poster - International Conference of Agricultural Engineering

Example of the manufactured pans of
galvanized steel with 57 cm diameter
and 30 cm height
Cover using wire mesh fixed on
steel frames and pieces of wood
base its height is 10 cm
The water level measured by ruler
scale and a pointer mounted on a
steel rod ended by a float on water.
Class A pan Defferent types of pan: very small pan (VS= Ф 16), small pan (S= Ф 32),
medium pan (M= Ф 40), larger pan (L= Ф 57)
FIGURE 1: Examples of the different pan sizes and class A pan used in the Pan development
and field experimental setup.
Measuring the Pan Water Levels
Water levels in an evaporation pan can be most accurately measured using a stilling well (as
illustrated in Figure 1) with a hook gauge and micrometer. The stilling well provides a fixed
reference point for accurate measurements even if the pan is not perfectly level. The hook
gauge and micrometer permit depths to be measured to the nearest 0.001 inches, which is
much greater accuracy than required for irrigation scheduling. Accuracies sufficient for irrigation
scheduling can be obtained using a finely-graduated ruler or meter stick. However, the water
depth measurement must always be made at the same location, and care must be taken to
assure that the ruler is held vertically when measurements are made. For ease of
interpretation, evaporation measurements should be made at about the same time each day.
Normally, early morning or late evening measurements are most convenient. Early morning
measurements permit yesterday's ET to be estimated and today's irrigations to be scheduled.
Late evening measurements can be used to estimate today's ET for irrigations to be scheduled
tonight or early tomorrow morning.
Determination of K pan (Pan Coefficients)
When using the evaporation pan to estimate the ET o , in fact, a comparison is made between
the evaporation from the water surface in the pan and the evapotranspiration of the standard
grass. Of course the water in the pan and the grass do not react in exactly the same way to the