TAKING A BOW, BUT MOVING ON!

Professor Jim Shuttleworth

The SAHRA Center, Dept. of Hydrology and Water Resources

When Tom Meixner contacted me saying “You have to give a seminar on the science that won you the International Hydrology Prize”, I responded negatively because giving such a seminar is not really my style and, although a significant scientific contribution to hydrology is expected, other things also come into the deliberations of the panel that awards this prize. Eventually Tom and I reached a compromise. Recognizing that others might enjoy seeing the actual award ceremony in Paris in July this year, in this seminar I plan first to show a short 10-15 minute video of the actual presentation made by Matt Brailey of the UA Engineering News team. Then, to take care of Tom’s request to overview my scientific contribution (at least in the area of evaporation), I will distribute copies on an extensive review paper on the topic of natural evaporation that I recently wrote which is called Putting the ‘VAP’ into Evaporation. This paper overviews this topic and, by default, therefore also describes my own contribution to it. The paper is, I hope, a good jumping off point for students interested in this topic. I prefer then to move on to speak of something new, specifically the campaign I am currently fighting to correct an anomaly that has been bugging me for more than two decades, i.e. the inconsistency between present day understanding of the evaporation process and the methodology that is still recommended by the United Nations’ Food and Agriculture Organization (FAO) for estimating the water requirements of irrigated crop based on estimates of reference crop evaporation and crop factors.

The remainder of this seminar therefore describes theoretical analyses that facilitate the alternative use of the Penman-Monteith equation to make a one-step estimate of crop water requirements in preference to the FAO recommendation. Reluctance to using a Penman-Monteith based estimate results from two outstanding issues, both of which are addressed. First, no method has hitherto been defined to handle the problem that routine climate measurements are commonly available only at 2 m above the ground while, when using the Penman-Monteith equation, they are required at some level above the evaporating crop. To resolve this, a blending height is defined in the atmospheric boundary layer where meteorological conditions are independent of underlying crops. Expressions are derived to calculate the aerodynamic resistances to, and the vapor pressure deficit at this blending height from climate variables at 2 m. Consequently, 2 m climate data can now be used in the Penman-Monteith equation, either to estimate transpiration from surface resistance or to calculate surface resistance from measured transpiration. Second, no table of effective values currently exists for the surface resistance of different crops equivalent to that for the crop coefficient. This talk calls for field studies to address this need. However, recognizing the need for an interim source of crop-specific surface resistances, a methodology is given for translating the crop coefficient into equivalent surface resistance. To make this translation, it is necessary to specify the relationship between the radiative and aerodynamic energy inputs to evapotranspiration when the crop coefficients were field calibrated. Insight from coupled surface-atmosphere modeling studies is used to define this relationship. Finally, a Penman-Monteith-based, one-step estimation equation is derived which makes proper allowance for the different aerodynamic characteristics of crops in all conditions of atmospheric aridity, and which estimates crop evaporation for any crop of specified height from existing crop coefficients using standard 2 m climate data.