CASA Model Pulse Response Respiration Functions

These pulse response functions can be used to compute the mean residence time of carbon in 
any 1 x 1 degree grid cell of the land surface. They can also be used to predict the 
isotopic composition of heterotrophic respiration, as well as perturbations in net 
ecosystem production (NEP) arising from a trend in net primary production (NPP). 

The pulse functions were generated by allowing NPP to vary according to typical CASA model 
behavior for the first year.  In all subsequent years of the 201-year model simulation, 
NPP was set equal to zero in each grid cell. The pulse functions during each month consist
of the sum of heterotrophic respiration arising from the flow of carbon through plant, 
litter, and soil carbon pools. The only source of carbon during the full 201 year simulation
was the NPP that occurred in year 1.  

The first year of the simulation (when x is equal to 0) includes heterotrophic respiration 
losses in each grid cell from NPP in the prior months.  For example, in July 
(filename resp.0.07), the respiration flux reflects NPP inputs from January to June of that 
first year.  After December and for the rest of the model simulation, NPP was set equal to zero.  
Even though NPP is cut off in the model, respiration continues as carbon cycles from plants to 
litter and ultimately to soil organic matter pools. 


Reading the Data:


One directory contains NPP and the other (RESP) contains the respiration pulse functions.  
For the respiration pulse functions, they are labeled resp.x.y, where x is the year and y 
is the month.  x varies between 0 and 200 (we saved the first 201 years of respiration 
losses).  y varies between 1 (Jan) and 12 (Dec). 

These files have a 1 degree lat by 1 degree lon gridded resolution (180 x 360). The first 
pixel is centered at 89.5N, 179.5W, the last pixel at 89.5S, 179.5E. Each file is IEEE 
floating point (4 bytes) so that the size is 180 x 360 x 4 = 259200 bytes.

The units are in gC/m2/month.

In each grid cell, the sum of the 12 NPP monthly values should be close to, but slightly 
less than the sum of all the respiration files.

These pulse functions were generated from the CASA model as described by the following 
reference: 

Randerson, J. T., M. V. Thompson, T. J. Conway, I. Y. Fung, and C. B. Field. 1997. The 
contribution of terrestrial sources and sinks to trends in the seasonal cycle of 
atmospheric carbon dioxide.  Global Biogeochemical Cycles. 11: 535-560.

More detail on the use of biogeochemical pulse functions can be found in:

Thompson, M.V. and J.T. Randerson. 1999. Pulse response functions of terrestrial 
carbon models: method and application. Global Change Biology. 5: 371-394.