Some aspects of measured and estimated evaporation in the Sudan

Abstract

The present work analyses, for the Sudan, the methods of evaporation measurement by Piche evaporimeter and Clas ‘A' pan, together with the estimation of open-water evaporation (EO) from Penman's formula (1948) and potential evapotranspiration (PE) by Thornthwaite's method (1948). Measured Class 'A' pan and Piche evaporation tend to follow similar fluctuations during the various seasons. Monthly and seasonal fluctuations of the measured water loss are large compared with the computed evaporation which displays limited variations. During the dry season, measured evaporation is greater than the computed, but, in the wet season, the measured values are slightly exceeded by the (PE) and even more so by the (EO). Regression analysis shows a close correlation between measured and computed evaporation at some stations in northern Sudan where the correlation coefficients are large (over 0.70). At some stations in central and southern Sudan the correlation coefficients are low (under 0.50), and the correlation is rather poor and may not be statistically significant. In a comparative study between Penman's (EO) and Thornthwaite's (PE), the former usually gives larger values than the latter. But the disparity between them is relatively small in the dry season. The final aspect of the thesis discusses the distribution of average annual, seasonal and monthly Piche evaporation over the Sudan. Annual values reveal a steady decrease from north to south, and evaporation isolines seem to run roughly along latitudes from east to west. This pattern of isolines is interrupted by the uplands of Jebel Marra, the Red Sea hills, and Nuba mountains where evaporation tends to decrease with high altitudes.