MAPPING DEPTH TO BEDROCK IN A TROPICAL PRE-MONTANE WET FOREST IN COSTA RICA

Accounting for all components of the water balance in a watershed includes an estimate of soil water storage, which in turn depends on the depth to bedrock. The soils in this transitional tropical forest contain large amounts of amorphous material from the saprolitic tuff thus classifying the soils as Andisols. Measuring the depth to bedrock in tropical montane environments is complicated by aspect, elevation, slope, landslides, slumping and other mass wasting events. As part of a larger study, Texas A& M Costa Rica REU aimed to close the water budget for a tropical pre-montane forest, the focus of this study is to generate a map of the depth to saprolitic tuff and topographical information for the purpose of estimating the volume of soil water storage in the Howler Monkey Watershed at Texas A&M University Soltis Center for Research and Education, San Isidro de Peñas Blancas, Costa Rica. A map of the depth to saprolitic tuff was created using 101 hand- augured holes (over 2.63 ha) spatially distributed throughout the watershed. Saprolitic tuff was defined as being 50% of the sample and containing grittiness and cobble sized chunks. To characterize the soils throughout the watershed, soil horizons at three sites were described and 22 cores for particle size. The cores consisted of over 40-55% clay classifying them as clayey or clayey loam. The samples also ranged from 50-73% water content. A map showing the slopes within the watershed also shows the relationship of soil depth above the bedrock within the watershed. The slopes across the watershed vary from 12-65 degrees but only have a 24% correlation with the depth to saprolitic tuff. Results suggest that the depth of the saprolitic tuff is quite sensitive to small scale topographic variability. Soil with such high water content becomes an integral part of the water budget since a significant portion of the water is maintained within the soil. Depth to bedrock provides necessary data to estimate the total volume of soil within the watershed and ultimately the hydrology of the area.