[Bfast-commits] Trend extraction

[Brian J. Buma]

I have a couple questions regarding bfast trend extraction- hopefully I'm not missing something super obvious.

I'm looking at a system which experienced several disturbances in a short time period, and attempting to look at the NDVI response, specifically the magnitude of the disturbance impact (the final disturbance, that is) in relation to previous disturbances, and also to look at NDVI recovery post-disturbance.


The first question involves the seasonal component.  I have a situation where a disturbance dropped the NDVI in 2002 (using MODIS), so I have two years or so pre-disturbance.  The recovery period is on-going.  Bfast fits the seasonal component over the entire time series, right, so the more severe disturbances are fitted with a lower seasonal component because the years following the disturbance are lower, thus comparing the magnitude of the break around the disturbance itself doesn't work- the more severe disturbances have a reduced breakpoint magnitude, because their seasonal component is lower overall.  Hopefully that makes sense.  Any ideas for making comparisons more equitable would be appreciated- I'm thinking of attempting to get bfast to fit the seasonal component on only the pre-disturbance years, and then use that for the following recovery as the "baseline."  Is that possible?


I'm trying to extract recovery rates in NDVI post-disturbance.  I'd really like to get the slope of the trend line component in bfast.  Is there a way to extract that number specifically?  Since I'm covering a large extent (1500+ time series) it needs to be automated.

Thanks for all your help!

-brian



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Brian Buma
PhD Student
Dept. of Ecology and Evolutionary Biology 
University of Colorado, Boulder
Brian.Buma at colorado.edu