NbS: Calculate forest biomass change over time

Modified on Mon, 27 Mar 2023 at 12:10 PM


This template project looks at historical forest biomass within Cambodia and tabulates the values for the defined project areas. This template will enable you to determine biomass changes since 2007 and compare total carbon content between project areas.


TABLE OF CONTENTS


Demonstration Video


What should I use this template for? 

You should use this template project to examine historical trends in forest biomass or to compare the total forest carbon between projects. This could be a large area (as in this example, where we are looking at Cambodia) or in one or more project sites (such as the 5 different project sites we have in this example). You can look at forest loss between the years 2007 to 2020, but note that the years 2011-14 are not covered by the source data.


What data has been used?

Data from the Japanese radar satellites PALSAR and PALSAR-2 are used in this template.  The global 25m PALSAR/PALSAR-2 mosaic is a seamless global SAR image created by mosaicking strips of the SAR imagery. Some data is missing within the mosaic, and in those cases where the availability was limited data were necessarily selected from the year before or after, including from 2006. The SAR imagery was ortho-rectified and slope corrected using the 90m SRTM Digital Elevation Model. PALSAR stopped operating in 2011 and PALSAR-2 didn't begin until 2015.  There is therefore a gap of 3 years in the data.  Note also that in 2019 there was a gap in the data collection so no data is available for one strip across Cambodia. 


What do the outputs show me? 


Map Outputs

The map output shows the biomass density (in Mg per ha) for the whole of Cambodia for each year of data.  You can press the "play" button on the data layer to see the animated sequence through the time period. 

Dashboard Outputs

The dashboard show a table and chart of the mean forest biomass density for each of the 5 project areas. A second table displays the total forest above ground carbon for each project.   The table data can be exported as CSV. 




What are the blocks used in the workflow? 


The main workflow is the one on the left. The workflow on the right uses the Hansen 2000 forest cover map and saves it as "2000 Forest Cover" so that it can be used to mask out everywhere that is not forest. 


The main workflow uses the following blocks:

  • The first block is a Use This Dataset container block to fetch the PALSAR radar mosaic. This gives a global data set of radar data that is sensitive to biomass. 
  • The next two blocks are for the Area of Interest (in this case, Cambodia) and the time period. 
  • The Mask Out Part of Image block uses the saved Hansen forest cover map from 2000 to remove all areas that are not forest. This ensures that urban area or water bodies are not included. 
  • The first Calculator block uses a relationship between the radar data and forest biomass that is from the published literature. It has an uncertainty of around 20 Mg/ha and was calibrated for Cambodia. The calculation looks like this: 
  • This saves a band called Biomass-CAMBODIA. The Add Map Layer block then displays this band on the map. 
  • The Add Table block then generates a table of average biomass density for each of the 5 project areas. 
  • The second Calculator block determines the carbon content per hectare by dividing the biomass by 2. 
  • The final Add Table block then tabulates the total carbon for each project area. 


Footnote:  the algorithm used comes from Avtar et al 2013, PLOS ONE. "PALSAR 50 m Mosaic Data Based National Level Biomass Estimation in Cambodia for Implementation of REDD+ Mechanism"


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