DOI: 10.5066/p9pgfny2
title: MODFLOW-NWT, MT3D-USGS, and VS2DH models of 6 hypothetical 1-dimensional variably saturated systems to demonstrate the accuracy of new heat transport capabilities in MT3D-USGS

description: Six hypothetical 1-dimensional models are used to verify and demonstrate new unsaturated-zone heat transport functionality added to MT3D-USGS (version 1.1.0). Because the governing equations describing groundwater solute transport and heat transport have a similar form, MT3D-USGS may be applied to heat transport problems. Published examples of MT3DMS, from which MT3D-USGS is derived, as a heat transport modeling tool have previously been limited to the saturated zone. However, with the publication of MT3D-USGS which added unsaturated zone solute transport capabilities, some additional support (i.e., new source code) is necessary to enable its use as a heat transport simulator where the unsaturated zone also is going to be modeled. The first five scenarios represent a 30 meter thick unsaturted zone. Of these, the first scenario is referred to a "quasi-steady-state" model because it divides a 16-year simulation period into 4 steady flow periods. In other words, this proof-of-concept model is setup using a free drainage lower boundary and transient stress periods that repeat the same boundary conditions over a four-year period before changing to a new set of steady flow and transport conditions that are subsequently repeated for another four-year period. This approach was adopted because the period of transition from one steady flow period to the next is of most interest in this scenario. Scenarios 2 through 5 simulate a 100-year transient period, wherein both the infiltration and temperature assigned to the infiltration vary daily. Scenario 2 serves as the baseline to which scenarios 3, 4, and 5 are compared. Briefly, scenarios 3, 4, and 5 investigate how warm-up at the model surface, intended to represent warmer atmospheric temperatures, manifest at different depths within the unsaturated zone. Scenarios 3, 4, and 5 vary in how they represent warm-up at land surface. Scenario 3 applies a gradual 3 degree Celcius (C) warm-up between the twentieth and fiftieth years. Scenarios 4 and 5 apply 'shock' (instantaneous) warm-ups of 1.5 and 3.0 degrees C, respectively, at the twentieth year. The sixth scenario returns to using the scenario 1 setup, but instead of applying a free drainage condition at the bottom of the active model domain, it uses a specified-head boundary condition that fixes a water table at 12 m deep. The purpose of the water table is to investigate the inter-play between convection, conduction, and dispersion in the presence of a water table. All of the 1D models use grid cells that are 1 meter on a side and 15 cm thick. Rainfall and the temperature assigned to the rainfall are simulated with the unsaturated-zone flow (UZF1) and unsaturated-zone transport (UZT) packages, respectively. All six scenarios are simulated first with MODFLOW-NWT and then with MT3D-USGS, but the latter model does not simulate groundwater flow nor variably-saturated flow. Thus, MODFLOW-NWT must be run prior to running MT3D-USGS to generate all the cell-by-cell flows required by MT3D-USGS. In addition, an identical model setup was created for VS2DH for each scenario to verify the new variably-saturated heat transport functionality within MT3D-USGS. This USGS data release contains the input and output data files for the six hypothetical 1-dimensional models used to demonstrate new functionality of MT3D-USGS. Model input files were developed from published information; no new datasets were collected as part of the modeling study associated with this data release. Details on data sources and processing for developing model input and output files are documented in the associated journal article (https://doi.org/10.1111/gwat.13256)