Measuring and modeling event-based environmental flows: An assessment of HEC-RAS 2D rain-on-grid simulations.

There is an immediate need to use available modeling tools to quantify environmental flows targets where changing climate and human activity has altered hydroecologically important streamflow regimes. A model performance assessment was undertaken using observed data collected from five nested gauging sites in a mixed land use watershed of the central US. An integrated modeling approach was used to couple The Soil and Water Assessment Tool (SWAT version 2012), and The Hydrologic Engineering Center's River Analysis System (HEC-RAS version 5.0.7). SWAT was used to generate effective rainfall needed to run HEC-RAS rain-on-grid two-dimensional hydrodynamic simulations. Model calibration results showed the potential usefulness of coupling SWAT and HEC-RAS using an integrated modeling approach. For example, PBIAS of 8.3%, NSE value of 0.84, and coefficient of determination (R) value of 0.80 at a highly urbanized monitoring site used for model calibration. Split-site validation results showed PBIAS values that ranged from 10.4 to 33.8%, NSE values that ranged from 0.33 to 0.92, and R values that ranged from 0.86 to 0.97. Results showed that 2D rain-on-grid HEC-RAS simulations can produce realistic simulations of stage hydrograph response when: (1) areal effective precipitation is used for 2D HEC-RAS rain-on-grid forcing's, (2) HEC-RAS is calibrated to observed data during the event of interest, (3) there are not substantial sources of backwatering from outside the models geometric data, and (4) during saturated antecedent soil moisture conditions surface DEM's adequately describe overland flow paths. This model performance assessment is among the first, if not the first, to show calibration and validation results associated with 2D HEC-RAS rain-on-grid simulations at a watershed scale. Results highlight the need for time-varying roughness coefficients to account for soil moisture conditions, and point to the efficacy of using a SWAT/HEC-RAS integrated modeling approach to generate event-based environmental flows information.