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The spatial patterns and trends of various categories of dry and wet spells were analysed from 1981 to 2010 in the Middle Belt of Nigeria. Daily rainfall (mm) data were obtained from eight synoptic weather stations spread across the region. The spatial variation and temporal trend of spells were analysed using the coefficient of variation (CV) and Pearson’s correlation coefficient (r). The result reveals that spatially, dry spells varied from 12.8 to 110.1%, while wet spells varied from 11.7 to 192.5%. The longest dry spell length by station ranged from 14 days (Jos) to 37 days (Yola), while the longest wet spell ranged from 7 days (Bida, Ibi and Makurdi) to 11 days (Ilorin and Jos). Both dry and wet spells exhibited positive and negative trends. Significant trends of dry spells include negative trends of categories 2–4 days (Ibi), 8–10 days (Yola), 11 days+ (Ilorin and Yola); and positive trends of categories 2–4 days (Lokoja) and 8–10 days (Ilorin). For wet spells, only positive trends were significant. The study concludes that the south western (northern) part of the region recorded the highest (least) annual frequency of dry spells and least (highest) annual frequency of wet spells, respectively.
Faculty of Environmental Sciences, Department of Geography, Benue State University, Makurdi, Nigeria
Michael Terver Iwan
Department of Geography, College of Natural and Applied Sciences, Kwararafa University, Wukari, Nigeria
*Address all correspondence to: btyubee@bsum.edu.ng
1. Introduction
The study of daily rainfall occurrence, especially dry and wet spells, is of high climatologic interest across the world. During the past few decades, rainfall distribution in sub-Saharan Africa has been observed to have high spatial and temporal variability [1]. Dry and wet spells are two main physical characteristics of rainfall, the distribution of which influences the volume of rainfall in a geographical area [2, 3, 4, 5]. Also, events of dry and wet spells have a critical influence on the duration of rainy season, mostly, in terms of false onset or early or late cessation of the season [6]. Thus, rain spells have been a topical issue of increasing importance globally. Global climate models have indicated a rising frequency of summer droughts due to global warming, even in regions where droughts were previously rare events, while at the same time, devastating floods in Nigeria, Europe and other parts of the world have increased the interest in wet spells and heavy precipitation occurrences [7, 8, 9, 10, 11]. Moreover, the persistence properties of the daily precipitation process are governed by spell lengths. Therefore, spells of dry days can reveal significant changes in the structure of drought, while identifying changes in the trend of both dry and wet spells as well as their persistency will provide useful information in predicting future climate events since these variables are closely related to extreme weather events such as drought and flood [12, 13].
The distribution of dry and wet spells remains an important applicative approach to rainfall statistics as it provides useful knowledge and information for many areas particularly agriculture [14, 15, 16, 17, 18], irrigation and water resource management [19, 20]. Knowledge of the period of occurrences, frequencies and duration of dry and wet spells will minimise unexpected damage due to drought or flood. Information on rainfall probabilities is also vital for the design of water supply management, supplementary irrigation schemes and the evaluation of alternative cropping systems for effective soil water management plans [21]. Such information can also be beneficial in determining the best-adapted plant species and the optimum time for seedlings to re-establish vegetation on deteriorated rangelands. Consequently, several studies were focused on dry (wet) spells since these variables greatly influence agriculture, drought and flood [10, 13, 22, 23, 24, 25].
Synoptically, rain spells are associated with large-scale atmospheric circulation patterns and coincide almost always with the presence of specific meteorological features such as depressions, cyclonic systems, fronts and troughs, whereas the absence of these phenomena is almost tantamount to the absence of rain [26, 27, 28]. For instance, the 2–3-, 4–5- and 6–9-day wet spells are reported to be associated with the propagation of the African Easterly Waves, while the 10–20-day wet spells are linked to the coupled land-atmosphere interaction in the African Monsoon rainfall [1, 29]. On the other hand, lack of rainfall (dry spell) is associated with anticyclone systems that can last from 1 day to many days or semi-permanent or seasonal anticyclones [30]. Therefore, knowledge of the characteristics of rain spells, from the synoptic view point, is relevant in recognising rain-bearing characteristics of these weather systems, while the rain spells themselves appear a more “natural” time unit for analysis of rainfall than the conventional ones [26].
Like other tropical regions, rainfall in the Middle Belt region and Nigeria at large, greatly varies temporally and spatially. Its onset and cessation, frequency, intensity, amount and sequences of spells also vary greatly. While rainfall characteristics such as onset, cessation, duration, intensity, rain days, seasonality, amount, persistence and periodicity are well researched and documented in the Middle Belt region [31, 32, 33, 34], studies on spells are rather scarce. A recent study in the region on dry and wet spells [15] focused on their frequency distribution and probabilities based on the Markov chain model, to guide agricultural cop planning in the region. Another more recent study [18] on dry spell prediction covered the whole of Nigeria, with nine study stations of which three (Ilorin, Lokoja and Makurdi) fall within the Middle Belt region. The focus of this study was on the prediction of critical dry spells in rain-fed maize crop production for various locations using an artificial neural network.
However, in addition to agricultural production, knowledge of dry and wet spells in the Middle Belt region is crucial to hydrologists, environmentalists, agriculturists, planners and climate-induced disaster managers. This chapter, therefore, analyses the spatial patterns and trends of both dry and wet spells for the Middle Belt region. The findings of this work are relevant in serving the information needs of the aforementioned interest groups and also adding to the existing knowledge and literature on rainfall climatology of the region.
2. Data collection, definition and categorisation spells
2.1 Data collection
Daily rainfall (mm) data were obtained from the Nigerian Meteorological Agency (NiMet) eight synoptic weather stations spread across the Middle Belt region for a 30-year period (1981–2010). The stations are Bida, Ibi, Ilorin, Jos, Lokoja, Makurdi, Minna and Yola. The choice of the stations was due to data quality and availability for the study period, and geographical spread.
2.2 Definition of spells
A spell is defined, in the study, as a consecutive number or group of days each with at least 1.2 mm of rainfall (wet spell) or less than 1.2 mm of rainfall (dry spell). As Hern’aeza and Martin-Vide [35] noted, the definition of thresholds is important because it plays a key role later on in the outcomes of the study. Fischer et al. [36] specifically pointed out that for agricultural applications a threshold of 1 mm day−1 may be more appropriate. This study, however, uses 1.2 mm as a threshold for description of dry/wet days, which translates to a weekly threshold of 8.4 mm. These thresholds relate more closely to the weekly crop water requirement for Nigeria, which is 8 mm [37, 38]. The 1.2 mm daily threshold also takes care of measurement errors associated with light rains due to direct evaporation at manual rain gauges [39, 40]. Thus, a day is reckoned as wet if it has rainfall equal to or more than 1.2 mm, while days with less than 1.2 mm are considered as dry. Similarly, a wet week is one with rainfall equal to or more than 8.4 mm, and a dry week has less than 8.4 mm.
2.3 Categorisation of spells
The dry and wet spells are categorised into four and two classes, respectively (Table 1). Though dry and wet spells vary in duration, the categories of dry spells are chosen with reference to the water stress on various crop types caused by dry spells of varying durations [41], and those of wet spells are chosen to correspond to the different synoptic systems causing rain in West Africa. The wet spells lasting 2–4 days are associated with the so-called “3–5 days” African Easterly Waves (AEWs), while those lasting for 5 days and above are related to the “6–9 days” African Easterly Waves [29].
3. Determination of onset and cessation of the rainy season
Gitau [42] made a useful suggestion that the actual onset and cessation dates of the rainfall season should be determined before the frequency distribution of wet and dry spells is derived. Since the scope of this study is limited to the rainy season, onset and cessation of the season have been defined and empirically determined from daily rainfall data. Based on Tarhule and Woo [39] method, the onset of the rainy season is taken to be the first wet day of the year after which there is no dry spell longer than 12 days in-between the subsequent 4 wet days, while the dry season is considered terminated on any wet day after which there occurred a dry spell longer than 12 days during the last 4 wet days of the year. The condition of having no dry spell of more than 12 days in-between 4 consecutive wet days following the onset day is to eliminate the possibility of including a false onset of the rainy season. The duration of the season is the number of days between the onset and cessation dates.
To determine the onset date of the rainy season for a given year, the daily rainfall data were observed from the beginning of the year to identify a wet day, then the next 4 consecutive wet days were also identified, ensuring that no dry spell of duration longer than 12 days occurred between them. The first wet day after which no dry spell longer than 12 days occurred, before the next 4 consecutive wet days were counted, marked the onset of the rainy season. On the other hand, cessation of the season was determined by observing the daily rainfall data backward from the end of the year to identify the wet day after which a dry spell longer than 12 days occurred during the last 4 wet days of the year.
To analyse the spatial variation of spells in the Middle Belt (MB) of Nigeria, isolines and coefficient of variation (CV) of the annual frequency of spells from 1981 to 2010 were used. The analysis was based on categories of dry and wet spells.
4.1.1 Spatial variation in dry spells
The total annual frequency of 2–4-day dry spells decreases from the southern to the northern part of the region. However, Lokoja and Jos recorded the highest and lowest annual frequency of 682 and 510 spells (Figure 1a). Makurdi (with 649 spells) comes second after Lokoja in the order of high occurrences but lies within the same zone with Bida and Ilorin where the occurrence number is between 600 and 650 spells. There is an enclave of low occurrences towards the eastern and north eastern parts of the region where Ibi and Yola have 552 and 534 spells, respectively, while Minna in the north western part is isolated with 596 spells. The CV values showed that Jos has the least station variation (CV = 13.4%), while Ibi has the largest station variation with CV values of 26.7%, respectively (Table 1).
Figure 1.
Spatial variation annual frequency of 2–4-day (a) and 5–7-days (b) dry spells in MBR.
For the 5–7 days category, all the Niger-Benue valley areas of Lokoja, Makurdi, Ibi and Yola, plus Ilorin in the south west fall within the same zone with a total number of occurrences between 150 and 200 dry spells (Figure 1b). There is a decrease in the annual frequency of occurrence from this zone towards the north central plateau where, again, the lowest value is at Jos, with a total annual occurrence of 83 dry spells. The CV values, however, showed that Ilorin has the least station variation (CV = 33.5%), while Jos has the largest station variation with CV values of 67.6%, respectively (Table 1).
The spatial pattern of annual frequency of 8–10-day dry spells category showed a zonal pattern with an increase from the southern part, along Ilorin-Lokoja-Ibi axis to the northern part (Jos). Similarly, Ilorin and Lokajo recorded the highest annual total of 50 and 60 dry spells, whereas Jos recorded the least frequency of 18 dry spells (Figure 2a). The CV values showed that Lokoja has the least station variation (CV = 55.8%), while Jos has the largest station variation with CV values of 120.7%, respectively (Table 1).
Figure 2.
Spatial variation in annual frequency of 8–10-day (a) and 11-day+ (b) dry spells in MBR.
The spatial pattern of annual frequency of 11 days+ dry spells (Figure 2b) is similar to the 8–11-day dry spells (Figure 2a). However, the annual total frequency was lower relative to 8–10-day dry spells with the highest and least annual frequency of 38 and 11 dry spells observed in Ilorin and Jos/Minna, respectively (Figure 2b). For station-to-station variation, the CV values showed that Ibi has the least station variation, with a CV value of 72.3%, while Ibi has the largest station variation with CV values of 182.4%, respectively. Generally, it can be inferred from the spatial distribution pattern that the annual frequency of the four categories of dry spells was higher in the southern part and lower in the northern part when compared to the frequency of the average region (Table 2).
Station
Duration of spell
2–4 days
5–7 days
8–10 days
11 days+
Total
Mean
CV
Total
Mean
CV
Total
Mean
CV
Total
Mean
CV
Bida
617
21
18.2%
138
5
50.0%
40
1
77.1%
15
1
114.5%
Ibi
534
18
26.7%
199
7
39.3%
53
2
65.9%
34
1
72.3%
Ilorin
601
20
21.8%
153
5
33.5%
56
2
81.8%
38
1
82.8%
Jos
510
17
13.9%
83
3
67.6%
18
1
120.7%
11
0
182.4%
Lokoja
682
23
21.5%
155
5
41.9%
56
2
55.8%
26
1
94.5%
Makurdi
649
22
15.4%
155
5
42.6%
42
1
85.1%
28
1
92.1%
Minna
596
20
16.4%
112
4
45.6%
44
1
114.3%
11
0
151.7%
Yola Region
552 593
18 20
17.1% 18,20%
152 143
5 5
43.0% 44.18%
43 44
1 1
74.8% 82.27
24 23
1 1
100.6% 111.22%
CV Range
12.8%
34.1%
64.9%
110.1%
Table 2.
Descriptive statistics of dry spells in the Middle Belt of Nigeria (1981–2010).
The result suggests that the annual number of dry spells decreases from the southern to the northern part of the region. However, the annual frequency and CV of dry spells have increased from shorter duration (2–4 days) to longer duration (11 days+) with Jos recording the least annual frequency and least station variation for all the categories of dry spells.
4.1.2 Spatial variation in wet spells
Spatial variation of wet spells in the region showed that there is an increase in the total annual frequency from south to north. For the 2–4-day category, Jos (north) and Lokoja (south) have the highest and least number of 2–4-day wet spells of 557 and 329 spells, respectively (Figure 3a), whereas Jos (north) and Ilorin (south) have the highest and least annual frequency of 100 and 40 spells for the 5-day+ category (Figure 3b). The results of coefficient of variations of the various spell categories (Table 3) showed that the CV ranges from 17.3% (Ilorin) to 27.9% (Ibi) for 2–4-day wet spells and from 31.7% (Jos) to 224.2% (Yola) for the 5-day+ wet spells, respectively. For wet spells, only the annual frequencies of Jos, Minna and Ilorin were higher than the areally average region for both the 2–4- and 5-day+ wet spells. (Table 3).
Figure 3.
Spatial distribution of total number of 2–4 days and 5-day+ wet spells in MBR.
Station
Duration of spell
2–4 days
5 days +
Total
Mean
CV
Total
Mean
CV
Bida
440
15
19.5%
20
1
113.7%
Ibi
381
13
27.9%
10
0
164.0%
Ilorin
459
15
17.3%
40
1
74.6%
Jos
557
19
18.3%
100
3
31.7%
Lokoja
430
14
27.1%
26
1
120.2%
Makurdi
422
14
23.0%
20
1
113.5%
Minna
490
16
21.2%
49
2
74.5%
Yola
329
11
34.0%
14
0
224.2%
Region
439
15
22.78%
35
1
192.50%
CV range
16.7%
192.5%
Table 3.
Descriptive statistics of wet spells in the Middle Belt of Nigeria (1981–2010).
4.2 Trend in dry and wet spells
4.2.1 Trend in dry spells
The correlation coefficients, which show the strength and significance of trend in the four categories of dry spells in the Middle Belt, are presented in Table 4. For the 2–4-day dry spell category, two areas within the Niger-Benue valley manifested significant opposite trends, which are Ibi, with a negative and significant trend, and Lokoja with a positive and significant trend. The other areas within the valley had positive but insignificant trends. The case of Bida is almost steady but increased slightly. The rest of the region, from the northwest to the north central plateau of Jos, and the south western parts (Ilorin), showed a negative but insignificant trend.
Correlation coefficients of dry spells in the Middle Belt of Nigeria (1981–2010).
Significant trend at 95% confidence level.
The trend of 5–7-day dry spell category showed that the Niger-Benue valley areas (Ibi, Makurdi and Yola) had a positive but insignificant trend except for Lokoja and the region where the trend was negative but insignificant. The rest of the region experienced an insignificant decreasing trend, except the north central plateau zone which manifested positive but insignificant. This means that there had only been slight changes in the occurrences of the medium dry spell category (5–7 days) in the MB region, from 1981 to 2010.
The 8–10 days dry spell category showed equal decreasing and increasing trends among the eight stations (Table 4). The Lokoja, Makurdi and Ibi-Yola, and the averaged region experienced a decreasing trend which, however, was only significant at Yola, whereas the northwest (Minna), north central (Jos) and south western parts (Ilorin) witnessed an increasing trend, but it was only significant in the south west. The directions of these trends are shown in Table 4.
The 11 days and above dry spell category decreased significantly over Ilorin, Yola and the averaged region, while the north central plateau (Jos) and the Niger-Benue confluence area (Lokoja) experienced decreasing but insignificant trend. Elsewhere, the trend was positive but insignificant (Table 4). The correlation result (Table 4), which showed a diffused situation whereby the probability of increasing trend of annual frequency of dry spells (positive correlation), points to severe drought and decreasing trend of annual frequency of dry spell (negative correlation), and points to a mild drought in the region.
4.2.2 Trend in wet spells
The trend in annual frequency of the 2–4 days wet spells showed that only Jos and Minna have negative trends (Table 5). The rest areas of Yola, Lokoja, Makurdi and Ibi, and Ilorin, and the areally-averaged region had positive trends. However, only the positive trend of Yola was significant.
Correlation coefficients of wet spells in MBR (1981–2010).
Significant trend at 95% confidence level.
For the long-wet spell category (5 days and above), there was a generally positive trend in the Middle Belt region, except for Ibi. However, only the positive trends of Jos, Yola and the region were significant (Table 5).
The orographic effect of the Jos Plateau has resulted not only in higher rainfall amounts but also increase in rain days and wet spells on the Plateau. The effect of the Jos Plateau, which is clearly discernible in the spatial pattern of dry and wet spells in the Middle Belt, is also documented in other rainfall characteristics such as on onset, cessation and duration of rainy season, daily rainfall organisation and intensity and annual rainfall [31, 32, 33, 34]. The Jos Plateau accelerated convective activities given rise higher annual number of rain days recorded in the region [32]. The zonal spatial pattern of increase (decrease) from the southern to the northern part of the region may be related to the proximity to the rain-bearing southwest trade wind, which advects moisture from the Atlantic Ocean. A similar zonal pattern is observed in the spatial distribution patterns of eight rainfall characteristics in the Middle Belt region [31].
For the entire region, there are more increasing trends of wet spells than decreasing trends. This is significant to agriculture and water resources management, especially soil erosion and flood control and management. In addition, as rain spells are related to synoptic systems such as depressions, cyclonic systems, fronts and troughs [1, 26, 29], the increasing trends of wet spells in the Middle Belt region could be an indication of increased activities of synoptic systems, especially the African easterly wave (AEW) that favours rainfall occurrence in central Nigeria.
The annual frequencies of 2–4- and 5-day+ wet spells were higher in the southern part than the northern part of the Middle Belt region. Conversely, the annual frequencies of the four categories of dry spells showed higher values in the northern part of the region. However, there is a station-to-station variation in annual frequencies of both dry and wet spells with Jos having the highest frequencies, especially for the wet spells due to the orographic effect of the Jos Plateau with an elevation of about 1500 m.
The region experienced a generally increasing trend in wet spells compared to dry spells. An increase in the frequency of wet spells may potentially increase the risk of flooding in the region, especially along the banks of Rivers Niger and Benue. An increase in the frequency of dry spells may aggravate drought and desertification, especially in the northern parts of the region. Sustainable water and land management practices may mitigate the dual threat of drought and flooding in the Middle Belt region.
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Written By
Bernard Tarza Tyubee and Michael Terver Iwan
Submitted: 10 August 2023Reviewed: 24 October 2023Published: 12 June 2024
Open access peer-reviewed chapter
