Trends in reference evapo-transpiration in the North East Arid Zone of Nigeria, 1961–91

The North East Arid Zone of Nigeria has experienced a decline in mean annual rainfall over the period 1961–90. This paper seeks to establish whether changes have occurred in the mean value of other climate parameters that would affect reference evapo-transpiration during the growing season. It was shown that for the three stations tested, monthly mean daily maximum, and particularly minimum, temperatures have increased over the period 1961–91 by up to 1•5°C. Although monthly mean daily relative humidity has been declining, this has been due to the increased temperature, and monthly mean daily vapour pressure has remained stable. As sufficient climate data to use the Penman–Monteith equation were not always available, other radiation and/or temperature based equations were tested. However, none of the methods used produced results that were comparable to the Penman–Monteith equation. The closest agreement was obtained using the Penman–Monteith equation with long-term monthly mean values of the missing parameters (where these themselves were not subject to change over time). The observed increases in temperature have not be great enough to result in significantly increased reference evapo-transpiration. Only at Maiduguri did the calculated reference evapo-transpiration show any significant trend over the period, which was primarily due to increases in recorded wind speed. An inconsistent and broken wind speed record at Maiduguri casts doubt over the validity this increase.