Sunday, September 26, 2010

Beetroot 2: how much water goes on?

The beetroot were cooked up in a spicy sauce and served on top of rice for the 300 or so lunches that the kitchen serves up every day (meat is too expensive, so the children live largely on bread and rice topped with a vegetable-based sauce). It tasted good, but our question is how well was it grown? How much water and nutrients did it take to grow? Could we grow much more beetroot with the same inputs?

The irrigation system was fitted with a tap timer, and there were small taps at the head of each drip line. So each drip line could be individually turned on and off. Theoretically we should know how long each drip line was run for and hence the application of water to the beetroot. Then we could look up the potential evapotranspiration for Maputo, make adjustments for the size of the crop, and we should be able to answer the question above - at least as far as water goes.

In practice it’s not that easy. First, no one keeps good enough records of the on/off times of the taps. Second, the application rate of the drippers was not constant. Although rated at 1.0 L per emitter per hour, we were running the system at lower than recommended pressure, and the more lines open the more the pressure fell. Actual application rates varied between 0.5 and 0.7 L/h

We rigged up a simple flow meter as in the picture below, with graduation marked on the bottle. We checked the uniformity of 20 drippers down the line, and compared this to the dripper we collected from. The system was uniform. So that solved the first question – how much water went on to the crop.

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