Evapotranspiration modules for crop growth simulation

Abstract

Calculation of evapotranspiration is essential for the estimation of crop water use or for studying the effect of drought stress on crop performance with simulation models. Several methods are available for calculation of evapotranspiration. This report describes three different methods : the Penman method (1948) and the approaches of Makkink (1957) and Priestley-Taylor (1972). The modules described in this report are developed for use in general crop growth models for water-limited conditions, such as in Van Laar et al. (1992). The application of the modules, however, is not limited to the SUCROS type models. The Penman method is important for the general understanding of evapotranspiration from surfaces both in more advanced models (such as greenhouse models) and in more simple approaches. When considered over longer periods of time 10 days), the Penman method calculates crop water loss with a reasonable degree of accuracy. It can be considered the best among the simple approaches. All three methods have in common that they estimate evapotranspiration of short , well-watered crops, however, they differ in their data requirements. Makkink and Priestley-Taylor require fewer meteorological observations because they are based on the observation that in many climates, the radiation-driven part of evapotranspiration is much more important than the part driven by vapour pressure deficit and wind speed. In the Priestley -Taylor equation, evapotranspiration is proportional to net radiation, while Makkink evapotranspiration is proportional to short-wave radiation. The Penman method requires daily values of radiation, temperature, vapour pressure and wind speed. The Makkink and Priestley-Taylor equations require only radiation and temperature. The Priestley-Taylor equation is used world-wide, e.g. in the IBSNAT network, but regional calibration can be necessary since it is based on the assumption that a constant relation exists between the evaporative demand by radiation and by wind. The same holds for the Makkink equation which is calibrated for use during the growing season in The Netherlands. In The Netherlands, Makkink and Priestley -Taylor should be used only during the growing season. An important finding is that the Makkink and Priestley -Tailor methods are valid for a larger part of the year in areas closer to the equator. This more or less justifies the use of these simple methods in agro-ecological zonation studies in these areas. The Penman formula calculates evapotranspiration by assuming that the surface temperature is not very different from the air temperature. Under normal circumstances this is indeed the case, but under extreme conditions surface temperatures can differ much from air temperatures, resulting in unwanted errors. To avoid this situation, the Penman module, as described here, can iteratively search for the equilibrium surface temperature and give an improved estimate of surface water loss.