SEDIMENT CHARACTERISTICS OF TWO CONIFEROUS AND BROADLEAF FORESTS IN KURAIYAMA, JAPAN

Abstract

The aim of this study was to compare the sediment characteristics of two small forest catchments, artificial coniferous (AC) and natural broadleaf (NB), utilizing sediment rating curves (SRCs) through the suspended load and hysteresis loops. The SRC equation was derived from the relationship between the water discharge value (Q) with the suspended sediment concentration (SS) or the suspended load (L) data from the recorded rainfall events. The erosivity index of AC was found to be higher than that of NB (a-values of 7.30 and 1.20 for AC and NB, respectively), showing that the erosion in the coniferous forest was greater than that in the broadleaf forest. AC also had a positive erosivity power (b-value) of 0.57, while NB had a negative value of -0.186. However, even though the b-value for NB is negative, it is close to 0. This means that even if the discharge increases, the suspended sediment concentration does not change enough to make the relationship between Q and SS for NB very low. Both catchments generated a weak coefficient of determination (R²). Nevertheless, it seems that there is a correlation between the R² values and the hysteresis loops, where the weaker the R², the wider the hysteresis loop can be captured. The suspended sediment of NB was finer and darker than that of AC, representing richer organic matter. The richer nutrients of NB catchment may have been brought about by the greater amount of leaf litter that flowed down due to heavy rainfall events, leading  to sediment of a darker color. It was concluded that coniferous forest catchment tended to be more erosive than broadleaf forest catchments.