I was not going to let crappy internet slow me down, so early in the morning I went to the International House and made the internet buzz. My crazy friend Bob wants to write a paper on “agrogeology”, and has tasked me with discussing, preferably in three pages or less, how hydrogeology relates the agriculture. Given that I teach a whole year sequence about water supply the task appeared impossible. So I figured I would talk about the ideas that roll in my head when I see countries like Morocco or Mexico struggling to provide enough water for agriculture, and then take a warp-drive leap into the cutting edge of technology used for water management in a place like California, where there are very large numbers of users tapping into the same groundwater basin.
As far as “basic” water harvesting schemes, I cast my mind back to a trip I made more than 15 years ago to the
of Tehuacán, in Mexico, where
the domesticators of corn built the El Purrón Dam (coordinates 18.176844,
-97.117449 in Google Earth). The dam was
completed by the Middle to Late Formative (ca. 650 - 150 B.C.), and was built
using an ancient system of caissons, where separate “cells” formed by walls of
stacked stones were filled in with soil. El Purrón Dam measured 400 meters
long, 100 meters wide, and nearly 25 meters high. Workers transported by hand
some 2.64 million cubic meters of earth to build it! Just as impressive are the
hundreds of kilometers of canals and aqueducts (locally called tecoatls) that
conveyed water from the dam and nearby springs to more than 330 square
kilometers of cropland. All this happened over 2,500 years ago!
The other approach I observed in
Morocco 3 or 4 years ago, through
the use of qanats or puquios. The invention of this method is
credited to Iranian farmers, but it soon spread through the Islamic world,
including Morocco and Al
Andaluz in southern Spain,
and from there it was brought by the Spanish explorers into Latin America,
where some still remain in use (e.g., Aguascalientes
A qanat is a long gallery that
extends from the upper portions of alluvial fans, where the water table
typically has a steep slope, down to the distal portion of the alluvial fan and
even down to the floor of the adjacent plains. The first step in building a qanat is for Farmer A to sink a hole in
the upper portion of the alluvial fan until groundwater is found. A second hole
is then excavated by Farmer B, about 20 m down the slope from the well of
Farmer A, to the same depth as that of the water bearing hole. Once the final
depth is reached Farmer B excavates a nearly horizontal tunnel to join one well
to the other until eventually the same groundwater is reached and water flows
into the channel. Now two farmers have wells with water in them. Farmer C then
repeats the operation, about 20 m downgradient of Farmer B’s well. One by one
downslope farmers “connect” into the ever growing underground “pipeline” until
eventually 50 to 100 farmers have local access to water.
Interestingly, the technique of the qanat was independently discovered by the Nazca civilization (ca. 100 BC to 800 AD) of the Atacama Desert, in
The Atacama Desert is one of the driest places
on Earth. Yet, the Nazca people managed to farm that land for hundreds of years
by using puquios, in all aspects
similar to qanats. A good example can
be observed in Google Earth by entering the coordinates
I felt particularly proud of being able to find these places through Google Earth, because I was going at it based on very faint memories of places I visited many years go. Travels do teach you a few useful things J
Full of myself I decided to tackle the more mundane task of finding a good place to have lunch before my class started at 1 pm. Theophilus, one of our fabulous Ghanaian students suggested I could try the Bush Cantine, just outside of the perimeter of the university (“turn left, then again left, then right, then left and go out of the university through the little door, and finally turn right until you see the entrance.”). Unbelievably the instructions made perfect sense once I started walking, and in no time whatsoever I found myself in a small market with a wide food court. Many of the stalls had pictures of the food they sold, and I selected one that offered beans with plantains. The dish, I learned, is called red-red, and starts with a generous serving of beans to which is added a handful of dry fufu, a scoop of some tasty red sauce, avocado pieces, and a good serving of fried plantains. It was delicious! In the days to come I plan to go from stall to stall, taking whatever they want to serve me, and in this way sample the best of Ghanaian cuisine.
After lunch I wandered through the market stalls, looking at knick knacks, until I came to the stall of a seamstress. On a whim I went in and asked if she could do me a shirt with colorful African fabric. No problem. So she took my measurements, had me choose the fabric, and in a week I should be able to disguise myself as a sage African professor. All this for the equivalent to US$ 15! If I like the shirt I plan to order one or two more.
Class was OK, but two of my three students mentioned that there was a lot of math in this class. More like arithmetic, really, but I have to remember that oh so many college students are math phobic, and see little value in trying to estimate quantities and rates. Where did we go wrong?
I think I will have an enormous, juicy, delicious mango for supper.