Micro-level water-resources engineering—9: Your enemy no. 1 is…

I am not sure how the elections affect the actual, on-the- ground activities related to the water conservation efforts, this year. However, the point I want to emphasize here is urgent—and it is technical in nature. It is also of very real consequences. I have made this same point several times over the past few years, but still find that, unfortunately, it still remains worth repeating even today. The point I want to remind you is the following:


Regardless of the scale of your water conservation project (whether farm-pond, small check-dam, big check-dam, KT weir, percolation tanks, dams, etc.), and regardless of whether it’s the building of a new structure or just the maintenance of an old one, remember that:

Evaporation loss is the least appreciated but also a most real factor that is actually operative in India.

Expect that depth-wise, water body that is about 8–10 feet deep will simply get evaporated away in a single year. There is nothing you can do about it. (So far, no suitable technology has ever been invented to cost-effectively counter or circumvent the evaporation losses.)

Also, realize that

A small pond (say 5 feet by 5 feet in area) and a large dam (say 1 km by 5 km in area) both lose the same height of water in the same time period.


For ease in visualization, remember, 10 feet is the height of a typical single storey building.

10 feet also is the height of a typical passenger bus.


Thus, if your farm-pond has water 20 feet deep when fully filled (say at the end of a monsoon), then expect that it will come to hold only about 8–10 feet deep water during the month of May next year—even if no one has taken even a single liter of water out of it, for any use whatsoever.

Further, realize that in any water-conservation structure, you are going to have some clearance in between the top level of the water-body and the top level of the dam-wall (or the pond-wall).

Thus, to have a water body that is at least 20 feet deep, you must have the top of the wall at a height of about 24–25 feet or more, when measured from the bottom of the water body. In contrast:

If the wall of your farm-pond or check-dam itself is only about 12 feet tall, then expect it to go absolutely completely dry during summer.

Don’t blame the failure of a shallow check-dam on any one. Most of all, don’t blame it on the vagaries of nature, don’t blame it on a lack of enough rain-fall “last year.” Blame it squarely on your own ignorance, your own poor design choices.

If your check-dam is not deep enough so as to fully overcome the evaporation loss, and further hold some additional useful depth of water, then it is by design going to be completely useless, absolutely non-functional. It is going to be a pure waste of money.


So, this year even if you are planning to undertake only the maintenance of older structures, drop from your list all those structures which won’t have at least 20 feet deep water body when fully filled (or 25 feet tall walls).

Remember, a penny saved is a penny earned. The same money can be used for building check-dams at better geographical sites, or even doing away with the whole idea of building check-dams (if no suitable site exists nearby a given village, as often happens in the Marathwada region of Maharashtra) and instead going in for just a set of farm-ponds—of sufficiently deep water bodies.

Just throwing money at schemes—whether by government agencies, or NGOs, or even by private parties—is not going to help, if you don’t pay attention to even simplest technical points like the minimum depth of water body.


Foreign authors don’t always adequately highlight this factor of the evaporation loss, because is not very significant in their climates. But it is, to us, in India.


Bottom-line:

If you are in water conservation, remember:

In India, your enemy no. 1 is not a lack of enough rain-fall. It is not even the uneven or non-uniform pattern of the rain-fall, though these certainly are a matter of concern. But they are not your enemy no. 1.

In water resources engineering in India, your enemy no. 1 is: the evaporation loss.

And realize, no feasible technological solution has ever been found to counter it.

All that you can do is to just build farm-ponds or check-dams that are deep enough—that’s all. … Having deep enough water bodies is the most intelligent way of going about it.


I wish all of you ample water supply at least during the next summer—if you spend money intelligently, this summer.

My two cents.


Addendum: My past blog-posts dealing with the topic of water resources may be found here: [^]. In general, the posts which appeared earlier in the series are more technically oriented; the posts that appeared later have been more in the nature of topical repetitions. The post with a high technical content—and also a simplest Python script to estimate evaporation losses—was this one [^]. Also see the next one in the series, here [^].


A late thought: A good project for ME/MTech in water resources engineering:

Given a geographical area (such as a state, region, district, or otherwise, a region defined via watershed areas), estimate the extent of floods that occur every monsoon. Then, estimate the potential amount of storage possible, and the amount actually realized. Be realistic for the second estimate—include seepage and evaporation losses, as well as cost considerations. Develop methodologies for making estimates of all kinds (flooding, seepage and groundwater storage, total on-surface storage potential, the potential that is realized). In the end, consider whether the following statement is defensible: So long as news of floods keep flooding in, we cannot say that the root-cause of water scarcity is the lack of sufficient rains, or uneven (in time) and non-uniform (in space) patterns of rainfall.

 

 

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