Comparison of the effects of rainfall and snowmelt on the carbon discharge of a small, steep, forested watershed in Hokkaido, northern Japan

We measured the carbon discharge from a small, forested watershed that is covered with deep snow in winter. Snowmelt and rainfall both cause high flows, and we compared their effect on carbon discharge characteristics. The particulate organic carbon (POC) concentration was stable (around 5 mgC/l) at times of low flow and increased rapidly during high flow periods. The maximum POC concentration exceeded 80 mgC/l during rainfall, but it did not exceed 20 mgC/l during snowmelt, even though the rate of water discharge was high for both events. The dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) concentrations fluctuated seasonally, and also temporarily during high flow events. The DIC concentration decreased from 4·0 to 1·4 mgC/l as the spring thaw progressed. It increased progressively to a high of 7 mg/l in August and September, and then gradually decreased again. The DOC increased rapidly from 2 to 4 mg/l at the beginning of the thaw, then plunged to 2 mg/l when the snowpack disappeared. It increased from July until October (>5 mgC/l) and then decreased once more. The DOC and DIC concentrations were not clearly synchronized during the snowmelt. During rainfall, the DIC concentration decreased in rising limbs of the hydrograph and increased in falling limbs, whereas the reverse was the case for the DOC concentration. In late November, however, both the DOC and DIC concentrations decreased with increasing water discharge. Values for the annual transport of water, POC, DIC, and DOC were 925 mm, 21 kgC/ha, 19 kgC/ha, and 33 kgC/ha, respectively. The fractions transported during the snowmelt, which spanned 62 days, were 69% for water, 54% for POC, 58% for DIC, and 65% for DOC. Excluding the snowmelt period, the fractions transported in the peak 15 days were 38% for water, 63% for POC, 30% for DIC and 46% for DOC. We discuss the effects of snowpack and of respiration by roots and heterotrophs on the carbon discharge patterns of the watershed. Copyright © 1999 John Wiley & Sons, Ltd.