Abstract
In relation to the water pollution of the Kasumigaura lake, the water quality and nutrient load of rivers in the lake basin have been investigated. However those investigations did not reveal the water quality and nutrient loads in case of flood runoff, and the problem remained untouched whenever the total nutrient load flowing into the Kasumigaura lake was estimated. Therefore, the water quality and nutrient load of flood runoff have been investigated for three years in the Hanasashi stream which is a small river in agricultural area.
1) When the discharge becomes large, the concentrations of SS, COD and T-P are higher, where as, that of NO3-N is often low (Figs. 3-6).
2) When the concentration increases with the discharge, usually the peak time of the concentration comes earlier than that of the discharge.
3) In the relation between the discharge and the concentrations, hysteresis is observed. With the decreasing discharge, the concentrations of SS, COD, and T-P become lower as compared with the same dischage value when the discharge is increasing.
4) Hysteresis is also observed in the relation between the discharge and the loads of the nutrients. The loads of SS, COD, and T-P become lower when discharge is decreasing than when it is increasing.
The load of T-N has the inverse tendency (Fig. 14).
5) The loads in period of rainfall are heavily influenced not only by rainfall and discharge but also by the rainless days before the rainfall and manuring period. The loads of SS and COD depend heavily on the rainless days and that of N and P on the manuring period (Table 4).
6) The mean concentration of direct runoff water CD is usually higher than the concentration of base flow water, CB. Especially the concentration ratio, fc of phosphorus is large, hence the load of P flows remarkably during the period of rainfall (Fig. 19).
