If I understand correctly, the historic trend is used both to estimate local subsidence/uplift (the difference between global and local SLR) and to reconcile the DEM to the NWI photo date. The global trend is measured over 100 years (1900-2000), while the difference between my DEM and NWI photo is only 6 years, and much more recent. So the local SLR measured over the time 1900-2000 is much lower than that measure for the period between the NWI and DEM dates (something like 2.8 vs 6.8 mm/yr). How would you advise choosing just one historic trend number (or is there a way to specify more than one, for each specific use)?
As a follow-up, it turns out that our local SLR projections already account for the subsidence in our study area. So what we are thinking of doing to solve this problem is to input 1.7 mm as the historic trend, so that SLAMM doesn't make any adjustments for local subsidence. Then, we would make our own adjustment to our DEM to bring it to the same date as our NWI data (using our recent, local SLR trend), so that SLAMM wouldn't use the 1.7 mm there (because if the dates are the same it presumably wouldn't make any adjustment). Does this seem like a reasonable approach? Is the historic trend used in any other parts of the model that we should worry about? Thanks for any input!
Nava -- when using SLR estimates that account for local subsidence and other local factors (RSLR scenarios as opposed to eustatic), setting the historic trend to 1.7 mm/year is the correct approach. (Just as a side note In more recent versions of the model we have allowed the user to specify the historic eustatic trend, recognizing that local historic trend data may have a different time period than 1900-2000 which is the time-period for the 1.7 mm/year estimate.)
That is the only place that historic trend is sued in the model so it is our practice to use 1.7 mm/year when using RSLR estimates.
Furthermore, if you use 1.7 mm/year, no subsidence effects will be calculated as a function of the difference between NWI and DEM dates. Unless your dates are off by a lot and your subsidence estimate is great it is probably not worth the effort to adjust the DEM based on subsidence. The amount of change in elevations is probably less than the model would show sensitivity to. Good luck in your modeling!
Thanks Jonathan, this is very helpful!