WikiJournal Preprints/Spatiotemporal Analysis of Onset, Cessation and Length of Rainy Season in Benue State

Introduction
Rainfall is a precursor to both vegetation and farming operations e.g., types of cultivated crops, vegetation cover and the quality of soil, such that a change in the rainfall pattern in the timing of the onset, growing season and cessation affect both the vegetation and agricultural productivity of a region. A deeper insight on rainfall characteristics is indispensable to rain-fed agriculture where a change in the pattern of rainfall in terms of the onset, growing season and cessation poses considerable risk to the timing of the planting, growing and harvest outcome of farm produce. When the accumulated rainfall received is more and less than 51mm in a season, it is referred to as the onset and cessation of rainfall, the window between the onset and cessation is called the length of the growing season. The sufficiency of the accumulated rainfall determines the crops to be cultivated and a reduction in the accumulated rainfall is a pointer to the end of the growing season. The advent of the rainy season is manifested when potential evapotranspiration is more than half.

The computation of the onset, growing season and cessation of rainfall is a challenging task due to the spatial variability of rainfall patterns. For example, forward and backward method of accumulation of mean daily rainfall was used in Odisha India to evaluate the onset and cessation of rainfall, the study in Hindol, Gondia and Kankadahad, revealed the onset of monsoon rain between 17 to 18th of June at 115 to 120 rainy days with the highest in Hindol at 122 days and the minimum in Odapada at 113 days while cessation of rain was observed in all areas around 10 to 11th October, in Botswana, artificial neural network (ANN) analysis used to predict the onset and cessation of the rainy season revealed an R-value of 99% for both onset and cessation at Sowa Pan and Letlhakane locations within five months from November to March, the study further revealed that onset occurs in November and spreads between November, December and January while cessation commences in February and March. The earliest Onset is the 28th and 29th of November at Pandamatenga and Francistown, while cessation of rainfall was observed earliest on the 27th and 30th of March in Pandamatenga and Shakawe with less variability as compared to Onset of Rainfall, but in West Africa, the rainfall pattern is influenced by Inter-Tropical Discontinuity, Land sea thermal contrast and sea surface temperature.

The rainy season in Nigeria is marked with rapid interannual variability due to the shift in sea surface temperatures in the eastern and western equatorial Pacific and the el-nino southern oscillation phenomenon. Consequently, Nigeria experiences 4 months of the onset of rainfall with an average spatial range of 137 days from February to June (Feb. 12, Warri to June 29, Nguru) and a rainfall cessation of 2 months from September to November with a spatial range of 82 days between September 4, in Nguru to November 25, in Calabar. Across the regions of Nigeria, rainfall characteristics are invariance in onset, cessation and growing seasons e.g. in Gombe and Dadin Kowa, the 25th and 30th of May are onset dates with a +10 and +15 days variability, cessation dates are October 12 for both Dadin Kowa and Gombe town with a +10 days variability and growing season for Gombe is 140 days and a +10 days variability and 137 days with + 15 days variability for Dadin Kowa, In Akure, the mean onset date is March 8, cessation date is October 21 with a variability of ± 21 days and 186 to 291 days of growing season length, In Anambra onset and cessation dates of the rainy season is characterized by early cessation of annual rainfall, frequent drought and flooding and changing the onset of rainy and growing seasons. In Northern Nigeria, the mean rainfall amount is 763 mm, mean rainfall onset is May 29 (149), mean rainfall cessation September 21 (264) and mean length of the rainy season is 4 months. The variation in the onset, cessation and length of growing season between the North and south is reflected in the vegetation and soil diversity of both regions which are savannah in the north and the forest in the south and as such the differences in the cultivated crops.

An understanding of the rainfall characteristics of Benue state is important for Agricultural planning especially when previous studies on rainfall patterns have only focused on the impact of rainfall on crop yield in selected areas of the state without investigating the robustness of the impacts in the entire state which necessitate this study to evaluate the rainfall characteristics (onset, cessation and length of growing season) of Benue state where intensive agriculture is practised. Also, the predictability in the onset of the rainy season determines the length of the growing and cessation seasons which is fundamental for agricultural planning and development of Benue state.

Study Area
Benue State is straddled between the Savannah Belt North and the Rain Forest South of Nigeria with a blend of climate, pedology and vegetation. It is situated on Latitudes 6o25’N and 8o8’N and longitudes 7o47’E and 10o0’E with a landmass of approximately 34,059 km2, a projected population of 7,097,863 in 2020 from 2006 and politically bifurcated into 23 local council areas including the capital in Makurdi. It is bordered to the North by Nasarawa State, Taraba State to the East, Cross River State and the Republic of Cameroun to the South and South East, and Kogi and Enugu States to the West and South West of Nigeria with an ethnically homogeneous Tiv, diverse Idoma, Igede, Etulo, Jukun and immigrant’s Ibo, Hausa, Yoruba and other Nigerians.

Benue state experiences both the dry and wet seasons; the dry season starts with a harmattan from November to January and ends in March with heat while the wet season commences in April with high humidity and precipitation and ceases in October. Light showers are also common in January, February and March due to the East-West line squalls to signal the preparation of the planting season. The area has recorded annual rainfall of between 1200mm to 1500mm, average maximum and minimum diurnal temperatures of 35°C and 21 °C during the wet season and 37°C and 16°C during the dry season, relative humidity of 74.88% and diurnal sunshine of 6.2 hours.

The landscape is defined by oxisols and ultisols tropical ferruginous soil in the North, lateritic and forest vegetation cover in Oju, Obi, Ogbadibo, Oturpo and Vandekiya areas in the South, entisols, inceptisols with young soils on hill slopes and recent alluvium and Eutrophic Brown earth and volcanic parent materials in Gbajimba, deep gullies in Ogbadibo an extension of the eastern Nigerian, metasedimentary deep gully system and other gully sites in Makurdi North Bank area, Tse Mker and Gbem in Vandeikya, and Gbajimba town. Streambank erosion in Gboko town and incised streams on slochallenging ous to Anwase, Kyogen, Abande ranges in Kwande LGA are peculiar environmental landscape and problems. The state is drained by the Benue River and tributaries from the Cameroonian mountains which makes a confluence with the Niger River in Lokoja.

Onset
According to Walter in 1967, onset of rainfall is the time of accumulation of rainfall of 51mm.

The actual date is given by the formula:

Days in the Month x   51   ‒     Accumulated Rainfall in the Previous Month $$/$$Total Rainfall for the Month

The date of cessation is the date after which no more than 51mm of rainfall is expected. The formula above is applied in reverse order by accumulating the total rainfall backwards from December to obtain the actual date of cessation of the rains. Length of Rainy Season is the intervening period between the dates of onset and cessation of rains. It is the difference between the cessation date and the onset date. It is calculated by subtracting the onset date from the cessation date.

Coefficient of Variation (CV)
The coefficient of Variation (CV) was computed for the mean monthly and annual rainfall amount, onset and cessation dates, length of the rainy season and rainfall seasonality index.

According to Sharma and Singh in 2019, CV is:

CV =    S $$/$$ V  x  100%

Where CV is the Coefficient of variation, S is the standard deviation and x̅ mean of rainfall, a higher value of CV indicates higher spatial variability, and vice versa.

Mann-Kendall test
The Mann-Kendall test is a non-parametric statistical test used to test for the presence of an increasing or a decreasing trend in the considered time series. Mann-Kendalll test statistic was done using R Language. Studies have employed the Mann–Kendall test for the detection of trends in hydrological time series data   and it has proven to be more efficient for detecting a trend in a skewed data distribution.

The computations assume that the data are independent and are identically distributed.

The mathematical equation for calculating Mann-Kendall statistics is given as;

S = $$\sum_{k=1}^{n-1}\sum {n}/j= k+1$$ $$sgn\bigl(X_j - X_k \bigr)$$

Where n is the number of data, x is the data point at times j and k (k>j), and the sign function is given as:

$$sgn = \bigl( X_j = X_k\bigr)= $$ $             \begin {cases} +1  if X_j - X_k = 0 \\ 0   if  X_j - X_k  =  0  \\ -1 \   if X_j - X_k = 0 \end {cases}$

Where S = Mann-Kendall test statistic, Sgn = an indication function, xj and xk are the annual values in years j and k, j > k, respectively. A positive S value indicates an increasing trend while a negative S indicates a decreasing trend in the data series.

In a situation where there are ties (i.e., equal values in the x values), the variance of S is estimated and given by:

Var (S) = [n (n-1) (2n+5)]-∑mi=1 ti(ti-1) (2ti+5)] $$            /$$18                        

Where m = number of tied groups in the data set and ti is the number of data points in the ith tied group.

For a time, series such as this with n longer than 10 (n > 10), S approximates a standard normal distribution, ZMK. ZMK is computed to test the presence of a statistically significant trend. The Z test statistics are given as;

Z =

A positive value of Z indicates an increasing trend while a negative value indicates a decreasing trend.

The Julian days defined as the particular day of the year in which a date fall was used. For example, the 28th day of February marks the 59th day of the year from January 1st.

Theil-Sen’s Slope
Theil-Sen’s Slope was used to estimate the slope of an existing trend that is, the change in rainfall (mm) per year. It is used where the trend is assumed to be linear. It is insensitive to outliers and has been called the most popular nonparametric techniques for estimating a linear trend. The trend magnitude is computed;

Xj-Xi                

                        Β =   Median        ₍             ₎            

                                                       Tj-Ti

Where xj and xi = values at times tj and ti respectively

Exponential Population Projections
Nt = P0 e rt

Where; Nt = number of people at a future time, P = number of people at the beginning time

E = base of the natural logarithms at 2.71828, r = rate of increase divided by 100 and

t = time involved.

Onset Dates of Rainfall
The earliest onset dates for all the stations fall within February (2nd to 18th of February) in 2003 for all stations as revealed in Table 1 and figures 2, 3 and 4. The actual onset dates occur between March and April with an exception in Makurdi station where the latest onset date was observed to have taken place on the 1st of May in 2011. The mean onset date for all stations falls within March (between 11th to 31st of March). The lowest value in the mean onset date occurred in Vandeikya, followed by Igumale on the 21st March while the highest mean value of the onset date occurred at the Makurdi station on the 31st of March. Following the same trend, the Zaki-Biam station with an onset date of 30th March is followed by Bopo Station on the 28th March. Of significance is the Otukpo, Gboko and Katsina-Ala stations with latitudinal similarities of latitude (7o16’2”, 7o19’43”, and 7o10’10” respectively) all having near-average onset dates of 23rd, 22nd and 24th March respectively.

'''Table. 1: Descriptive Statistics of the Onset Dates of Rainfall''' Source: Author's Analysis (2019) The latest onset dates over the period are between the 7th of April in Vandeikya to 1st of May in Makurdi as shown in figure 1. The Otukpo station recorded the highest value in the onset date on the 28th of April next to the Makurdi station and followed by Zaki-biam and Katsina-Ala stations. The shift in the mean, earliest and latest onset dates vary according to latitude difference as one move from the southern to the northern region of the state. The mean onset dates increase northward from the south as shown in the earliest onset date of 11th March in Vandeikya and the latest onset date of 31st March in Makurdi. Also, the earliest onset dates were observed in February 2003 for all stations e.g., 2nd February in Vandeikya, followed by Igumale on 7th February and 18th February in Makurdi. The variations observed in the latest onset dates across all stations e.g., 7th April in Vandeikya in the south and 1st of May in the northern station of Makurdi shows an increasing northward latitudinal trend in the mean, earliest and latest onset dates of rainfall across all stations. The northern part of the state experienced more delayed onset dates in rainfall than the southern stations which explain the relationship between variation in rainfall and movement of the Inter-Tropical Convergence Zone (ITCZ) (which dictates the distribution of wind and precipitation) across the tropics. The inter-annual variation in coefficient of variation in rainfall between cessation and length of the rainy season revealed the onset dates with the highest inter-annual variation. The average onset CV of the study area is 16.4% when compared to 3.04% and 6.87% for the cessation and the length of the rainy season respectively. However, there is a major regional pattern in the degree of variation in the onset date among the stations as the Vandeikya and Igumale stations in the south had higher variation in onset dates than the Zaki-biam station. This is a pointer to the fact that the CV decreases northward suggesting a positive correlation in rainfall decrease between the south and north direction. Also, stations to the east in the state (Katsina-Ala and Zaki-Biam) have less variation than the stations in the west which corroborates with the findings of Ayoade that higher rainfall amount has a lower variabilitonset and cessation of rainfall (1993) both reported higher inter-annual variability of the onset than the cessation of rainfall e.g., Odekunle (2004) reported onset range as high as 70 days for a station which is similar to the findings of this study where three stations out of the eight revealed a range of 70 days, others recording over 60 days and Zaki-Biam the least at 56 days. Nigeria experiences 4 months of the onset of rainfall with an average spatial range of 137 days from February to June (February 12, Warri to June 29, Nguru), in Gombe and Dadin Kowa areas the 25th and 30th May are onset dates with a +10- and +15-days variability fall, In Akure, the mean onset date is 8th of March, In Botswana, earliest Onset is the 28th and 29th of November at Pandamatenga and Francistown In East Africa, all observed high inter-annual variability in onset dates relative to cessation dates. For example, Emielu specified a 14.5 days’ inter-annual variation in onset date than 10.3 days for cessation while in Hindol, Gondia and Kankadahad, cities in India, onset of monsoon rain was observed between 17 to 18th of June at 115 to 120 rainy days with the highest recorded in Hindol at 122 days and the minimum in Odapada at 113 days.

Cessation Dates of Rainfall
The mean date of cessation for study varies from early November in the Southern part of the state to late October in the northern zone as shown in Table 2 and figures 5, 6 and 7. This shows that the cessation date for all the locations has a short range of fewer than two dekads (i.e., from 25th October to 7th November). Vandeikya station in the south records the highest value in mean date of cessation followed by Gboko and Katsina-Ala in the central region while Igumale and Otukpo at the western region record the same date of cessation of 30th October. However, Bopo and Makurdi station in the north records the least value showing earlier rainfall cessation averagely over the study period. The earliest cessation date over the study period was observed in Vandeikya (in 1995) followed by Gboko station on the 2nd of October 1994. The earliest cessation date for Otukpo and Igumale was observed almost at the same period of the year. Makurdi station records the highest value as regards the earliest cessation date. The latest cessation date also shows a similar pattern of variation as the least value is found in Vandeikya (Southern Benue) while the highest value is found in Makurdi and Bopo. All stations experienced the latest cessation in 1990 except Katsina-Ala and Gboko station which occurred in 1994 and 2015 respectively.

Table 2: Descriptive Statistics of the Cessation Dates of Rainfall Source: Author's Analysis (2019) The earliest cessation is found in the south of the state in early October when the ITCZ begins to retreat to the south while the latest cessation date is found to be at the central region (Otukpo, Katsina-Ala, and Gboko) in late November extending to the north in Zaki-biam and Makurdi in mid-November. Cessation date has the lowest CV rate in all the stations ranging from 2.3% to 4.4%. The CV values is an indication of a higher spatial coherence across all stations with no discernable variation in inter-regional pattern. However, as regards the latitudinal position of the stations. The south has a higher CV of approximately 4.4% in Vandeikya (Benue south) which decreases northward to 2.3% in Makurdi (Benue north). This result is consistent with the findings of other studies on cessation of rainfall in other regions of Nigeria where cessation fell within the same period e.g., Nigeria experiences a rainfall cessation of 2 months from September to November with a spatial range of 82days between (Sep. 4, Nguru to Nov. 25, Calabar), in Gombe and Dadin Kowa cessation dates are 12th October for both Dadin Kowa and Gombe town with a +10days variability. In Akure, the cessation date is 21st of October with variability of ± 21 days and coefficient of variation of 13%, In Botswana, cessation of rainfall was observed earliest on the 27th and 30th of March in Pandamatenga and Shakawe with less variability, and in Odisha, India, cessation of rain was observed in all areas around 10 to 11th October.

Length of the Rainy Season
The mean length of the rainy season is shortest in the northern regions with less rainfall amount as compared to the southern region as revealed in Table 3, and figures 8,9 and 10. For example, the Makurdi station in the north records the least value of the mean length of the rainy season. An increase in the mean length of the rainy season can be observed in the south in Otukpo with 221 days (latitude 7o 16’ 2”) and 241 days in Vandeikya (latitude 6o 47’ 10’’). The northern stations recorded the shortest length of the rainy season while the southern region recorded the longest rainy season length with a range of 32 days between the two regions.

Worthy of note regarding the temporal trend in the length of the rainy season is the occurrence of the longest rainy season length in 2003 and 2004 for all the stations except Igumale while Vandeikya showed variation in 2010. According to, the length of the rainy season in Benue State ranged between 6-7 months with the southern regions experiencing 7 months while the northern region with a 6 months’ duration. This corroborates with the findings of Ileoje (2004) and Emielu (2008) who stated that an average of six to seven months of the rainy season is dominants in the Guinea savanna region of Nigeria where Benue state is situated and that the length of the rainy season in Nigeria decreases from the South to the north. The variation in the length of the rainy season according to the coefficient of variation for all stations is 7%. Furthermore, in Gombe and Dadin Kowa revealed that the growing seasons for Gombe town are 140days and a +10days variability while Dadin Kowa is 137 days with a +15 days variability. In Akure, 186 to 291 days of growing season length was recorded and an 11% of interannual variability coefficient (Mosunmola, et al. 2020), In Botswana, the longest duration of 120 days of the rainy season was recorded at Pandamatenga and the least of 45days at Tsabong.

Table 3: Descriptive Statistics of the Length of Rainy Season Source: Author's Analysis (2019)

Trend Analysis of Onset and Cessation Dates of Rainfall and the Length of Rainy Season
The Mann-Kendall trend test and the Sen’s Slope estimate was used to detect the nature of the trend in the onset, cessation dates and the length of the rainy season respectively as shown in Tables 4, 5 and 6. It was also used to test the null hypothesis which states that "there is no significant trend in the mean dates of onset, cessation and length of the rainy season at 0.05 level of significance.

Upward trends were observed in the mean onset dates of rainfall in all stations while cessation dates of rainfall showed a downward trend for all stations except Vandeikya and Gboko with an observed upward trend. The length of the rainy season revealed a downward trend for all stations except Vandeikya station.

The Sen’s slope values confirmed the magnitude of trend in line with the Mann-Kendall trend for all stations. The onset P-value for all the stations is greater than 0.05 (with a positive sign), thus, the upward trend in the mean onset dates of rainfall in all the stations was insignificant. Also, both upward and downward trends observed in the cessation dates of rainfall were insignificant for all the stations.

The trends (downward) observed in the mean length of rainy seasons for all the stations were insignificant while the only upward trend observed (for Vandeikya station) was significant. For Bopo and Zaki-Biam, the downward trend in the mean length of rainy seasons is significant as their respective P-values are less than 0.05. Thus, the null hypothesis that there is no significant trend for Bopo and Zaki-Biam is rejected. There is a significant trend in the mean length of the rainy season for Bopo and Zaki-Biam stations while the rest of the stations shows an insignificant trend.

Table 4: The Mann-Kendall Trend Test and Sen’s Slope Estimate of Onset Dates *Significant at the 0.05 significance level

Source: Author's Analysis (2019)

Table 5: The Mann-Kendall Trend Test and Sen’s Slope Estimate of the Cessation Dates *Significant at the 0.05 significance level

Source: Author's Analysis (2019)

Table 6: The Mann-Kendall Trend Test and Sen’s Slope Estimate of the Length of Rainy Season *Significant at the 0.05 significance level

Source: Author's Analysis (2019)

An increasing trend in the onset of the rains implies that rainfall is becoming progressively delayed and tends to occur much later than usual. The upward trend implies that the onset dates of rainfall are getting later over the years and this poses a great danger to agricultural activities as late planting dates will result in poor crop output due to a shorter period of rainfall.

The decreasing trend in the onset on the other hand reveals that the rainfall has become rather earlier than usual and an increasing trend in the cessation dates indicates that there is a progressive delay in the retreat of rainfall. An increasing or a decreasing trend in the length of the rainy season indicates that the period of the rainy season is increasing or decreasing concerning the stations.

Conclusion
The mean dates of onset of rainfall for all the stations was observed in March while October and November witnessed the mean dates of cessation. Expectedly, the onset dates of rainfall increase with increasing latitude while the cessation dates decrease with increasing latitude. There exists an upward trend in the onset dates of rainfall in all the stations with significance for Zaki-Biam and Bopo stations. While the cessation date is characterized by a downward trend for all stations except Gboko and Vandeikya though insignificant for all the stations. There is a downward trend in the length of the rainy season for all the stations except Zaki-Biam and Bopo stations with significant downward trends. A high inter-annual variation was observed in the onset dates of rainfall in contrast to the cessation dates which showed little inter-annual variation using the coefficient of variation while the length of the rainy season showed an intermediate variation between the onset and cessation dates.

Competing interests
The authors all declared a conflict of no interest.