How the MBDI Compares
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The relationship between the Moisture Balance Drought Index (MBDI) and the number of acres burned in Arizona wildfires is inconsistent, as is often the case when considering the influence of climate variables on wildfires. An inconsistency of the effect of fuels – such as the dry grass and dead wood that carry fires – is likely part of the reason for the inconsistency in matching fire to climate.
The analysis described here involved a comparison between MBDI and the number of acres burned annually during wildfire seasons from 1980 to 2004. For this comparison, the state was divided into four quadrants, with seasonal values for acres burned and MBDI averaged for each quadrant. For each quadrant, the eight seasons with the most acres burned were compared to the eight seasons with the least acres burned. Results reported here emphasize how the values relate during the months of May, June and July.
Few consistent differences showed up. For this comparison, the southwestern Arizona quadrant best reflected the expectation that big fire years would occur when MBDI values were relatively low (i.e., drier conditions). “Big fire years” are years with large numbers of acres burned relative to the historic average.
At the 12-month scale, MBDI values almost always averaged below 0.5 during big fires years, and near or above 1.0 during low fire years. Even greater differences in range occurred when considering the 6-month scale, while comparable differences held for the 3-month scale.
In the southeastern Arizona quadrant, MBDI values averaged 0.10 or below during big fire years, and above 0.4 during low fire years at the 12-month scale. At the 1-month scale, values were consistently negative during big fire years and positive during low fires years. Little difference for MBDI values occurred at the 6-month and 3-month scale.
In both quadrants in northern Arizona, average values for MBDI were inconsistent. MBDI values were higher during low fire years at the 3-month scale for both quadrants, and at the 6-month scale for the northeastern quadrant as well. The lack of consistency is not surprising, considering the factors that can affect wildfire.
For instance, forest wildfires tend to burn most extensively during dry years (when MBDI is low), but grassland fires can become more widespread during wet years (when MBDI is high), or years with moderate MBDI values, but large amounts of grass fuels carried over from a previous wet summer. This complexity is illustrated by comparison of two years with large numbers of acres burned in Arizona, 2005 and 2002, described below.
In response to the wet winter of 2004-2005, grasses and other fine fuels grew abundantly during the early spring, leaving extensive fodder for grassland fires that occurred following several dry, hot months later that spring. More than 700,000 acres burned during the summer of 2005, chiefly in rangeland environments northwest of Phoenix, Arizona. That amount surpassed even the approximately 500,000 acres burned in the 2002 season, when extreme drought contributed to large-scale wildfires in forests in northern Arizona.
Further, even in forests, wet years can encourage the growth of ground cover that can contribute to big fires during dry seasons in subsequent years (Swetnam and Betancourt, 1998). Finally, the number of acres burned also responds to management practices, including efforts at suppressing wildfires.
For example, strategic placement of firefighting resources and personnel in areas with high fire potential even before the fire season starts can help firefighters more effectively stamp out fires before they gain a hold on the landscape. Making the public aware of the risk can help reduce the number of human-caused fire starts, which in turn could keep firefighters from being spread too thin in their efforts to contain and control wildland fires.
Analysis continues to untangle some of the complexities in relating the MBDI to the number of acres burned in wildfires.
Caption for intext table/graphic: The average MBDI value for the Arizona quadrant in question (see map above) is shown in this table. Low values reflect dry years of high evaporative demand, while higher values reflect relatively wet years with less evaporative demand.
Credit: Table and graphic design by Jorge Arteaga.