Numerical model development of coupled surface-groundwater interactions in the Nooksack Basin

Surface-groundwater interactions are simulated using a loose coupling (3 steps) of the Topnet-WM surface water model and the MODFLOW groundwater model. Data included here are draft results of the surface model and final results of the coupled surface-groundwater model. Draft surface model outputs (recharge) was used as a model drivers for the groundwater model (Step 1). The groundwater model was run, informed by surface water management, to generate depth to water table outputs (Step 2). The depth to groundwater from Step 2 was used to update the surface model subsurface state variable twice per year. The steady state groundwater level per drainage with no irrigation was used to update the model each March 1, to improve saturated winter subsurface conditions. The steady state groundwater model with irrigation was used to update the surface water model subsurface state variable each October 1. Final model results are available for streamflow and water budget components on a daily timestep from 1955-2010 as part of the project “Development of a Numerical Groundwater Model for the Lynden/Everson/Nooksack/Sumas (LENS) Area of Whatcom County” for Phase 4 – Numerical Model Development for the WRIA 1 Watershed Management Project Joint Board (Joint Board).

This work was developed to provide updated model code and recharge estimates of Bertrand and WRIA Drainage from Surface Water Modeling with Water Management for coupling to the MODFLOW groundwater model. Subsurface recharge is a surface water model (Topnet-WM) output that can be used as an independent constraint on the groundwater recharge that is specified for the groundwater model (MODFLOW). This will help to ensure that the surface water and groundwater models are internally consistent. The surface water modeling will help to identify a longterm period of representative climatic conditions and resulting groundwater recharge, which can serve as input over a representative time period and for the calibration of a steadystate groundwater model. The surface water modeling also provide important insights about stream baseflows for calibrating the groundwater model. The analysis of surface water and groundwater conditions is conducted over a large model domain with progressively increasing resolution (e.g. with increased focus in Bertrand) where we are addressing specific questions of drainage impacts and groundwater withdrawal impacts from wells on surface water. The current model domain of the surface water flow model, Topnet-WM, includes the entire WRIA 1 watershed, but calibration and water use inputs have only been refined for the Lower Nooksack Subbasin portion of the basin with additional model development was previously completed in the Bertrand Creek drainages resulting in the surface model resolution being increased from a single watershed average to 46 sub-drainages per watershed. The following steps estimate groundwater recharge in the conceptual/numerical groundwater model:

A distribution of recharge was developed for the entire conceptual/numerical model domain WRIA1, inclusive of the LENS domain. Model simulations for GW model test coupling with outputs for Water Management On. Of the 172 modeled Nooksack drainages, those with water management data available are primarily in the Lower Nooksack.

A distribution of recharge was developed for the entire conceptual/numerical model domain in Bertrand Creek. Model simulations for GW model test coupling with outputs for Water Management On. The greater resolution groundwater recharge estimates for Bertrand Creek sub-drainages developed from the 2016 Topnet-WM model are also used as input to the groundwater model.

For developers: the original Topnet-WM model written in Fortran was recoded in C++ and new model output files were designed for groundwater model coupling. Model software code and related files are available at https://github.com/ChristinaB/Topnet-WM.