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 para meters that would affect reference evapo-transpiration during the grow ing season. It was shown that for the three stations tested, monthly m ean daily maximum, and particularly minimum, temperatures have increas ed over the period 1961-91 by up to 1.5 degrees C. Although monthly me an daily relative humidity has been declining, this has been due to th e increased temperature, and monthly mean daily vapour pressure has re mained stable. As sufficient climate data to use the Penman-Monteith e quation were not always available, other radiation and/or temperature based equations were tested. However, none of the methods used produce d results that were comparable to the Penman-Monteith equation. The cl osest agreement was obtained using the Penman-Monteith equation with l ong-term monthly mean values of the missing parameters (where these th emselves were not subject to change over time). The observed increases in temperature have not be great enough to result in significantly in creased reference evapo-transpiration. Only at Maiduguri did the calcu lated reference evapo-transpiration show any significant trend over th e 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. (C) 1998 Academic Press Limited.