Fortran is the workhorse for much of the numerical and scientific modelling community, no doubt in large part because of its ability to handle matrices and arrays, the quality of its optimisers, and the substantial volume of legacy code. It also neatly interfaces with languages like R and Python.
There are many texts and online manuals that can help you get inducted into the world of Fortran. I’ve found the text Modern Fortran Explained (by Michael Metcalf and John Reid) to be very helpful, and the most recent edition covers Fortran 95, Fortran 2003 and Fortran 2008. Of course there will no doubt be plenty of other code available.
Here is a (very) partial list of open source hydrological models written in Fortran:
|GR4J||A parsimonious four parameter conceptual rainfall-runoff model that has been shown to work well across a diversity of catchments. Code can be downloaded here, and a detailed description of the model can be found in this paper. This model operates at a daily timestep, although an hourly version (GR4H) has also been developed. An R version of this model is also available via the hydromad package.|
|SWAT||The SWAT (Soil and Water Assessment Tool) model is available both as an executable and as Fortran code. The model is semi-distributed, based on the concept of hydrological response units, and operates at a daily timestep.|