This is the code for the CAESAR-Lisflood model with a proposed method for water source tracing added (software version 1.9j-WS). The code is based on version 1.9j of the CAESAR-Lisflood model which is available here. This additional methodology will be included in a future version of the main software branch. The methodology, example applications, and computational benchmarking are described in the accompanying paper by Wilson & Coulthard [https://doi.org/10.5194/gmd-2021-340]. The methodology developed allows additional insights into flood dynamics to be gained by accounting for flow pathways and is independent of the hydraulic formulation used. The software provided here includes the easy to use GUI of the main CAESAR-Lisflood software, with additional functionality added to enable water source tracing and for visualisation. Along with the code, an example application is provided of 15 m version of the Carlisle model. This is in the folder Carlisle Example Getting started: run the software and open (using Config File > Open) either the Carlisle 15 m case study (carlisle15m load file.xml). These are all set up and you should be able to click load data then Start. The additional GUI components for water source tracing are as follows: Enable water source tracing in the Hydrology tab, with the checkbox under the reach input variables. Note that each source file specified will be counted as one source, even if it is used in multiple grid cells. If you are using rainfall as an input, you can also provide a rainfall zonation map to split the tracing of rainfall depending on where it falls (e.g. in different catchments). Under Save Options, optionally place a check next to the water tracers and rain zone tracers outputs. Set up the rest of the model in the same way as any other simulation, then load data and press Start. To visualise the water sources during a simulation: an extra water tracer visualisation control box is added to the GUI. To view the fraction of depth from each source, assign numbers to each of the red (R), green (G) and blue (B) colour options. Note that sources 1 and 2 are reserved for downstream stage and rainfall, so the fluvial sources begins at 3 and increases in the order specified in the reach input variables. The enhance slider control changes the value of beta used for emphasising lower water source fractions (see equations 11 and 12 in Wilson & Coulthard). The point info window checkbox opens an extra dialog which allows a user to click on the map and obtain information about depth, flow velocity etc. The fraction of each water source in the cell is included in this dialog - not that the total water source tracer should sum to 1.0: very small deviations from this are possible due to numerical errors. Additional examples are available in these videos (applications as described by Wilson & Coulthard), each produced with output from this software: Carlisle flood model, UK (5 m version). Avon-Heathcote shallow estuary model, New Zealand. Amazon-Purus confluence, Brazil. Planar test case. For details of each of these, please see the Wilson & Coulthard paper [https://doi.org/10.5194/gmd-2021-340]. Video files are also available. For details of in general use of the CAESAR-Lisflood software, please see the main software website here.