Analysis of Empirical Rainfall Data Covering 1979-2020 as a Guide to Agriculture and Water Resources Management in Ondo State, Nigeria

: The aim of this paper was to evaluate daily empirical rainfall data covering 1979-2020 as a guide to agriculture and water resources management in Ondo, State, Nigeria by collecting data from the Nigerian Meteorological Agency, Abuja, using various standard methods. Results showed that, the area observed rainfall throughout the year with March (86.4 mm) as the rainfall onset, November (56.4 mm) was the cessation, double maximum rainfall occurred in July (254.2 mm) and September (282.1 mm) with the climatological mean of 1752.2 mm. Also, the percentage (%) difference rainfall showed seven (7) positive and four (4) negative occurrences. The highest positive % difference occurred in September (10.4), while the lowest occurred in July (2.6). The highest negative % difference was -26.5 (November), while the lowest was -5.9 (August). The stations deviation from zonal average monthly rainfall showed that, Igbokoda, Ilaje LGA deviated in all the months negatively, while Okitipupa, Okitipupa LGA and Oka-Akoko, Akoko South West LGA deviated positively in eleven (11) months each. The study concluded that, rainfall varied and had declined generally over the study area. Hence, it was recommended that, the Seasonal Climate Prediction (SCP) by Nigerian Meteorological Agency (NiMet) should be down-scaled early to serve as a guide to farmers and hydrologists; irrigation should be encouraged; hybrid crops should be cultivated and more water reservoirs should be constructed.

Nigeria, as a tropical and agrarian country relies on sufficient, efficient and reliable rainfall at the beginning of farming season which allows for "serious" farming activities to commence.This sufficient, efficient and reliable rainfall at this period is described as its onset which is crucial in agriculture.Sometimes, the onset rainfall is always truncated with a "break" which Ibrahim et al. (2018) refers to a pentad with less than 5 mm of rainfall beginning from 1 st May (25 th pentad of the year from 1 st January of a given year) in the Guinea Savanna Zone of Nigeria.Onset of rainfall varies across different agro-climatic zones.Audu et al. (2019) observed that, the onset of rainfall in Gombe and Dadin Kowa were 25 th and 30 th May with variability of ± 10 days for Gombe and ± 15 days for Dadin Kowa.If the onset of rain delays, farming activities delay as well because in Nigeria, agriculture is strongly tied to rainfall.Salawu et al. (2014) observed that, temperature and rainfall have greater impacts on human settlements and human activities both directly and indirectly than other elements in terms of development, health and agriculture.Umar (2016); Umar et al. (2021) both cited in Umar and Aliyu (2022) stated that, rainfall and length of growing season (LGS) are the most important factors affecting the sustainability of rainfed crop cultivation in the semi-arid areas of northwestern Nigeria.Rainfall varies over both space and time which affects agriculture and water resources.Writing about the correlation study on developmental stages in soya bean (glycine max) and prevailing agro-meteorological indices in forest savanna eco-climatic zone of Nigeria, Adetayo (2013) discovered that, relatively low amount of rainfall is required for fruiting, while occasional showers with high temperature is needed for ripening and drying for good quality seeds.Variability is an attribute of rainfall in Nigeria which also affects its derived parameters.Variability is observed in rainfall cessation and hydrological growing season which directly affect agriculture.The effectiveness of rainfall in terms of its onset, cessation, hydrological growing season (HGS), amount and distribution are crucial to the success of crop production.The alternation of these attributes is injurious to crop development, maturation and yields.A balanced rainfall condition is required in every cropping season for better growth, development and yield.However, these conditions are not always favourable on daily, pentad, monthly, annual and inter-annual basis due to the incidences of delayed onset, false onset, early and late cessations, shortened HGS, rainfall spells, dry spells, little dry season (LDS) and droughts in its various forms.In a study by Umar and Aliyu (2022), it was discovered that; the cultivation of drought resistant crops such as pearl millets and sorghum is being threatened by the occurrence of frequent dry spells and extended water deficient conditions in the semi-arid northwestern Nigeria.Audu et al. (2022) opined that, late onset and early cessation pentads rainfall shortened length of hydrological growing season (LHGS) thereby affecting effective cropping, growth of grasses for domestic uses, ground water recharging, river flow and other uses.
On water resources, rainfall is a major source of both surface and underground water.Evaporation from water sources and wet surfaces is an important ingredient in hydrological cycle (Abubakar, 2015).Adequate water resource is a panacea to irrigation.On the contrary, excessive rainfall has caused and still causing hydro-meteorological hazards in Nigeria.Annually, cases of massive water erosion, unprecedented flooding, huge landslides as well as severe water pollution are reported.Lawal et al. (2022) observed that, if past trends of rising global temperature continue; the type, frequency and intensity of precipitation storm characteristics are bound to change leading to extreme events such as severe drought, intense water shortages and in some cases, flooding.Matazu (2021) remarked that, in 2021; Nigeria like other countries of the world experienced some extreme weather events including flooding, dry spells and heat waves.These hazards are injurious to agriculture and water resources.Several studies have been carried out on rainfall attributes and the practice of farming in the Southwestern part of Nigeria.Adejuwon (2010) noted that, little dry season occurs in the midst of wet season in southwestern Nigeria.Onyejekwe et al. (2016) perceived that, in the last two (2) decades; there have been anomalies in the incidence and distribution of rainfall leading to very erratic onset, cessation, duration and distribution.Despite these studies, there is need for regular studies on rainfall as it relates to agriculture and water resources so as to ascertain its current state because weather is highly dynamic.This has called for this research with the aim of analyzing rainfall to guide farmers and water resources management in Ondo State, Nigeria.Hence, the objective of this paper was to evaluate daily empirical rainfall data covering 1979-2020 as a guide to agriculture and water resources management in Ondo State, Nigeria.

MATERIALS AND METHODS
Study Area: According to Akanbi (2022), Ondo State (study area) is one of the six (6) states that make up the South West geopolitical zone of Nigeria.It lies between latitudes 5 o 05'-7 o 10' North of the Equator and longitudes 4 o 30'-6 o 0' East of the Greenwich Meridian (Figure 1).Akinbode et al. (2006) (Akanbi, 2022).Awomeso et al. (2015) observed that, the study area is bordered by 80 km out of the 853 km total coastlines of Nigeria which is derived from the Atlantic Ocean.

Methods of Data Analysis:
To determine the monthly mean rainfall in the study area, daily empirical rainfall data were first computed to derive the monthly rainfall as follows: The station monthly rainfall was calculated using Equation 2, as follows: The station monthly mean rainfall was calculated using Equation 3, as follows: =  ℎ The regional monthly mean rainfall was calculated using Equation 4, as follows: The station monthly mean percentage (%) rainfall difference was calculated as:

% 𝑑𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑐𝑒 = (𝑠 𝑘 𝑚 𝑗+1
̅̅̅̅̅̅̅̅̅ −     ̅̅̅̅̅̅)100 (   +11 ̅̅̅̅̅̅̅̅̅̅ −    +10 ̅̅̅̅̅̅̅̅̅̅)100 5 Station deviation from zonal monthly mean rainfall was calculated as thus: The month under reference is that in which the accumulated rainfall is in excess of 51 mm.The cessation month of the rains is that in which not more than 51 mm of rain is expected.

RESULTS AND DISCUSSION
In Figure 2  In Figure 4, Igbokoda, Ilaje LGA, Ondo State, Nigeria receives mean rainfall throughout the year.Onset rainfall month is March (71.9 mm) and cessation is October (213.7 mm).Least mean rainfall is December (8.9 mm), highest is September (268.6 mm), double maxima in June (215.7mm) and September, September peak (254.6 mm) is higher than the 1 st peak in June.There is a little dry season (LDS) otherwise called "August break" which begins in July (208.3mm) through August with monthly mean rainfall of 186.3 mm.These rains are lower than those of the two (2) peaks.There are eight ( 8) months (March-October) of wet season and four (4) months (January, 9.6 mm; February, 28.9 mm; November, 39.0 mm; and December, 8.9 mm) of dry season.In Figure 5, Okitipupa in Okitipupa LGA, Ondo State, Nigeria receives monthly mean rainfall throughout the year.The onset rainfall month is March (109.8mm) and its cessation is November (64.8 mm).Least monthly mean rainfall occurs December (11.1 mm), highest is September (309.0 mm), double maxima in July (266.3mm) and September with September peak (2 nd peak)  In Figure 6, Oka Akoko, Akoko South West LGA, Ondo State, Nigeria observes monthly mean rainfall throughout the year.Onset rainfall occurs in March (101.0mm) and its cessation is November (94.1 mm).Least monthly mean rainfall is December (15.6 mm), highest is September (342.8mm), double maxima in July (328.5 mm) and September (342.8mm) with September peak higher than the 1 st peak in July.Little dry season (LDS) also called "August break" occurs with mean rainfall of 283.8 mm which is lower than the July and September rainfall.Nine ( 9) months (March-November), observe wet season; while three (3) months (January, 17.3 mm; February, 32.5 mm; and December, 15.6 mm),are dry season months.LGA, Ondo State Source: Authors' computation, 2023 In Figure 7, Isua Akoko, Akoko South East LGA, Ondo State, Nigeria receives monthly mean rainfall throughout the year.The onset rainfall occurs in March (70.1 mm) and its cessation is October (190.7 mm).Lowest monthly mean rainfall is January (11.8 mm), highest is in September (278.2mm), double maxima are in July (262.1 mm) and September with September peak higher than the 1 st peak in July.There is little dry season (LDS) otherwise called "August break" (236.8 mm) which is lower than the rainfall in July and September.Eight (8) months (March-October) make up the wet season, four (4) months (January, 11.8 mm; February, 36.1 mm; November, 43.9 mm and December, 16.5 mm) constitute the dry season months.In Figure 9, the monthly state average percentage rainfall difference in Iju-Itaogbolu, Akure North LGA indicates both positive and negative differences.The positive value is higher in number (7), while the negative value is four (4).The highest positive value occurs in September (11.5), while the lowest is 3.5 (July).On the other hand, the highest negative difference is -27.5 (November), while the lowest is -5.2 (December).The negative percentage difference of -7.0 in August is a sign of little dry season (LDS).The monthly state average percentage rainfall difference in Owena, Idanre LGA indicates both positive and negative differences.The positive value is higher in number (7), while the negative value is four (4).The highest positive value occurs in September (13.0), while the lowest is 2.7 (February).On the other hand, the highest negative percentage difference is -22.3 (November), while the lowest is -6.3 (December).The negative difference of -11.5 in August is a sign of little dry season (LDS).Figure 9 shows monthly state average percentage rainfall difference in Igbokoda, Ilaje LGA has both positive and negative differences.The positive value is higher in number (6), while the negative value is five (5).The highest positive value occurs in September (14.3), while the lowest is 3.4 (January).On the other hand, the highest negative % difference is -30.2 (November), while the lowest is -1.3 (July).November and December are dry season months.The negative difference of -1.3 and -7.0 in July and August is a sign of little dry season (LDS).It means LDS begins in July at this station.The monthly state average percentage (%) rainfall difference in Okitipupa, Okitipupa LGA indicates both positive (7) and negative (4) differences.Highest positive value occurs in May (10.7), while the lowest is 0.4 (July).On the other hand, the highest negative difference is -23.4 (November), while the lowest is -6.0 (August) which is a sign of LDS.In Figure 9, the monthly state average percentage rainfall difference in Oka-Akoko, Akoko South West LGA indicates both positive (7) and negative (4) differences.The highest positive value occurs in September (11.5), while the lowest is 3.5 (July).On the other hand, the highest negative difference is -27.5 (November), while the lowest is -5.2 (December).November and December are dry season months.The negative % difference of -7.0 in August is a sign of LDS.The monthly state average percentage rainfall difference in Isua-Akoko, Akoko South East LGA shows both positive (7) and negative (4) differences.The highest positive value occurs in April (10.8), while the lowest is 4.2(February).On the other hand, the highest negative difference is -25.4(November), while the lowest is -4.4 (August) which is the LDS.The monthly average percentage rainfall difference in the Ondo State, Nigeria (study area) indicates both positive (7) and negative (4) differences.The highest positive value occurs in September (10.4), while the lowest is 2.6 (July).On the other hand, the highest negative difference is -26.5 (November), the lowest is -5.9 (August) and signifies the LDS.In Figure 10, all the data collection points do not observe zero (0) deviation which would have been a perfect result in this study.This implies that, all the stations deviate from the regional average both positively and negatively.Igbokoda deviates in all months negatively, while Okitipupa and Oka-Akoko deviate positively in eleven (11) months each.In both positive and negative deviations, the values that are large, high or far from zero (0) are the biased estimators of the zonal average monthly rainfall which cannot be relied upon.On the other hand, the good ones are those values close to zero (0) which are called unbiased estimators which can be relied upon.There

Fig 1 :
Fig 1: Map of Nigeria showing Ondo State Source: National Space Research and Development Agency (NASRDA, 2023) Materials: Daily empirical rainfall data which covered from 1979-2020 (42 years) for six (6) meteorological stations across Ondo State, Nigeria were collected from the Nigerian Meteorological Agency (NiMet), Abuja and analysed for this study.These stations include Iju-Itaogbolu and Owena in Rain Forest Zone; Igbokoda and Okitipupa in Mangrove Swamp Forest as well as Oka-Akoko and Isua-Akoko in Guinea Savanna Zone.

Fig 3 :
Fig 3: Monthly mean rainfall in Owena, Idanre LGA, Ondo State, Nigeria Source: Authors' computation, 2023 1 st peak in July.There is a little dry season (LDS) otherwise called "August break" with monthly mean rainfall of 251.7 mm which is lower than July and September rainfall.Wet season lasts nine (9) months (March-November), while dry season lasts three (3) months (January, 15.6 mm; February, 48.1 mm and December, 11.1 mm).

rainfall in mm Months seems
Ishiyaku (2006)y in the onset month of rain across the data collection points which is March.However, there is variability in the monthly mean rainfall across the six (6) data collection points in the onset and cessation months, rain amount, rain amount at the peak (September), months with the least rainfall and values as well as the value of rain during the LDS.It is worth nothing that, all the data collection points have their highest monthly mean rainfall in September with variability in the rain amounts.Okitipupa has the highest monthly mean rainfall at the peak of the rains which is September (309 mm), while Owena in Idanre LGA has the least of 239 mm.Iju-Itaogbolu in Akure North LGA has 254.6 mm, Igbokoda in Ilaje LGA has 268.6 mm, Oka Akoko in Akoko South West LGA has 267.7 mm and Isua Akoko in Akoko South East LGA has 278.2 mm.Regarding the month with the least mean rainfall, Iju-Itaogbolu in Akure North LGA and Isua-Akoko in Akoko South East LGA observe the least rainfall in January in the order of 11.9 mm and 11.8 mm.Igbokoda in Ilaje LGA, Owena in Idanre LGA, Okitipupa in Okitipupa LGA and Oka Akoko in Akoko South West LGA observe it in December in the order of 8.9 mm, 13.4 mm, 11.1 mm and 15.6 mm.This corroborated the findings of Eludoyin et al. (2017) which opined that, Ondo State received its highest monthly mean rainfall in September with the least in January.The little dry season (LDS) which occurs mainly in August also exhibits variability in mean rainfall amount across the data collection points.Iju-Itaogbolu in Akure North LGA has 190.6 mm, Igbokoda in Ilaje has 186.3 mm, Owena in Idanre LGA has 163.4 mm (least), Okitipupa in Okitipupa LGA has 251.7 mm, Oka Akoko in Akoko South West has 283.8 mm (highest) and Isua-Akoko in Akoko South East LGA has 236.8 mm.These amounts of rains are clear indications that, LDS is not a total absence of rainfall or that the rains are insufficient for agricultural purpose.The study by AdeJuwon and Odekunle (2006) discovered the occurrence of LDS along the coast of Southwestern Nigeria from mid-July to mid-September.In this study, variability is detected in monthly and climatological averages, % difference as well as the deviation from mean values.The negative percentage difference is observed across all stations and at regional level in the month of August.Variability is a typical attribute of rainfall in Nigeria.The study ofTubosun (2022)discovered significant variations in rainfall mostly in 1994 and 2001 in Lower Niger River Basin Development Authority Catchment Area.The positive rainfall anomaly in the dry season months of January and February is an indication of increasing trend.Ishiyaku (2006)in a study discovered severe thunderstorms in January over Akure, Nigeria.This might have been responsible for the positive rainfall trend in the study area as observed in this study.The study area observes rainfall throughout the year, March (86.4 mm) is the rainfall onset, November (56.4 mm) is the cessation, double maximum occurs in July (254.2mm) and September (282.1 mm) and mean rainfall is 1752.2mm.In conclusion, the area experiences variability and decline in rainfall.Hence, it is recommended that, NiMet should down-scaled the Seasonal Climate Prediction (SCP) early to guide farmers and hydrologists; irrigation should be encouraged; hybrid crops should be cultivated and more water reservoirs should be constructed. Conclusion: