Seven unsaturated-zone solute-transport models were tested with two data sets to select models for use by the Agricultural Chemical Team of the U.S. Geological Survey's National Water-Quality Assessment Program. The data sets were from a bromide tracer test near Merced, California, and an atrazine study in the White River Basin, Indiana. In this study the models are designated either as complex or simple based on the water flux algorithm. The complex models, HYDRUS2D, LEACHP, RZWQM, and VS2DT, use Richards' equation to simulate water flux and are well suited to process understanding. The simple models, CALF, GLEAMS, and PRZM, use a tipping-bucket algorithm and are more amenable to extrapolation because they require fewer input parameters. The purpose of this report is not to endorse a particular model, but to describe useful features, potential capabilities, and possible limitations that emerged from working with the model input data sets. More rigorous assessment of model applicability involves proper calibration, which was beyond the scope of this study.

Uncalibrated ("cold") simulations were run using all seven models to predict the transport of bromide (Merced) and the transport and fate of atrazine and three of its transformation products (White River Basin). Among the complex models, HYDRUS2D successfully predicted both the surface retention and accumulation of bromide at depth at the Merced site, whereas RZWQM and VS2DT predicted only the latter. RZWQM predictions of atrazine were closest to observed values at the White River Basin site, where preferential flow has been observed. LEACHP predicted smaller solute concentrations than observed at both the Merced and White River Basin sites. Among the simple models, CALF predicted the highest values of atrazine and deethylatrazine at the measurement depth of 1.5 meters. CALF includes the Addiscott flow option for preferential flow, and also accepts user-specified dispersivity. PRZM underpredicted solute concentrations, probably because control of dispersion is a problem with this model. GLEAMS has a maximum simulation depth of 1.5 meters, which is limiting for mass-balance purposes because it creates a potential disconnect between unsaturated-zone transport and the water table.

Of the models tested, RZWQM, HYDRUS2D, VS2DT, GLEAMS and PRZM had graphical user interfaces. Extensive documentation was available for RZWQM, HYDRUS2D, and VS2DT. RZWQM can explicitly simulate water and solute flux in macropores, and both HYDRUS2D and VS2DT can simulate water and solute flux in two dimensions. The version of RZWQM tested had a maximum simulation depth of 3 meters. The complex models simulate the formation, transport, and fate of degradates of up to three to five compounds including the parent, with the exception of VS2DT, which simulates the transport and fate of a single compound.

Contents
Abstract 
Introduction
     Purpose and Scope
Approach
Results
     Merced, CA
     White River Basin, IN 
Model Summaries
     Complex models
          LEACHP
          HYDRUS2D
          RZWQM
          VS2DT
     Simple models
          CALF
          GLEAMS
          PRZM
Summary
References 
Appendix—Published Model Comparison Studies

Figures

1a. Fit of van Genuchten moisture-retention function to field data at Merced, California, for sediment type 1 (0-125 cm 
and 175-300 cm depths) 
1b. Fit of Brooks-Corey moisture-retention function to van Genuchten values derived from grain size data at the White 
River Basin (1.45 meters) 
2. Moisture content with depth at Merced, California
3. Matric potential with depth at Merced, California
4. Bromide concentration with depth at Merced, California
5. Matric potential with time at White River Basin, Indiana 
6. Atrazine concentration with time at White River Basin, Indiana
7. Desethylatrazine concentration with time at White River Basin, Indiana
8. Didealkylatrazine concentration with time at White River Basin, Indiana

Tables

1. Model summary
2. Simulation layers and sediment properties
3. Model parameters associated with chemical transport of bromide, atrazine, and atrazine degradates