Hillslopes in Humid-tropical Climates Aren’t Always Wet: Implications
for Hydrologic Response and Landslide Initiation in Puerto Rico
Abstract
The devastating impacts of the widespread flooding and landsliding in
Puerto Rico following the September 2017 landfall of Hurricane Maria
highlight the enhanced hazard potential from increasingly extreme storms
in mountainous humid-tropical climate zones. Long-standing conceptual
models for hydrologically driven hazards in Puerto Rico posit that
hillslope soils remain wet throughout the year and antecedent soil
wetness imposes a negligible effect on hazard potential. Our post-Maria
in situ hillslope hydrologic observations indicate that while some
slopes remain wet throughout the year, others exhibit appreciable
seasonal and intra-storm subsurface drainage. Therefore, we used
receiver-operating characteristic analysis and the Threat Score (TS)
skill statistic to evaluate the performance of hydro-meteorological
(soil wetness and rainfall) versus intensity-duration (rainfall only)
hillslope hydrologic response thresholds that identify the onset of
positive pore-water pressure, a predisposing factor for widespread slope
instability in this region. We found that the hydro-meteorological
thresholds outperformed intensity-duration thresholds for a seasonally
wet, coarse-grained soil (TS = 0.8 vs. 0.6, respectively), although they
did not outperform intensity-duration thresholds for a perennially wet,
fine-grained soil (TS = 0.2 vs. 0.2, respectively). These soils types
may also produce radically different stormflow responses, with
subsurface flow being more common for the coarse-grained soils underlain
by intrusive rocks versus infiltration excess and/or saturation excess
for the fine-grained soils underlain by volcaniclastic rocks. We
conclude that variability in soil-hydraulic properties, as opposed to
the humid-tropical climate zone, is the dominant factor that controls
runoff generation and modulates the importance of antecedent soil
wetness for our hillslope hydrologic response thresholds. Our findings
encourage further deployment of continuous in situ hydrologic monitoring
to facilitate the development of empirical hillslope hydrologic response
and landslide thresholds for regional-scale hazard warning systems that
must account for spatially variable soil types.