The Drought Fingerprint Plot

Most meteorological drought indices depend on conditions during a fixed time interval (such as the Standardized Precipitation Index), a fixed range of times (such as an SPI calculated with precipitation weighting), or are autoregressive with a predominant time scale (such as the Palmer Drought Severity Index). The abrupt transitions of the SPI are problematic, and it can be difficult to reconcile different index values when they are simultaneously sensitive to different aspects of drought and different time scales of drought.

The Drought Fingerprint Plot (DFP) is a way of overcoming these limitations to provide insights into the time scales driving existing droughts. The plot shows accumulated precipitation, working backward from the present (or any desired date) over as much as five years of precipitation. To interpret the plot in a drought context, the background is shaded according to historical percentiles of accumulated precipitation from the same station or a nearby long-term reference station. The shading corresponds to the US Drought Monitor percentiles, so that in a sense the DFP depicts the D-level interpretation of the SPI continuously at all possible relevant time scales. It is not quite the SPI, because the SPI uses a fitted probability distribution and the DFP uses raw percentiles (excluding the precipitation instance being plotted), but the interpretation is the same. Additional background colors represent the higher percentiles of precipitation analogously to the D-levels. Completing the depiction are lines corresponding to the median of accumulated precipitation as well as the historical minima and maxima of accumulated precipitation.

The accompanying image shows an example from the Louisiana flash drought of 2023. Despite about an inch of recent rain, the accumulated precipitation at Lake Charles at the end of August hovered around the D3-D4 boundary as far back as early May, including record low accumulated precipitation since early July. Contributing to the drought impacts may have been a separate spell of dry conditions in late 2021 and early 2022, such that overall accumulated precipitation since late 2021 also met the D3 criterion. The record high temperatures during the summer of 2023 break the tie in favor of D4 for Lake Charles, as correctly depicted in the US Drought Monitor on August 29, 2023. This example illustrates how the DFP can provide a comprehensive picture of rainfall deficits over time at a given location, facilitating accurate drought assessment.