Locus: Windhoek, Namibia (22.5609 S, 17.0658 E)
With the exception of its north-eastern regions, Namibia is, predominantly, a hot, dry country with a climate second only to the Sahara Desert in aridity. Due to losses in rainfall as a result of high evaporation levels, the country is classified as drought-prone with an estimated 90% of annual rainfall lost to evaporation.
The Ministry of Agriculture, Water and Forestry (MAWF) manages water resources in Namibia, and NamWater, a state-owned enterprise under the purview of the MAWF, operates the reservoirs, dams, pipelines, boreholes, and water-treatment plants throughout the country.
Windhoek, the capital city of Namibia, is situated in a basin between the Khomas, Eros, and Auas mountains on the Central Plateau of southern Africa. The city lies 1,680 m above sea-level and is home to a little more than 460,000 inhabitants.
As the economic, political, judicial, and administrative centre of Namibia, Windhoek relies on several reservoirs, such as the Von Bach, Swakoppoort, Omatako, and Goreangab dams, water reclamation projects, and boreholes to supplement its water supply.
A widely accepted fact is that the mean annual precipitation (MAP) for Windhoek, for the “last twenty (20) years”, is 370 mm with the highest precipitation (or rainfall) occurring annually between January and May, and little to no rainfall from June to September. The source for this mean annual precipitation (MAP) calculation is unknown.
At 200 litres per capita per diem (per person, per day), Windhoek has the highest water consumption in Namibia. Water consumption varies considerably between neighbourhoods with low-income areas consuming significantly less water per capita per diem than high-income areas, not least because water is scarce and access to water is restricted in low-income areas by local municipalities.
Despite a diversified network of supplementing water resources, the City of Windhoek municipality imposed water restrictions at least five (5) times during the last ten (10) years. From media reports it was evident that the municipality blamed human behaviour, negligence, business activities, leaks, and wasteful habits for the city's water woes, its utterances and responses clearly based upon the assumption that rainfall levels in Windhoek remained constant throughout, but that water consumption and waste instead had increased.
But, was that an accurate reflection of the situation?
Being “water wise” and careful with consumption in Windhoek is appropriate and sensible, it could even be considered ethical, under the circumstances. However, human behaviour and consumption alone could not account for the occurrence of three (3) troubling events within a short space of time...
Drought
A drought is defined as “an event of prolonged shortages in the water supply, whether atmospheric (precipitation), surface or ground water.” Another definition of drought is “...a prolonged period marked by a lack of precipitation resulting in a water shortage.”
Droughts have many causes: some are natural, some man-made, others are caused by fluctuating ocean and land temperatures, altered weather patterns, reduced soil moisture, and finally, climate change.
Global warming is defined as the rise, or increase, in global temperature indices and the primary cause behind climate change. When the average temperatures in a region increase, even slightly, the effects can be devastating. For example, in wetter areas, the warmer air absorbs more moisture and creates extraordinary rainfall and weather events such as floods and storms. In arid areas, such as Namibia, warmer air increases evaporation from plants, soil, surfaces, and the atmosphere creating drought conditions.
Droughts have a cumulative impact on affected areas. Not only do higher evaporation rates adversely affect agriculture which, in turn, contributes to food insecurity, droughts also decimate general plant cover and suppress regional rainfall rates to the extent that “atmospheric rivers”, streams of moisture in the atmosphere, are altered and / or interrupted.
According to local media reports, Namibia experienced “extreme drought” conditions in 2015, 2017, and, again, in 2019. Extreme drought is category D3 and corresponds to areas marked by severe agricultural and plant cover losses, at high risk of frequent and extended wild fires, and widespread water shortages that require restrictions. The highest category is D4, known as the “exceptional drought.”
Statement of Purpose
This study was inspired by media reports, municipal attitudes, and government responses that treated three (3) extreme drought incidents as unrelated, unusual events, the results of human behaviour, and local water consumption.
This study focused on mean annual precipitation (MAP) data for Windhoek over a ten-year period (2010 to 2020), so as to establish the real mean annual precipitation (MAP), to determine standard deviation from the mean annual precipitation (MAP) for the period under review, and to determine dominant trends in precipitation.
Therefore, the premises for this study are:
1) That the frequencies of extreme droughts that had affected the supply of water to Windhoek are significant, but not unusual, during the period under review;
2) That the frequencies of extreme droughts that affected the supply of water to Windhoek, during the period under review, are indicative of a pattern or trend in precipitation, unrelated to human behaviour or local water consumption;
3) That the frequencies of extreme droughts that affected the supply of water to Windhoek were typical events within a continuous event, i.e. long-term drought.
Long-term drought is defined as drought conditions that persist for longer than six (6) consecutive months.
1. Mean Annual Precipitation (MAP) Data for Windhoek, Namibia: 2010 - 2020
Mean annual precipitation (MAP) is defined “as the sum of rainfall for a given year divided by the total number (n) of values.” Mean is the arithmetic synonym for “average.” Please note that precipitation (or rainfall) is measured in millimetres (mm), i.e. distance, with a rain gauge, the standard measurement instrument used by meteorologists, not in millilitres (ml).
Table 1: Mean annual precipitation (MAP) for Windhoek, per annum, 2010 to 2020
Source: Namibia Weather, www.weather.namsearch.com
The number (n) of mean annual precipitation (MAP) values in this study equals eleven (11). The upper limit of the data is 1221.8 mm for 2011, and the lowest is 109.6 mm for 2019. The upper and lowest data limits establish a range of reference for the mean annual precipitation (MAP) for Windhoek, over a ten-year period. The table shows data in non-ascending but chronological order, for every year, using the following scale: 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020 (n = 11).
Graph 1: A line graph presentation of the mean annual precipitation (MAP) data for Windhoek, Namibia, 2010 to 2020, including trendline
The independent variables, years under review, were plotted along the x-axis of the graph (n = 11). The dependent variable, mean annual precipitation (MAP) for every year, was plotted along the y-axis of the graph, at intervals of 100 mm (see Table 1), above and below the upper and lowest data limits. Both independent and dependent variables equal n = 11.
Graph 1 shows eleven (11) values for eleven (11) years. Line graphs show minor or major changes between points of value and highlight changes in a single variable over a period of time. The data values in Graph 1 show a consistent decrease in mean annual precipitation (MAP) for Windhoek.
Graph 1 also shows an outlier, or anomaly. Mean annual precipitation (MAP) in 2011, at 1221.8 mm, was significantly higher than for any other year, during the period under review.
A trendline is a linear representation that demonstrates the relationship between dependent and independent variables using linear prediction based on available data. Graph 1 shows that the precipitation trend, over time, in this case, years, is a downward trendline. It can, therefore, be inferred, from the trendline, that precipitation (or rainfall) decreased significantly between 2010 and 2020. Down trendlines indicate fundamental changes in variables*.
2. Real Mean Annual Precipitation (MAP) for Windhoek: 2010 to 2020
By using the data in Table 1, the mean annual precipitation (MAP) for Windhoek, Namibia, during the period under review, i.e. 2010 to 2020, was established to be 403.75 mm, 8.35% higher than the widely used, unverified mean annual precipitation (MAP) of 370 mm. An accurate mean annual precipitation (MAP) is critical to determine the standard deviation in rainfall, for the period under review.
3. Standard Deviation from the Mean Annual Precipitation (MAP)
Table 2: Standard Deviation Calculation of Mean Annual Precipitation (MAP)
With the mean annual precipitation (MAP) established at 403.75 mm for the period under review, deviation from the MAP was calculated for every year (2010 to 2020). The deviation values in red are negative, relative to the mean annual precipitation (MAP) at 403.75 mm, meaning less than (<) or below the mean annual precipitation (MAP). The deviation values in black are positive relative to the mean annual precipitation (MAP) for Windhoek, meaning equal to (=) or more than (>) the mean annual precipitation at 403.75 mm. Black values (positive) equal n = 4. Red values (negative) equal n = 7. The standard deviation in mean annual precipitation (MAP) for the period under review was 287 mm.
Table 3: The Normal Distribution of MAP Data arranged in Ascending Order for a Standard Deviation Curve or Bell Curve Data Distribution
Table 3 shows the normal distribution data arranged in ascending order, from lowest to highest, for a standard deviation curve. Given that the mean annual precipitation (MAP) for the period under review was 403.75 mm, it is evident from the normal distribution table that the first seven (7) values, 109.6 mm to 313.4 mm, or seven (7) years, in non-chronological order, out of eleven (11), experienced rainfall below the mean annual precipitation (MAP) of 403.75 mm.
Graph 2: Bell-shaped Data Distribution Curve of Mean Annual Precipitation (MAP) Deviations for Windhoek, between 2010 and 2020
The mean annual precipitation (MAP) for Windhoek, between 2010 and 2020, was 403.75 mm. On a distribution graph, or bell-shaped curve, 403.75 mm corresponds to 1 on the y-axis, and to 403.75 mm on the x-axis. Ceteris paribus the majority of the distribution points cluster near the mean annual precipitation (MAP) of 403.75 mm (= 1), or fall within a standard deviation of 287 mm, on either side of the mean annual precipitation (MAP).
Graph 2 shows that most deviations are positioned to the left of the mean annual precipitation (MAP), on the x-axis. Again, this shows that the majority of years, during the period under review, had lower than average annual rainfall with the exception of one anomalous year, that being 2011. The anomaly that distorts the bell distribution curve is more than three (3) times the value of the standard deviation (287 mm).
4. Premise 1: The frequencies of extreme droughts that had affected the supply of water to Windhoek were significant, but not unusual, during the period under review.
According to media reports, Windhoek experienced “extreme drought” conditions on three (3) separate occasions; in 2015, early in 2017, and again in 2019. Aside from the fact that all of the aforementioned years ended in prime numbers, a fact that has no statistical relevance, the occurrences of three (3) extreme drought events at intervals of two (2) years, are significant, but not unusual.
It is the finding of this study that the consistent, sustained downward trend in mean annual precipitation (MAP), during the period under review, correlates positively to the occurrence of drought, and for extreme drought conditions to materialise.
5. Premise 2: The frequencies of extreme droughts that had affected the supply of water to Windhoek, during the period under review, are indicative of a pattern or trend in precipitation, unrelated to human behaviour or local water consumption.
Table 1 and the downward trendline of Graph 1 proved a pattern, or trend, of decreasing precipitation in Windhoek, during the period under review. Logic dictates that the correlation coefficient for precipitation and water supply is positive, meaning, an increase in precipitation correlates positively to an increase in water supply.
It is the finding of this study that a downward trend (or decrease) in precipitation (or rainfall), during the period under review is clearly illustrated by Graph 1, which is indicative of a pattern or trend showing a decrease in precipitation independent of human behaviour or local water consumption.
6. Premise 3: The frequencies of extreme droughts that had affected the supply of water to Windhoek were not three (3) unrelated events, but, were typical within a continuous event, i.e. long-term drought.
Table 2 showed that 63% of the mean annual precipitation (MAP) data points were negative, or red; meaning, that for more than half the years during the period under review, annual rainfall was lower than the mean annual precipitation (MAP).
As opposed to short-term drought, long-term drought has a cumulative effect. Long-term droughts are caused by changing weather conditions, or atmospheric circulation patterns that become entrenched, and lead to precipitation shortfalls that last for several months or years. It is possible for the very same weather, or atmospheric circulation patterns responsible for long-term droughts, to create short-term, exceptional rainfall events, also.
Two (2) significant rainfall anomalies in Windhoek, during the period under review, demonstrate the aforementioned: 1221.8 mm in 2011, and 109.6 mm in 2019, representing the highest and lowest mean annual precipitation (MAP) data points, respectively.
It is the finding of this study that Windhoek experienced three (3) events of “extreme drought” (D3) within an extended, long-term drought period, caused by months and years of accumulated precipitation shortfalls, evident in the decrease in mean annual precipitation (MAP) which resulted in rainfall anomalies and contributed to the occurrence of three (3) extreme drought events. As explained earlier in the study, long-term drought has a cumulative impact.
Conclusion
It is cause for concern that precipitation, or rainfall, for Windhoek and surrounding areas, is trending downward and that the city experienced three (3) events of extreme drought in ten (10) years. Given the historical data, on which this study is based, it is highly likely that Windhoek will experience similar precipitation deficits, in the coming decade (2020 to 2030).
It is, therefore, recommended that the local authority and municipality prioritise planning, budgeting, and putting in place drought management and mitigation measures as a matter of urgency. Drought mitigation measures, supported by a permanent, appropriate institutional framework at municipal level, should make provision for the implementation of drought monitoring systems, water supply increase, demand reduction, and drought impact minimisation measures.
Contrary to local perceptions and opinions which, in the past, had blamed water shortages on increased, negligent or wasteful consumption, this study shows that not human behaviour, nor consumption levels, but an external, environmental variable, instead, i.e. precipitation (or rainfall), had significantly changed, and consistently decreased, over a period of ten (10) years.
In other words, even if the inhabitants of Windhoek consume the same amount of water per capita per diem for ten (10) years, and even if the inhabitants of Windhoek consume less water per capita per diem for ten (10) years, droughts will continuously occur, regardless, as a result of changing weather and atmospheric circulation patterns.
This study proves that precipitation for Windhoek had dropped to such alarmingly low levels, between 2010 and 2020, that long-term drought, as a result of changing weather patterns, cannot be excluded as a major contributing factor to the events of extreme drought that had occurred, during the period under review.
References
1. The Story of Water in Windhoek: https://www.mdpi.com/2073-4441/10/10/1366/htm#:~:text=CoW%20purchases%20bulk%20water%20from,by%20the%20City%20of%20Windhoek.
2. Major Dams Drying Up: https://reliefweb.int/report/namibia/major-dams-drying
3. Climate and Weather of Namibia: https://www.info-namibia.com/info/namibia-weather#:~:text=The%20annual%20precipitation%20average%20of,for%20Windhoek%20is%20370%20mm.
4. Rainfall in Windhoek, Namibia: http://www.windhoek.climatemps.com/precipitation.php
5. Drought: https://en.wikipedia.org/wiki/Drought
6. Tree Rings provide Snapshots of Earth's Past Climate: https://climate.nasa.gov/news/2540/tree-rings-provide-snapshots-of-earths-past-climate/#:~:text=One%20light%20ring%20plus%20one,local%20climate%20in%20the%20past.&text=This%20field%E2%80%94the%20study%20of%20past%20climates%E2%80%94is%20called%20paleoclimatology.
7. Tree Rings, Carbon Dioxide, Climactic Change: https://www.pnas.org/content/94/16/8350
8. Drought and Climate Change: https://www.c2es.org/content/drought-and-climate-change/
9. Drought: Degrees of Drought: https://www.ncdc.noaa.gov/news/drought-degrees-drought-reveal-true-picture#:~:text=The%20extreme%20drought%20category%2C%20D3,can%20be%20expected%20requiring%20restrictions.&text=This%20is%20part%20of%20a,conditions%20across%20the%20United%20States.
10. Measuring Drought: https://www.ncdc.noaa.gov/monitoring-references/dyk/measuring-drought#:~:text=The%20Palmer%20Z%2DIndex%20measures,agriculture%20during%20the%20growing%20season.
Originally published: 4 January, 2021
Written and researched by Anya Namaqua Links: anyalinks@gmail.com. No portion of this article may be reproduced without the express written permission from the author. Acknowledgement is required for quotations and referencing.
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