S.R. Evett, USDA-ARS, Conserv. and Prod. Res. Lab., Bushland, TX, 79012; and R.J. Lascano, Texas A&M Univ., Agric. Res. and Ext. Ctr, Lubbock, TX 79401. Joint contribution from USDA-ARS, Conserv. and Prod. Res. Lab., and the Texas A&M Univ., Agric. Res. and Ext. Ctr. Received 22 Nov. 1991. *Corresponding author. Published in Agron. J. 85(3):763-772 (1993) Abbreviations: ASCII, American Standard Code for Information Interchange; CPU, central processing unit; CSMP, Continuous System Modeling Program; DOS, disk operating system; ET, evapotranspiration; LAI, leaf area index; and PC, personal computer. ENWATBAL.BAS: A Mechanistic Evapotranspiration Model Written in Compiled BASIC S.R. Evett and R.J. Lascano 1 ABSTRACT 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-independent variable data pairs. The BASIC version is highly modular and thus easier to read and maintain. The verification of 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 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. 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 complex mechanisms of evapotranspiration. imulation models are useful tools for investigating S questions which are difficult to examine via field studies, for designing better field experiments, and for gaining insight into how the parts of a complex system interact. ENWATBAL (Energy and Water Balance), a mechanistic evapotranspiration (ET) model, was originally written in the CSMP simulation language (Continuous System Modeling Program, e.g., Speckhart and Green, 1976). It was used to predict cotton ET (Lascano et al., 1987) and sorghum ET (Van Bavel and Lascano, 1987) at Lubbock, TX. More recently Ritchie and Johnson (1990) compared ENWATBAL to the functional CERES-Maize model for predicting sorghum ET. Krieg and Lascano (1990) used ENWATBAL to predict sorghum ET at Brownfield, TX, and Lascano (1991) used the model to predict the effects of N on the water use of irrigated and dryland sorghum at Lubbock, TX. The main purpose of ENWATBAL is the separate calculation of soil and crop evaporation as a function of crop development and weather via numerical modeling of the energy and water balances of the soil - plant - atmosphere system. ENWATBAL is not a crop growth and development model; however, because it predicts soil water content and temperature, it can be used to investigate agronomic implications, such as seedling emergence, irrigation strategies, and other processes depending upon soil water and temperature. The primary purpose of this work was to rewrite ENWATBAL to more flexible and more widely used than CSMP. Therefore, ENWATBAL.BAS was developed to extend the model application to personal computers (PCs) using the BASIC language.