Abstract

The growing season indices (start, end, and length) are highly dependent on rainfall in areas where agricultural production is led by rain-fed agricultural systems. Planning agricultural activities may require reviewing statistics on the onset and cessation of the main rainy season, particularly to decide when to prepare the field for sowing and planting. The main objective of this study was to ascertain the characteristics of growing season over the Upper Blue Nile (UBN) basin. For this study, data from the National Meteorological Agency of Ethiopia for 16 stations between 1987 and 2018 has been taken. A threshold based method relating rainfall and potential evapotranspiration has been used to evaluate the growing season. The results show that the starting date of the growing season on the scale of stations shows a decreasing and an increasing trend from the eastern part to the western part below and above 100 N, respectively. The northern and central parts of the basin start the growing season later than the other regions, with a high amount of variability around the eastern part and a low amount of variability around the southern part. The end dates of the growing season show an eastward decline trend from the other sides of the basin. Most of the northern and eastern parts show a positive trend, with the positivity inclined towards the north. The variability of the end of the growing season has been low around the northern and central parts and high around the eastern and western parts. The shortest length of the growing season was observed around the eastern and northern parts of the study area, while the longest mean length of the growing season was seen over the western and southern parts. The advancement of LGS seems to happen towards the south from the north-eastern part of the UBN basin. On stations in some eastern, north-eastern, and northern parts of the UBN basin, a positive trend (enhancement) was observed, whereas the rest of the basin showed a negative trend (decrement). In addition, stations in most of the study area show strong variability for the LGS, which conveys the message that caution, should be taken when planting cultivars in the study area. In general, within the study period, there exist spatial as well as temporal variations on all parameters of the growing season over the UBN basin.