# Calculating the vertical exchange time

I am a EE user and want to calculate the vertical exchange time using the EE, I know the water age in the EE is usually used to calculate the horizontal exchange time. Therefore, I want to know which code (SUBROUTINE) is employed to calculate the water age, if possible, would you tell me that how I recode the existing code to meet the requirement for calculating the vertical exchange time, thank you.

Hi,

The age of water currently computed in EFDC_DSI / EFDC_Explorer reports the residence time in teh system. Age of water allows us to understand how long the water stays into the system. So, the age reported in EE takes into account all the water coming from all the boundaries and it does include density effects as well. Age is calculated in subroutine calconc.f90. Not sure what you are referring as vertical exchange time here. Can you explain it more ?

Best,
Janesh Devkota

I am sorry for not expressing the vertical exchange time clearly, the age of water in EE usually employed to calculate horizontal exchange time of a system, of which the water usually comes from a river upstream and downstream. The vertical exchange time represents the elapsed time since
the water was last in contact with the water surface. The resulting water age at any location represents the transport time required for the water parcel to be transported from the water surface to that location, regardless of its pathway. The model boundaries wereusually set in the water surface and bottom during the simulation process. I am looking forward for your answers. Thank you.

Hi,

I know the vertical exchange time you are talking about and it is extensively used in marine sciences to find the age of water from the surface to the desired bottom location. However, currently there is no direct way to compute such vertical exchange time in EFDC+ / EFDC_Explorer. There are several papers out there which follow the approach of dye concentration and age concentration and then find the age of water as the ration of age concentration to the dye concentration. You will have to do code modifications in order to implement that approach. That approach still has issues when you are diving the age concentration by very small dye concentration.
Some of the papers that you might want to read are as follows:
Liu, W.-C., Chen, W.-B., and Hsu, M.-H. (2011). “Using a three-dimensional particle-tracking model to estimate the residence time and age of water in a tidal estuary.” Computers & Geosciences, 37(8), 1148-1161.
Mercier, C., and Delhez, E. J. M. (2010). “Consistent computation of the age of water parcels using CART.” Ocean Modelling, 35(1–2), 67-76.
Shen, J., and Lin, J. (2006). “Modeling study of the influences of tide and stratification on age of water in the tidal James River.” Estuarine, Coastal and Shelf Science, 68(1-2), 101-112.
Shen, J., and Wang, H. V. (2007). “Determining the age of water and long-term transport timescale of the Chesapeake Bay.” Estuarine, Coastal and Shelf Science, 74(4), 585-598.
Devkota, J., Fang, X., and Fang, V. Z. (2013). “Response characteristics of Perdido and Wolf Bay system to inflows and sea level rise.” British Journal of Environment and Climate Change, 3(2), 229-256.