Effects of storm flow samplings on the evaluation of inorganic nitrogen and sulfate budgets in a small forested watershed

摘要

To investigate the effects of storm flow samplings on the evaluation of DIN and SO_4~(2-) budgets, we conducted storm flow samplings and discrete interval (weekly) samplings of stream water, throughfall, and stem flow. This investigation lasted for three years and took place in a small forested watershed in western Japan. Annual exports were calculated by the sum of the load (ΣL_(lotal))- sum of the discharge (ΣQ_(lotal))) relationships obtained by storm flow samplings. These were then compared to calculated ones that were based on LQ relationships obtained by discrete interval sampling. The results of discrete interval sampling showed that the relationships of DIN or SO_4~(2-) concentration to the discharge amount found in stream water differed from those of storm flow sampling. This was especially notable in years with higher annual precipitation and subsequent higher annual discharges. Also, exports of DIN and SO_4~(2-) during storm flow periods accounted for 83% and 78% of annual exports on 3-year averages, respectively. These results confirmed that storm flow samplings are essential for estimating annual exports of DIN and SO_4~(2-). Annual exports that were calculated based on just discrete interval sampling (46% and 17%, respectively) overestimated DIN and SO_4~(2-) from discrete interval and storm flow samplings on a 3-year average. These overestimates were pronounced in years of higher discharge, and critically affected the rates of net-retention or export of DIN and SO_4~(2-). The relationship of smaller input than output of DIN was reversed if storm flow sampling was included.