Using Temporal "Rubber-Sheeting" to Match an Annual Phenological Profile to a Reference Profile for Direct Comparison of Vegetation Status and Health
Abstract
Interannual differences in timing of phenology make direct comparisons of vegetation health and performance between years difficult, whether at the same or different locations. Considering one phenology curve to be a reference, time intervals within a second phenology profile can be "rubber-sheeted" to fit that baseline as well as possible, just by moving observations temporally in the second curve. Time intervals within the second phenology profile are compressed or dilated such that the sum of squared differences between the two curves is minimized. Similar to "rubber-sheeting" to georectify a map inside a GIS, both curves match at a pair of control points, which can be any recognizable phenological events or milestones. Temporal rubber-sheeting shows how many days the phenological development is ahead or behind the seasonality of the reference vegetation at every time interval. Rubber-sheeting can be used to impute missing NDVI values that were unobservable due to clouds or snow, since time-warped NDVI values can be used for that location on days for which no observations exist.
Temporally adjusting phenologies can quantify vegetation impacts from frost, drought, and insects and diseases, by permitting quantitative comparisons with unaffected or pre-disturbance vegetation. NDVI amplitude differences between the two annual profiles increase the sum-of-squared-errors, but do not necessarily produce differences in phenological timing. Because a flat profile will show no timing differences from a non-flat profile, even catastrophic disturbances like tornadoes, hurricanes and wildfires show strong and and increasing inital temporal departures, as an increasingly early "fall" drop, before resuming temporal synchrony. Only the growing inital departures in phenological timing are shown, despite lasting post-disturbance decreases in NDVI. Rubber-sheeting every grid cell's post-disturbance annual phenology curves across an entire map produces a series of maps through time showing disturbance in terms of days early/late relative to pre-disturbance annual phenology. Although every cell's temporal sequence is adjusted independently, patterns of days early/late are cohesive across space and through time. An example shows impacts of the May 2007 frost affecting much of the southeastern United States.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.B32E..02H
- Keywords:
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- 0414 Biogeochemical cycles;
- processes;
- and modeling;
- BIOGEOSCIENCES;
- 0439 Ecosystems;
- structure and dynamics;
- BIOGEOSCIENCES;
- 0480 Remote sensing;
- BIOGEOSCIENCES;
- 1630 Impacts of global change;
- GLOBAL CHANGE;
- 1632 Land cover change;
- GLOBAL CHANGE;
- 1637 Regional climate change;
- GLOBAL CHANGE;
- 1640 Remote sensing;
- GLOBAL CHANGE