A mathematical model of pan evaporation under steady state conditions
Date
2016-09
Authors
Lim, Wee Ho
Roderick, Michael L.
Farquhar, Graham D.
Journal Title
Journal ISSN
Volume Title
Publisher
Elsevier
Abstract
In the context of changing climate, global pan evaporation records have shown a spatially-averaged trend of ∼−2 to ∼−3 mm a⁻²over the past 30–50 years. This global phenomenon has motivated the development of the “PenPan” model (Rotstayn et al., 2006). However, the original PenPan model has yet to receive an independent experimental evaluation. Hence, we constructed an instrumented US Class A pan at Canberra Airport (Australia) and monitored it over a three-year period (2007–2010) to uncover the physics of pan evaporation under non-steady state conditions.
The experimental investigations of pan evaporation enabled theoretical formulation and parameterisation of the aerodynamic function considering the wind, properties of air and (with or without) the bird guard effect. The energy balance investigation allowed for detailed formulation of the short- and long-wave radiation associated with the albedos and the emissivities of the pan water surface and the pan wall. Here, we synthesise and generalise those earlier works to develop a new model called the “PenPan-V2” model for application under steady state conditions (i.e., uses a monthly time step). Two versions (PenPan-V2C and PenPan-V2S) are tested using pan evaporation data available across the Australian continent. Both versions outperformed the original PenPan model with better representation of both the evaporation rate and the underlying physics of a US Class A pan. The results show the improved solar geometry related calculations (e.g., albedo, area) for the pan system led to a clear improvement in representing the seasonal cycle of pan evaporation. For general applications, the PenPan-V2S is simpler and suited for applications including an evaluation of long-term trends in pan evaporation.
Description
Keywords
Pan evaporation, Aerodynamic function, Net irradiance, Short-wave irradiance, Long-wave irradiance
Citation
Collections
Source
Journal of Hydrology
Type
Journal article
Book Title
Entity type
Access Statement
Open Access