Influences of Weather Conditions and Daily Repeated Upstream Releases on Temperature Distributions in a River-Reservoir System

Abstract

A calibrated three-dimensional environmental fluid dynamics code model was applied to simulate temperature distributions under various hypothetical weather conditions and daily repeated large releases (DRLRs) from the upstream boundary in a river-reservoir system in Alabama. Both the duration of DRLRs and weather conditions affect and control the formation and spread of density currents along channel bottom at downstream locations, which affect the bottom-layer water temperatures. The daily drop rates of bottom-layer temperature at the Gorgas upstream monitoring station (GOUS) under 6-h DRLRs are 0.3, 0.5, and 0.4°C/day for assumed 2°C/day air temperature drop lasting 2, 4, and 6 days, respectively. The average bottom-layer temperature at the river intake under 4-h DRLRs is 2.3°C lower than one under 2-h DRLRs and only 1.1°C higher than one under 6-h DRLRs in the whole simulation period. The daily drop rate and dropping duration of bottom-layer temperature are almost the same for 2-, 4-, and 6-h DRLRs because of the same drop and rise pattern for weather conditions. The maximum differences between the stationary weather scenario and the 11-day drop-rise weather scenario range from 3.1 to 4.2°C under different release durations. The lower bottom-layer temperatures at GOUS and the river intake are primarily because of the lower air temperatures and solar radiation during the 11 days and less affected by the release pattern. Bottom-layer temperature dynamics in the riverine portion are more affected by flow momentum of DRLRs and, in the deeper reservoir portion, primarily controlled by weather conditions.