Rainfall is considered the primary input for crop yield in Sub-Saharan Africa, any significant variability in the amount of annual rainfall could have an equally significant impact on agricultural production (Ati et al., 2009). Rainfall shows a more complex structure over time and space. The rainfall is highly variable both in amount and distribution across regions and seasons (Mersha, 1999). The Intergovernmental Panel on Climate Change(IPCC, 2001) pointed out that rainfall in Sub-Saharan Africa exhibits high interdecadal variability, a pattern of continuous aridity since the late 1960s. The degree to which rainfall amounts vary across an area or through time is an important characteristic of the climate of an area. This subject area in meteorology/climatology is called rainfall variability (Nouaceur and Murarescu, 2016).

Impacts of rainfall variability are felt among farmers in many regions and developing countries including Nigeria. Rainfall variability constitutes a serious adverse socio-economic and environmental problem in Northeastern Nigeria. It has resulted into seasonal drought, flood, soil erosion, climate change, biodiversity loss among others (Adejuwon and Odekunle, 2006). The natural resources base on which farmers depend on are altered, traditional socio-economic livelihoods stressed and the potential for future agricultural development are affected by rainfall variability (Jost et al. 2016).

Moreover, over the past years concerns have grown on increased rainfall variability across seasons resulting in large yield variability and thus becoming an apparent determinant on the performance and adaptation of sor-ghum varieties (Traore et al., 2016). Meteorological data have shown that rainfall pattern in Nigeria has changed in the past decades. Oladipo (1995) reported that decline in the rainfall in Nigeria started in the beginning of the 1960s when a decade of relatively wet years ended. Fluctuations in rainfall and other manifestations of the impacts of climate change are direct threats to the livelihoods of Nigerians, and indeed a direct threat to farmers. As rainfall variability increases, its impact on livelihoods of indigenous farmers bites harder. Some of the consequences of this are that farmers are forced to cultivate more lands to marginally increase yield to make up for the shortfall (Agwu and Okhimamhe, 2009).

Total annual rainfall at a location is influenced by se-veral variables including the frequency of rainfall events, the duration of the rainy period and the intensity of rainfall of individual events. Inhomogeneities in the annual rainfall therefore reflect changes in these contributory variables. Adejuwon et al. (1990) fitted linear trends to the annual rainfall series of several locations in Nigeria for the entire period of available data which, in some cases, began in 1922. Olaniran (1991) analyzed the fluctuations in the series of rain days of three rainfall categories (low, moderate and heavy intensity). On the other hand, Dammo et al. (2015) examined quantitative analysis of rainfall variations in north-eastern region of Nigeria based on the standard climatic normal periods. Apart from this, assessment of the trends and changes in different categories of rainfall variability and its effects on the yield of sorghum in Potiskum Local Government Area (LGA), Yobe State, Nigeria appears in the literature.

Sorghum (Sorghum bicolor L. Moench) is a local grain cultivated predominantly in the semiarid savannah and grassland areas of Northern Nigeria and other parts of the world. Sorghum is viewed as a possible replacement crop for corn cultivated for grain and silage, in part due to sorghum’s lower input requirements and costs, and its drought tolerance. It is nutritionally rich and serves as a staple food in most parts of Northern Nigeria. The grain has assumed commercial relevance lately, especially in the food and beverage industry. It has been found to be a valuable ingredient next to malted barley used in the beverage industry. Sorghum is an annual grass, 5–7 feet tall, similar in appearance to maize (corn). Through breeding efforts, newer varieties now have 2-3 dwarf genes, resulting in a plant 2-4 feet tall and easier to harvest. The most extensively grown and best-known sorghum species in Nigeria is vulgare and S. bicolor (L.) Moench., locally called guinea corn,. Both varieties can be white or yellow (FAO, 2012).

Sorghum requires rainfall of about 600-1,000 mm for a duration of at least five months spread over eighty rainy days (Odjugo, 2009). That means where rainfall totals fall below 600 mm, effective sorghum production will be affected. It also implies that sorghum production will also be affected where the amount of rainfall exceeds the required maximum. The development of the crop takes 110 to 170 days, and is frequently considered to have 3 stages: emergence, floral initiation to flowering, and flowering to physiological maturity. The crop thrives better under dry and cool conditions. Sorghum is a thermophilic (26ºC-40ºC), drought-resistant plant, which grows slowly at 16-20ºC, and stops growing under 14ºC. Its productivity is dependent on quantity of rains during pre-sowing season and water holding capacity of soil (Iren, 2004).

Sorghum is widely grown both for food and as a feed grain. It plays a significant role in food security in northern Nigeria. The stems are used for fuel and building fences and local huts, hence, the reason for its wide cultivation in northern Nigeria. Nigeria must double its production capacity and equally address the challenges facing the agricultural sector, particularly those associated with rainfall variability in order to meet the high demand of sorghum. According to Schaefer (2001), crop yields are affected by climatic variables, uneven rainfall distribution and prolonged arid period which lead to further development of soil erosion and loss of fertile soil. Thus, this study succinctly assesses the relationship between rainfall variability and the yield of sorghum in Potiskum LGA of Yobe State, Nigeria. Results from previous studies/experiments on the production of sorghum show that sorghum yields vary among varieties and across locations and season.

Study area

Potiskum Local Government Area (LGA) is one of the seventeen LGA of Yobe State, Nigeria. It is located on the A3 highway and geographically referenced approximately on 11°33’40??N to 11°50?30??N of the equator and 11°3?30??E to 11°11?55?E of the Greenwich meridian. It has an approximate total area of 559 Km². According to National Population Census (2009), Potiskum LGA has a total population of 244,050 people with a population density of about 436.6 people per km² (National Bureau of Statistics, 2019). Potiskum LGA shares boundaries with Fune LGA to the East, Fika LGA to the South, Nangere LGA to the North-West (Figure 1). The ethnical composition of Potiskum is heterogeneous which includes both indigenous and settlers. The indigenous ethnic groups include the Ngizim, Kare-Kare and Bolewa, whereas the settlers’ ethnic groups include the Hausa, Fulani, Babur, Kanuri, Igbos, Yoruba and Shuwa-Arabs. Agriculture is the predominant occupation among the people (NBS, 2019).

The annual rainfall series for Potiskum was subjected to the normality test using the coefficients of Skewness (Z₁) and Kurtosis (Z₂) as illustrated in Table 1.

Both results showed that the series was negatively skewed and had no significant deviation from the normal curve (1.96) at 95% confidence level showing normal distribution of data since the results were below 1.96. The Coefficient of variation showed that rainfall was moderately variable at 24.66%. The annual range of rainfall was lengthier for 29 years at 751.38 mm which raises serious concerns of rainfall irregularity and variability.

The trend in annual rainfall series from 1956 to 2018 as presented in Figure 2 generally showed that rainfall was oscillating and declining. The linear equation is:

y= -1.5219x + 722.29

This shows a negative trend line which signifies a below normal scenario. There were notable increases in TAR above the mean from 1956-1964, 1979-1981, 1999-2001 and from 2008-2013. Other yearly increases were short-lived especially in 1967, 1972, 1974, 1986, 1988 to 1989, 1991, 1994, 1996, 2003 and 2005. According to Ati et al. (2009), 5-year running mean show annual rainfall at Potiskum of above the long term mean from 1953–1960. The results further showed that rainfall amount was above the long-term mean from the late 1960s to the early 1990s. Declining total annual rainfall at Potiskum LGA contradicts findings by Mamman et al. (2008) who reported an upward trend in mean annual rainfall in similar ecological zones of Bauchi, Maiduguri, Nguru and Yelwa. FAO (2019) further observed an increasing average rainfall trend (1999–2016) for Potiskum (Figure 2). This finding also disagrees with findings by Audu (2016) who reported an upward trend in annual rainfall in Yobe State with an increasing rainfall of an approximately 74.6 mm at the rate of 1.86 mm annually.

However, in other years, TAR greatly dropped below normal condition especially in the mid-1960s to the mid-1970s, early- to late-1990s and from 2014 to 2018.

Generally, these findings depict that Potiskum suffered the great northern Nigerian drought years and rainfall decrease as reported by Oladipo (1995), Ati (2006), Sawa (2010) and Nnachi (2014). These studies showed that declining rainfall in northern Nigeria started in the early 1960s when a decade of relatively wet years ended. Of no doubt, this decline in TAR has a significant impact on crop production with manifestation in frequent crop failures and poor yields in the study area.

Figure 3 indicates an increasing trend in sorghum yield (ton/ha) in the study area with the trend line equation:

y = 0.0348x + 0.6406.

The graph shows a positive trend in the sorghum yield. The highest yields (tons/hectare) were recorded in 2003, 2010 and 2011, while the lowest yield was in 1993. With the decline in rainfall amounts in the study area, farmers would have expected a decline in the sorghum yield; rather the result shows that sorghum yields increased. Several factors could have resulted in this outcome. According to the Kano Agricultural and Rural Development Agency (KNARDA, 2008), there has been appreciable increase in the land area and output of sorghum in Nigeria which could be attributed to advanced agricultural technology and use of viable adaptation strategies such as the introduction of high yielding/ climate resilient crop varieties, application of fertilizers, irrigation, improved agronomic practices and provision of services such as seminars and workshop to rural farmers coupled with the provision of agricultural facilities and seeds. Moreover, the changing climate pattern must have given rise to the need to adopt improved agronomic practices as agriculture is more susceptible to climate change. Seydou et al. (2022) reported that farmers that have access to credit, highly resilient crop varieties, climate information and contact with extension services are likely to have increased crop yield despite the effect of climate change on crop production. Today, sorghum is a staple food for millions of Nigerians.

However, Saleh (2014) reported an increasing trend in annual rainfall and sorghum yield in Potiskum between 1981 and 2010. This result further corroborates the findings of Ikpe (2014) who reported an increase in sorghum yield in Goronyo LGA, Sokoto State, Nigeria.

Table 2 shows the computation of the Pearson’s product moment correlation coefficient (r) calculated for both TAR and sorghum yield. The results (r=0.364) indicated that there was a negative correlation between rainfall and sorghum yield. Although there was a negative trend line in annual rainfall, sorghum yield was positive implying an increase. This is the reason why researchers like Sawa (2002) described sorghum as a drought resistant economic crop which thrives even when there is decline in rainfall.