Use of piche evaporation in a Stevenson screen to estimate reference evapotranspiration in the Sudan

Abstract EN

Direct measurements of evaporation from open water have not been satisfactory since its adoption in the early days.

The class A pan proved to be unsatisfactory especially in dry areas due to high advected energy from the surrounding areas.

The ‘Piche’ tube needs a correction factor.

Scientists worked actively to try to estimate evaporation from climatic data.

Many formulae were produced.

Most of them fitted the humid zones.

Even the best known Penman formula has a problem with its wind term.

Monteith and Unsworth worked out the wind term based on the physics of evaporation.

Then they developed a formula for estimating reference evapotranspiration (ETo), which is nowadays the basis for estimating crop water requirements.

The data needed to use the resulting Penman-Monteith formula for ETo are available at only a limited number of stations in Sudan.

Whereas Piche evaporation and relative humidity data, as measured in a Stevenson screen, are available in many more stations.

The objective of this paper is to develop a relationship between Penman-Monteith based ETo and Stevenson screened Piche evaporation.

An exponential relationship was established (R2= 66%) between the ratio of ETo and Piche evaporation and relative humidity using the most recent climatic normals of 1981- 2010 provided by the Sudanese Meteorological Authority.

The validation using Wadmedani station datasets for two consecutive years (2010 and 2011) resulted in acceptable root mean square errors of 0.2 mm and 0.5 mm, respectively.

Based on the mean bias error (MBE) indicator, the validation over-predicted ETo resulted in acceptable values of 0.55 and 0.60 mm, respectively.