I recently installed a well-point irrigation network at our home in Fish Hoek. The irrigation network is fed by the well-point pump or the Municipal water supply. A small 15mm pipeline is what we get from our fathers in City Hall. Of interest I found that the well point pump is a 750 watt unit delivering a flow rate of about 25 litres per minute at a pressure of 1 bar. This flow rate is substantially less than the pump capacity. Poor delivery is simply because the well point supply is weak. Switching to the Municipal supply, I noticed that the supply and pressure was significantly more generous than the pump. To maximize water usage I have had to install a holding tank. Slow top up and fast relief. To match the Municipal supply, I have two pumps. Both are 750 watts. The one tops up the holding tank from the well point, the second provides the irrigation requirement.
So it appears that our water supply can be replaced by two small pumps with an electrical connection. Lets explore this thought some more. The irrigation cycle lasts 80 minutes per day - using a 750 watt pump, the volume of water made available for irrigation is slightly more than 6000 litres. At this point I need to mention that for this application, the area under irrigation is 150 square meters. The irrigation rate is therefore 40 litres per square meter per day. Because the well point supply is weak, the time required to provide a top up supply of 6000 litres is approximately 260 minutes per day. In order to irrigate at a rate of 40 litres per square meter per day, our irrigation arrangement requires an energy supply of 750 watts running for 340 minutes per day. At the current electrical tariff our well point water supply costs slightly more than R1 per kilolitre. Typically the monthly cost to supply water for irrigation purposes is in the region of R200. (Assuming a 22 day irrigation cycle, the electrical cost is roughly R1.15 per square meter per month).
And what a gift we receive from City Hall. Pressurized, potable water for our gardens at such a a small price. Two other factors leap out when making these ever such rudimentary calculations; (1) a not too insignificant initial cost required for the installation of the well point and irrigation pump arrangement and (2) the availability of regular potable quality water. The ground water supply in Fish Hoek is not entirely sediment free. Fortunately the iron content is low, conductivity and pH are within acceptable limits. Other areas of Cape Town are not so fortunate.
Talking of fortunate, un or otherwise. Is it not a slight anomaly that a very middle class garden in Fish Hoek uses 6000 litres per day - admittedly in summer time only - for a very ordinary garden. How many people in this country have access to 100 litres of water per day? Could 60 people live in my house?
Closing thought. Civil society is alive and well in this part of the world, especially for those with irrigated gardens.
