In December of 2024, OneAfrica television published an article on Facebook written by Leonard Witbeen and informed by the weekly Dam Bulletin that most dams and reservoirs in Namibia show significant drops in water levels due to evaporation, water transfers, and limited rain. The water levels in two (2) dams, the Swakoppoort and Von Bach, both of which serve as primary reservoirs for the capital city, Windhoek, had decreased to 16.4% and 10.4%, respectively, of their total capacity, by Christmas, 2024. Both dams rely on rainfall to replenish water reserves.
In a previous blogpost, mean annual precipitation (MAP), or average annual precipitation, 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.” It was also clarified in the same blogpost, that precipitation (or rainfall) is measured in millimetres (mm), i.e. distance, not millilitres (ml), and using a rain gauge, the standard measuring instrument used by meteorologists.
Using MAP data compiled by Namibia Weather (https://weather.namsearch.com/wdhrainsummary.php), the real mean annual precipitation or rainfall for the capital city of Windhoek in the Khomas Region, Namibia, over a ten-year period, 2010 to 2020, was established at 403.75mm, and not the widely accepted and widely quoted mean annual precipitation of 370mm. It is to date unknown by whom or how the current mean annual precipitation of 370mm for Windhoek had been established.
In this blogpost, the mean annual precipitation (MAP) data for Windhoek will again be used to:
1. Determine a more accurate average precipitation for Windhoek for the last sixteen (16) years, 2008 to 2024;
2. Establish the dominant trend for annual precipitation in Windhoek;
3. Determine the cause for low water reserves in the reservoirs for Windhoek.
2. Establish the dominant trend for annual precipitation in Windhoek;
3. Determine the cause for low water reserves in the reservoirs for Windhoek.
1. Real Mean Precipitation for Windhoek: 2008 - 2024
The table below, Table 1, tabulates the mean annual precipitation (MAP) for every year since 2008 to 2024. The table also shows the total precipitation and the mean (or average) precipitation for the period under review. The mean precipitation was calculated by dividing the total precipitation with the number of values (n = 17).
MAP Data provided by Namibia Weather (https://weather.namsearch.com/wdhrainsummary.php)
The total number of values (n) equal seventeen (17). The upper limit of the data is 1221.8mm (2011) and the lower limit of the data is 109.6mm (2019). The upper and lower limits establish the range of reference for annual rainfall in Windhoek. The total precipitation (i.e. 6,840.3mm) is divided by 17 to calculate the real mean or real average precipitation for the period 2008 to 2024. It is, therefore, correct to declare that the average precipitation for the period 2008 to 2024 was 402.4mm. The latter is 8% higher than the widely accepted average rainfall of 370mm for Windhoek and it is, therefore, safe to say that the average or mean annual precipation for Windhoek is around 400mm.
2. Dominant Trend for Annual Precipitation in Windhoek: 2008 - 2024
The graph below, Graph 1, plotted the mean annual precipitation for the years under review in columns with data labels for each year and a trendline to illustrate the trend for annual precipitation in Windhoek.
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