Román Alvarez

TROPICAL MEXICO'S RECENT LAND‐USE CHANGE: A REGION'S CONTRIBUTION TO THE GLOBAL CARBON CYCLE

We applied modeled biomass density estimates to changes in land use/land cover (LU/LC) statistics for the intensively impacted and highly fragmented landscape of tropical Mexico to estimate the flux of carbon (C) between terrestrial ecosystems and the atmosphere between 1977 and 1992. Biomass densities were assigned to hybrid LU/LC classes on vegetation maps produced by Mexican governmental organizations and, by dif- ferencing areas and biomass C pools, net C flux was calculated in the eight-state tropical region of southeast Mexico. These states, representing tropical Mexico, experienced a mean annual deforestation rate of nearly 559 000 ha/yr, or 1.9%, between 1977 and 1992. The total area of closed forests decreased by 26%, open/fragmented forests decreased by 31%, and agroecosystem areas increased by 64%. Total mean biomass densities ranged from a high of 265 Mg/ha in the Veracruz state tall/medium tropical evergreen forest class to a low of 12 Mg/ha in the cultivated land class (several states). We estimate that a total of 280 Tg C were released from the terrestrial biosphere during the 15-yr period covered by our study, equal to nearly 20% of the regions 1977 biomass C pool. The study region, while comprising just 24% of Mexicos surface area, contributed 36% of the net national C emissions from LU/LC change.

DEM generation by contour line dilation

Abstract A method is developed to generate digital elevation models by means of the dilation of contour lines stored in a raster grid. An iterative procedure produces an extension of contour lines by applying alternative four-connected and eight-connected erosions of the background until the produced surfaces become contiguous. The limits between these surfaces correspond to a new family of intermediate contour lines that are added to the raster grid. This procedure stops when no new contour lines can be generated. C subroutines are given. The algorithm is simple to apply and has a low computational cost.

Linear fractal analysis of three Mexican soils in different management systems

Abstract The purpose of this study was to document the fractal nature of three soils of Mexico with contrasting genesis and marked differences in morphology and to estimate the fractal dimensions of their sets of aggregates and pores. These dimensions were estimated along lines and were called linear fractal dimensions. A single, ‘ideal’ fractal dimensionality was detected in the three soils studied. The soil linear fractal dimensions, calculated from macro and micromorphological data, had larger values than the dimension of the Cantor fractal dust model, but were less than unity. It was shown, that the fractal structure of the soil pore space could not be described by the same dimension as that of the aggregates. The linear fractal dimensions of soils of distinct genesis, were significantly different on all scales compared, but the differences fluctuated between 0.4% and 9.1%.

Assessment of seasonal forest fire risk using NOAA-AVHRR: a case study in central Mexico

A logistic model was constructed to assess the risk of forest fire and tested over the central region of Mexico. The model incorporates both static and dynamic predictive variables: elevation, aspect, slope, vegetation type, precipitation, Normalized Difference Vegetation Index (NDVI), land surface temperature (LST), and cloud cover. The latter three variables were derived from National Oceanic and Atmospheric Administration (NOAA) Advanced Very High Resolution Radiometer (AVHRR) images from the four months (November to February) before the fire seasons. Actual forest fires were detected on NOAA-AVHRR images from the four fire seasons (March to May) from 1997 to 2000. Variables included in the model were chosen following a stepwise strategy. Statistically, the January NDVI, the February LST, vegetation type and slope had the greatest influence on the distribution of forest fires; however, elevation and precipitation were also included in the final model. The probability of forest fire occurrence for each fire season from 1997 to 1999 was mapped. The accuracy of the model was estimated to be 79.8% with reference to sensitivity, specificity and receiver operating characteristic (ROC) curves. Model predictions were validated against data from the 2000 fire season. The fire occurrence probability map is useful for designing large-scale management strategies for wildfire prevention not only in the test area of this study but also in regions where the static and dynamic variables can be similarly defined.

Hybrid maps: Updating Mexico's forest cartography using landsat tm imagery and land use information

Abstract Mexican forests represent a surface of about 49 million hectares. Traditional methods of mapping are expensive and time consuming when applied to extensions of such magnitudes. An alternative is presented, in which visual interpretation of 74 satellite images is performed in order to update existing land use cartography at the 1:250,000 scale. Based on previous vegetation classifications, and on the possibility of distinguishing among several classes on satellite imagery, a new classification of 39 land classes is proposed. With a geographic information system, the old and the updated cartographic information are joined in a single product, called a hybrid map”;. Forest areas which changed into farmlands or grasslands are defined as perturbed. A graphic solution is adopted to indicate the different sources of the final cartographic product. The method is applied to a pilot area of 4800 km2 in the southeastern part of Mexico in order to evaluate the quality of the product and to show its possibili...

The elusive Rivera-Cocos plate boundary: not diffuse

Abstract Inland and offshore gravimetric determinations in the Southern Colima rift in western Mexico allowed for the construction of a Bouguer anomaly map of the area. Four submarine canyons of tectonic relevance are located in the offshore area. Gravimetric models of the oceanic subducting slab were calculated from the residual Bouguer anomaly along six trench-parallel lines and three trench-perpendicular lines including the area of the canyons. The former lines show considerable distortion of the slab that we attribute to compression, while the latter show distinct dipping angles of the oceanic slab at distances of around 75 km from the trench: the westernmost line shows a dip angle of 55° between depths of 20 and 70 km, while the easternmost shows a dip angle of 32° at depths between 18 and 50 km. We submit that the former represents a section of the subducting Rivera plate, and the latter represents a section of the Cocos plate. Extracting coordinates of representative points on the surface of the nine slab models allowed for a reconstruction of the slab surface: the transition from the Rivera to the Cocos plate is marked by topographic gradients in the modelled slab surface suggesting the trajectory of the boundary between the plates. We propose a tectonic model that includes a transpression zone involving the marine platform and the Southern Colima rift and a transtension zone in the Northern Colima rift: the Colima Volcanic Complex is located in the transition zone between them.

Geophysical determination of buried geological structures and their influence on aquifer characteristics

Abstract Many coastal plains in the semi-arid regions of Mexico become fertile lands when properly irrigated. In the last thirty years extensive drilling in several places has disturbed the natural equilibrium of the aquifers; this is partly due to poor knowledge of their distribution and properties, as well as lack of adequate exploitation strategies. This study constitutes a case history of the valley of Guaymas in northwestern Mexico, in which three sets of data are considered: (a) a set of 262 wells, (b) four telluric lines of approximate total length of 90 km, and a set of 326 randomly distributed gravimetric stations. The valley dimensions are 20 km by 50 km; two aquifers have been located, one above 160 m and the other below 320 m. Models have been computed for the four telluric lines and four gravimetric sections. They suggest that sediments on the south-central portion of the valley have a thickness of 800 m. The basement becomes shallow toward the north and south portions of the valley, reaching depths ranging from 200 to 300 m. The valley is flanked by two buried depressions oriented in NNE-SSW direction; these regions reach depths of over 1000 m in some places and apparently constitute reservoirs in which the surface recharge waters are maintained relatively free of contamination from hydrothermal fluids. Such fluids are extracted from shallow wells (under 200 m) in some areas in which the basement approaches the surface. It is concluded that performing geophysical studies on the aquifers location, in order to determine its regional geological characteristics, is a cost-effective procedure, that allows the establishment of timely extraction strategies.

Adubação da cana-de-açúcar: VII - Ensaio preliminar de adubação N-P-K no arenito de Bauru

The results obtained in a 33 factorial experiment on the applications of the three major elements for the sugar cane are presented in this paper. The fertilizer levels used were, as follows: (1) Nitrogen, applied aa ammonium sulphate at the rates of 0.90 and 180 k/ha; (2) phosphorus, applied as superphosphate at the rates of 0.80 and 160 k/ha of P2O5; and (3) Potassium, as potassium chloride at the rates of 0.100 e 200 kg/ha of K2O. All cultural operations were caried out as in commercial plantings. The results indicated that phosphorus was the element that gave the highest yield increases, improved also the sugar content of the canca. Nitrogen also increased the yield linearly, but the effects were less noticeable than those of phosphorus. The increase in yield due to potassium was lower than those of phosphorus and nitrogen.

A volcanic centre in Mexico's Pacific continental shelf

Abstract On the continental platform of western Mexico, there is a young island of volcanic origin: Isla Isabel. It is the only volcanic manifestation in the region, contrasting with Islas Tres Marías, which are not volcanic. We aim at characterizing the source of the volcanic activity present in this particular location. Under Isla Isabel there is a bathymetric bulge that rises 60–80 m above the platform, and extends 20 km in the NW–SE direction and 17 km in the perpendicular direction. Isla Isabel is the only emerged portion of the bulge, extending only 1.8 km in the NW–SE direction. The island shows Plio-Pleistocene volcanic activity, including the formation of maars and the presence of mantle xenoliths. Using independent 2D modelling and 3D inversion methods for the gravity and magnetic fields, we analyse the nature of the bulge and its surroundings. A magnetotelluric station yields information about the electrical resistivity under the island, with penetration depths of approximately 20 km. The models are consistent with the presence of dense bodies of varying magnetizations that are interpreted as intrusive bodies. Results support the presence of an intrusion that locally has raised the ocean-floor topography. Volcanic activity projected from the bulge created Isla Isabel; the existence of additional, submerged volcanic centres in the area is most probable. We are inclined to identify the Isabel Bulge as a laccolith.

Geophysical modelling of Isla Isabel: a volcanic island on the Mexican continental margin

Abstract Isla Isabel is a small island located on top of a 20 km-long bathymetric bulge, part of the continental shelf of Nayarit in the vicinity of the mouth of the Gulf of California; it is of volcanic origin with tuff cones, basaltic flows and Surtseyan-type explosion craters exposed. Geophysical surveys were carried out in order to model the geological bodies associated with the local gravity and magnetic fields. 2D and 3D models are presented, and a 1D inversion model is derived from magnetotelluric data. Analysis of the density models of the upper 1400 m suggests two growth stages for Isla Isabel in this depth range: the first one reaching 800 m in depth and the second one projecting from that depth to the surface. The corresponding magnetic susceptibility models concur with this observation. The bodies projected to the surface appear to correspond to diatremes. The 1D magnetotelluric inversion shows two conductive anomalies within the crust between 3–5 and 8–17 km, followed by a resistive substratum that coincides with the seismically derived limit of the Earths crust at the islands position.