“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.
The Blog by Javier 
estimado javier
your observations and calculations on olive trees per ha dependency on rain almost exactly matches mine as reported in an article in agricultural and human values (nr 19,pp173-87, 2002) , see p.181 wth yields per tree and tree densities in spain(750 mm) and sbeitla (tunisia,only 160 mm). note that with the high densities in spain yields are zero where rains are as low as 250 mm , in exceptionally dry years. so, farmers everywhere know exactly what to do.
javier, spent the day reflecting and googling on olives ,yields and water, and guessed that most important complcating aspect must be that, the less rain, the less water usefully spent in transpiration of trees, due to increasing impact of soil evaporation , or inevitable losses due to sparse trees and less shade..
once more on olives and rain; imagine for a shortmoment that all fallen rain is used for olive growth and ripening, that means at rains of 10 mm a total of 100.000 liter/ha, olives use rougly 1000 lt to produce 1 kg fruits, this would mean one tree of 100 kg harvest per ha, or distance of 100×100 mt in an orchard, of course not realistic (see only above, e.g., roots cant reach all that fallen rain etc)) but a nice sum to do indeed.
a little bit ,but not much,more realistic would come to 4 trees with 25kg olives per ha at 10 mm rain per year, or tree pattern of 50×50 mts.
closer even to reality: 50% water loss due to evaporation at 200 mm rain per yr, or tree pattern of 12×20 mts at harvests of 25 kg/tree.
Hi Dirk,
I’m glad that you liked the post. I appreciate your comments. I’ll have to go through your numbers in detail, as know I see your line of argument but would love to have the picture and order of magnitudes well ingrained 🙂
yes, javier, it was really a surprise to see you wondering and calculating on olives at different tree densities,it is now some 10 yrs ago that I struck on such figures frm Jaen and mid tunesia, and used them to show that with some clever manipulation in densities you can solve a lot, but never got any positive respons to it (specialists prefer to irrigate and invest as much as possible). but, be careful, apart of my one consideration above, there are many more in practice (percolation, varying evaporation and transpiration, etc), but as you remarked, one can start with a simple model, like also in economics at large (e.g. start with assuming that people are always rational beings), but then comparing results with statistics and adapt your model. so, once again, it was fun to meet a colleague in observations and thinking, greetings dirk
the reason I struck on you was, surfing the net on olives in tunisia after reading in Kaplan’s Mediterranean Winter that in carthago a certain Mago wrote, even B.C., that trees should be planted there at 20x20mts, for Kaplan a sign of how fertile the soils there were (in Greece distances were 7×7 mt), now, you and me and maybe others know better than Kaplan on this, whom, nevertheless, I esteem very highly, even after this mistake. dirk