A study on low streamflow and its severity for water resources preservation in forested river basin.

Clarification of the status of low streamflow and its severity in a river basin as one of the most important considerations for achieving sustainable water resources in adequate quantity (referred to as streamflow discharges), quality (referred to as suspended sediment production) and the timing distribution of water resources in a river basin was discussed. Water deficiency was shown to be closely related to the subjects of hydrology, meteorology and agriculture, of which this study emphasized low streamflow as a kind of water deficiency related to the field of hydrology. Index of low streamflow severity (SIL) was referred to as the annual low streamflow severity (ALS) in which greater SIL grade indicates a more severe low streamflow severity. ALS lower than 200mm (SIL-1, SIL-2) were slight and fair, whereas those greater than 200mm (SIL-3, SIL-4, SIL-5) were moderate, hard and very hard severe of low streamflow respectively. The Atsuma River Basin, identified as SIL-3, was selected for this study because it is one of the representative low streamflow area in Hokkaido. Potential streamflow discharges and suspended sediment production were used for expressing the hydro-orological conditions. The relation between ALS and specific streamflow discharges was used to estimate ALS in each selected Sub-Basin. A simple model assessment of the hydro-orological condition was established by comparing streamflow discharge and suspended sediment production. The rank of hydro-orological conditions, in descending order are as follows: (1) Shoshiutsu, (2) Shuruku, (3) Chikapeppu, (4) Atsuma, (5) Shoroma, (6) Merukunnai, (7) Ukuryu, (8) Onikishibe and (9) Habiu Sub-Basin. Precipitation has a significant influence on low streamflow, while other factors were geological structures, morphological features and local river basin characteristics. The augmentation of low streamflow was accepted as an important consideration for improving the hydro-orological conditions. Several river basin management practices have been carried out in the Atsuma River Basin. A great fluctuation of streamflow discharges might be reduced or at least minimized so as to control and improve the seasonal distribution of water yields.