The origin of the Nicaraguan depression

A conspicuous graben extends for 800 kilometers through El Salvador and western Nicaragua to the Caribbean Sea in northeastern Costa Rica. Like the smaller but structurally similar Semangko and Toba Depressions of northern Sumatra, the trough is clearly related to voluminous volcanic eruptions during Late Tertiary time. In the region around Lakes Managua and Nicaragua, where the depression is best defined and reaches its greatest dimension, a thick series of Tertiary sediments and volcanic rocks provides a means of interpreting the Cenozoic history of the region. Following a long period of intermittent volcanic activity and sedimentation, extensive sheets of andesitic and dacitic ignimbrites were erupted during Late Miocene time from fissure sources which appear to have been located near the now-subsided central portion of the graben. Near the coast, ignimbrites flowed across a flat lagunal shore overwhelming and burying the tropical vegetation and finally coming to rest in shallow water. Unusual textures and chaotic mixtures of pumice with sediments and silicified wood characterize the bases of many of the water-laid ignimbrites. Subsidence of the graben does not appear to have occurred concurrently with the ignimbrite eruptions but followed them closely near the end of the Miocene or the beginning of Pliocene time. Subsequent activity has been confined to relatively smaller eruptions from central vents near the boundary faults of the graben.     

Evaluating GCM Output with Impact Models

This study uses empirical agricultural impact models to compare the U.S. climate change predictions of 16 General Circulation Models (GCMs). The impact analysis provides a policy-relevant index by which to judge complex climate predictions. National aggregate impacts vary widely across the 16 GCMs because of varying regional and seasonal patterns of predicted climate change. Examining the predicted impacts from the full set of GCMs reveals that the seasonal detail in the GCM predictions is so noisy that it is not significantly different from a constant annual change. However, a consistent regional pattern does emerge across the set of models. Nonetheless, aggregating climate change across seasons and regions within the United States, using a national-annual climate change provides a reasonable and efficient approximation to the expected impact predicted by the 16 GCM models.     

A finite-element study of fold propagation in a viscous layer

A finite-element model of a viscous layer contained in a viscous matrix and undergoing layer-parallel compression is used to examine the hypothesis that a long chain of folds, as found in real rocks, can originate from one initial perturbation to the layer geometry. This hypothesis is tested by determining the velocity with which a perturbation spreads along layers of various viscosities.An insight is gained into the roles played by local strain and local layer strength in the folding mechanism. The results show that for layers with viscosity ratios comparable with those of real rocks it is impossible for long chains of folds to originate from one perturbation. The authors conclude that rock layers contain many initial perturbations and folding originates at all perturbation sites simultaneously. The growth of such folds depends on the amplitude and shape of the initial perturbation and on subsequent interference between folds.     

Development of a continuous soil moisture accounting procedure for curve number methodology and its behaviour with different evapotranspiration methods

The curve number (CN) method is widely used for rainfall–runoff modelling in continuous hydrologic simulation models. A sound continuous soil moisture accounting procedure is necessary for models using the CN method. For shallow soils and soils with low storage, the existing methods have limitations in their ability to reproduce the observed runoff. Therefore, a simple one‐parameter model based on the Soil Conservation Society CN procedure is developed for use in continuous hydrologic simulation. The sensitivity of the model parameter to runoff predictions was also analysed. In addition, the behaviour of the procedure developed and the existing continuous soil moisture accounting procedure used in hydrologic models, in combination with Penman–Monteith and Hargreaves evapotranspiration (ET) methods was also analysed. The new CN methodology, its behaviour and the sensitivity of the depletion coefficient (model parameter) were tested in four United States Geological Survey defined eight‐digit watersheds in different water resources regions of the USA using the SWAT model. In addition to easy parameterization for calibration, the one‐parameter model developed performed adequately in predicting runoff. When tested for shallow soils, the parameter is found to be very sensitive to surface runoff and subsurface flow and less sensitive to ET. Copyright © 2007 John Wiley & Sons, Ltd.