Comparison of Rainfall Season and Snowmelt Season
in Relation to Carbon Discharge in a Small Steep Forested Watershed
in Hokkaido, Northern Japan
Tomoki Sakamoto1, Masamichi Takahashi1, Tomomi Terajima1,
Yuichiro Nakai1, and Yojiro Matsuura1
We measured carbon discharge from a small steep forested watershed and discussed differences
between rainfall floods and during snowmelt. The water samples were passed once through the
0.106-mm mesh screen and filtered through 0.001-mm glass fiber filters. The filters were dried and
the residual matter was weighed and then ignited. The further weight loss due to combustion of the
organic matter was measured. Particulate Organic Carbon (POC) was estimated as 50% of the
further loss weight. Dissolved Organic Carbon (DOC) and Dissolved Inorganic Carbon (DIC) con-
centrations in the filtrate were analyzed using a total organic carbon analyzer.
POC was around 5 mg/L and increased in floods. Its maximum exceeded 80 mg/L in rainfall floods,
but did not exceed 20 mg/L in snowmelt, during which water discharges were more. DIC and DOC
fluctuated seasonally and temporarily in floods. The maximum DOC was 5.2 mg/L in snowmelt
and 15.4 mg/L in rainfall season. DIC decreased from 4.0 mg/L to 1.4 mg/L while snowmelt pro-
gressed. After that, it increased progressively to 7 mg/L in August and September, and decreased
gradually. DOC increased rapidly from 2 mg/L to 4 mg/L in the beginning of snowmelt and plunged
to 2 mg/L when snow disappeared. It increased gradually to exceed 5 mg/L from July to October
and decreased gradually.
DIC and DOC were not synchronized with daily fluctuation of water discharge during snowmelt.
DIC decreased in rising limb and increased in falling limb in rainfall floods. DOC increased in
rising limb and decreased in rainfall floods. In November, however, DOC fluctuated like DIC. The
annual discharges of water, POC, DIC, and DOC were 925 mm, 21 kg/ha, 19 kg/ha, and 33 kg/ha,
respectively.
Annual organic carbon discharge was 2.73.6% of carbon quantity of litters produced by the forest.
Sixty nine percent, 54%, 58% and 65% of the annual discharge of water, POC, DIC, and DOC
respectively, flowed out in snowmelt (62 days). In the rain season, carbon discharge was extremely
concentrated in flood events.
These results indicated the following: 1) There were relationships between DIC and base flows, and
between DOC and direct flows; 2) DIC originated from carbon dioxide with respiration of plant
roots and microorganism; and 3) DOC corresponded to resolution of organic matters in the soil.
1
Hokkaido Research Center, Forestry & Forest Products Research Institute, Hitsujigaoka 7, Toyohira-ku,
Sapporo 062-8516, Japan
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