PROGNOSTIC EVALUATION OF CHANGES OF THE OXYGEN REGIME AND WATER QUALITY IN THE MOZHAISK RESERVOIR UNDER EXTREMELY HOT WEATHER

Abstract

Quantitative assessment of possible changes of the hypoxia zone (with dissolved oxygen concentrations less than 2 mg/l) depending on water amount and synoptic conditions of the vegetation season was carried out for the Mozhaisk reservoir, the main element in theMoskvaRiverwater supply system ofMoscow. Particular attention was given to the analysis of unfavorable hydrometeorological situations, such as combination of the low level of seasonal flood, hot anticyclone weather and small water input into the reservoir during the low-flow period. The spatial-temporal pattern of dissolved oxygen distribution was calculated using the hydrological model of the reservoir (GMV—MGU) with additional ecological unit. The oxygen unit of the model was verified basing on the data of 10 hydrological-hydrochemical surveys. Standard deviation of calculated values of dissolved oxygen concentrations from 609 measurements during the whole period of modeling (361 days) was ±2,1 mg/l and mean absolute error of model calculation was –0,6 mg/l. Scenario model calculations revealed different parameters of the hypoxia zone in relation to the timing of hot weather. A combination of medium flow, hot spring and low seasonal flood results in much lower oxygen concentrations in the reservoir as compared to a typical medium-flow year, because of the small amount of diatoms, earlier formation of stable thermal stratification and lower concentrations of dissolved oxygen under high water temperatures. Extremely hot summer weather may cause the formation of vast hypoxia zones in the hypolimnion layer (up to 36% of the total water amount) and affect water quality in the reservoir.