Models

A spheroidal weathering model coupling porewater chemistry to soil thicknesses during steady-state denudation

Fletcher, R.C., Buss, H.L., Brantley, S.L., 2006.Aspheroidal weathering
model coupling porewater chemistry to soil thicknesses during
steady-state denudation. Earth Planet. Sci. Lett. 244, 444–457.

Abstract: 
Spheroidal weathering, a common mechanism that initiates the transformation of bedrock to saprolite, creates concentric fractures demarcating relatively unaltered corestones and progressively more altered rindlets. In the spheroidally weathering Rio Blanco quartz diorite (Puerto Rico), diffusion of oxygen into corestones initiates oxidation of ferrous minerals and precipitation of ferric oxides. A positive ΔV of reaction results in the build-up of elastic strain energy in the rock. Formation of each fracture is postulated to occur when the strain energy in a layer equals the fracture surface energy. The rate of spheroidal weathering is thus a function of the concentration of reactants, the reaction rate, the rate of transport, and the mechanical properties of the rock. Substitution of reasonable values for the parameters involved in the model produces results consistent with the observed thickness of rindlets in the Rio Icacos bedrock (≈2–3cm) and a time interval between fractures (≈200–300 a) based on an assumption of steady-state denudation at the measured rate of 0.01cm/a. Averaged over times longer than this interval, the rate of advance of the bedrock–saprolite interface during spheroidal weathering (the weathering advance rate) is constant with time. Assuming that the oxygen concentration at the bedrock–saprolite interface varies with the thickness of soil/saprolite yields predictive equations for how weathering advance rate and steady-state saprolite/soil thickness depend upon atmospheric oxygen levels and upon denudation rate. The denudation and weathering advance rates at steady state are therefore related through a condition on the concentration of porewater oxygen at the base of the saprolite. In our model for spheroidal weathering of the Rio Blanco quartz diorite, fractures occur every ∼250yr, ferric oxide is fully depleted over a four rindlet set in ∼1000yr, and saprolitization is completed in ∼5000yr in the zone containing ∼20 rindlets. Spheroidal weathering thus allows weathering to keep up with the high rate of denudation by enhancing access of bedrock to reactants by fracturing. Coupling of denudation and weathering advance rates can also occur for the case that weathering occurs without spheroidal fractures, but for the same kinetics and transport parameters, the maximum rate of saprolitization achieved would be far smaller than the rate of denudation for the Rio Blanco system. The spheroidal weathering model provides a quantitative picture of how physical and chemical processes can be coupled explicitly during bedrock alteration to soil to explain weathering advance rates that are constant in time.

Mudslide-caused ecosystem degradation following Wenchuan earthquake 2008

Ren, D., et al., 2009. Mudslide-caused ecosystem degradation following Wenchuan earthquake 2008. Geophysical Research Letters 36, L05401.

Abstract: 
We have applied a scalable and extensible geo-fluid model that considers soil mechanics, vegetation transpiration and root mechanical reinforcement, and hydrological processes to simulate two dimensional maps of the landslides occurrence following the 2008 Wenchuan earthquake. Modeled locations and areas generally agree with observations. The model suggests that the potential energy of earth was lowered by 1.52×1015 J by these landslides. With this, the vegetation destroyed transfer ~235 Tg C to the dead respiring pool and transforms 5.54×10-2 Tg N into unavailable sediments pools and the atmosphere. The cumulative CO2 release to the atmosphere over the coming decades is comparable to that caused by hurricane Katrina 2005 (~105 Tg) and equivalent to ~2% of current annual carbon emissions from global fossil fuel combustion. The nitrogen loss is twice as much as that released by the 2007 California Fire (~2.5×10-2 Tg). A significant proportion of the nitrogen loss (14%) is in the form of nitrous oxide, which can affect the atmospheric ozone layer.

Hurricane Effects on Soil Organic Matter Dynamics and Forest Production in the Luquillo Experimental Forest, Puerto Rico: Results of Simulation Modeling

Hurricane Effects on Soil Organic Matter Dynamics and Forest Production in the Luquillo Experimental Forest, Puerto Rico: Results of Simulation Modeling
Robert L. Sanford, Jr., William J. Parton, Dennis S. Ojima and D. Jean Lodge
Biotropica
Vol. 23, No. 4, Part A. Special Issue: Ecosystem, Plant, and Animal Responses to Hurricanes in the Caribbean (Dec., 1991), pp. 364-372

Abstract: 
The forests and soils at Luquillo Experimental Forest (LEF), Puerto Rico, are frequently disturbed by hurricanes occurring at various frequencies and intensities. We have derived a forest version of the Century soil organic matter model to examine the impact of hurricanes on soil nutrient availability and pool sizes, and forest productivity in the tabonuco forest at Luquillo. The model adequately predicted aboveground plant production, soil carbon, and soil nitrogen levels for forest conditions existing before Hurricane Hugo. Simulations of Hurricane Hugo and of an historical sequence of hurricanes indicated a complex pattern of recovery, especially for the first 10 yr after the hurricanes. After repeated hurricanes, forest biomass was reduced, while forest productivity was enhanced. Soil organic matter, and phosphorus and nitrogen mineralization stabilized at higher levels for the LEF than for hurricane-free tabonuco forest, and organic soil phosphorus was substantially increased by hurricanes. Results from these simulations should be regarded as hypotheses. At present there is insufficient data to validate the results of hurricane model simulations.

REDUCTION OF BEDROCK BLOCKS AS CORESTONES IN THE WEATHERING PROFILE: OBSERVATIONS AND MODEL

Fletcher RC, Brantley SL. 2010. Reduction of bedrock blocks as corestones in the weathering profile: observations
and model. Am. J. Sci. 310:131–64

Abstract: 
the Espiritu Santo and Mameyes rivers within the Luquillo Experimental Forest (Puerto Rico) are interpreted as corestones, reduced from initial joint-bounded bedrock blocks by subsurface weathering. Maximum corestone size, expressed as the geometric mean of the three dimensions, S 3 abc, shows a smooth envelope when plotted against elevation. We postulate that, for each catchment, they represent in situ corestones within a stratified weathering profile, many tens of meters in thickness, that has been subsequently exhumed by younger erosion. We formulate a simplified one-dimensional model for reduction in corestone size within a steady-state weathering profile that incorporates: (i) vertical fluid transport of the reactant and the soluble products of chemical weathering; (ii) linear kinetics of corestone reduction; and, subsequently, (iii) erosion. The rate of advance of a steady-state weathering profile is a statement of the mass balance between entering reactants and weathering components, here idealized as H and albite. The mathematical relations, tie the laboratory-determined rate constant for dissolution of albite (k) to a generalized kinetic constant for the rate of decrease (K) in corestone diameter to the advance rate of the weathering profile (V ). The last parentheses contain an effective roughness at the scale of the weathering profile, where S0 is the maximum size of initial bedrock blocks, inferred to be set by initial bedrock fracture spacing, and 3L* is the profile thickness. The laboratory scale roughness value, , is the ratio of the surface area accessed by BET analysis to that of the corestone grain scale. In the model, erosion is not coupled with weathering, although the presence of corestones of finite size, SE>0, exiting at the erosional surface may be postulated to affect the erosional flux. The thickness of the corestone weathering profile derived for the model for the distance between bedrock and a corestone-free saprolite cap is approximately This expression is the product of the effective pH buffering-adjusted input reactant flux per unit area times a stoichiometeric factor linking this to net albite dissolution, divided by the rate of corestone size reduction at the input concentration of protons. Further, the profile thickness scales with the input “particle” size, S0. The model fit, which yields the ratio is consistent with a rate constant for albite dissolution that lies between laboratorymeasured and field-estimated values. Sensitivity to the reaction order of albite dissolution with respect to H, N, is small, except near the base of the profile. This model yields insights into the relationship between fracture spacing and the evolution of particle size and chemistry in weathering profiles.

Modelling the impact of recent land-cover changes on the stream flows in northeastern Puerto Rico

Wu W, Hall CAS, Scatena FN. Modelling the impact of recent
land-cover changes on the stream flows in northeastern Puerto
Rico. Hydrol Process 2007; 21: 2944-2956.

Abstract: 
We investigated the influence of recent and future land-cover changes on stream flow of a watershed northeastern Puerto Rico using hydrological modeling and simulation analysis. Monthly and average annual stream flows were compared between an agricultural period (1973–1980) and an urbanized/reforested period (1988–1995) using the revised Generalized Watershed Loading Function model. Our validated results show that a smaller proportion of rainfall became stream flows in the urbanized/forested period compared with the agricultural period, apparently because of reforestation. Sensitivity analysis of the model showed that evapotranspiration, precipitation, and curve number were the most significant factors influencing stream flow. Simulations of projected land-cover scenarios indicate that annual stream flows would increase by 9Ð6% in a total urbanization scenario, decrease by 3Ð6% in a total reforestation scenario, and decrease by 1Ð1% if both reforestation and urbanization continue at their current rates to 2020. An imposed hurricane event that was similar in scale to the largest recent event on the three land-cover scenarios would increase the daily stream flow by 62Ð1%, 68Ð4% and 67Ð1% respectively. Owing to the environmental setting of eastern Puerto Rico, where sea breezes caused by temperature differences between land surface and the ocean dominate the local climate, we suggest that managing local land-cover changes can have important consequences for water management. Copyright  2007 John Wiley & Sons, Ltd.

Spatial modelling of evapotranspiration in the Luquillo experimental forest of Puerto Rico using remotely-sensed data

Wu, Wei; Hall, Charles A.S.; Scatena, Frederick N.; Quackenbush, Lindi J. 2006. Spatial modelling of evapotranspiration in the Luquillo experimental forest of Puerto Rico using remotely-sensed data.. Journal of Hydrology 328, 733- 752.

Abstract: 
Actual evapotranspiration (aET) and related processes in tropical forests can explain 70% of the lateral global energy transport through latent heat, and therefore are very important in the redistribution of water on the Earth’s surface [Mauser, M., Scha¨dlich, S., 1998. Modelling the spatial distribution of evapotranspiration on different scales using remote sensing data. J. Hydrol. 212–213, 250–267]. Unfortunately, there are few spatial studies of these processes in tropical forests. This research integrates one Landsat Thematic Mapper (TM) image and three Moderate Resolution Imaging Spectroradiometer (MODIS) images with a hydrological model [Granger, R.J., Gray, D.M., 1989. Evaporation from natural nonsaturated surfaces. J. Hydrol. 111, 21–29] to estimate the spatial pattern of aET over the Luquillo Experimental Forest (LEF) – a tropical forest in northeastern Puerto Rico – for the month of January, the only month that these remotely sensed images were acquired. The derived aETs ranged from 0 to 7.22 mm/day with a mean of 3.08 ± 1.35 mm/day which were comparable to other estimates. Simulated aET was highest in the low elevation forest and decreased progressively toward higher elevations. Because of differences in solar radiation at different elevations, aspects and topographic positions, aET tended to be higher on south slopes and along ridges than on north slopes and in valleys. In addition, the Bowen ratio (the ratio of sensible heat to latent heat) varied across different vegetation types and increased with elevation, thus reflecting differences in the distribution of net solar radiation incident on the earth’s surface. Over a day, the highest simulated aET occurred at around noon. We also applied this model to simulate the average monthly aET over an entire year based on the cloud patterns derived from at least two MODIS images for each month. The highest simulated aET occurred in February and March and the lowest in May. These observations are consistent with long term data. The simulated values were compared with field measurements of the sap flow velocity of trees at different elevations and in different forest types. These comparisons had good agreement in the low elevation forest but only moderate agreement in the elfin forest at high elevations. ª 2006 Elsevier B.V. All rights reserved.

Modeling the spatial and temporal variability in climate and primary productivity across the Luquillo Mountains, Puerto Rico

Wanga, Hongqing; Halla, Charles A.S.; Scatenab, Frederick N.; Fetcherc, Ned; Wua, Wei 2003. Modeling the spatial and temporal variability in climate and primary productivity across the Luquillo Mountains, Puerto Rico.. Forest Ecology and Management 179 :69-94l.

Abstract: 
There are few studies that have examined the spatial variability of forest productivity over an entire tropical forested landscape. In this study, we used a spatially-explicit forest productivity model, TOPOPROD, which is based on the FORESTBGC model, to simulate spatial patterns of gross primary productivity (GPP), net primary productivity (NPP), and respiration over the entire Luquillo Experimental Forest (LEF) in the mountains of northeastern Puerto Rico.We modeled climate variables (e.g. solar insolation, temperature, rainfall and transpiration) using a topography-based climate model, TOPOCLIM. The simulated GPP ranged from 8 to 92 t C/ha per year with a mean of 51 t C/ha per year. The simulated NPP ranged from 0.5 to 24 t C/ha per year with a mean of 9.4 t C/ha per year. The simulated plant respiration ranged from 31 to 68 with a mean of 42 t C/ha per year. Simulated GPP and respiration declined with increased elevation whereas simulated NPP increased from low to middle elevation but decreased from middle to high elevations. Statistical analyses indicate that variation in solar insolation, which decreases with increase in elevation, is the most important factor controlling the spatial variation of forest productivity in the LEF. Validation with the limited spatial empirical data indicated that our simulations overestimated GPP by 2% for a middle elevation test site, and by 43% for a mountain peak site. Our simulations also overestimated NPP in the middle elevation Colorado forest and higher elevation Dwarf forest by 32 and 36%, respectively, but underestimated NPP in the Tabonuco and Palm forests at low to middle elevations by 9–15% and 18%, respectively. Simulated GPP and NPP would decrease under CO2 doubling as projected temperatures increase and precipitation decreases. Different forest types respond differently to potential climate change and CO2 doubling. Comparison with other tropical forests suggests that the LEF as a whole has higher GPP (51 tC/ha per year versus 40 t C/ha per year) but lower NPP (9.4 t C/ha per year versus 11 t C/ha per year) than other tropical rain forests.

Bivergent thrust wedges surrounding oceanic island arcs: Insight from observations and sandbox models of the northeastern Caribbean plate

ten Brink U, Marshak S, Granja JL (2009) Bivergent thrust wedges
surrounding oceanic island arcs: insights from observations and
sandbox models in the north-eastern Caribbean plate. Geol Soc
Am Bull 121:1522–1536

Abstract: 
At several localities around the world, thrust belts have developed on both sides of oceanic island arcs (e.g., Java-Timor, Panama, Vanuatu, and the northeastern Caribbean). In these localities, the overall vergence of the backarc thrust belt is opposite to that of the forearc thrust belt. For example, in the northeastern Caribbean, a north-verging accretionary prism lies to the north of the Eastern Greater Antilles arc (Hispaniola and Puerto Rico), whereas a south-verging thrust belt called the Muertos thrust belt lies to the south. Researchers have attributed such bivergent geometry to several processes, including: reversal of subduction polarity; subduction-driven mantle fl ow; stress transmission across the arc; gravitational spreading of the arc; and magmatic infl ation within the arc. New observations of deformational features in the Muertos thrust belt and of fault geometries produced in sandbox kinematic models, along with examination of published studies of island arcs, lead to the conclusion that the bivergence of thrusting in island arcs can develop without reversal of subduction polarity, without subarc mantle fl ow, and without magmatic infl ation. We suggest that the Eastern Greater Antilles arc and comparable arcs are simply crustalscale bivergent (or “doubly vergent”) thrust wedges formed during unidirectional subduction. Sandbox kinematic modeling suggests, in addition, that a broad retrowedge containing an imbricate fan of thrusts develops only where the arc behaves relatively rigidly. In such cases, the arc acts as a backstop that transmits compressive stress into the backarc region. Further, modeling shows that when arcs behave as rigid blocks, the strike-slip component of oblique convergence is accommodated entirely within the prowedge and the arc—the retrowedge hosts only dip-slip faulting (“frontal thrusting”). The existence of large retrowedges and the distribution of faulting in an island arc may, therefore, be evidence that the arc is relatively rigid. The rigidity of an island arc may arise from its mafi c composition and has implications for seismic-hazard analysis.

Meteorological Impacts of Land Use Change in the Maritime Tropics

Abstract: 
The island Puerto Rico is the smallest of the Greater Antilles. Measuring roughly 180 x 60 km, it is situated in the eastern Caribbean at 18o15’N and 66o30’W (figure 1.1). Apart from the coastal plains on the north and south coast, the island is mountainous, with the highest peaks in the central and north-eastern part of the island rising to elevations of 1000-1300 ma:s:l: (figure 1.2).

Hydrological Processes in a humid Tropical Rain Forest: A Combined Experimental and Modelling Approach

Schellekens, J. 2000. Hydrological processes in a humid tropical rain
forest: a combined experimental and modeling approach. Ph.D.
Thesis, Free University of Amsterdam, Amsterdam University
Press, 158 p.

Abstract: 
With populations growing explosively in the tropical parts of the world, and the per capita water demands increasing where living standards improve, optimisation of water resources is becoming increasingly important [Bonell et al., 1993]. Similarly, the strong demands for industrial wood (pulpwood, saw and veneer logs), fuelwood and charcoal, require the establishment of large areas of fast-growing plantation forests, often on land that is currently not forested [Evans, 1992; Brown et al., 1997]. Coupled with (i) the continued indiscriminate clearing of the world’s tropical forests [Jepma, 1995; Nepstad et al., 1999] which in many areas serve as the traditional supplier of high quality water; (ii) the associated deterioration of soil and water quality due to erosion and pollution [Oldeman, 1994], plus (iii) the possibility of gradually less dependable precipitation inputs and (in certain ‘maritime’ tropical areas away from the equator) an increasing frequency of devastating hurricanes due to ‘global change’ [Wasser and Harger, 1992], a sound understanding of the hydrological functioning of tropical forests is arguably even more important nowadays than ever before [cf. Bruijnzeel, 1990, 2000a]. Bruijnzeel and Abdul Rahim [1992] suggested that in a time of dwindling resources, additional forest hydrological research in the humid tropics could best be carried out at a limited number of carefully selected data-rich key locations that could be loosely joined together in a network that captures the environmental variability encountered in the humid tropics. Furthermore, Bruijnzeel [1993] and Bonell and Balek [1993] considered a catchment-based approach to offer the best framework for such research as this allows for the integration of hydrological, geomorphological, pedological and ecological observations in a spatial context, particularly if supplemented by process studies and physicallybased distributed modelling.
Syndicate content