducting the intensive field campaign during the spring of REFERENCES Giesbrecht, M. A. & Woo, M. (2000) Simulation of snowmelt in a subarctic spruce woodland. 2. Open woodland model. Water Resour. Res. 36, Granger, R. J. & Male, D. H. (1978) Melting of a prairie snowpack. J. Appl. Met. 17, Hardy, J. P., Davis, R. E., Jordan, R., Li, X., Woodcock, C., Ni, W. & McKenzie, J. C. (1997) Snow ablation modeling at the stand scale in a boreal jack pine forest. J. Geophys. Res. 102, Jordan, R. (1991) A one-dimensional temperature model for a snow cover. In: Special Report 91-16, US Army Corps of Engineers, Hanover, New Hampshire, USA. Koivusalo, H. & Kokkonen, T. (2002) Snow processes in a forest clearing and in a coniferous forest. J. Hydrol. 262, Kojima, K. (1979) Snowmelt mechanism and energy balance. In: Snowmelt and Avalanche Japanese Met. Res. Note no. 136, Meteorological Society of Japan.

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