## 2.24.3. Deforestation fires[¶](#deforestation-fires "Permalink to this headline")
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CLM focuses on deforestation fires in tropical closed forests. Tropical closed forests are defined as grid cells with tropical tree (BET and BDT tropical) coverage \\(>\\) 60% according to the FAO classification. Deforestation fires are defined as fires caused by deforestation, including escaped deforestation fires, termed degradation fires. Deforestation and degradation fires are assumed to occur outside of cropland areas in these grid cells. Burned area is controlled by the deforestation rate and climate:

(2.24.34)[¶](#equation-23-34 "Permalink to this equation")\\\[A\_{b} = b \\ f\_{lu} f\_{cli,d} f\_{b} A\_{g}\\\]

where \\(b\\) (s\-1) is a global constant; \\(f\_{lu}\\) (fraction) represents the effect of decreasing fractional coverage of tree PFTs derived from land use data; \\(f\_{cli,d}\\) (fraction) represents the effect of climate conditions on the burned area.

Constants \\(b\\) and \\({f}\_{lu}\\) are calibrated based on observations and reanalysis datasets in the Amazon rainforest (tropical closed forests within 15.5 °S \\(\\text{-}\\) 10.5 °N, 30.5 ° W \\(\\text{-}\\) 91 ° W). \\(b\\) = 0.033 d\-1 and \\(f\_{lu}\\) is defined as

(2.24.35)[¶](#equation-23-35 "Permalink to this equation")\\\[f\_{lu} = \\max (0.0005,0.19D-0.001)\\\]

where \\(D\\) (yr\-1) is the annual loss of tree cover based on CLM land use and land cover change data.

The effect of climate on deforestation fires is parameterized as:

(2.24.36)[¶](#equation-23-36 "Permalink to this equation")\\\[\\begin{split}\\begin{array}{ll} f\_{cli,d} \\quad = & \\quad \\max \\left\[0,\\min (1,\\frac{b\_{2} -P\_{60d} }{b\_{2} } )\\right\]^{0.5} \\times \\\\ & \\quad \\max \\left\[0,\\min (1,\\frac{b\_{3} -P\_{10d} }{b\_{3} } )\\right\]^{0.5} \\times \\\\ & \\quad \\max \\left\[0,\\min (1,\\frac{0.25-P}{0.25} )\\right\] \\end{array}\\end{split}\\\]

where \\(P\\) (mm d \-1) is instantaneous precipitation, while \\(P\_{60d}\\) (mm d\-1) and \\(P\_{10d}\\) (mm d \-1) are 60-day and 10-day running means of precipitation, respectively; \\(b\_{2}\\) (mm d \-1) and \\(b\_{3}\\) (mm d \-1) are the grid-cell dependent thresholds of \\(P\_{60d}\\) and \\(P\_{10d}\\); 0.25 mm d \-1 is the maximum precipitation rate for drizzle. [Le Page et al. (2010)](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#lepageetal2010) analyzed the relationship between large-scale deforestation fire counts and precipitation during 2003 \\(\\text{-}\\)2006 in southern Amazonia where tropical evergreen trees (BET Tropical) are dominant. Figure 2 in [Le Page et al. (2010)](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#lepageetal2010) showed that fires generally occurred if both \\(P\_{60d}\\) and \\(P\_{10d}\\) were less than about 4.0 mm d \-1, and fires occurred more frequently in a drier environment. Based on the 30-yr (1985 to 2004) precipitation data in [Qian et al. (2006)](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#qianetal2006). The climatological precipitation of dry months (P < 4.0 mm d \-1) in a year over tropical deciduous tree (BDT Tropical) dominated regions is 46% of that over BET Tropical dominated regions, so we set the PFT-dependent thresholds of \\(P\_{60d}\\) and \\(P\_{10d}\\) as 4.0 mm d \-1 for BET Tropical and 1.8 mm d \-1 (= 4.0 mm d \-1 \\(\\times\\) 46%) for BDT Tropical, and \\(b\\)2 and \\(b\\)3 are the average of thresholds of BET Tropical and BDT Tropical weighted bytheir coverage.

The post-fire area due to deforestation is not limited to land-type conversion regions. In the tree-reduced region, the maximum fire carbon emissions are assumed to be 80% of the total conversion flux. According to the fraction of conversion flux for tropical trees in the tree-reduced region (60%) assigned by CLM4-CN, to reach the maximum fire carbon emissions in a conversion region requires burning this region about twice when we set PFT-dependent combustion completeness factors to about 0.3 for stem \[the mean of 0.2\\({-}\\)0.4 used in [van der Werf et al. (2010)](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#vanderwerfetal2010). Therefore, when the burned area calculated from equation [(2.24.36)](#equation-23-36) is no more than twice the tree-reduced area, we assume no escaped fires outside the land-type conversion region, and the fire-related fraction of the total conversion flux is estimated as \\(\\frac{A\_{b} /A\_{g} }{2D}\\). Otherwise, 80% of the total conversion flux is assumed to be fire carbon emissions, and the biomass combustion and vegetation mortality outside the tree-reduced regions with an area fraction of \\(\\frac{A\_{b} }{A\_{g} } -2D\\) are set as in section [2.24.1.3](#fire-impact).

