An oxygen equivalent model for water quality dynamics in a macrophyte dominated river

A water quality modeling study was performed in a macrophyte dominated river. A computer model, MACRIV (macrophyte growing river) was formulated to incorporate diurnal dissolved oxygen variations and nutrient uptake/recycle caused by all aquatic plants, including macrophytes. In this model, the aquatic plant is represented as an equivalent amount of dissolved oxygen by reach, which varies diurnally as well as seasonally depending upon solar radiation, temperature, and nutrients. The solar radiation is computed in the model based on the latitude, Julian day, and real-time of day. The model simulates seven coupled state variables (BOD (CBOD or 5-day BOD), DO (daily average value or diurnal variation), organic nitrogen, ammonia nitrogen, nitrite/nitrate nitrogen, total organic phosphorus, dissolved inorganic phosphorus (DIP)) and two non-coupled variables (a first-order decay substance, and a conservative substance). Through a stoichiometric relationship between plant protoplasm and physiological processes including photosynthesis, respiration, and death, the water quality variables interacting with aquatic plants are computed in the model. The model was calibrated and verified against data collected from an impounded river in which macrophytes play an important role in defining water quality dynamics. There were reasonable agreements between model predictions and the field measurements both for steady-state water quality and diurnal dissolved oxygen. The overall study demonstrated that the developed MACRIV model can simulate the most important parameters involved in waste load allocation studies in macrophyte growing rivers, such as diurnal DO variation, BOD (autochtonous and allochtonous), ammonia toxicity, and nutrient dynamics.