Assessment of rainfall variability, rainwater harvesting potential and storage requirements in Odeda Local Government Area of Ogun State in South-western Nigeria

Figures & data

Figure 1. Map depicting location of Odeda LGA in South-west Nigeria.

Source: Isaac Idowu Balogun.

Table 1. Different Global Seasonal classifications of climate from the literature

Class	(1)		(2)		(3)		(4)
Seasons	Months	Seasons	Months	Seasons	Months	Seasons	Months
Spring	Mar, Apr and May	Pre-monsoon	MAM	Spring	Apr, May and Jun	Spring	Feb, Mar and Apr
Summer	Jun, July and Aug	Monsoon	JJAS	Summer	Jul, Aug and Sep	Summer	May, Jun and Jul
Autumn	Sep, Oct and Dec	Post-monsoon	Oct,Nov, Dec	Fall	Oct, Nov and Dec	Autumn	Aug, Sep and Oct
Winter	Dec, Jan and Feb	Winter	Jan, Feb	Winter	Jan, Feb and Mar	Winter	Nov, Dec and Jan

Table 2. Seasonality index (SI) (Kanellopoulou, 2002) and annual standardized precipitation index (SPI) (McKee et al., 1993)

Seasonality index		Standardized annual precipitation index
Rainfall regime	SI	Classification	SPI
Very equable	≤ 0.19	Near normal	−0.99 to 0.99
Equable but with a definite wetter season	0.2–0.39	Moderately wet years	1.0 to 1.49
Rather seasonal with a shorter drier season	0.40–0.59	Moderately dry years	−1.0 to −1.49
Seasonal	0.60–0.79	Very wet	1.5–1.99
Markedly seasonal with a long drier season	0.80–0.99	Severely dry years	−1.5 to −1.99
Most rain in three months or less	1.00–1.19	Wet extreme	≥ +2.0
Extreme, almost all rain in one–two months	≥ 1.20	Dry extreme	≤ −2.0

Figure 2. Climograph of seasonal rainfall in Odeda LGA in Ogun State, South-west, Nigeria.

Table 3. Descriptive characteristics of seasonal rainfall in Odeda LGA, Ogun State

Parameters	Spring	Summer	Autumn	Winter
Mean	141.41	214.68	297.44	162.54
Maximum	206.68	353.76	406.86	277.08
Minimum	61.15	71.94	163.85	108.43
SD	45.56	82.67	71.24	48.2
COV (%)	32	39	24	30
Csx	−0.08	−0.19	−0.11	1.17

Figure 3. Line graph of seasonal variation of rainfall in Odeda LGA, Ogun State, Nigeria.

Figure 4. SPI for annual rainfall from 1995 to 2012 for the study area.

Table 4. Classification of annual rainfall based on SPI

Classification	Year
Near normal	1995, 1996, 1997, 1998, 1999, 2000, 2002, 2004, 2006, 2007, 2008, 2012
Moderately dry	2003
Moderately wet	2009, 2011
Severely dry	2001, 2005
Extremely wet	2010

Figure 5. Graph of monthly rainfall in Odeda LGA, south-west Nigeria.

Table 5. Descriptive characteristics of monthly rainfall in Odeda, Ogun State, Nigeria.

Parameters	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Mean (mm)	45.8	41.3	47.4	52.7	63.4	69.5	81.8	97.9	101.9	97.7	67	48.6
Max (mm)	73.5	65.8	73.7	84.8	107.6	121.7	126.8	140	148.8	150	143.1	76.9
Min (mm)	17	18.2	17	8.2	23.9	23	33	66	56.9	41.4	40.5	31
SD	15.7	12.7	16.8	19.4	26.1	28.8	29.5	25.4	27.7	28.7	26	15.6
COV (%)	34	31	35	37	41	41	36	26	27	29	29	32
Csx	0.38	0.46	−0.15	−0.50	0.12	−0.13	−0.25	−0.26	−0.07	0.04	1.58	0.93

Figure 6. Line graph of monthly rainfall variation for the study period.

Table 6. Trend results for Mann–Kendall, linear regression, SPI Tests and Hoeffding’s D statistics for monthly rainfall

	Mann–Kendall				Linear Regression		SPI	Hoeffding’s D Statistics
	N	tm	rm	Trend	Slope	Rcorr	Slope	D	P{D}	ρn
Jan	18	0.18	±0.15	Significant increase	1.55	0.34	0.098	0.1159	4.5517	0.0661
Feb	18	0.27	±0.14	Significant increase	1.19	0.44	0.109	0.0925	3.9058	0.0661
Mar	18	0.31	±0.13	Significant increase	1.57	0.68	0.096	0.0740	3.3952	0.0661
Apr	18	0.31	±0.13	Significant increase	1.74	0.32	0.092	0.0765	3.4642	0.0661
May	18	0.36	±0.12	Significant increase	2.36	0.27	0.093	0.0740	3.3952	0.0661
Jun	18	0.33	±0.13	Significant increase	2.28	0.25	0.085	0.0742	3.4008	0.0661
Jul	18	0.19	±0.15	Significant increase	1.95	024	0.068	0.0878	3.7761	0.0661
Aug	18	0.05	±0.17	Insignificant increase	0.78	0.07	0.032	0.1144	4.5103	0.0661
Sep	18	0.02	±0.17	Insignificant increase	0.20	−0.07	0.001	0.1247	4.7945	0.0661
Oct	18	0.29	±0.13	Significant increase	0.62	−0.09	0.022	0.1258	4.8249	0.0661
Nov	18	0.19	±0.15	Significant increase	2.27	0.15	0.090	0.1001	4.1156	0.0661
Dec	18	0.28	±0.13	Significant increase	1.48	0.35	0.098	0.0885	3.7954	0.0661

Note: R_corr= correlation coefficient.

Figure 7. Comparison of monthly SPI from 1995 to 2012.

Table 7. Trend results for Mann–Kendall, linear regression, SPI Tests and Hoeffding’s D statistics for seasonal rainfall

	Mann–Kendall				Linear regression	SPI	Hoeffding’s D statistics
Seasons	N	tm	rm	Trend	Slope	Slope	D	P{D}	ρn
Spring	18	0.62	± 0.26	Significant increase	4.500	0.102	0.0988	4.0797	0.0661
Summer	18	0.62	± 0.26	Significant increase	6.70	0.083	0.0356	2.3354	0.0661
Autumn	18	0.09	± 0.16	Insignificant increase	1.59	0.023	0.1178	4.604	0.0661
Winter	18	0.35	± 0.13	Significant increase	5.44	0.116	0.0741	3.398	0.0661

Figure 8. Comparison of Seasonal SPI from 1995 to 2012.

Figure 9. Seasonal residual rainfall for Odeda LGA from 1995 to 2012.

Figure 10. Annual residual rainfall for Odeda LGA from 1995 to 2012

Figure 11. Monthly residual rainfall for Odeda LGA from 1995 to 2012.

Table 8. Monthly harvestable rainwater (MHRW) based on maximum error estimate of $\overline{R_m}$

Limits	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
QUCL (m^3)	1.55	1.37	1.61	1.81	2.22	2.44	2.79	3.17	3.32	3.22	2.32	1.63
QMEAN (m^3)	1.28	1.16	1.33	1.48	1.78	1.95	2.29	2.74	2.85	2.74	1.88	1.36
QLCL (m^3)	1.01	0.94	1.04	1.14	1.33	1.46	1.78	2.31	2.38	2.25	1.43	1.09

Table 9. Monthly harvestable rainwater (HRW) based on coefficient of variation limits

Limits	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
QUCL (m^3)	1.72	1.51	1.79	2.02	2.50	2.74	3.11	3.45	3.62	3.53	2.61	1.80
QLCL (m^3)	0.85	0.80	0.86	0.93	1.05	1.15	1.47	2.03	2.08	1.94	1.14	0.93

Table 10. Water demand for various applications

Applications	Water demand (lpcd)	Sources
Bathing	6	WHO (2005)
Toilet flushing	10	WHO (2005)
Dishwashing	4	Authors^1
Laundry (weekly)	10	WHO (2005)
Drinking and cooking	7.5	Sojobi et al. (2015)

¹ Gurung and Sharma (2014) recommended a value of 2.5 lpcd which was considered low for a typical Nigerian setting.

Table 11. Domestic rainwater harvesting potential (DRHP) based on MEE limits

Scenarios	Total annual HRW (m^3)	% Annual NPWD	% Annual PWD	% Total annual HHWD
UCL	27.45	49.64	141.20	36.73
Mean	22.45	41	115.48	30.04
LCL	18.16	32.84	93.42	24.30

Table 12. Domestic rainwater harvesting potential (DRHP) based on COV limits

Scenarios	Total annual HRW (m^3)	% Annual NPWD	% Annual PWD	% Total annual HHWD
UCL	30.40	54.91	156.38	40.67
LCL	15.23	27.51	78.34	20.38

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