Abstract
Abstract. This guide to estimating daily and monthly actual, potential, reference crop and pan evaporation covers topics that are of interest to researchers, consulting hydrologists and practicing engineers. Topics include estimating actual evaporation from deep lakes and from farm dams and for catchment water balance studies, estimating potential evaporation as input to rainfall-runoff models, and reference crop evapotranspiration for small irrigation areas, and for irrigation within large irrigation districts. Inspiration for this guide arose in response to the authors' experiences in reviewing research papers and consulting reports where estimation of the actual evaporation component in catchment and water balance studies was often inadequately handled. Practical guides using consistent terminology that cover both theory and practice are not readily available. Here we provide such a guide, which is divided into three parts. The first part provides background theory and an outline of the conceptual models of potential evaporation of Penman, Penman–Monteith and Priestley–Taylor, as well as discussions of reference crop evapotranspiration and Class-A pan evaporation. The last two sub-sections in this first part include techniques to estimate actual evaporation from (i) open-surface water and (ii) landscapes and catchments (Morton and the advection-aridity models). The second part addresses topics confronting a practicing hydrologist, e.g. estimating actual evaporation for deep lakes, shallow lakes and farm dams, lakes covered with vegetation, catchments, irrigation areas and bare soil. The third part addresses six related issues: (i) automatic (hard wired) calculation of evaporation estimates in commercial weather stations, (ii) evaporation estimates without wind data, (iii) at-site meteorological data, (iv) dealing with evaporation in a climate change environment, (v) 24 h versus day-light hour estimation of meteorological variables, and (vi) uncertainty in evaporation estimates. This paper is supported by a Supplement that includes 21 sections enhancing the material in the text, worked examples of many procedures discussed in the paper, a program listing (Fortran 90) of Morton's WREVAP evaporation models along with tables of monthly Class-A pan coefficients for 68 locations across Australia and other information.
Highlights
Actual evaporation is a major component in the water balance of a catchment, reservoir or lake, irrigation region, and some groundwater systems
We refer heavily to these texts in this paper which is aimed at improving the practice of estimating actual and potential evaporation, reference crop evapotranspiration and pan evaporation using standard daily or monthly meteorological data
Where EDL is the evaporation from the deep lake, Rn is the net radiation at the water surface (MJ m−2 day−1), Ea is the evaporation component due to wind, is the slope of the vapour pressure curve at air temperature, γ is the psychrometric constant, λ is the latent heat of vaporization (MJ kg−1), and H is the change in heat storage (MJ m−2 day−1)
Summary
Actual evaporation is a major component in the water balance of a catchment, reservoir or lake, irrigation region, and some groundwater systems. For major reservoirs in Australia, actual evaporation losses represent 20 % of reservoir yield Compared with precipitation and streamflow, the magnitude of actual evaporation over the long term is more difficult to estimate than either precipitation or streamflow. This paper deals with estimating actual, potential, reference crop and pan evaporation at a daily and a monthly time step using standard meteorological data. McMahon et al.: Estimating evaporation of the use of remotely sensed data to estimate actual evaporation is outside the scope of this paper but readers interested in the topic are referred to Kalma et al (2008) and Glenn et al (2010) for relevant material
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