4.6 The Fumigants module

The Fumigants module implements multiple additional options required to simulate processes related to fumigant applications and transport. This module allows users to specify an additional injection of fumigants into the transport domain at a specifed location and time, consider the pres­ence or absence of a surface tarp, allow the tarp properties to depend on temperature, and permit tarp removal at a specifed time. The Fumigants module was recently used to investigate the effect of different application scenarios, such as tarped broadcasting, tarped bedded shank injection, tarped drip line-source application, and other options (e.g., having different initial water con­tents and tarp permeability) on fumigant volatili-zation.¹¹⁹ Figure 9 illustrated an application.

5 Conclusions

In this paper we provided an overview of the HYDRUS codes and various specialized modules that were recently added to the computer software packages. We believe the that HYDRUS models have provided, and will continue to provide, an important service to the soil environmental and hydrological communities, especially but not lim­ited to vadose zone processes. This is refected by their frequent use in a wide range of applications in both basic and applied research and engineer­ing. For example, the HYDRUS website (http:// www.pc-progress.com/en/Default.aspx) lists over one thousand publication in which the HYDRUS codes have been used. The codes cover a large number of processes, from relatively simple one-dimensional solute transport problems to multidi­mensional fow and transport applications at the feld scale, including relatively complex problems involving a range of biogeochemical reactions. Examples of the latter are the HP1/HP2 modules that couple the HYDRUS software packages with the PHREEQC geochemical code, and the Wet­land module that includes two biokinetic model formulations (CW2D and CWM1). The need for codes such as HYDRUS is refected further by the frequency of downloading from the HYDRUS web site. For example, HYDRUS-1D was down­loaded more than 10,000 times in 2012 by users from some 50 different countries. The HYDRUS web site receives on average about 700 individual visitors each day.

The wide use of the HYDRUS models is in large part due also to their ease of use because of the implementation of relatively sophisti­cated interactive graphical user interfaces, which are continuously being updated in attempts to make the codes as attractive as possible for non­expert users. Feedback from users is continu­ously being used also to improve the codes, to identify particular strengths and weaknesses of the models, and to defne additional processes or features that should be included in the models. We believe that the codes and associated manu­als hence are serving an important role in the transfer of vadose zone research and develop­ment technologies to both the scientifc commu­nity and practitioners.