Enjoy it while it lasts

Dear Estelle,

Evaporation from 10 to 16th January 5-1,7, 4, 5, 5-2, 4, 2-17 My historic average evaporation for January is 2.3mm a day, so we were still experiencing evaporation much higher than ‘normal’.

My title refers to electricity – not to rainfall. It seems we are no longer being subjected to the full 8 hours a day power cuts, yet the Zambezi flow is still well below average and we currently have 60 days generating capacity in Kariba (and 90 on the Kafue). Add to that the decision to delay the Power Factor penalty to April and keeping the tariff for domestic consumption at the old price and we seem to have done everything possible to make sure we run out of electricity as soon as possible. The website http://www.wxmaps.org/pix/soil10.html shows that (apart from Mwinilunga itself) the rest of the Zambezi catchment above Kariba is not exactly flush with moisture. Kafue the same. So, those of us who are lucky enough to have irrigation water (not me) are unlikely to have adequate power to irrigate with.

Some more good news (genuine, to go with last week’s news of a 100mkg shortfall in Brazilian tobacco): Chris Aston reports that his VFDrives have reduced his diesel consumption from about 24 litres an hour to about 16. He reports that learning how to use VFDs is not easy, but it is obviously a technology that we are going to have to get to grips with. Also in the paragraph on good news is the fact that (after 35 years – Duh) I think I have finally come up with a solution to water hammer in my irrigation pipes. It really needs a diagram (which I will try to prepare for next week) but it involves putting the pressure relief valve under water on the suction side. This enables it to be set at a much lower pressure (so that it releases the hammer quickly) but also it then doesn’t matter if it leaks slightly as it cannot suck air into the system. With water being so short we CANNOT afford leaking pipes any longer and managing water hammer should help reduce those leaks.

Finally on water; I did mention about 5 years ago (when I was last critically short of water) that it was important to calculate our profit per megaliter of water. Time to dust off and update those figures. As Chris Aston pointed out (at the time) it is important to include the evaporation losses. A simple look at how much we pump onto irrigated tobacco, might suggest that it gives a better return than wheat. But, by the time we have irrigated our tobacco our dams have lost a lot more through evaporation and wheat, using the water earlier, might actually have been the better option.





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