1.1 Interpretation of results
The results of this quantitative analysis support two major findings: that UAS aerial ignition burns were more efficient in terms of ha day− 1 burned, and that those burns were of a comparable standard to hand crew methods. While superficially the UAS appeared to be increasing ha day− 1 on a programmatic scale, statistical analysis provided further evidence that this could be attributed to the UAS. The decorrelation between ha burned and days burned following the use of the UAS is arguably one of the most noteworthy takeaway from our perspective. It communicates the difficulty of burning more ha day− 1 under normal circumstances (r = 0.96 from 2012–2018), and the magnitude of UAS aerial ignition’s impact following its incorporation (r = 0.515 from 2012–2021). The context and scope of our burn program is important when considering these results. Our prescribed fire operations are conducted primarily in Texas and Louisiana, frequently where wildland urban interface occurred, usually in forested understory or native prairie for conservation purposes, during any time of year, and with personnel and equipment resources that were smaller than that of government agencies. While the increase in ha day− 1 burned is emphasized in our analysis, it is not the only underlying goal of our adopting UAS aerial ignition. This new technology also helped mitigate those major challenges mentioned earlier; climate change, the WUI problem, and public perception of fire.
1.2 Impact of UAS aerial ignition on prescribed fire
Where and how UAS aerial ignition mitigated these challenges was highly circumstantial, and specific to each burn project. For example, treatment areas that were remote and unencumbered by WUI and smoke mitigation issues, benefited from the ability to burn much larger areas with the UAS. Additionally, the ability to relocate personnel away from interior ignition responsibilities, and focus them on igniting and patrolling firelines, further enabled larger unit size while still conducting a safe burn. Prior to incorporating the UAS, the largest unit size burned by Raven between 2012–2018 was 172 ha, compared to 453 ha after its use. Where large area burns were possible, this increase in capability enabled burn objectives in less than half as many days, in some cases. Because prescribed fire is heavily dependent on specific weather conditions, this jump in efficiency greatly enabled management goals in the limited number of suitable days available.
Burns of this larger scale were infrequent, though, with smoke management and WUI often constraining unit size. For example, Raven burned 121 ha or more in a day 8.7% of the time, and 202 ha or more in a day 1.5% of the time between 2012–2021. During that same period, the average unit size was 64 ha for all burning. The UAS and its impact may not have been a dramatic increase of ha burned on every occasion, but it enabled safer and more effective prescribed, regardless of unit size. This is especially true because of the previously mentioned ability to reallocate personnel from interior ignition duties to other responsibilities. This not only enables line-holding efforts, but it eliminated the inherit hazards of hand crew interior ignition methods. Those hazards include dehydration, navigation challenges in thick understory vegetation, tripping and falling hazards, and crewmembers entrapping one another while igniting. The UAS could also navigate more precisely, quickly, and with greater awareness than that of hand crews (Beachly, E., et al. 2017b). In the event a fire emergency occurred, namely a spot-over, the UAS ensured that personnel were already well-poised to respond.
These increases in safety helped to mitigate other major prescribed burning challenges. Namely, landowner concern regarding the risk of escaped fires was arguably reduced because of personnel reallocation (Weir et al. 2019). Furthermore, burning with the UAS created new approaches to interior ignition timing, and therefore better smoke management. Throughout a burn day, temporal weather patterns typically allow for the best atmospheric lift of the smoke before 3pm, or 4pm when using aerial ignition (Waldrop and Goodrick 2012). Completing the interior ignition phase of a burn within this favorable window was often crucial, particularly where WUI and smoke sensitive receptors are nearby. The UAS could perform this task in a window of time previously unobtainable when using hand crew methods, allowing for more targeted timing of interior ignition and a decreased likelihood of negatively impacting surrounding areas with smoke.
These new igniting capabilities could be leveraged and strategized in numerous ways outside of smoke management, too. Igniting timing and techniques could be altered quickly in response to changing environmental conditions and fire behavior (Beachly et al. 2017a, 2018). This flexibility enabled fuel management goals in several ways. For example, when fire behavior was excessive and threaten to damage forested overstory, a smaller ignition window could be moved to a less impactful time of day. On one occasion, an early and brief afternoon shower temporarily raised relative humidity, a situation that might normally eliminate burning altogether. However, the UAS’s efficiency still allowed the burn to commence after some drying, and valuable time and resources were not lost.
For burn programs of a larger scale, such as the U.S. Forest Service or Bureau of Land Management, UAS aerial ignition provides an alternative to manned aircraft operations. The risks involved in these operations have a history of fatal accidents, with one occurring on the Sam Houston National Forest as recently as 2019 (Gabbert 2022). The UAS and IGNIS platform used in this study might not be capable of completing substituting for manned operations, with some prescribed burns being on the scale of well over 405 ha (Waldrop and Goodrick 2012), but it can minimally displace some portion of them. Any prescribed fire where a UAS has replaced the role of manned operations is safer, and therefore a worthwhile consideration. The cost savings of UAS versus manned operations is also significant and can help agencies accomplish more burning objectives with the same financial resources.