Effect of land cover on runoff in the USVI

The figure below shows computer-simulated runoff in three USVI watersheds that results from an identical 2.54 inch, 9-hour rainfall recorded at the Cyril E King airport on St. Thomas on 11/8/2018. Runoff simulations were done using 1985 land cover and 2018 land cover for each watershed. A detailed explanation of the figure is included below the figure.

The x-axis, at bottom, indicates the number of hours following the beginning of a rainstorm that dropped 2.54 inches of rain at the Cyril E King airport in St. Thomas on 11/8/2018. The left-side y-axis is the amount of rainfall recorded per hour, and relates to the gray line on the chart. Note the gray line on all three charts is identical.

The right-side y-axis indicates runoff, as a proportion of the maximum runoff in the 1985 simulation. Runoff using 1985 land cover is shown in blue, runoff using 2018 land cover is shown in red. Thus, 1985 runoff peaks at 100% in all three charts. Runoff increased when model inputs were changed from 1985 to 2018 land cover. Therefore, 2018 runoff peaks at 143%, 397%, and 240% in the simulations for Queens, Bethlehem, and Guinea Guts, respectively. The difference between the blue and red lines represents the runoff attributed solely to the change in land cover from 1985 to 2018.