Abstract:Radiative characteristics in a forested drainage basin during the snowmelt season were examined in order to better understand and predict snowmelt runo in the basin. A method for estimating net radiation in a forest (R nf ) was presented using the total sky view factor (P) and the sun path sky view factor (Q). Solar radiation, albedo, atmospheric radiation and air temperature observed at an open site were also required. The total and the sun path sky view factors were determined from all-sky photographs. Q was expressed as a linear function of P for 0 . 15 5 P 5 0 . 86 regardless of forest type. For P 5 0 . 15, Q was set to zero, and for P 4 0 . 86, Q was equal to unity. The short-wave radiation budget at the forest¯oor (S nf ) increased with P, whereas the long-wave radiation budget (L nf ) decreased with P. R nf increased with P for 0 . 15 5 P 5 0 . 86, and changed little with P for P 5 0 . 15 and P 4 0 . 86, as the increase in S nf was oset by the decrease in L nf . The forest eect on R nf was diminished under cloudy or high albedo conditions, because S nf was easily oset by L nf . This estimation method was extended to the whole basin, and R nf was obtained over a watershed covered by trees. At the beginning of the snowmelt season when the albedo remained high, the forest eect became null because the decrease in S nf was balanced by the increase in L nf . As the albedo gradually lowered with the advance of the snowmelt season, the decrease in S nf owing to forest covers exceeded the increase in L nf , and the forest eect to decrease R nf became evident.
Characteristics of the water balance were investigated in an experimental watershed where there is a seasonal snow cover. To estimate the basin-wide snowmelt by the heat balance method, seven observation sites were established in the watershed and the dependencies of meteorological parameters on elevation, slope, and forest density were examined. In the snowmelt season, the net radiation and sensible heat flux increased with elevation because of the decrease in forest density, an increase in wind speed, and the temperature inversion phenomenon, which resulted in an increase in the snowmelt rate with elevation. The solid precipitation accounted for 53% of the annual precipitation and snowmelt runoff accounted for 45% of the annual runoff. The runoff/precipitation ratio was 0.75 in the snowmelt season. Evaporation was negligibly small during the winter season, but it increased during the snow-free period. The total water loss due to evaporation was about a quarter of the annual precipitation.
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