Near-surface geophysical methods at multiple scales of measurement to understand variation in erosion rates and water storage in the Luquillo CZO, Puerto Rico
Abstract
We used a unique array of geophysical methods to image the critical zone in the Luquillo Mountains of Puerto Rico, and thereby provide information on belowground features at different resolution and sampling volumes, including shallow roots, regolith thickness, spatial extent of fractures, and depth to the water table and/or to the bedrock. Methods included ground-penetrating radar, terrain conductivity, capacitively-coupled resistivity, and shallow seismic and were constrained with direct cores and outcrops where available. Earlier studies in the area provide evidence that spatial variation in erosion rates over a chemically homogenous bedrock can explain variations in saprolite thickness, soil mineralogy and structure, forest composition and above ground biomass. Variations in erosion rate are here triggered by landscape rejuvenation, in the form of upstream-migrating waves of erosion, such that soils and saprolites thicken abruptly in the upstream direction. Rejuvenation may add its effects to an independent trend in uphill-soil thickening driven by upslope increase in orographic precipitation. Deep soils and saprolites at high elevations, may then store a substantial part of the orographic precipitation high in the mountain and release it year-around, rather than let it be lost rapidly to the sea through shallow runoff. In order to quantify how spatial variation in erosion rate controls groundwater retention and underground biomass distribution, we conducted geophysical surveys over areas with highly varying erosion rates supporting different plant communities. Results show that more slowly eroding areas are not only underlain by thicker soil but also by very thick saprolite, which includes vadose and saturated regions. This study shows the importance of multi-method geophysical approaches for better understanding and imaging the critical zone at different scales.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMNS41B0821C
- Keywords:
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- 0416 Biogeophysics;
- BIOGEOSCIENCESDE: 0999 General or miscellaneous;
- EXPLORATION GEOPHYSICSDE: 1824 Geomorphology: general;
- HYDROLOGYDE: 1835 Hydrogeophysics;
- HYDROLOGY