How wildfire risk is mapped in Portugal: a naïf, flawed, and very effective approach

Departamento de Engenharia FlorestalInstituto Superior de Agronomia

José M.C. PereiraJoão M.B. Carreiras

MODIS Rapid Response System

Number of fires and area burned in southern Europe, 1980 – 2004, absolute values (European Commission, 2005)

Nº fires Area burned

Number of fires and area burned in southern Europe, 1980 – 2004, per unit area (European Commission, 2005)

Nº fires / 100ha Burned area density

BACKGROUND

1997 - 2005

Night-time hotspots detected with the Along Track Scanning Radiometer, onboard the European Remote Sensing Satellite.

BACKGROUND

BACKGROUND

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50000019

75

1977

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1981

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2005

area burned (ha)

BACKGROUND

Kernel density interpolation of burned area marked point data, using 25 nearest neighbors, Gaussian kernel, and 1km2 grid.

All fires > 5ha mapped from Landsat satellite imagery. About 35000 fires, during the period 1975 – 2005.

Fire risk mapping

- Fire risk map developed by request of the Portuguese Forest Service, to replace outdated map, from the late 1970s.

- Map is used to support decisions dealing with location / allocation of fire-fighting manpower and equipment, routing ground patrols, prioritise afforestation / reforestation projects.

- Why not simply use historical fires map (previous slide) to characterise risk?

Fire risk mapping

– There are areas that never burned during the study period, but which have environmental characteristics similar to those that did burn. They are high risk areas that need to be identified and mapped.

– Once an area burns, it takes a few years for vegetation to regrow, so that the area can burn again.

– An area may be very fire prone, in the long run perspective. But, in the short run, it is unlikely to burn because it lacks fuel.

Fire risk mapping

- Our approach:

–Model long-term (10-year) spatial fire pattern with regression tree, using KD-interpolated fire map as dependent variable and slow-changing environmental factors (climate, topography, and population) as independent variables.

–Model rapid fuel availability dynamics as a function of time

(years) since last fire.

–Model fast-changing relationship between plant moisture status and fire incidence, using late Spring satellite imagery.

Burnt area1995-2004

interpolation

- population density- slope- Nº days Tmax > 25 ºC- Nº days R (5-9) > 1 mm

Slow Response Variables

Slow Response VariablesFire Risk Map

(5 classes)Long-term

CART (r2 = 0.66)

Rapid Response Variables

MODIS 2000-2004pre-fire NDVI incidence

(weibull)

MODIS 2005pre-fire NDVI

5 levels NDVI incidence(mean +- 0.5, 1.0, 1.5,

2.0, 2.5 stdev)

Burnt area1990-2004

Nº years sincelast fire(2005)

CLC2000

- forest, shrub- agriculture- urban/barren/water

5 levels biomassaccumulation(nat. breaks)

Equation biomassaccumulation

Rapid Response VariablesFire Risk Map

(5 classes)Short-term

Combined Slow/RapidResponse Variables

Fire Risk Map(5 classes)

Fire perimeters mapped from Landasat satellite imagery,with a pixel size of 30m.

The area burned in this period is approximately 1.500.000 ha (17% of the total area of Portugal).

Area burned, 1995 - 2004

- Centroids of fire perimeters (1995-2004).

- Points marked with area value.

- Kernel density interpolation to 1km2 grid.

Proportion of area burned (1995 – 2004), in each grid cell:

P = 1 – (1 – 1/b)n,

P - Probability of burning, over a period of n years, in each grid cell.b – Fire cycle: time required for burning 100ha in a grid cell.

Burning probability over a 30 year period

Slope (%) Population density (inhab/km2)

# days Tmax > 25 ºC # days P > 1mm (May-Sept)

Definition of 5 risk classes:

Very low: P < 0.10Low: 0.10 < P < 0.20Medium: 0.20 < P < 0.30High: 0.30 < P < 0.49Very high: P > 0.49

Fire risk – Slow variables

- Fire incidence over pre-fire values of the normalized difference vegetation index, from Terra/Aqua MODIS images 2000-2004.

- Fitted Weibull distribution: mean value of 0.599 and standard deviation of 0.102.

- Definition of 5 risk classes, centred on the mean value of the Weibull distribution:

Very low: µ ± 2.5 sLow: µ ± 2.0 sMedium: µ ± 1.5 sHigh: µ ± 1.0 sVery high: µ ± 0.5 s

Fire risk – NDVI

- Area burned 1990-2004: calculation of the # years since last fire.

- CLC2000 (+ fogos 2000-2004): classes de ocupação do solo.

- Post-fire fuel accumulation curve (for shrublands):

B = 31.6614 (1 – e –0.0917t),

B – biomass (t/ha), t – time (years)

- Definition of 5 risk classes associated with fuel accumulation):

Very low: B < 5 t/haLow: 5 < B < 10 t/haMedium: 10 < B < 15 t/haLow: 15 < B < 20 t/haVery low: B > 20 t/ha

Fire risk – Fuel accumulation

- Two-way table and map combining NDVI class and post-fire fuel accumulation class.

- Integrates fuel quantity and moisture status. Represents “fast” vegetation variables.

Fire risk – NDVI x Fuel accumulation

- Two-way table and map combining the slow variables (slope, climate, population) and the fast vegetation variables.

Fire risk – Slow x Fast variables

Validation

- Calculation of relative fire incidence:

RI =Portugalin x classrisk proportion

xclassrisk burned area proportion

Fire risk & area burned

Fire perimeters

County borders

- This was a “quick-and-dirty” approach, combining previously existing datasets, and methodologies we were familiar with.

- We want to improve it, make it more rigorous. We’d like to represent both fast and slow risk components in a probabilistic framework, eliminating the need for the two-way tables to combine incommensurate datasets.

- The concept of a slow risk component, excluding the vegetation factor, and a fast risk component, representing vegetation dynamics (fuel depletion / regrowth) is valuable and should be kept.

- Formal improvement should not reduce effectiveness of the results.

Acknowledgements:

- The Wildifre Risk Map for 2005 was developed at the Departamento de Engenharia Florestal (DEF), Instituto Superior de Agronomia (ISA), in collaboration with the Direcção-Geral dos Recursos Florestais (DGRF), and funded by COTEC Portugal - Associação Empresarial para a Inovação, under the Forest Fires Initiative.