Many strategies for water quality conservation in Lake Biwa are being carried out mainly by reducing runoff pollutant loads into the lake. But influence of the runoff load reduction on the water quality in Lake Biwa has not been clarified enough so far. This study is aimed at discussing methodology to estimate water quality distribution in Lake Biwa using runoff pollutant loads from its basin. The runoff loads from the basin are calculated by Macro Model with GIS database of the Lake Biwa basin, and the water quality distribution in the lake is estimated by spline technique with the calculated runoff loads. As a result, it has been proved that the methodology has enough reproducibility to estimate the water quality distribution in Lake Biwa and is also available to examine the water quality in the lake.
