ENWATBAL.BAS: a Mechanistic Evapotranspiration Model Written in Compiled Basic

Abstract

AbstractENWATBAL, a mechanistic energy, water balance model originally written in the Continuous System Modeling Program (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‐independent variable data pairs. The BASIC version is highly modular and thus easier to read and maintain. The verification of ENWATBAL.BAS against ENWATBAL, with identical input data, discretization, and time integration steps, indicated no appreciable differences between the two versions. Runtimes for a seasonal simulation (100 d) were nearly the same (about 5 h) for the compiled BASIC version with a 20 MHz, 80386‐based PC and the CSMP version with 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 analysis 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. 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 complex mechanisms of evapotranspiration.