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Agriculture on the outer part of an alluvial fan

December 6, 2008

A reader from Germany alerted me to a cool image from the Terra satellite. This is from southern Iran and shows how they are dealing with the lack of fertile ground and water in this desert region.

The greenish colors in the image below highlight the distribution of agricultural land on the depositional landforms in the valley (clicking on the image will take you to a much larger [3 MB] version of the image directly from the NASA Terra site).

The image below zooms in to the beautiful alluvial fan. Note how the agricultural land is out on the outer part of the fan. In most alluvial fans there typically an overall decrease in grain size in a downfan direction — that is, coarser-grained material in the updip (proximal) areas of the fan and finer-grained material in the outer (distal) fan. What the exact grain size distribution is for this fan depends a lot on the rock types in the mountainous area being eroded but the trend from proximal to distal is common.

In this case, my educated guess is that the farmers are taking advantage of the finer-grained deposits out on the outer part of the fan. It would be fun to get on the ground and test that idea. I also wonder how they deal with water. The gray bifurcating threads in the upper-left part of the fan are the currently-active channels — perhaps they capture water during active periods and then distribute to other regions?

Perhaps some of you know of other examples of alluvial fan agriculture?

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9 Comments leave one →
  1. December 6, 2008 8:40 pm

    Brian-

    great photo. if you do a search on google maps of Leh, India and pan around the central valley you will find a lot of variation in the use of alluvial deposits for agriculture. some have agriculture along the proximal to distal transect. some are limited to the distal end. most of the agriculture is clustered around the central meandering Indus river.

    regarding the fan shaped agricultural pattern seen in the image you posted. I just wonder if it reflects community agreement on ownership of water. each landowner has ownership (priority use) of a stream. since the streams are arranged in a radial fashion, land ownership patterns evolved that way too.

  2. December 7, 2008 8:42 am

    Suvrat says: “I just wonder if it reflects community agreement on ownership of water.”

    That is interesting — I wonder if there is some knowledge passed down about the history of the fan — where the active channels are (where the most water is being delivered)?

  3. December 7, 2008 9:05 am

    I was just guessing. any idea how often channels get abandoned or shift in such fan systems?

  4. December 7, 2008 9:24 am

    Suvrat … great question … there are some studies that address that, I’d have to dig them up. It can be quite variable. The active part that is on the left part of fan now (looking downstream) might switch over to the right part after 100s-1000s of years. But that’s a guess, I’d have to look up some studies. Eric (Dynamic Earth) might know that off the top of his head.

  5. December 7, 2008 5:11 pm

    The property lines remind me of the French “long lots” (still extant around St. Louis, MO, and in parts of Louisiana) that give every owner access to the road. Notice the road at the uphill end of these. And as families grow and divide land, these would get skinnier and skinner.

  6. CamArchGrad permalink
    December 8, 2008 11:28 am

    My feeling is that you’re seeing the farms outline a porosity change in the sedimentary package. As noted fans go from coarse to fine as distance from the source increases.This affects water distribution as coarse material loses water quickly while fine material provides a barrier to water transport. The farms outline the change as the underlying sediment which provide the ideal balance between water retention and water loss.

  7. December 8, 2008 12:25 pm

    As I understand desert agriculture in this part of the world, the fields rely on water delivered by gently sloping tunnels, or qanats, from the apex of the fan. I’d imagine the water table is highest at the foot of the fan, too, and the risk of flood lowest.

  8. December 9, 2008 8:41 am

    Couldn’t they have just built an irrigation canal around the inner edge of the cultivated area? That way, wherever the channels shift, the water would still be available to all. There is a curvilinear feature, more reflective than the road, in about the right place on this image.

  9. December 9, 2008 9:15 am

    Chris, I think you’re right … if you go to the NASA site w/ the high-res version and zoom in, you can kind of see what you are describing.

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