State is in the South East geopolitical zone of Nigeria. The major occupation of the people in this region is trading and farming, which depends on rainfall and other climatic factors. This paper presents statistical and trend analyses of the rainfall in some selected stations in Anambra State, which includes Ifite-Ogwari, Awka, Onitsha and Ihiala. Rainfall data for a period of 1971-2010 were obtained from Climate Research Unit (CRU). The existence of trend and statistical analyses was conducted on monthly total rainfalls using non-parametric techniques. The study revealed that overall averages of yearly and monthly total rainfall were 5798.78 mm and 1739.62 mm in Ifite-Ogwari, 6051.8 mm and 1815 mm in Awka, 6288.87 mm and 1886.88 mm in Onitsha, and 6637.19 mm and 1997.1 mm in Ihiala. Yearly total rainfall has Mann-Whitney of 26 and 41 between 1971 and 1990, 1991 and 2010 respectively in Ifite-Ogwari, 32 and 42 between 1971 and 1990, 1991 and 2010 respectively in Awka, 42 and 39 between 1971 and 1990, 1991 and 2010 respectively in Onitsha, and 33 and 45 between 1971 and 1990, 1991 and 2010 respectively in Ihiala. These parameters show that there are significant trends in the rainfall in term of yearly total for the period. Sen’s estimator revealed that there were significant downward trends for yearly total (-0.775 mm/year) and (-0.094 mm/year) within the period of 1971-1990 and 1991-2010 in Ifite-Ogwari. There was an upward trend of yearly total (1.841 mm/year) between 1971 and1990, whereas there was a downward trend of yearly total (-0.211) between 1991 and 2010 in Awka. It was concluded that there was a significant downward trend in the yearly total and mean rainfalls in Ifite-Ogwari, Awka, Onitsha and Ihiala in the last four decades (40 years), which could be attributed to climate change.
Trend detection is an active area of interest for both hydrology and climatology in order to investigate climate change scenarios and enhance climate impact research. Therefore, trend detection in precipitation time series is crucial for planning and designing regional water resources management. Several recent studies on climatologic trends conclude that trends in observed precipitation comprise a complex function of climatic environment, precipitation intensity and season [
The purpose of this work is to investigate the trend of total amount of rainfall in Ifite-Ogwari, Awka, Onitsha and Ihiala in Anambra State by detecting precipitation changes in the temporal structure for the period 1971-2010.
Anambra State of Nigeria was created on 27th August, 1991 out of the old Anambra State with its state capital as Awka and lies at Latitude 6˚20'N and Longitude 7˚00'E. The land area is approximately 4844 km2, its annual population growth rate of 2.21% per annum [
For this study, the following towns were selected; they are Awka, Onitsha, Ihiala and Ifite-Ogwari. Awka lies between 6˚21'N and 7˚61'E, Onitsha lies between 6˚17'N and 6˚78'E, Ihiala lies between 5˚86'N and 6˚86'E and Ifite-Ogwari lies between 6˚60'N and 6˚95'E (
The data used in this study include the records of the precipitation obtained from Climatic Research Unit (CRU). The precipitation records includes reanalysis observation spanning from 1971 to 2010 and cover a period of 40 years, which is considered to be long enough for a valid mean statistic [
Statistical tools commonly used to detect significant trends in climatic and hydrological time series is either or the non-parametric test such as Mann-Whitney U, Wilcoxon W, Mann-Kendall or Spearman’s rank correlation and the parametric test such as student’s t-test [
where,
Mann-Whitney U-test
Where,
Wilcoxon Rank Sum Test
Where,
Sen’s Slope Estimator Test: The magnitude of trend is predicted by Sen’s estimator.
Here, the slope
where
Sen’s estimator is computed as
dence interval and then a true slope can be obtained by the non-parametric test. Positive value of
Models that relate yearly total rainfall to the year were developed from the trend analysis using Statistical and logical techniques.
Results of this study are discussed in the following categories: statistical summary of rainfall (annual mean precipitation); monthly rainfall trend and sequential analyses.
Months | January | February | March | April | May | June | July | Aug. | Sept. | Oct. | Nov. | Dec. |
---|---|---|---|---|---|---|---|---|---|---|---|---|
Mean | 6.18 | 23.2 | 70.15 | 132.9 | 203.16 | 230.25 | 277 | 250.9 | 292.2 | 209.5 | 35.91 | 8.27 |
Maximum | 30.9 | 107.1 | 205 | 286.5 | 376.1 | 330.7 | 423.7 | 597.9 | 573.6 | 336.9 | 163.5 | 53.9 |
Minimum | 0 | 0 | 14.6 | 43.6 | 128.5 | 137.1 | 144.7 | 89.8 | 187.2 | 60.4 | 0 | 0 |
Median | 3.45 | 17.25 | 59.2 | 122.35 | 193.8 | 231.35 | 271.1 | 219.8 | 287.1 | 208.3 | 31.4 | 4.65 |
Standard Deviation | 8.64 | 24.04 | 43.27 | 55.42 | 51.47 | 39.63 | 75.5 | 111.1 | 72.8 | 68.6 | 35.79 | 11.4 |
Coefficient of Variation | 139.8 | 103.65 | 61.69 | 41.7 | 25.34 | 17.21 | 27.24 | 44.28 | 24.93 | 32.74 | 99.66 | 137.85 |
Months | January | February | March | April | May | June | July | Aug. | Sept. | Oct. | Nov. | Dec. |
---|---|---|---|---|---|---|---|---|---|---|---|---|
Mean | 7.04 | 16.87 | 71.2 | 147.61 | 209.4 | 239.7 | 292.6 | 266 | 298.4 | 217.5 | 40.59 | 8.64 |
Maximum | 51.3 | 94.9 | 222.3 | 331.2 | 403.1 | 392.2 | 587.7 | 487.8 | 582.7 | 345.6 | 179.9 | 50.4 |
Minimum | 0 | 0 | 7.3 | 26.7 | 96.2 | 90.2 | 113.1 | 88.5 | 152.3 | 64.6 | 0 | 0 |
Median | 2.45 | 13 | 65.4 | 145.3 | 202.9 | 236.4 | 293.5 | 245.9 | 286.1 | 218.2 | 24.5 | 2.35 |
Standard Deviation | 12.12 | 19.33 | 48.97 | 58.72 | 66.3 | 63.7 | 95.5 | 112.3 | 82.5 | 74.7 | 47.31 | 13.1 |
Coefficient of Variation | 172.26 | 114.59 | 68.78 | 39.78 | 31.68 | 26.56 | 32.63 | 42.23 | 27.66 | 34.36 | 116.6 | 151.54 |
cant rainy months. The month of July has the highest magnitude of monthly rainfall with 587.7 mm followed by month September with 582.7 mm.
The result of trend analyses are discussed in the following ways: trend of yearly rainfalls with the sequences of the monthly and yearly rainfalls.
Months | January | February | March | April | May | June | July | Aug. | Sept. | Oct. | Nov. | Dec. |
---|---|---|---|---|---|---|---|---|---|---|---|---|
Mean | 8.38 | 23.37 | 78.59 | 151.6 | 215.6 | 244.88 | 299.8 | 267.3 | 309.9 | 228 | 48.25 | 11.01 |
Maximum | 58 | 131.8 | 215.7 | 352.6 | 434.3 | 385 | 524.8 | 535.2 | 615.6 | 359.5 | 257 | 62.9 |
Minimum | 0 | 0 | 10.3 | 15.3 | 119.1 | 100.8 | 115.2 | 100.7 | 188.1 | 34.2 | 0 | 0 |
Median | 4.85 | 19.8 | 69.05 | 137.4 | 208.3 | 242.85 | 304 | 230.6 | 311.1 | 228.9 | 34.65 | 4.7 |
Standard Deviation | 13.36 | 24.48 | 48.29 | 64.1 | 63.8 | 54.1 | 93.1 | 113.6 | 80.3 | 75.4 | 53.29 | 15.25 |
Coefficient of Variation | 159.38 | 104.74 | 61.44 | 42.3 | 29.59 | 22.09 | 31.06 | 42.51 | 25.91 | 33.05 | 110.4 | 138.44 |
Months | January | February | March | April | May | June | July | Aug. | Sept. | Oct. | Nov. | Dec. |
---|---|---|---|---|---|---|---|---|---|---|---|---|
Mean | 12.7 | 28.94 | 91.55 | 161.18 | 223.55 | 258.14 | 314.5 | 265.9 | 321.3 | 244.4 | 59.22 | 15.72 |
Maximum | 86.6 | 154.7 | 226.9 | 352.3 | 391.7 | 440.2 | 536.7 | 527.6 | 648 | 414 | 289 | 74.9 |
Minimum | 0 | 1.3 | 14.4 | 31.7 | 129.5 | 118.7 | 140.7 | 94.1 | 203.9 | 50.6 | 0.4 | 0 |
Median | 7.15 | 25 | 86.25 | 155.05 | 219.65 | 246.9 | 317.9 | 254.5 | 318.3 | 241.7 | 47 | 9.35 |
Standard Deviation | 17.69 | 27.99 | 47 | 58.54 | 62.24 | 62.64 | 102.1 | 105.5 | 85.8 | 82.6 | 53.67 | 19.15 |
Coefficient of Variation | 139.31 | 96.7 | 51.34 | 36.32 | 27.84 | 24.27 | 32.47 | 39.68 | 26.72 | 33.81 | 90.63 | 121.81 |
Year | Mean | Mann-Whitney U | Wilcoxon | Z-Statistics | Asymptotic Significant (2 Tailed) | Sen’s Estimator | Number of Samples |
---|---|---|---|---|---|---|---|
1971-1980 1981-1990 | 12.9 8.1 | 26 | 81 | −1.814 | 0.070 | −0.775 | 10 10 |
1991-2000 2001-2010 | 11.4 9.6 | 41 | 96 | −0.680 | 0.496 | −0.094 | 10 10 |
and 2010. Asymptotic significant (probability) of these rainfall parameters were found to be 0.070 and 0.496 for these periods respectively. SAI provides an area average index of relation rainfall yields based on the standard of total, mean, maximum and minimum rainfalls. These magnitudes of SAI
Sen’s estimator revealed that yearly total rainfalls have trends of −0.775 mm/year and −0.094 mm/year for a period of 1971-1990 and 1991-2010 respectively.
Figures 2(a)-(d) present sequential values of the rainfalls in Ifite-Ogwari. The figures apparently show decreasing trends in rainfalls.
Figures 2(a)-(d) illustrate the downward trends appearance of the yearly total, maximum, minimum and mean rainfall. From the graphs, it appears the strongest decreasing trend occurred between 1979 and 1986.
climate change, which manifest itself in the area as temperature is increasing and reduction in rainfall in the years 1971-1974, 1980-1985 and 1991-1994 as showed in
Year | Mean | Mann-Whitney U | Wilcoxon | Z-Statistics | Asymptotic Significant (2 Tailed) | Sen’s Estimator | Number of Samples |
---|---|---|---|---|---|---|---|
1971-1980 1981-1990 | 8.7 12.3 | 32 | 87 | −1.361 | 0.174 | 1.841 | 10 10 |
1991-2000 2001-2010 | 11.3 9.7 | 42 | 97 | −0.605 | 0.545 | −0.211 | 10 10 |
of total, mean, maximum and minimum rainfalls. These magnitude of SAI
Sen’s estimator revealed that yearly total rainfalls have trends of 1.841 mm/year and −0.211 mm/year for a period of 1971-1990 and 1991-2010 respectively.
Figures 3(a)-(d) present sequential values of the rainfalls in Awka. The figures apparently show decreasing trends in rainfalls.
Figures 3(a)-(d) illustrate the downward trends appearance of the yearly total, maximum, minimum and mean rainfall. From the graphs, it appears the strongest decreasing trend occurred between 1979 and 1983.
Sen’s estimator revealed that yearly total rainfalls have trends of 0.146 mm/year and 0.535 mm/year for a period of 1971-1990 and 1991-2010 respectively.
Figures 4(a)-(d) present sequential values of the rainfalls in Onitsha. The figures apparently show decreasing trends in rainfalls.
Figures 4(a)-(d) illustrate the downward trends appearance of the yearly total, maximum, minimum and mean rainfall. From the graphs, it appears the strongest decreasing trend occurred between 1979 and 1983.
Year | Mean | Mann-Whitney U | Wilcoxon | Z-Statistics | Asymptotic Significant (2 Tailed) | Sen’s Estimator | Number of Samples |
---|---|---|---|---|---|---|---|
1971-1980 1981-1990 | 11.3 9.7 | 42 | 97 | −0.605 | −0.545 | 0.146 | 10 10 |
1991-2000 2001-2010 | 11.6 9.4 | 39 | 94 | −0.832 | −0.406 | −0.535 | 10 10 |
orded highest magnitude of rainfall with 524.8 mm and 615.6 mm respectively. These trends can be attributed to climate change, which manifest itself in the area as temperature is increasing and reduction in rainfall in the years 1971-1977, 1979-1984 and 1991-1994 as showed in
Sen’s estimator revealed that yearly total rainfalls have trends of 1.437 mm/year and −0.023 mm/year for a period of 1971-1990 and 1991-2010 respectively.
Figures 5(a)-(d) present sequential values of the rainfalls in Ihiala. The figures apparently show decreasing
Year | Mean | Mann-Whitney U | Wilcoxon | Z-Statistics | Asymptotic Significant (2 Tailed) | Sen’s Estimator | Number of Samples |
---|---|---|---|---|---|---|---|
1971-1980 1981-1990 | 8.8 12.2 | 33 | 88 | −1.285 | 0.199 | 1.437 | 10 10 |
1991-2000 2001-2010 | 11 10 | 45 | 100 | −0.378 | 0.705 | −0.0232 | 10 10 |
trends in rainfalls.
Figures 5(a)-(d) illustrate the downward trends appearance of the yearly total, maximum, minimum and mean rainfall. From the graphs, it appears the strongest decreasing trend occurred between 1973 and 1976 and 1981 and 1984 respectively.
The application of this trend analysis framework revealed an overall downward precipitation trend. Ifite-Ogwari station indicated downward trends between 1971 and 1990 and 1991 and 2010, but Awka, Onitsha and Ihiala showed an upward trend between 1971 and 1990 and a downward trend between 1991 and 2010. However, this decrease was found to be statistically significant at 95% confidence level at the four stations. The trend revealed that August and October had downward trends.