Global Stream Temperature Simulation and Affected Ecosystem Factors Analysis with an Earth System Modeling Approach

Human and water management activities have continuously altered the spatiotemporal pattern of hydrometeorological characteristics in a watershed. Coastal ecosystems are affected by many factors including water quality of upstream land runoff. In this study, we leveraged the enhanced modeling capability of the river transport and temperature model in Energy Exascale Earth System Model (E3SM) to analyze streamflow, stream temperature, saturated dissolved oxygen (SDO) and decay rate of carbonaceous biochemical oxygen demand (CBOD) focusing on coastal areas. The global model was run at half degree resolution and evaluated between 1975 and 2004 using observed streamflow and temperature data. Inclusion of water management (wm) in the form of reservoir operation and water demand extraction improved the model performance (Nash-Sutcliffe Efficiency Coefficient, NSEC) on 32% (552 stations) and 21% (478 stations) of streamflow and temperature stations, respectively. On the other hand, the root mean square error (RMSE) improved for 23% and 19% of the stations with observation data. Only 32 streamflow and 21 stream temperature stations got worse NSEC performance (19 and 13, respectively for RMSE). Drought events (identified using standardized runoff index) were alleviated or temporally shifted with water management. The SDO indicated a decreasing trend while CBOD decay rate has increased slightly with significant effect during identified drought periods, which may exacerbate the adverse impacts of extreme events. Identifying the trend and rate of these ecological factors plays an important role in determining nutrient levels and biogeochemical reactions in coastal areas and inland water systems.