ENWATBAL.BAS: A Mechanistic Evapotranspiration Model Written in Compiled BASIC

ENWATBAL, a mechanistic energy, water balance model originally written in the CSMP simulation language, was largely incompatible with personal computers (PCs). ENWATBAL.BAS was developed to extend the model application to PCs using BASIC which is widely available. BASIC functions or subprograms were provided to emulate CSMP language commands including integration, implicit root finding and generation of dependent variable values from tables of dependent-ind ependent variable data pairs. The BASIC version is highly modular and thus easier to read and maintain. The verification o f ENWATBAL.BAS against ENWATBAL, using identical input data, discretization and time integration steps, indicated no appreciable differences between the two versions. Runtimes for a seasonal simulation (100 days) were nearly the same (about 5 hours) for the compiled BASIC version using a 20 MHz, 80386 based PC and the CSMP version using a MicroVAX II, and four times faster for a 33 MHz, 80486 based PC. Discretization analysis showed that soil layer thickness should be no larger than 0.002 m for the surface layer although thickness may increase to as much as 0.2 m for subsurface layers. Parameter sensitivity analysi s showed that evapotranspiration estimates changed as expected in response to changes in parameter values for surface roughness length, maximum crop water potential, soil albedo, and crop hydraulic resistance . execute in a computer language which is more accessible, Many model changes were made to the BASIC version subsequent to the speed comparisons resulting in a more advanced and flexible model. ENWATBAL.BAS is a useful tool for investigating the comple x mechanisms of evapotranspiration. imulation models are useful tools for investigating

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