ΔOxstr=(κ2∗(Oxsat−Oxstr)+(α3∗μa−α4∗ρa)∗algae−κ1∗cbod−1000∗depthκ4−α5∗βN,1∗NH4str−α6∗βN,2∗NO2str)∗TT
where ΔOxstr is the change in dissolved oxygen concentration (mg O2/L), κ2 is the reaeration rate for Fickian diffusion (day−1 or hr−1), Oxsat is the saturation oxygen concentration (mg O2/L), Oxstr is the dissolved oxygen concentration in the stream (mg O2/L), α3 is the rate of oxygen production per unit of algal photosynthesis (mg O2/mg alg), μa is the local specific growth rate of algae (day−1 or hr−1), α4 is the rate of oxygen uptake per unit of algae respired (mg O2/mg alg), ρa is the local respiration or death rate of algae (day−1 or hr−1), algae is the algal biomass concentration at the beginning of the day (mg alg/L), κ1 is the CBOD deoxygenation rate (day−1 or hr−1), cbod is the carbonaceous biological oxygen demand concentration (mg CBOD/L), κ4 is the sediment oxygen demand rate (mg O2/(m2.day) or mg O2/(m2.hr)), depth is the depth of water in the channel (m), α5 is the rate of oxygen uptake per unit NH4+ oxidation (mg O2/mg N), βN,1 is the rate constant for biological oxidation of ammonia nitrogen (day−1 or hr−1), NH4str is the ammonium concentration at the beginning of the day (mg N/L), α6 is the rate of oxygen uptake per unit NO2− oxidation (mg O2/mg N), βN,2 is the rate constant for biological oxidation of nitrite to nitrate (day−1 or hr−1), NO2str is the nitrite concentration at the beginning of the day (mg N/L) and TT is the flow travel time in the reach segment (day or hr). The user defines the rate of oxygen production per unit algal photosynthesis, the rate of oxygen uptake per unit algal respiration, the rate of oxygen uptake per unit NH4+ oxidation and rate of oxygen uptake per unit NO2− oxidation. Section 7:3.1.2.1 describes the calculation of the local growth rate of algae while equation 7:3.1.17 describes the calculation of the local respiration rate of algae. The rate constant for biological oxidation of NH4+ is calculated with equation 7:3.2.5 while the rate constant for NO2− oxidation is determined with equation 7:3.2.9. The CBOD deoxygenation rate is calculated using equation 7:3.4.2. The calculation of depth and travel time are reviewed in Chapter 7:1.
κ4=κ4,20∗1.060(Twater−20) 7:3.5.2
where κ4 is the sediment oxygen demand rate (mg O2/(m2.day) or mg O2/(m2.hr)), κ4,20 is the sediment oxygen demand rate at 20°C (mg O2/(m2.day) or mg O2/(m2.hr)), and Twater is the average water temperature for the day or hour (°C).