Forecasting the changes in lake water quality in response to climate changes, using past relationships between meteorological conditions and water quality

In order to forecast the effects of global warming on the water environment , the relationship between meteorological conditions and lake water quality was investigated statistically using 17 years of monitoring data obtained from a shallow eutrophic lake, Lake Kasumigaura. The usefulness of the DPY (difference from the previous year) method was confirmed for removing the w atershed change (e.g. land cover, population, etc.). From the analysis of t he relationships between air temperature and water temperature on a monthly basis, the delay of time was seen to be negligible, but the gain was reduc ed due to the high frequency of change. As the amount of precipitation affe cted their relationship on a yearly basis, the slopes of 1.0-1.2 degrees C water temperature/degrees C air temperature were determined with the DPY me thod by excluding the combinations of the years having large difference in annual precipitation. The deterioration of lake water quality, such as incr eases in COD (chemical oxygen demand) and decreases in transparency, was qu antitatively assessed as corresponding to an increase in air temperature. I n addition, we found that higher precipitation led to high nitrogen concent rations on a monthly basis, as well as on a yearly basis, probably induced by both the runoff of soilwater having high concentrations and the lowering of residence times of lake water. Copyright (C) 2000 John Wiley & Sons, Lt d.