# Water age analysis

Hi
i am using EFDC to model an estuarine system and one task to calculate water age for some ecological modeling works. i used one year as spin up to stablize the “water age”. It seems long enough for the system to reach an equilibrium. however, if i use two years to spin up , the water age end up at a higher “eqilibrium” stage. it seems to me that the longer i run the model, the older the water will be since some water will never leave the system. If so , how can i estimate the water age properly , how long should i spin up the model for? Anycomments would be much appreciated!

Thanks!!!

Hi Garry, in answer to your question, if some water never leaves system then it is possible that you could get a result where the longer you run the model the older the water age. However, if you want a meaningful result you should calculate the mass weighted time series average over the entire water body, or a specified region (using a polygon). You should not just extract a time series.

The way you extract the data depends what you trying to achieve. If you want the peak residence time for each year you could use peak age of water, but note that this will vary from season to season.

Please refer to our recently update guide to use of Age of Water in EE. Age of Water Guide

Thank you very much for you help. one more question , how to calculate the mass weighted age of the water in the EE . Aain , thanks for you help.

Hi Garry, please look at Step 7 “Generate Time Series of Mass Weighted Age” in the link for the Age of Water Guide above. If this doesn’t answer your question please let us know.

Hi EFDC Development Team,

Would you please let us know what equations does EFDC Explorer uses to calculate the water age ? If you could write the equations for the water age calculations that would be great. I would also like to know the physical meaning of water age that is calculated by EFDC Explorer. Thanks.

John

We have now updated the Age of Water guide to contain the equations used for this analysis: Age of Water