Trend Analysis of Precipitation in Some Selected Stations in Anambra State ()
1. Introduction
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 [1] [2] . Relevant reviews on trend analysis in precipitation time series include the studies of [3] - [7] .
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.
2. Methodology
2.1. Study Area
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 [8] . Anambra State has over 60% of its people living in urban areas making it one of the most urbanized places in Nigeria [9] . Since then, the state has being witnessing immense growth in the size of built-up areas, number of immigrants, transportation and commercial activities. It experiences warm humid tropical climate, with average rainfall between 1520 and 2020 mm per annum. Minimum and Maximum temperatures range between 25.4˚C and 30.6˚C and its vegetation is the tropical forest type [10] .
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 (Figure 1).
2.2. Materials and Methods
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 [11] . Moreover, [12] states that a minimum record length of 25 years ensures statistical validity of the trend results in climate change research.
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 [13] [14] . The non-parametric test is considered better because it is a function of the ranks of observation and it displays much insensitivity to outliers unlike the parametric counterpart. Slope 
of the existence trend were determined using Sen’s estimator and overall slopes (Tan 
were computed as follows:
where, 
rank of group 
rank of group 
number of samples in group 
number of samples in group 
(1)
where, 
slope of group 
slope of group 
![]()
slope of group ![]()
![]()
angle of group ![]()
![]()
Figure 1. Map of anambra State showing the selected stations.
![]()
angle of group ![]()
![]()
angle of group ![]()
(2)
Mann-Whitney U-test
![]()
(3)
![]()
(4)
Where, ![]()
is the number of observations in the first group, ![]()
is the number of observations in the second group, ![]()
is the sum of the ranks assigned to the first group and ![]()
is the sum of the ranks assigned to the second group. In other words, both ![]()
equations can be understood as the number of times observation in one sample precede or follow observation in the other sample.
Wilcoxon Rank Sum Test
![]()
(5)
Where, ![]()
![]()
![]()
sample size
![]()
sum of ranks for smaller sample size ![]()
![]()
smaller of sample sizes
![]()
larger of sample sizes
![]()
and
Sen’s Slope Estimator Test: The magnitude of trend is predicted by Sen’s estimator.
Here, the slope ![]()
of all data pairs is computed as [15]
![]()
(6)
where ![]()
and xk are considered as data values at time ![]()
and ![]()
![]()
corresponding. The median of these ![]()
values of ![]()
represented as Sen’s estimator of slope which is given as:
![]()
--![]()
is odd (7)
![]()
--![]()
is even (8)
Sen’s estimator is computed as ![]()
if ![]()
appears odd, and it is considered as ![]()
if ![]()
appears even. At the end, ![]()
is computed by a two sided test at ![]()
confi-
dence interval and then a true slope can be obtained by the non-parametric test. Positive value of ![]()
indicates an upward or increasing trend and a negative value of ![]()
gives a downward or decreasing trend in the time series.
Models that relate yearly total rainfall to the year were developed from the trend analysis using Statistical and logical techniques.
3. Results and Discussion
Results of this study are discussed in the following categories: statistical summary of rainfall (annual mean precipitation); monthly rainfall trend and sequential analyses.
3.1. Statistical Monthly Rainfall Summary
3.1.1. Ifite-Ogwari
Table 1 presents monthly mean, median, maximum and minimum rainfalls in Ifite-Ogwari. The table revealed that the rainy period is between February and November, with May, June, July, August, September and October as significant rainy months. The month August has the highest magnitude of monthly rainfall with 597 mm followed by month September with 573 mm. This shows that for farming activities in rainy seasons crops with maximum water demands month of six months are best situated for the station.
3.1.2. Awka
Table 2 presents monthly mean, median, maximum and minimum rainfalls in Awka. The table showed the rainy period is between March and October, with April, May, June, July, August, September and October as signifi-
![]()
Table 1. Statistical summary of monthly rainfall in Ifite-Ogwari.
![]()
Table 2. Statistical summary of monthly rainfall in Awka.
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.
3.1.3. Onitsha
Table 3 presents monthly mean, median, maximum and minimum rainfalls in Onitsha. The table revealed that the rainy period is February and September, with March, April, May, June, July, August and September as significant rainy months. The month of September has the highest magnitude of monthly rainfall with 615.6 mm followed by month August with 535.2 mm.
3.1.3. Ihiala
Table 4 presents monthly mean, median, maximum and minimum rainfalls in Ihiala. The table showed that the rainy period is March and October, with April, May, June, July, August, September and October as significant rainy months. The month of September has the highest magnitude of monthly rainfall with 648 mm followed by month of July with 536.7 mm.
3.1.4. Trend Analysis
The result of trend analyses are discussed in the following ways: trend of yearly rainfalls with the sequences of the monthly and yearly rainfalls.
3.2. Ifite-Ogwari
3.2.1. Trend of Yearly Rainfalls
Table 5 presents results of non-parametric analyses (mean rank, Mann-Whitney, Wilcoxon, standard anomaly index (SAI:Z), Asymptotic Significant, Sen’s estimator. From these analyses presented in the table, it was observed that they are downward trend in the total, mean, maximum and minimum rainfall in Ifite-Ogwari between 1971 and 1990, but these trends were not statistically significant at 95% confidence level. Between 1991 and 2010, there are downward trends in the rainfall parameters with SAI’s of −1.814 and −0.680 for between 1991
![]()
Table 3. Statistical summary of monthly rainfall in Onitsha.
![]()
Table 4. Statistical summary of monthly rainfall in Ihiala.
![]()
Table 5. Non-parameter test of summarized statistical data in Ifite-Ogwari.
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 ![]()
and probabilities show that the downward trends are statistically significant at 95% confidence level. Also the sign ![]()
revealed that the values were less than expected overall mean rainfall and that there was more dryness in the period between 1971 and 1990 than the period between 1991 and 2010.
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.
3.2.2. Sequence Analysis of the Rainfalls
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.
Figure 2(c) clearly shows yearly maximum rainfall from 1971 to 2010 which implies that 1987 and 2004 recorded highest magnitude of rainfall with 579.9 mm and 573.6 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 1971-1974, 1980-1985 and 1991-1994 as showed in Figure 2(d).
Figure 2(e) clearly shows the month of August in 1987 recorded the highest magnitude of rainfall with 597.9 mm followed by the month of September in 2004 with 573.6 mm of rainfall amounts. The month of October in 1992 recorded the lowest magnitude of rainfall with 60.44 mm.
3.3. Awka
3.3.1. Trend of Yearly Rainfalls
Table 6 presents results of non-parametric analyses (mean rank, Mann-Whitney, Wilcoxon, standard anomaly index (SAI:Z), Asymptotic Significant, Sen’s estimator. From these analyses presented in the table, it was observed that they are upward trend in the total, mean, maximum and minimum rainfall in Awka between 1971 and 1990, but these trends were not statistically significant at 95% confidence level. Between 1991 and 2010, there are downward trends in the rainfall parameters with SAI’s of −1.361 and −0.605 for between 1991 and 2010. Asymptotic significant (probability) of these rainfall parameters were found to be 0.174 and 0.545 for these periods respectively. SAI provides an area average index of relation rainfall yields based on the standard
![]()
Table 6. Non-parameter test of summarized statistical data in Awka.
of total, mean, maximum and minimum rainfalls. These magnitude of SAI ![]()
and probabilities show that the downward trends are statistically significant at 95% confidence level. Also the sign ![]()
revealed that the values were more than expected overall mean rainfall and that there was more wetness in the period between 1971 and 1990 than the period between 1991 and 2010.
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.
3.3.2. Sequence Analysis of the Rainfalls
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.
Figure 3(c) clearly shows yearly maximum rainfall from 1971 to 2010 which implies that 1990 and 2004 recorded highest magnitude of rainfall with 587.7 mm and 582.7 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 1971-1975, 1979-1983 and 1991-1992 as showed in Figure 3(d).
Figure 3(e) clearly shows the month of July in 1990 recorded the highest magnitude of rainfall with 587.7 mm followed by the month of September in 2004 with 582.7 mm of rainfall amounts. The month of October in 1983 recorded the lowest magnitude of rainfall with 64.6 mm.
3.4. Onitsha
3.4.1. Trend of Yearly Rainfalls
Table 7 presents results of non-parametric analyses (mean rank, Mann-Whitney, Wilcoxon, standard anomaly index (SAI:Z), Asymptotic Significant, Sen’s estimator. From these analyses presented in the table, it was observed that they are upward trend in the total, mean, maximum and minimum rainfall in Onitsha between 1971 and 1990, but these trends were not statistically significant at 95% confidence level. Between 1991 and 2010, there are downward trends in the rainfall parameters with SAI’s of −0.605 and −0.832 for between 1991 and 2010. Asymptotic significant (probability) of these rainfall parameters were found to be −0.545 and −0.406 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 ![]()
and probabilities show that the downward trends are statistically significant at 95% confidence level. Also the sign ![]()
revealed that the values were more than expected overall mean rainfall and that there was more wetness in the period between 1971 and 1990 than the period between 1991 and 2010.
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.
3.4.2. Sequence Analysis of the Rainfalls
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.
Figure 4(c) clearly shows yearly maximum rainfall from 1971 to 2010 which implies that 1990 and 2004 rec-
![]()
Table 7. Non-parameter test of summarized statistical data in Onitsha.
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 Figure 4(d).
Figure 4(e) clearly shows the month of September in 2004 recorded the highest magnitude of rainfall with 615.6 mm followed by the month of August in 1971 with 535.2 mm of rainfall amounts. The month of October in 1983 recorded the lowest magnitude of rainfall with 34.2 mm.
3.5. Ihiala
3.5.1. Trend of Yearly Rainfalls
Table 8 presents results of non-parametric analyses (mean rank, Mann-Whitney, Wilcoxon, standard anomaly index (SAI:Z), Asymptotic Significant, Sen’s estimator. From these analyses presented in the table, it was observed that they are upward trend in the total, mean, maximum and minimum rainfall in Ihiala between 1971 and 1990, but these trends were not statistically significant at 95% confidence level. Between 1991 and 2010, there are downward trends in the rainfall parameters with SAI’s of −1.285 and −0.378 for between 1991 and 2010. Asymptotic significant (probability) of these rainfall parameters were found to be 0.199 and 0.705 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 ![]()
and probabilities show that the downward trends are statistically significant at 95% confidence level. Also the sign ![]()
revealed that the values were more than expected overall mean rainfall and that there was more wetness in the period between 1971 and 1990 than the period between 1991 and 2010.
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.
3.5.2. Sequence Analysis of the Rainfalls
Figures 5(a)-(d) present sequential values of the rainfalls in Ihiala. The figures apparently show decreasing
![]()
Table 8. Non-parameter test of summarized statistical data in Ihiala.
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.
Figure 5(c) clearly shows yearly maximum rainfall from 1971 to 2010 which implies that 1990 and 1971 recorded highest magnitude of rainfall with 536.7 mm and 470.9 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 1973-1976, 1981-1984, 1986, 1989, 1991 and 1999 as showed in Figure 5(d).
Figure 5(e) clearly shows the month of September in 2004 recorded the highest magnitude of rainfall with 648 mm followed by the month of September in 2006 with 536 mm of rainfall amounts. The month of October in 1983 recorded the lowest magnitude of rainfall with 0.5 mm.
4. Conclusion
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.
NOTES
*Corresponding author.