Geomorphology

Banana Crop Expansion and Increased River-borne Sediment Exports to the Gulf of Uraba, Caribbean Coast of Colombia

Blanco-Livreros, J.F. 2009. Banana Crop Expansion and Increased River-borne Sediment Exports to the Gulf of Uraba, Caribbean Coast of Colombia. AMBIO: A Journal of the Human Environment 38(3):181-183. 2009. doi: 10.1579/0044-7447-38.3.181.

Abstract: 
Sedimentation is a major environmental issue in the Gulf of Uraba´ (southern Caribbean coast of Colombia) (1), yet driving forces and influences on coastal ecosystems are poorly known by local scientists and natural resource managers. This is partially a result of the small temporal and spatial windows employed in scientific studies and management strategies that hide the complex interactions among hydrologic, geographic, and socioeconomic processes operating at basin level. For instance, sediment accumulation in the nearshore has been managed at a local scale by dredging access channels in shoaling areas and by constructing barriers to prevent littoral erosion at other sites, with little recognition of sediment sources and transport (2). Moreover, causes of such problems have not been investigated throughout entire river basins, several of which are world-class producers of banana, and therefore become potential sources of sediments to be transported to the coastal zone. Elsewhere, conversion of native forest to agriculture lands has been implicated as a major driver of increased land erosion and sediment yields (3). Specifically, deforestation and urbanization may have accounted for a positive trend in sediment delivery to the Caribbean Sea through the Magdalena River basin, Colombia’s largest system (4). Agricultural lands doubled in area between 1970 and 1990, while sediment loads increased in 17 out of 32 river subbasins. Maximum water discharge and a deforestation index explained 96% of the variation in sediment yield across the upper Magdalena basin. Here, I discuss the hypothesis that sediment yields in the Uraba´ region are greater in river basins with extensive banana crops than in basins under other land covers.

Geomorphology, disturbance, and the soil and vegetation of two subtropical wet steepland watersheds of Puerto Rico

F.N. Scatena, Ariel E. Lugo 1995. Geomorphology, disturbance, and the soil and vegetation of two subtropical wet steepland watersheds of Puerto Rico.. Geomorphology 13 :199-213.

Abstract: 
Relationships between landforms, soil nutrients, forest structure, and the relative importance of different disturbances were quantified in two subtropical wet steepland watersheds in Puerto Rico. Ridges had fewer landslides and treefall gaps, more above-ground biomass, older aged stands, and greater species richness than other landscape positions. Ridge soils had relatively low quantities of exchangeable bases but high soil organic matter, acidity and exchangeable iron. Valley sites had higher frequencies of disturbance, less biomass, younger aged stands, lower species richness and soils with more exchangeable bases.Soil N, P, and K were distributed relatively independently of geomorphic setting, but were significantly related to the composition and age of vegetation. On a watershed basis, hurricanes were the dominant natural disturbance in the turnover of individuals, biomass, and forest canopy. However, turnover by the mortality of individuals that die without creating canopy openings was faster than the turnover by any natural disturbance. Only in riparian areas was forest turnover by treefall gaps faster than turnover by hurricanes. The same downslope mass transfer that links soil forming processes across the landscape also influences the distribution of landslides, treefall gaps, and the structure and composition of the forest. One consequence of these interactions is that the greatest aboveground biomass occurs on ridges where the soil nutrient pools are the smallest. Geomorphic stability, edaphic conditions, and biotic adaptations apparently override the importance of spatial variations in soil nutrients in the accumulation of above-ground biomass at this site.

An Introduction to the Physiography and History of the Bisley Experimental Watersheds in the Luquillo Mountains of Puerto Rico

Scatena, F. N. 1989. An introduction to the physiography and history of the Bisley Experiment Watersheds in the Luquillo
Mountains of Puerto Rico. General Technical Report SO-72:1–
22. U.S. Department of Agriculture, Southern Forest Experiment
Station, New Orleans.

Abstract: 
This paper summarizes the physiographic setting and historical uses of the Bisley experimental watersheds. These watersheds are the site of long-term watershed studies in the Luquillo Experimental Forest of Puerto Rico. Each of these watersheds drains deep, clayey soils that overlie a highly dissected terrain underlain by volcanoclastic sandstones. The drainages are covered by secondary tabonuco type forests and receive about 3,500 mm&r of rainfall. Since European settlement, about 490 years ago, the study area has been explored for precious ores, cultivated, and selectively logged. The major obstacle to the exploitation of the resources of the watersheds has been inaccessibility. High rainfall, steeply sloping terrain, and slippery clay soils combine to make transportation in the area difficult. The most rapid change to the Bisley landscape occurred at the end of the 19th century and the beginning of the 20th century. During this time, local agricultural activity was at a maximum, timber was being exported from the region, and copper mines were active in the Rio Blanc0 area. In addition to human activity, major hurricanes occurred in 1892 and 1932. Human-induced disturbance in the watersheds has been selective in both space and time. The pattern of disturbance is contrary to that described in other temperate and tropical forests. Anthropogenic disturbance in these watersheds has apparently increased the spatial heterogenity of the forest. The success of both natural and induced regeneration in the area suggests that the impact of human disturbance was greater on forest structure than on its long-term productivity.

Riparian Nitrogen Dynamics in Two Geomorphologically Distinct Tropical Rain Forest Watersheds: Subsurface Solute Patterns

Riparian Nitrogen Dynamics in Two Geomorphologically Distinct Tropical Rain Forest Watersheds: Subsurface Solute Patterns
William H. McDowell, William B. Bowden and Clyde E. Asbury
Biogeochemistry
Vol. 18, No. 2 (1992), pp. 53-75

Abstract: 
Nitrate, ammonium, dissolved organic N, and dissolved oxygen were measured in stream water and shallow groundwater in the riparian ones of two tropical watersheds with different soils and geomorphology. At both sites, concentrations of dissolved inorganic N (DIN: NH-4+ and NO-3-N) were low in stream water lt 110 mu-g/L). Markedly different patterns in DIN were observed in groundwater collected at the two sites. At the first site (Icacos watershed), DIN in upslope groundwater was dominated by NO-3-N (550 mu-g/L) and oxygen concentrations were high (5.2 mg/l). As groundwater moved through the floodplain and to the stream, DIN shifted to dominance by NH-4+-N (200-700 mu-g/L) and groundwater was often anoxic. At the second site (Bisley watershed), average concentrations of total dissolved nitrogen were considerably lower (300 mu-g/L) than at Icacos (600 mu-g/L), and the dominant form of nitrogen was DON rather than inorganic N. Concentrations of NH-4+ and NO-3- were similar throughout the riparian zone at Bisley, but concentrations of DON declined from upslope wells to stream water. Differences in speciation and concentration of nitrogen in groundwater collected at the two sites appears to be controlled by differences in redox conditions and accessibility of dissolved N to plant roots, which are themselves the result of geomorphological differences between the two watersheds. At the Icacos site, a deep layer of coarse sand conducts subsurface water to the stream below the rooting zone of riparian vegetation and through zones of strong horizontal redox zonation. At the Bisley site, infiltration is impeded by dense clays and saturated flow passes though the variably oxidized rooting zone. At both sites, hydrologic export of nitrogen is controlled by intense biotic activity in the riparian zones. However, geomorphology appears to strongly modify the importance of specific biotic components.

SLOPEWASH, SURFACE RUNOFF AND FINE-LITTER TRANSPORT IN FOREST AND LANDSLIDE SCARS IN HUMID- TROPICAL STEEPLANDS, LUQUILLO EXPERIMENTAL FOREST, PUERTO RICO

Larsen, M.C., Torres-Sánchez, A.J., and Concepción, I.M., 1998, Slopewash, surface runoff, and fine-litter transport in forest and landslide scars in humid-tropical steeplands, Luquillo Experimental Forest, Puerto Rico [abs] EOS, Transactions American Geophysical Union, vol. 80.

Abstract: 
Slopewash, surface runoff, and fine-litter transport in forest and landslide scars in humid-tropical steeplands, Luquillo Experimental Forest, Puerto Rico Rainfall, slopewash (the erosion of soil particles), surface runoff, and fine-litter transport at humid-tropical steepland sites in the Luquillo Experimental Forest, Puerto Rico (18° 20' N, 65° 45' W) were measured from 1991 to 1995. Hillslopes underlain by: 1) Cretaceous tuffaceous sandstone and siltstone in subtropical rain (tabonuco) forest with vegetation recovering from Hurricane Hugo (1989); and underlain by 2) Tertiary quartz diorite in subtropical lower montane wet (colorado and dwarf) forest with undisturbed forest canopy were compared to recent landslide scars. Monthly surface runoff on these very steep hillslopes (24° to 43°) was only 0.2 to 0.5 percent of monthly rainfall. Slopewash was higher in sandy loam soils whose parent material is quartz diorite (averaging 46 g m-2 a-1) than in silty-clay loam soils derived from tuffaceous sandstone and siltstone where the average was 9 g m-2 a-1. Annual slopewash of 100 to 349 g m-2 on the surfaces of two recent, small landslide scars was measured initially but slopewash decreased to only 3 to 4 g m-2 a-1 by the end of the study. The mean annual mass of fine litter (mainly leaves and twigs) transported downslope at the forested sites ranged from 5 to 8 g m-2 and was lower at the tabonuco forest site, where post-Hurricane Hugo recovery is still in progress. Mean annual fine-litter transport was 2.5 g m-2 on the two landslide scars.

Infiltration on mountain slopes: a comparison of three environments

Harden,Carol P.; Scruggs, P. Delmas 2003. Infiltration on mountain slopes: a comparison of three environments.. Geomorphology 55 ;5 -24.

Abstract: 
Water is well established as a major driver of the geomorphic change that eventually reduces mountains to lower relief landscapes. Nonetheless, within the altitudinal limits of continuous vegetation in humid climates, water is also an essential factor in slope stability. In this paper, we present results from field experiments to determine infiltration rates at forested sites in the Andes Mountains (Ecuador), the southern Appalachian Mountains (USA), and the Luquillo Mountains (Puerto Rico). Using a portable rainfall simulator–infiltrometer (all three areas), and a single ring infiltrometer (Andes), we determined infiltration rates, even on steep slopes. Based on these results, we examine the spatial variability of infiltration, the relationship of rainfall runoff and infiltration to landscape position, the influence of vegetation on infiltration rates on slopes, and the implications of this research for better understanding erosional processes and landscape change. Infiltration rates ranged from 6 to 206 mm/h on lower slopes of the Andes, 16 to 117 mm/h in the southern Appalachians, and 0 to 106 mm/h in the Luquillo Mountains. These rates exceed those of most natural rain events, confirming that surface runoff is rare in montane forests with deep soil/regolith mantles. On well-drained forested slopes and ridges, apparent steadystate infiltration may be controlled by the near-surface downslope movement of infiltrated water rather than by characteristics of the full vertical soil profile. With only two exceptions, the local variability of infiltration rates at the scale of 10j m overpowered other expected spatial relationships between infiltration, vegetation type, slope position, and soil factors. One exception was the significant difference between infiltration rates on alluvial versus upland soils in the Andean study area. The other exception was the significant difference between infiltration rates in topographic coves compared to other slope positions in the tabonuco forest of one watershed in the Luquillo Mountains. Our research provides additional evidence of the ability of forests and forest soils to preserve geomorphic features from denudation by surface erosion, documents the importance of subsurface flow in mountain forests, and supports the need for caution in extrapolating infiltration rates.

ESTIMATING THE HUMAN IMPACT ON SUSPENDED SEDIMENT LOADS IN MOUNTAIN STREAMS OF EASTERN PUERTO RICO

Abstract: 
Worldwide, suspended sediment flux entering the oceans each year is increasing due to human activities. Land development in Puerto Rico is causing the island’s mean annual suspended sediment flux to increase. These high suspended sediment yields are unfavorable for human recreation, water treatment, as well as habitat protection. This study attempts to determine if land development in eastern Puerto Rico is significantly impacting suspended sediment (SS) levels. In eastern Puerto Rico lies the Luquillo Experimental Forest (LEF), which has been protected from land development since the 1940s. With 75 percent of the Island’s virgin forests, seven streams (Quebrada 1, Quebrada 2, Quebrada 3, Rio Mameyes Puente Roto, Quebrada Sonadora, Quebrada Guaba, and Rio Icacos) were chosen to represent “natural” suspended sediment conditions. Examining the relationships between drainage area, discharge, suspended sediment concentrations, slope and suspended sediment loads, these seven streams were compared to one stream outside of the national forest, Rio Mameyes Rt. 3, whose watershed is partially used for agriculture and urban purposes. Results indicate that humans are not significantly impacting suspended sediment levels in the Rio Mameyes Rt. 3. Because land development in the Rio Mameyes Rt. 3 watershed is relatively small, less than 20 percent, it is feasible that the agriculture and urban development are not drastically affecting SS levels or the sampling period did not capture the full range of variability. A principal component analysis of drainage area, suspended sediment load, discharge, slope and temperature of the eight streams shows that only one factor, which contains all components, can be extracted.

Morphology and sedimentation in the Caribbean montane streams: examples from Jamaica and Puerto Rico

Ahmad R, Scatena FN, Gupta A. 1993. Morphology and sedimentation in Caribbean montane streams: examples from Jamaica and Puerto Rico. Sedimentary Geology 85: 157–169.

Abstract: 
This paper presents a summary description of the morphology, sedimentation, and behaviour of the montane streams of eastern Jamaica and eastern Puerto Rico. The area is located within a 200 km wide seismically active zone of Neogene left-lateral strike-slip deformation which defines the plate boundary between the Caribbean and North American Plates. Tropical storms, occasionally strengthening up to hurricane force, affect the region periodically. This is an area of steep, mass-movement-scarred hillslopes which supply a large amount of coarse sediment to the rivers. From the description presented, we have constructed a model for the rivers of this region controlled by both neotectonics and periodic large floods. The drainage density is low with a near-rectangular stream network. The gradients are steep with boulder accumulations in the channels, their location at times related to the presence of large past landslides on hillslopes. Narrow, steep and confined channels occur in the mountains, but in wider sections and lower down near coastal plains, flood depositional forms appear in coarse valley alluvium. Small-scale deviations from the general pattern occur locally, controlled by variations in lithology, neotectonism, seasonality in flow, etc. This model for Caribbean montane streams differs considerably from the standard descriptions of alluvial rivers for which a number of detailed studies are available.

Rainfall-induced landslide susceptibility zonation of Puerto Rico.

Lepore, C. Kamal, S. A., Shanahan, P. Bras, R. L., Rainfall-induced landslide susceptibility zonation of Puerto Rico. Environmental Earth Science, 2011. DOI 10.1007/s12665-011-0976-1

Abstract: 
Landslides are a major geologic hazard with estimated tens of deaths and $1–2 billion in economic losses per year in the US alone. The island of Puerto Rico experiences one or two large events per year, often triggered in steeply sloped areas by prolonged and heavy rainfall. Identifying areas susceptible to landslides thus has great potential value for Puerto Rico and would allow better management of its territory. Landslide susceptibility zonation (LSZ) procedures identify areas prone to failure based on the characteristics of past events. LSZs are here developed based on two widely applied methodologies: bivariate frequency ratio (FR method) and logistic regression (LR method). With these methodologies, the correlations among eight possible landslide-inducing factors over the island have been investigated in detail. Both methodologies indicate aspect, slope, elevation, geological discontinuities, and geology as highly significant landslide-inducing factors, together with land-cover for the FR method and distance from road for the LR method. The LR method is grounded in rigorous statistical testing and model building but did not improve results over the simpler FR method. Accordingly, the FR method has been selected to generate a landslide susceptibility map for Puerto Rico. The landslide susceptibility predictions were tested against previous landslide analyses and other landslide inventories. This independent evaluation demonstrated that the two methods are consistent with landslide susceptibility zonation from those earlier studies and showed this analysis to have resulted in a robust and verifiable landslide susceptibility zonation map for the whole island of Puerto Rico.
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