Monthly Archives: September 2010

A ride into the Sahara (video)

During our holidays in Tunisia, we took a 2-days excursion to the South-West; into the Sahara. We had never been in a desert before.  We loved it.

In those two days we lived many new experiences. We felt like in a roller coaster. Yesterday, I posted a video about the amphitheater of El Djem. Today I am posting another video about some of the other things we did. I truly believe that it will give you a much better idea than a thousands words…

I chose UB-40 song “Higher Ground” for the sound track of the video because indeed this is what we listened during our ride in the Toyota Land Cruiser: plenty of UB-40 songs chosen by our Tunisian driver to our delight.

Regarding the cars, I would say that around 90% of those venturing into the desert were Toyota; the remainder, mainly Nissan. If you saw what they did to those cars… they must be reliable cars.

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Amphitheater of El Djem (video)

Last August Luca and I visited the amphitheater in El Djem, Tunisia. It is the biggest one in Africa and the 4th in the World. It could host up to 35,000 spectators who came mainly to watch chariot races and gladiator shows… and as some of you may know, it was used to film some parts of the movie “Gladiator“, by Ridley Scott.

As I have done before, instead of just sharing some pictures of the amphitheater, I prepared a small video, I hope you like it. I did enjoy preparing it.

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How rain determines olive tree economics in Tunisia

“The North of Tunisia is the most fertile region. There it rains about 1,000mm per year. In the middle about 200mm. The South is almost deprived from rain with only between 0-50mm of rain”. More or less these were the words we heard from Mohammed, our guide in Tunisia for 3 days, no less than 3 times. You can “see” that with Google Earth already.

Tunisia.

He also went on to explain that the olive leaves are a symbol of wealth and that Tunisia was one of the main producers (5th in the World, after Spain, Italy, Greece and Turkey). So, after hearing all these explanations and seeing so many olive trees in the fields along the road trips, I started to notice the difference between the olive trees in the North and the South, and mainly the difference in the distance in which they are planted from one another.

Seeing the landscape I thought that (even if they did it unconsciously) these people were using some scientific approach there. I must say that I have no clue about agriculture and olive trees, but let me elaborate.

  • “1 mm rain a year” means that in one square meter during one year 1 liter of water is collected.
  • The surface from which each of the trees is collecting water must be proportional to the distance (d) between them: (π/4)*d² [m²].
  • I assume the water (volume) one olive tree needs along the year must be proportional to its size (volume)… then, the water they can collect is limited by the rain (mm) and the distance among trees: k*(π/4)*d²*r. Where “r” is the quantity of rain measured in mm of water, “d” the distance and “k” a constant.
  • Their size may be limited by the rain (if in the South is too dry?), by the distance if they are too close to each other, by genetics of these kind of tree (?)…

So, imagine that we are in two regions in which the annual rain is over the minimum so the olive tree can realize to its “own potential” (olive trees having the same size), then:

  • The farmer in the region with less rain must be aware that he shall plant the trees with a distance (d2) between them of: d2 = d1*√(r1/r2), where “d1” is the distance in the rainy region [m], and “r1” and “r2” are the quantity of rain in each regions [mm].
  • So, if I see olive trees the same side in the North (1,000mm, region 1) and Middle (200mm, region 2) of Tunisia, the larger distance in the Middle region should be around √5 = 2.23 times the distance in the North.

As we go to drier regions (Middle, region 2, or South, region 3), it may be that the final size of the tree is smaller and the distance will have to be larger.

  • If the less than 0-50mm in the South was still enough to have large olive trees, then the distance should be over 20 times the distance that we see in the North. However, I cannot tell you, since we didn’t go the most Southern part of Tunisia.
  • If the 200mm of the Middle is not enough water to have that large olive trees then, you could either calculate the size of the tree by planting at different distances in relation to the sizes and distance in the North, or knowing the maximum size you may get you could get the distance at which you have to plant the tree…
    • tree2 = tree1*(r2/r1)*(d2/d1)²… if I guess the distance is about double (by seeing in the pictures), then (for a rain of r2=200mm) the tree2 near the Sahara would have a volume of about a tenth of the tree in the North. May well be true by seeing the pictures below.
    • If we knew beforehand that the tree would only get to reach a tenth of the size, we could calculate that the distance would need to be double.

Even though I am sure there are many more aspects impacting the growth and productivity of olive trees, if I were in Middle / South Tunisia starting from scratch and not knowing anything: I could start sizing the number of trees I could plant in my garden or how big they would grow, how much olives I would get from my land, etc…

Apologies to the experts in the field for the charade that I may have just written, but it was fun playing with the numbers.

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