# An Open-Source R Package for Building and Using US Environmentally-Extended Input-Output Models
## 4 Conclusions  
The USEEIO modeling framework requires model building tools to be free, open-source, and easily distributed, installed, and used in common computing environments. The programming languages behind the tools should have simple syntax and an active user community; support structured programs, and high-level, efficient matrix mathematical operations; use and produce human-readable, non-proprietary data formats; and be able to be maintained on GitHub, or a similar git-based cloud version control platform. Among suitable languages, Python was initially selected to create two packages, _ionb_ and _useeiopy_, for USEEIO assembly and organizing some of the USEEIO input data, respectively, but collecting and preparing the core economic data, the environmental data, and indicators was not performed by either package. To assist data acquisition and transformation, the R language, which has many similarities in data science applications to Python, was used to create a set of interdependent R scripts, coupled with the _useeiopy_ and _ionb_ packages, to retrieve and process larger environmental datasets and assemble the simplified two-region USEEIO state models. With a growing need for increased variety of USEEIO models for various applications, USEEIO was rebranded as a \"family of models\", and received significant redesign in the modeling process. In response to the needs for more transparent and reproducible model build paths from data acquisition through to final output, as well as embedded \begin{table}
\begin{tabular}{c c} \hline \hline
**Flow** & **Contribution** \\ \hline Sulfur dioxide/emission/air/kg & 56.1\% \\ Nitrogen dioxide/emission/air/kg & 40.0\% \\ Ammonia/emission/air/kg & 2.8\% \\ Sulfuric acid/emission/air/kg & 0.7\% \\ Hydrofluoric acid/emission/air/kg & 0.2\% \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Contribution from top 5 flows to total acidification potential in the Utilities sector. \begin{table}
\begin{tabular}{c c} \hline \hline
**Sector** & **Contribution** \\ \hline
111 CA/US—Farms & 92.1\% \\
22/US—Utilities & 4.7\% \\
311 FT/US—Food and beverage and tobacco products & 1.6\% \\
113 FF/US—Forestry, fishing, and related activities & 0.9\% \\
332/US—Fabricated metal products & 0.1\% \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Contribution from top 5 sectors to direct freshwater withdrawals in the food and beverage and tobacco products sector. model validation procedures, the _useeior_ R package was developed, along with a versioning scheme [4] for USEEIO models. _useeior_ simplifies and streamlines the modeling process and enables transparent and reproducible model construction. Users are provided with not only fundamental data and metadata to build default USEEIO models, but also great flexibility to customize models of their own. _useeior_ replaces the _iomb/useeiory_ tools, and now serves as a core component of the US EPA's USEEIO modeling framework, as it integrates the up-to-date IO tables prepared within itself with the environmental data generated by other tools within the framework, and then produces EEIO results in standard formats and software-ready LCI. Designed and created with the principles of open-source software, _useeior_ is continuously improved to be more comprehensive and up-to-date in a transparent and iterative way. Currently, _useeior_ is capable of building USEEIO models that reflect the US national average economic and environmental conditions. Users should be aware of the limitations of using the national average to estimate environmental impacts of goods and services produced in a sub-national scope. To address user demand for sub-national models, regionalized versions of the model are being incorporated into _useeior_ to enable construction of two-region (a US state and a rest of US region) USEEIO state models. Future improvements, such as physical hybrid models and linkage to global multi-regional input-output (MRIO) models are planned to facilitate additional extensions in _useeior_.