Statistical analyses of potential evapotranspiration changes over the period 1930-2012 in the Nile River riparian countries

Changes in potential evapotranspiration (PET) in the Nile Basin tend to be analyzed mostly based on short-term remotely sensed annual data. In this study, long-term country-wide series from 1930 to 2012 were used to assess changes in annual and seasonal PET in all the Nile River Riparian Countries (NRRCs). Variability was investigated using the nonparametric anomaly indicator method. Trend was assessed both graphically and statistically using the cumulative rank difference method. The PET totals from 1930s to 1970s (from around 1980 to early 2000s) were generally below (above) the mean of the long-term data. Moreover, for this period from around 1980 to early 2000s, both annual and seasonal PET totals in most of the NRRCs were characterized by an increase significant at 5% level. This increase in the PET influenced the long-term trend based on the full time series from 1930 to 2012 towards positive direction. For instance, the long-term annual PET exhibited increasing trend significant at 5% level in 2 of the 6 countries in the equatorial region. However, the positive trends in the PET of Sudan, Ethiopia, Eritrea and Egypt were insignificant at 5% level. It was found that the temporal changes in PET especially during rainy seasons can be explained at the significance level of 5% by the rainfall variation. Sampling uncertainties on the PET trend magnitudes are quantified and provided. The findings in this study are important for determining the crop water requirements especially in arid conditions where rainfall is unreliable and low in volume.