|Other Abstract||The study was conducted at the Kunnes River basin, and the air temperature, relative humidity, wind speed, radiation on snow surface, physical property of snow and snowmelt rate at open site on sunny slope (OPS) and beneath different forest canopy openness (BFC) was observed. The energy budget at snow surface, snow accumulation and ablation processes, the main influence factor of the snow ablation under different underlying surface were analyzed. And also, the radiation model on forest snow surface was constructed. A method to monitor the snow accumulation and ablation processes under complicated terrain and beneath forest canopy was proposed. The main results were displayed as follows:
(1) Due to the effect of forest canopy and terrain, the net shortwave radiation (K) and sensible heat flux (H) were energy source, and net longwave radiation (L) and latent heat flux (LVE) were energy sinks at OPS and beneath 80% forest canopy openness (80% BFC), but the K, H and L were energy sources, and LVE was energy sink beneath 20% forest canopy openness (20% BFC).The downward shortwave radiation on snow surface at BFC was markedly lower than that at OPS, and the downward shortwave radiation, snow surface albedo and K was increased with the forest canopy openness increase. However, the downward longwave radiation at BFC was lower than that at OPS, and the downward longwave radiation and L was increased with the forest canopy openness decrease. Due to the influence of wind speed, the H on snow surface at BFC was lower than that at OPS. Generally, the LVE were negative at all sites during snowmelt period. The loss of LVE at 80% BFC was higher than that at OPS, and LVE was increased with the forest canopy openness increase. Due to the clam wind at nighttime, the exchanges of H and LVE on snow surface at BFC were mainly occurred at daytime. The difference of energy supplied by precipitation under the different underlying surfaces was increased with the increase precipitation.
(2) According to Beer-Lambert Law, the downward shortwave radiation can be calculated using tree height, crown radius, crown depth, diameter of breast height and distance between trees. During snow accumulation period, snow albedo can be calculated using a combination of the new snow albedo and the number of days after fresh snowfall. In snowmelt period, snow albedo can be calculated using a combination of the number of days after the snowmelt began, the number of days after fresh snowfall and the accumulated wind speed after such fresh snowfall. The downward longwave radiation at BFC can be calculated using forest canopy openness, air temperature, downward shortwave radiation above forest canopy and the longwave radiation enhanced by adjacent terrain. The upward longwave radiation can be calculated using air temperature.
(3) The downward diffuse radiation and downward longwave radiation was not influenced by tree height, crown depth and diameter of breast height, the downward direct radiation was hardly influenced by the crown depth and diameter of breast height, but the downward direct radiation was significantly influenced by tree height and crown radius. With the decreased in tree height and crown diameter, the direct radiation were decreased and increased, respectively. With the forest canopy openness increased, the downward shortwave radiation and downward diffuse radiation were increased exponentially and linearly, respectively. The downward longwave radiation was increased with slope of adjacent terrain and air temperature. When air slope (air temperature) is relatively low, the longwave radiation enhanced by adjacent terrain is not sensitive to slope (air temperature), but the sensitivity increases with the decreasing snow cover area at OPS. And the effect is especially sensitive when the snow cover at OPS melted completely. The downward shortwave radiation was obviously influenced by the albedo of adjacent terrain and forest canopy, especially when the slope of adjacent was relatively large, but the influence of the albedo of adjacent terrain and forest canopy on downward shortwave radiation was not significant.
(4) The variation of snow depth and snowmelt rate beneath different forest canopy openness and under different slope has a same trend. With the increase in forest canopy openness, the interception efficiency of forest canopy decrease, the snow depth and snow water equivalent beneath forest canopy increase. The larger forest canopy openness, the later the start/end time of snowmelt. The value and the diurnal variation of snowmelt rate beneath different forest canopy openness were determined by the percentage K vis-à-vis L. The amount of water exchange due to sublimation and condensation was very small. The sublimation from interception snow was obviously bigger than sublimation from snow beneath forest canopy and at OPS. With the increased in forest canopy openness, the amount of sublimation losses in solid precipitation during whole snow period decreased.
(5) An increase in water vapor is the most important factor for the variation of the energy budget at snow surface and snow ablation process at OPS. The H increasing with air temperature rise is the second important factor. The influences of air temperature and humidty on energy budget at snow surface and snow ablation process at BFC were slight. The slope of the adjacent terrain and the percentage of snow cover area at OPS were the most important factors. Against the background of a warming and more humid climate in the study area, the snowmelt rate will inevitably increase, the risk of snowmelt flood will also increase, especially toward the end of the snowmelt period. However, such snowmelt rate increases will also be influenced by the temporal distribution of precipitation.
(6) According to the characteristics of snow thermal conductivity, the snow depth, snow density, snow water equivalent and cold content of snow under different underlying surface can be better estimated according to snow temperature of different heights above the ground surface. When the whole snow layers temperature increased to 0℃, the snowmelt rate drastically increased. When the cold content of snow at OPS and BFC were less than 0.1 MJ m-2 and 0.04 MJ m-2, respectively, the risk of snowmelt flood will increase.|