The Estimation of Evaporation: John Monteith and Some Subsequent Developments.

John Monteith made probably the greatest contribution to our understanding of evaporation from plant canopies of any scientist during the twentieth century; he used a rigorous theoretical physical science base to build on the advances made by Howard Penman, his former advisor, then colleague, at Rothamsted research station. A major feature of his approach was to recognize where approximations were needed and appropriate. In particular he developed the use of electrical resistance analogies in a way that allowed the accurate description of evaporation responses to stomatal and environmental factors and incorporated them into what is now known as the Penman-Monteith (P-M) equation. An equally important, but perhaps less widely recognized, advance was John’s introduction of the very useful approximation involved in the concept of a radiative conductance (or the equivalent concept of ‘isothermal net radiation’, defined as the net radiation that would be absorbed by a leaf/canopy if it were at air temperature). This provides us with a powerful tool to handle the feedbacks related to the fact that evaporation rate affects leaf temperature and hence the driving force for evaporation; this invaluable tool is now widely incorporated into our modelling of evaporation and the energy balance of vegetation.

John also pointed out the importance of crop coupling to the atmosphere. This concept was then taken forward by Paul Jarvis & Keith McNaughton, who formally partitioned the P-M equation into ‘imposed’ and ‘equilibrium’ components by the use of what they termed a decoupling factor. In this paper I will discuss some of the more recent applications of John Monteith’s work to estimation of evaporation from plant leaves, canopies, and regions including the widespread use of remote sensing technologies, whether they be based on satellite or ‘in-field’ sensing.