Abstract
Reliable estimates of reference evapotranspiration
(
ET0
)
are key elements for efficient water resource management, and estimating
ET0
, based on “Class ‘A’ pan evaporation” data is common in arid climates. A pan coefficient
(
Kp
)
, which depends on the distance (or fetch) of green vegetation or fallow soil around the pan
(F)
, wind run
(U)
, and relative humidity (RH), is used to convert from pan evaporation to
ET0
. Several researchers have developed models for estimating
Kp
values for pans surrounded by green vegetated fetch, but there is only one equation to estimate
Kp
values for dry fetch conditions. The equation is complex, so the objective of this research was to develop a new simple equation to estimate
Kp
under fallow soil fetch conditions. The new
Kp
equation and the more complex equation were compared with tabular values published by the United Nations Food and Agriculture Organization. The new equation performed slightly better at matching the tabular
Kp
values than the complex equation. The equation derivation and evaluation are presented.