The Mapping and Analysis of the Infrequent, Large-Scale Blowdown Event in the Colorado Front Range

Wind disturbance (blowdown, windthrow) results from particular meteorological conditions, where wind gust speed is a key factor. We map and analyze the 2020 blowdown in the Colorado Front Range (CFR), US. We (1) develop and test a tunable blowdown mapping workflow based on Sentinel 2 data and change vector analysis (CVA) and compare its output with the Global Forest Change (GFC) data, (2) explore soil disturbance patterns, and (3) analyze the impact of topography on blowdown occurrence. The CVA mapping is based on (1) the difference image computed using post- and pre-event images and (2) the parameters calculated using two bands of the difference image: magnitude representing the amount of change and direction referring to the type of change. The methodology is tunable for desired error characteristics, for example, true positive vs. false positive rates. For our test analysis, we balanced the CVA output and GFC data at a true positive rate (TPR) of 66%–67%, with a false positive rate (FPR) of 0.9% and 3%, respectively. The CVA can be adjusted to achieve a TPR up to 88.7%, which increases the FPR to 17.8%. In our test landscape, the blowdown led to soil disturbances, with root plate volumes of 0.1–0.8 m³.