On the proper employment of evaporation pans and atmometers in estimating potential transpiration

To investigate the feasibility of estimating Penman's potential transpiration ( E rp ) without recourse to sophisticated instrumentation, the rates of evaporation from a Class‐A pan and from a Piche atmometer exposed within a thermometer screen were analysed in detail using continuous micrometeorological measurements taken on the same site. Evaporimeter and other associated manual measurements were made not only on a routine daily basis but also on certain days from hour to hour. the latter measurements were of particular help in interpreting physically the behaviour of both evaporimeters. It is shown: that the evaporation from the Piche was closely proportional to the (complete) second term in Penman's ( E rp ) equation; that evaporation from the Class‐A pan was adequately described by a combination‐type equation including an effective psychrometric constant γ p = 2.1 γ (where γ = 0‐66mb(°C) −1 , near sea level) and the wind function f p (u) = 0.12(1 + 1.35 u) mmd −1 (mb) −1 ; that the deduced daily (24 h) net radiation absorbed by the pan was consistently 31% larger than that on the short, green (but unirrigated) grass covering the site; by applying (iii), that close to 8.5% of rainfall was lost by splashout from the pan, and that on applying (iv), wet‐day values of pan evaporation lost much of their supposedly inaccurate and paradoxical nature. The franework for the general application of conclusions (i) and (iii) in generating a synthetic series of values of E rp is introduced and discussed.