Title: **Significance of "Time Step (years)" in model execution**

Post by:**Ibe** on **July 23, 2015, 10:17:53 AM**

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This may be trivial but I am trying to understand that if I specify custom SLR to 1m and set time step of 10 years. And if I repeat it with 25 years time step then results vary. What is the overall role of time step? Model breaks down the 1m SLR to incremental rise (say for 25 year time step, it incrementally increase SLR from 0m to 1m) based on time steps specified or something else?

Please advise. Thanks.

Please advise. Thanks.

Title: **Re: Significance of "Time Step (years)" in model execution**

Post by:**marco.propato** on **July 24, 2015, 01:53:08 PM**

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There are several reasons why results may depend on the time step.

First, SLAMM does only one conversion at each time steps. So for example, it will still take at least 3 time steps to convert dry land to water because first it is converted to transitional marsh, then tidal flat and then water no matter the time step. So this may lead a different results for different time steps depending on how fast elevation is lost. But if elevation losses are so fast then one would question if model assumptions hold (see comment below).

Second, conversion is calculated only at the end of each time step. This also may lead different results because using a smaller time step a land conversion may occur earlier according to the elevation loss and after that land cover type will have a different decision tree and loss elevation rates until next time step.

Having more time steps is better, but you have to balance with the fact that SLAMM assumes land cover in quasi-equilibrium with SLR. Two time steps calculations in 25 years may be reasonable.

First, SLAMM does only one conversion at each time steps. So for example, it will still take at least 3 time steps to convert dry land to water because first it is converted to transitional marsh, then tidal flat and then water no matter the time step. So this may lead a different results for different time steps depending on how fast elevation is lost. But if elevation losses are so fast then one would question if model assumptions hold (see comment below).

Second, conversion is calculated only at the end of each time step. This also may lead different results because using a smaller time step a land conversion may occur earlier according to the elevation loss and after that land cover type will have a different decision tree and loss elevation rates until next time step.

Having more time steps is better, but you have to balance with the fact that SLAMM assumes land cover in quasi-equilibrium with SLR. Two time steps calculations in 25 years may be reasonable.