Short answer: The woods are very dry

by Don Moniak
June 27, 2024
(Updates: July 5, 2024: Latest national drought map shows that in the past week most of Aiken County has regressed into moderate drought and the rest of the county is “abnormally dry.” See new map below Figures 1 and 2. The KBDI index remains in the 500-600 range. August 13, 2024: Since July 5th, the area has received 12-16 inches of rain. Bush Field in Augusta measured 9.5 inches in July alone, and the rain from Tropical Storm Debbie brought anywhere from 2 to 6 inches to the Aiken area.).

On Monday, June 24th, the Graniteville-Warrenville-Vaucluse (GVW) Volunteer Fire Department responded to a wildfire near Old Sudlow Lake Road and adjacent to Hunter’s Glen subdivision, that originated from an escaped campfire.

The proximity of the fire to Hunter’s Glen subdivision further prompted a request for South Carolina Forestry Commission crews, who brought two tractors that were used to contain the fire at two acres.

What was unusual about this fire was not so much a neglected campfire next to a large subdivision, it was the response time of 4:30 a.m, a time of day characterized by higher humidity—and obviously no solar heating. Any fire activity during this time of day is a powerful indication of how dry the woods are across Aiken County.

National Weather Service data shows only one day with more than a half inch of precipation (June 17) in the past three weeks, and no rain in the past ten days. Temperatures have steadily risen from the low 90’s to above 100 degrees and relative humidity has steadily declined towards the thirty percent, which is low for this region. Add to this the desiccation of vegetation further caused by intense solar radiation during the longest days of the year, and the net result is high wildfire danger.

The current pattern is similar to the period of May 17 to June 7, 2019, which featured 24 days without rain, nineteen consecutive days over 90 degrees, and several record setting days exceeding 100 degrees. Relative humidities dropped as low as the teens. Overall, the extremely dry conditions were later determined to constitute a “flash drought,” which the National Integrated Drought Information System defines as “the rapid onset or intensification of drought, set in motion by lower-than-normal rates of precipitation, accompanied by abnormally high temperatures, winds, and radiation.”

The key difference during the 2019 flash drought were high winds, which further dried out vegetation; helping to turn normally green fields brown and create an unusually high level of pine needlecast that was described as unprecedented in recent times.*

A significant event during that flash drought period was the 200-acre Gateway Fire along Interstate 20, a fire that spotted across the highway and burned actively during nighttime hours. The Gateway Fire triggered the closure of I-20 for more than a day and the evacuation of dozens of homes.

Long-term Drought Indices

Like 2019, the current level of drought is not well indicated by long-term drought indices. Last week’s U.S. Drought Monitoring Map (Figure 1) showed our area as normal, but not far from counties with “drier than normal” conditions. However, this week’s map (Figure 2) shows Aiken County as “drier than normal,” and not far from counties in moderate drought. This does provide some indication that a flash drought is developing or already in place, but in this case it is a rapid onset of drought rather than an intensification of existing drought.

The latter conclusion is supported by another long-term drought index, the Palmer Drought Severity Index (PDSI), which is “based on a simplified soil water balance and estimates relative soil moisture conditions.” The current PDSI actually shows moist soil conditions in this region (Figure 3).

Figures 1 and 2. June 20 and 27th Drought Monitoring Maps. (Click to enlarge.)

Update: July 5, 2024 map showing moderate drought status for most of Aiken County and much of the surrounding region.

A Short-Term Drought Index

One drought index that better reflects short-term drought conditions and the possibility or presence of a flash drought is the Keetch-Byram Drought Index (KBDI), which is widely utilized as an indicator of potential wildfire spread and intensity. The index best measures the moisture levels of the finest forest fuels, the litter and duff layers.

The South Carolina Forestry Commission (SCFC), which relies upon this index to monitor potential fire activity, describes KBDI as follows:

“The Keetch–Byram Drought Index (KBDI) is used to assess and predict wildfire potential and severity. Calculated once a day based on rainfall, air temperature and other meteorological factors, KBDI values represent the net effect of evapotranspiration and precipitation relating to the flammability of organic material in the ground. The index ranges from zero, the point of no moisture deficiency, to 800, the maximum drought that is possible. The higher the number, the greater potential of wildfires igniting easily and spreading rapidly.”

The Forestry Commission provides daily KBDI mapping updates and measurements from individual weather stations from across the state. In the past week, the KBDI has risen to a level showing the strong potential for dangerous and erratic wildfire behavior. While the fire risk has been somewhat mitigated by the absence of high winds, that could temporarily change with an approaching cold front or other unstable weather system.

The current map shows values in the 500-600 range. The National Wildland Fire Assessment System (WFAS) describes KBDI values of 400 to 600 as “Typical of late summer and early fall conditions. Lower litter and duff layers actively contribute to fire intensity and will burn actively.”

According to the WFAS, KBDI values of 600 – 800 are “Often associated with more severe drought with increased wildfire occurrence. Intense, deep burning fires with significant downwind spotting can be expected. Live fuels can also be expected to burn actively at these levels.” 

In the past day alone, an area of greater than 600 KBDI has emerged in the Columbia area, and a KBDI measurement at one of our closest weather stations, on the Southeast portion of Savannah River Site, increased from 564 to 591 (Figures 4 and 5).

This region is approaching KBDI values of 600, and Forestry Commission Firewise and Prevention Coordinator Andy Johnson is certain that “there are localized  areas with more pronounced drought conditions,” with KBDI values in excess of 600.

Figures 4 and 5: KBDI Index, June 26 and 27. (Click to enlarge)

With the Fourth of July and its associated fireworks festivities arriving next week, this dryness and the associated fire danger looms a little larger. Although weather forecasters are predicting increased humidity and a more normal pattern of a chance of scattered thunderstorms, no widespread rain is currently predicted.

The current level of dryness is contributing to the flaring of 25-30 wildfires a day (Figure 5), many of them escaped debris burns, which is putting a strain on SCFC firefighter resources, thus increasing the likelihood of firefighter injuries** during any fire as well as extensive property damage from larger fires.

Footnote

* Needlecast is the phenomenon in which older pine tree needles turn brown and drop to the ground. It is most notable in Longleaf Pine.

Needlecast is a natural response to dry conditions, and functions to conserve water. New growth of brighter green needles is more visible during needlecast. As such, it also provides an indication of live fuel moisture levels are somewhat reflected by the degree of observed pine needlecast; particularly Longleaf Pine.

This is more of a Fall phenomenom, but also occurs in response to drought conditions in late Spring and early Summer. 

In a 2021 article, SC DNR biologist Johnny Stowe wrote that, since the mid-2000’s he has “seen healthy longleaf pines drop needles before the summer solstice only once.” That was in 2019, when significant needlecast dotted the countryside. The dry sandy ridges found across Aiken County.

During this period, the drier sandy ridges of Aiken County developed a fall-like tannish hue, with patches of browner shades dotting the landscape.

When Longleaf Pine drops its needles, it provides a flashy fuel bed that will persist until there is significant rainfall.

** On April 4, 2024 U.S. Forest Service/Savannah River Site firefighter sustained second degree burns during a prescribed burn. The Occurrence Report for the accident stated that the firefighter “received second degree burns to approximately two percent of his body while attempting to clear a fire line around a dead tree within the boundary of Savannah River Site.”

Below: Example of Longleaf Pine needlecast. Note the proliferation of fresh green needles above the brown needles that will fall during strong winds or heavy rain.