Record-breaking short-term extreme heavy rainfall has occurred in the area around Nyuzen Town, Toyama Prefecture, Japan, with rainfall approaching 100 millimeters in one hour—a short-term intensity far exceeding the average for this time of year. When extensive surface runoff caused by heavy rainfall continuously washes away agricultural non-point source pollutants, scattered household waste, and surface soil from mountainous areas along the riverbanks and flows into the river, it will significantly increase the concentrations of suspended solids and nitrogen and phosphorus nutrients in the river water within a short period.
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