I finally got my hands on a digitized version of some of the rare 1:10,000 topo maps of Sri Lanka. Thank you Jonathan Rosenwasser of Harvard's map library!
The maps of Sri Lanka's Dry Zone, where I'll start my Fulbright in a few days, corroborated what I had seen in May. Many of the "tanks" (human-built irrigation lakes) have a dam with an exaggerated convex shape. Walking along these dams last spring it occurred to me that this dam shape is counter-intuitive. How do dams like this withstand the rush of water that fills the lakes after every dry season?
Other questions came to mind as well. It's well understood that the tanks separate out silt, salts, and alkaline water, and even distribute them differentially to
the rice fields below (designated as "P" (paddy) on these maps just north-northeast of the dam.
Thinking about these human-built ecosystems in the harsh Dry Zone of Sri Lanka it makes sense that they would be built the way they are, just because the convex dam collects the most water possible. But questions of dam integrity, as well as the complex filtering and functioning of the tank remain. I have wondered all along-are these tanks just impounding water or are they doing much more--taking advantage of water's characteristics to persuade it to behave in certain ways?
And if water just rushed into the tank as I diagrammed above, wouldn't this be an unsustainable proposition? But some of these tanks are more than 2000 years old!
Part of the answer seems to lie in vegetation that grows inside the tank (there's lots outside too). In the very oversimplified diagram below I've sketched out two vegetative features of the tank, the gasgommana and the perahana. From what I've read and what I saw last spring, the gasgommana, comprised of large trees that grow just inside the dam, shades the water, filters it, and most importantly eases its pressure on the dam. The perahana, a large reed bed near the catchment area, filters out silt, slows the initial flow into the tank, and provides habitat for all kinds of small fish, birds, and invertebrates.
There are other biotic (living) features of the tank. But I'll wait until I learn more before I write about them. Bottom line, I think there's a kind of interior girdle of living plants that protects and enriches the tank. This living girdle also contributes to the complex functions of the tank. And I assume it influences how water flows in the tank, something I hope to learn more about in the next few weeks.
Looking forward to sharing lots more of my findings. Also thanks to Tony Janetos of the Boston University Pardee Center for the Study of Longer Range Systems for starting me thinking about the interaction of land systems with human needs.