GREEN-Rgrid is an advanced R version of the model GREEN (Grizzetti, Bouraoui, and Aloe 2012), originally developed in Fortran (Aguzzi, Gasparo, and Macconi 1987) for estimating nutrient loads from diffuse and points sources in Europe. Using R we improved the original structure of the GREEN model ensuring a model fully open to scrutiny since there is a considerable reliance on proprietary software for environmental modelling assessment in Europe (Carslaw and Ropkins 2012). The GREEN-Rgrid model works on a grid cell discretization that the user can change depending on the purpose of the modelling. The model input consists of the latest and best available global data. The GREEN-Rgrid code integrates a landscape routing model to simulate nutrient fluxes of nitrogen-nitrates, total nitrogen, total phosphorous and phosphates across discretized routing units. The grid-based approach was adopted to adapt to the readily available global raster data that can be easily incorporated as model inputs providing a more homogeneous nutrient assessment between different areas of the world. With respect to the original GREEN model, the diffuse source DS was calculated as a function of the gross nutrient balance from agricultural land that is computed as the difference between the inputs (for N: fertilizer application, fixation, and atmospheric deposition; for P: fertilizer application, P release, P transported with sediment) and the output (crop nutrient uptake). A basin attenuation coefficient was applied (aP) to the diffuse sources while two additional coefficients aM and bM are used in the calculation of the in-stream retention (Venohr and et al. 2011). These parameters were calibrated in the model using a Latin Hypercube approach based on packages FME (Soetaert and Petzoldt 2010) and the best parameters set was selected comparing the predicted and observed annual loads using the package HydroGOF (Zambrano-Bigiarini and Bellin 2010). Other important packages used for specific purposes in the GREEN-Rgrid model include the package raster (Hijmans and van Etten 2012), data.table (Dowle et al. 2015) and sqldf (Grothendieck 2012). The structure of the model is organized in three folders: the inputs, where the input table and grid raster cell are stored; the Scripts that includes the master R file and the functions and the outputs where all tables and figures generated are saved. We applied the GREEN-Rgrid model in the Mediterranean area (about 8.066x103 km2) using a grid cell size of 5 arc-minute resolution (9.2 km at the equator) and three years of simulations 2005, 2006, 2007. We show the predicted fluxes and concentrations of nutrients in gauged and ungauged grid cells, showing plots and raster maps automatically generated. Finally, some considerations are given for future developments.
