Two recent Technical Reports released by NSCEE describe the conceptual underpinnings of a computer model of steady state infiltration of water into desert alluvial soils from ditches and trenches. The topic continues to be of considerable interest in theagri-cultural and environmental sciences. Examples of applications include the study of irrigation strategies for crops in arid regions and the analysis of the migration of water-borne pollutants from waste repositories in such regions. The reports describe the basic physics, mathematics, and numerical methods used in the model. At this point the model deals with a simplified 2-D geometry under assumptions of soil homogeneity. Nonetheless, the reports illustrate nicely the multidisciplinary nature of computer simulation. The latter refers to the use of computers, often high performance machines, in order to accurately mimic physical or other phenomena. The use of computer simulation is now recognized as a third basic methodology in science and engineering, joining theory and experimentation.
A feature of the project described in the reports deals with the use of parallel processing for some of the computation. Parts of the model required intensive throughput which would have used an inordinate amount of time on a conventional computing platform. The issue was resolved by using a tool available at NSCEE called the Parallel Virtual Machine system. PVM is a software package that permits a heterogeneous collection of Unix computers hooked together by a network to be used as a single large parallel computer. Thus large computational problems can be solved cost effectively by using the aggregate power and memory of many computers. The individual computers may be shared- or local-memory multi-processors, vector supercomputers, specialized graphics engine, or scalar workstations that may be interconnected by a variety of networks. Even the 10 nodes of the ORIGIN 2000 at NSCEE may be made to operate under the control of PVM. User programs written in C or Fortran are provided access to PVM through the use of calls to library routines for functions such as process initiation, message transmission and reception, and synchronization via barriers or rendezvous. The PVM system transparently handles message routing, data conversion for incompatible architectures, and other tasks that are necessary for operation in a heterogeneous network environment. This software package is indeed a versatile and cost-effective tool for computer simulations requiring intensive throughput.
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