Automating pan evaporation measurements for irrigation control

✍ Scribed by C.J Phene; R.B Campbell

• Book ID: 102980222
• Publisher: Elsevier Science
• Year: 1975
• Weight: 507 KB
• Volume: 15
• Category: Article
• ISSN: 0002-1571
• DOI: 10.1016/0002-1571(75)90003-5

✦ Synopsis

Class A pan evaporation measurements are reasonable estimates of evapotranspiration in humid climates when soil water is no t restricting plant growth. With the development of high-frequency irrigation systems where small quantities of water are applied often to replace evapotranspiration losses, frequent automatic water-level measurements are essential and can be used to control an irrigation system.

A class A pan evaporation measurement and control instrument was developed to automatically measure pan evaporation with electronic analog or digital recording equipment. The instrument was a LVDT with a stainless-steel float adapted to a standard cylindrical stilling well.

Measurement sensitivity of 4.542 + 0.001 V cm -l of evaporation and range of 5 cm was obtained with an input voltage of 24 V dc. The instrument's time response was 130 sec to a step change in water level of 0.27 cm. Automatic measurements obtained during a windstorm (average gusts of wind at 48 km h -l ) indicated that this instrument could measure pan evaporation more precisely under adverse wind conditions when simultaneous manual measurements with the hook gauge were not possible.

This instrument plus an electronic soil matric potential sensor was used in the field to control a high-frequency porous tube subirrigation system. Our results indicated that this control method could maintain the fluctuation of the soil matric potential in the root zone within a narrow range.