Micro-level water-resources engineering—8: Measure that water evaporation! Right now!!

It’s past the middle of May—the hottest time of the year in India.

The day-time is still lengthening. And it will continue doing so well up to the summer solstice in the late June, though once monsoon arrives some time in the first half of June, the solar flux in this part of the world would get reduced due to the cloud cover, and so, any further lengthening of the day would not matter.

In the place where I these days live, the day-time temperature easily goes up to 42–44 deg. C. This high a temperature is, that way, not at all unusual for most parts of Maharashtra; sometimes Pune, which is supposed to be a city of a pretty temperate climate (mainly because of the nearby Sahyaadris), also registers the max. temperatures in the early 40s. But what makes the region where I currently live worse than Pune are these two factors: (i) the minimum temperature too stays as high as 30–32 deg. C here whereas in Pune it could easily be falling to 27–26 deg. C even during May, and (ii) the fall of the temperatures at night-time proceeds very gradually here. On a hot day, it can easily be as high as 38 deg C. even after the sunset, and even 36–37 deg. C right by the time it’s the mid-night; the drop below 35 deg. C occurs only for the 3–4 hours in the early morning, between 4 to 7 AM. In comparison, Pune is way cooler. The max. temperatures Pune registers may be similar, but the evening- and the night-time temperatures fall down much more rapidly there.

There is a lesson for the media here. Media obsesses over the max. temperature (and its record, etc.). That’s because the journos mostly are BAs. (LOL!) But anyone who has studied physics and calculus knows that it’s the integral of temperature with respect to time that really matters, because it is this quantity which scales with the total thermal energy transferred to a body. So, the usual experience common people report is correct. Despite similar max. temperatures, this place is hotter, much hotter than Pune.


And, speaking of my own personal constitution, I can handle a cold weather way better than I can handle—if at all I can handle—a hot weather. [Yes, in short, I’ve been in a bad shape for the past month or more. Lethargic. Lackadaisical. Enervated. You get the idea.]


But why is it that the temperature does not matter as much as the thermal energy does?

Consider a body, say a cube of metal. Think of some hypothetical apparatus that keeps this body at the same cool temperature at all times, say, at 20 deg. C.  Here, choose the target temperature to be lower than the minimum temperature in the day. Assume that the atmospheric temperature at two different places varies between the same limits, say, 42 to 30 deg. C. Since the target temperature is lower than the minimum ambient temperature, you would have to take heat out of the cube at all times.

The question is, at which of the two places the apparatus has to work harder. To answer that question, you have to calculate the total thermal energy that has be drained out of the cube over a single day. To answer this second question, you would need the data of not just the lower and upper limits of the temperature but also how it varies with time between two limits.


The humidity too is lower here as compared to in Pune (and, of course, in Mumbai). So, it feels comparatively much more drier. It only adds to the real feel of a real hot weather.

One does not realize it, but the existence of a prolonged high temperature makes the atmosphere here imperceptibly slowly but also absolutely insurmountably, dehydrating.

Unlike in Mumbai, one does not notice much perspiration here, and that’s because the air is so dry that any perspiration that does occur also dries up very fast. Shirts getting drenched by perspiration is not a very common sight here. Overall, desiccating would be the right word to describe this kind of an air.

So, yes, it’s bad, but you can always take precautions. Make sure to drink a couple of glasses of cool water (better still, fresh lemonade) before you step out—whether you are thirsty or not. And take an onion with you when you go out; if you begin to feel too much of heat, you can always crush the onion with hand and apply the juice onto the top of your head. [Addendum: A colleague just informed me that it’s even better to actually cut the onion and keep its cut portion touching to your body, say inside your shirt. He has spent summers in eastern Maharashtra, where temperatures can reach 47 deg. C. … Oh well!]

Also, eat a lot more onions than you normally do.

And, once you return home, make sure not to drink water immediately. Wait for 5–10 minutes. Otherwise, the body goes into a shock, and the ensuing transient spikes in your biological metabolism can, at times, even trigger the sun-stroke—which can even be fatal. A simple precaution helps avoid it.

For the same reason, take care to sit down in the shade of a tree for a few minutes before you eat that slice of water-melon. Water-melon is nothing but more than 95% water, thrown with a little sugar, some fiber, and a good measure of minerals. All in all, good for your body because even if the perspiration is imperceptible in the hot and dry regions, it is still occurring, and with it, the body is being drained of the necessary electrolytes and minerals. … Lemonades and water-melons supply the electrolytes and the minerals. People do take care not to drink lemonade in the Sun, but they don’t always take the same precaution for water-melon. Yet, precisely because a water-melon has so much water, you should take care not to expose your body to a shock. [And, oh, BTW, just in case you didn’t know already, the doctor-recommended alternative to Electral powder is: your humble lemonade! Works exactly equivalently!!]


Also, the very low levels of humidity also imply that in places like this, the desert-cooler is effective, very effective. The city shops are full of them. Some of these air-coolers sport a very bare-bones design. Nothing fancy like the Symphony Diet cooler (which I did buy last year in Pune!). The air-coolers locally made here can be as simple as just an open tray at the bottom to hold the water, a cube made of a coarse wire-mesh which is padded with the khus/wood sheathings curtain, and a robust fan operating [[very] noisily]. But it works wonderfully. And these local-made air-coolers also are very inexpensive. You can get one for just Rs. 2,500 or 3,000. I mean the ones which have a capacity to keep at least 3–4 people cool.(Branded coolers like the one I bought in Pune—and it does work even in Pune—often go above Rs. 10,000. [I bought that cooler last year because I didn’t have a job, thanks to the Mechanical Engineering Professors in the Savitribai Phule Pune University.])


That way, I also try to think of the better things this kind of an air brings. How the table salt stays so smoothly flowing, how the instant coffee powder or Bournvita never turns into a glue, how an opened packet of potato chips stays so crisp for days, how washed clothes dry up in no time…

Which, incidentally, brings me to the topic of this post.


The middle—or the second half—of May also is the most ideal time to conduct evaporation experiments.

If you are looking for a summer project, here is one: to determine the evaporation rate in your locality.

Take a couple of transparent plastic jars of uniform cross section. The evaporation rate is not very highly sensitive to the cross-sectional area, but it does help to take a vessel or a jar of sizeable diameter.

Affix a mm scale on the outside of each jar, say using cello-tape. Fill the plastic jars to some level almost to the full.

Keep one jar out in the open (exposed to the Sun), and another one, inside your home, in the shade. For the jar kept outside, make sure that birds don’t come and drink the water, thereby messing up with your measurements. For this purpose, you may surround the jar with an enclosure having a coarse mesh. The mesh must be coarse; else it will reduce the solar flux. The “reduction in the solar flux” is just a fancy [mechanical [thermal] engineering] term for saying that the mesh, if too fine, might cast too significant a shadow.

Take measurements of the heights of the water daily at a fixed time of the day, say at 6:00 PM. Conduct the experiment for a week or 10 days.

Then, plot a graph of the daily water level vs. the time elapsed, for each jar.

Realize, the rate of evaporation is measured in terms of the fall in the height, and not in terms of the volume of water lost. That’s because once the exposed area is bigger than some limit, the evaporation rate (the loss in height) is more or less independent of the cross-sectional area.

Now figure out:

Does the evaporation rate stay the same every day? If there is any significant departure from a straight-line graph, how do you explain it? Was there a measurement error? Was there an unusually strong wind on a certain day? a cloud cover?

Repeat the experiment next winter (around the new year), and determine the rate of evaporation at that time.

Later on, also make some calculations. If you are building a check-dam or a farm-pond, how much would be the evaporation loss over the five months from January to May-end? Is the height of your water storage system enough to make it practically useful? economically viable?


A Song I Like:

(Hindi) “mausam aayegaa, jaayegaa, pyaar sadaa muskuraayegaa…”
Music: Manas Mukherjee
Singers: Manna Dey and Asha Bhosale
Lyrics: Vithalbhai Patel

Micro-level water-resources engineering—7: Dealing with the [upcoming] summer

Last monsoon, we’ve mostly had excess rain-fall in most parts of Maharashtra, even over India, taken as a whole.

Though the weather in Maharashtra still is, for the most part, pleasantly cool, the autumn season this year (in India) is about to get over, right this month.

Therefore, right now, i.e. right at the beginning of February, is the perfect time to empirically check the water levels in all those check-dams/farm-ponds you have. … That’s because, evaporation is going to happen at an accelerating pace from now on…

Between end-October (say Diwali) and March (say Holi), every solar year in India, the reduction in the levels of the stored water is dominated by the following two factors:
(i) seepage (i.e. the part which occurs after the rains cease), and
(ii) usage (i.e. the irrigation for the “rabbi” (i.e. the winter agricultural) season).

But from now on, the dominant factor is going to be the third one, namely, (iii) evaporation, and it is going to be increasingly ever more important throughout the upcoming summer, i.e., until the arrival of the next monsoon.

As I had earlier pointed out in this series  [^][^], in Maharashtra, the losses due to evaporation are expected to be about 5–8 feet (or 1 to 1.5 “puruSh”) deep.

Don’t take my word for it. … Go out and actually check it out. (Take snap-shots for your own record, if you wish.)

The beginning of February is also the perfect time to start executing on your plans for any maintenance- or new construction-activities on any check-dams/farm-ponds/residential water conservation that you might have thought of, in your mind. If you start executing on it now, you still have a very realistic framework of about 4–4.5 months left, before the next monsoon rains are slated to arrive [give or take about a half month here or there].

…Just a reminder, that’s all.


Keep in touch, best, and bye for now…


[As usual, I may come back and edit this post a bit after its publication, say, after a couple of days or so… I don’t know why, but things like that—viz., thinking about what I did happen to write, always happen to me. But the editing wouldn’t be too much. … OK. … Bye [really] for now.]

 

Micro-level water-resources engineering—6: Evaporation

As compared to the last year, public awareness about water resources has certainly increased this year. It has been a second drought-year straight in a row. None can miss it—the water issue—now. [Not even the breweries.]

There are several NGO initiatives involved in the awareness campaigns, as always. Even celebrities, now. Also politicians.

The heartening part this year is that there also is now a much greater participation of the common people.

Indeed, water conservation schemes are these days receiving quite a broad-based support, cutting across all political party-lines. People are actively getting into the building nallah-bunds, farm-ponds, and all. Good.

Good? … This is India, so how can anything be so straight-forwardly good?

With that question mark, I began taking a second look at this entire scene. It all occurred to me during a show that I saw on TV last week or so.

Well, that way, I don’t watch TV much. At least in India, TV has gone beyond being a stupor- or passivity-inducing device; it has become an active noise generator. So, the most I can put up with is only some channel-flipping, once in a while. [In my case it is typically limited to less than 15 minutes at a time, less than 7 times a week]. In one such episode [of flipping through the channels], I happened to catch a few minutes of a chat that some Marathi journos were having with Aamir Khan and Satyajit Bhatkal. [They should have been in awe of Bhatkal, but instead were, of Aamir Khan. [Journos.]]

Both Khan and Bhatkal were being all earnest and also trying to be all reasonable on that show, and in that vein, at one point, Bhatkal mentioned that there have been hundreds (or thousands) of KT-weirs, nallah-bunds and all, which have been implemented by the successive Maharashtra State governments. These are the structures or works which now have become defunct because of a lack of maintenance. Mentioning this point, he then added something like the following: [not his precise words, but as my casual impression of what he effectively was saying]:

For the best or the most optimum utilization of the available money, it would be better to begin with a revival or maintenance (like silt-removal/wall-repairs) of these thousands of the already existing structures, rather than building everything anew, because the latter would cost even more money.

Looks like quite sensible an approach to take, doesn’t it?

Well, yes, on the face of it. But not so, once you begin to think like an engineer about it. In fact, I do want to raise one flag here—one very big, red flag. [No, I am not a communist, just in case you have begun reading this blog only now.]

Let’s look at some hard facts—and also some simplest physical principles—first.


The only primary source of water is: the rainfall.

The two means of conserving water are: (i) surface storage, and (ii) ground-water recharge.

The two big [physical] enemies of water conservation are: (i) run-off and (ii) evaporation.

Run-off means: Rain-water running off the earth’s surface as floods (may be as flash-floods), without getting intercepted or stored anywhere. Evaporation means: the loss of the stored water due to ambient heat.

It’s good that people have gotten aware about the first part—the runoff factor. The by-now popular Marathi slogan: “paaNee aDavaa, paaNee jirawaa” [English: “block water, percolate water”] refers to this first factor. Unfortunately, it has come to refer to only the first factor.

People must also become fully aware about the second factor—namely, evaporation. It too is just as important in India, particularly in places like Maharashtra.

Evaporation is not always an acute concern in the cooler climates (think USA, Canada, Europe, Japan, Australia, New Zealand). But it is, in the hotter climates (think most of the third world). My focus is exclusively on India, mostly on Maharashtra. Since most of the advanced countries happen to lie in the cooler regions, and since in India we habitually borrow our engineering common-sense from the advanced countries rather than developing it individually here, I want to once again stress this point in this series.


As I mentioned in my last post in this series [^]:

“Evaporation is a really bad factor in hot climates like India. At the level of large-scale dams and even for check dams, there is precious little that can be done about it.”

There is a technological reason behind it: You can’t sprinkle some powder or so to cover the surface of a water body, and thereby arrest or slow down the evaporation losses, without also polluting water body in the process.

These days, you often see a layer of water hyacinth in dams/rivers. Thought the plant contiguously covers the water body, contrary to the naive expectation, it in fact accelerates evaporation. The plant sucks water from below and perspires it out via leaves. This rate of perspiration happens to be higher than that of the plain evaporation. Further, water hyacinth has big leaves. The total surface area of the leaves is many times greater than the area of the water body that the plant covers.

But, yes, the simple-minded idea is right, in a way. If instead of the water-sucking water-hyacinth, something else—something chemically inert and opaque—were to cover the water body, then it would cut down on the evaporation losses. People have tried finding such a material, but without success. Any suggested solutions are either not scalable, not economical, or both. That’s why, evaporation is a fact that we must simply learn to live with.


Let me continue quoting from my aforementioned post:

“Evaporation maps for Maharashtra show losses as high as 1.5 m to even 2.5 m per year. Thus, if you build a check-dam with a 3 m high wall, expect to lose more than half of the [stored] water to evaporation alone.

For the same reason of evaporation, most nallah-bunding and contour-trenching works [such as] those typically undertaken under the socialist programs like MNREGA don’t translate to anything at all for storage, or for that matter, even for seepage. Typically, the bunds are less than 1 m tall, and theoretically, water in them is expected to plain evaporate out right before December. Practically, that anyway is the observation! […] It is a waste of money and effort.”

That’s what I had said, about a year ago. It needs to be repeated.

Most people currently enthusiastic about water conservation simply don’t seem to have any appreciation as to how huge (and how hugely relevant) this factor of evaporation is. Hence this post.


To repeat: In Maharashtra, the range of evaporation losses is as high as 1.5–2.5 m. That is, about 5–8 feet, in terms of the height of water lost.

Thus, if you build or repair a nullah-bund that is about 10 feet tall (which is the typical height of a house), then you should expect to lose about 75% of the stored water to evaporation alone. Perhaps even 90% or more. After all, nullahs and rivers typically have a progressively smaller width as we go deeper, and so, the volume of the water body remaining at the bottom after evaporation is even smaller than what a simple height-based calculation tells you.

Coming back to the Khans and Bhatkals, and Patekars and Anaspures: If the small check-dam or Kolhapur-type of bund/weir you are repairing this summer is, say, 7–8 feet high, then what you should expect to see in the next March or April is: a dry river-bed with a few puddles of water perhaps still lingering here and there. Picture a stray dog trying to satisfy his thirst from a puddle that is relatively cleaner from among them, but with a vast patch of a darkish brown, rocky or parched land filling the rest of your visual field. In no case should you picture a large body of clean water extending a couple of kilometers or more upstream of the bund. The fallen rain-water would have got blocked by that bund, sure, but if your bund is only 7–8 feet tall, then all of it would have disappeared [literally] in the thin air through evaporation alone, by the time the summer arrives. [We are not even counting seepage here. And realize, not all seepage goes towards meaningful groundwater recharge. More on it, may be, later.]

Now, the fact of the matter is, many, many KT weirs and bunds, as built in Maharashtra, are hardly even 5–6 feet tall. (Some are as low as just 3–4 feet tall.) They are, thus, not even one (Marathi/Sanskrit word) “puruSh” deep. …

The next time you go for an outing, keep an eye for the bunds. For instance, if you are in Pune, take an excursion in the nearby Purandar taluka, and check out the series of the bunds built by the PWD/Irrigation department on the Neera river. Most of them are just 3–5 feet tall. None is as big as a “puruSh” tall. None ever shows any water left after December. [But don’t therefore go and talk to the PWD/Irrigation engineers about it. These engineers are smart. They will tell you that those are flood-control structures, not water-storage structures. You will thus come back non-plussed. You are warned.]

… In case you didn’t know what “puruSh” means: Well, it’s a traditionally used unit of depth/height in India. It is defined as the uppermost reach of a man when he stands upright and stretches his arms up. Thus, one “puruSh” is about 7–8 feet. Typically, in earlier times, the unit would be used for measuring the depth of a well. [During my childhood, I would often hear people using it. People in the rural areas still continue using it.]

So keep the following capsule in mind.

In most parts of Maharashtra, expect the evaporation losses to be about one “puruSh” deep.

If the water-body at a nallah-bund/check-dam/farm-pond is one “puruSh” deep during the monsoon, then expect its water body to completely dry up by the time the summer arrives the next year.

Therefore, an urgent word of advice:

If you are building farm-ponds or undertaking repairs of any bunds or KT weirs structures this year, then drop from your planning all those sites whose walls are not at least 2.0 “puruSh” tall. [If a wall is 2.0 purush tall, the water body will be about 1.5 purush deep.] Evaporation losses will make sure that your social-work/activity would be a complete waste of money. The successive governments—not just politicians but also social workers, planners, bureaucrats and engineers—have already wasted money on them. Let the wastage stop at least now. Focus from now on only on the viable sites—the sites where the depth of the water-body would be at least 12–15 feet or so.

If the nullah is not naturally deep, and if the local soil type is right, then you may think of deepening it (to a sufficient minimum depth), perhaps with machinery and all.

But in any case, keep the factor of evaporation in mind.


As pointed out in my earlier posts in this series, given the geological type of the top layers in most parts of Maharashtra, seepage is not a favorable option for water conservation planning.

The only exception is the patch that runs across Dhule, Jalgaon through Wardha, Nagpur. There, the top-layer is sufficiently sandy (as in Rajasthan.) Mr. Suresh Khanapurkar has done a lot of seepage-related work in this patch, and groundwater recharge indeed is a viable option there.

But remember: seepage is not viable for most of the remaining parts of Maharashtra (and in fact, it also is not, over very large patches of India). So, if your idea is to build shallower bunds with the expectation that it would help improve groundwater levels via seepage during and soon after monsoon (i.e., before evaporation kicks in the months following the monsoon), then that idea is not so much on the target, as far as Maharashtra is concerned. Engineering for seepage can be viable only if the local geology favors it.

For the general-purpose water conservation, in most parts of Maharashtra, we have to look for storage, not seepage. Therefore, evaporation becomes a more important factor. So, avoid all shallower sites.

In particular, when it comes to farm-ponds, don’t build the shallower ones even if government gives you subsidy for building them (including for the blue plastic sheet which they use in the farm-ponds to prevent the wasteful seepage). If your pond is shallow, it would once again be a waste of money, pure and simple. Evaporation would make sure of that.

That’s all for now, folks.


Yes, I have been repetitive. I don’t mind. I want to be repetitive, until the time that social workers and engineers begin to show a better understanding of the engineering issues involved in water conservation, esp. the factor of evaporation. Currently, an appreciation of this factor seems to be non-existent.


My blogging in the upcoming weeks will be sparser, because I have to re-write my CFD course notes and research related notes, simulation programs, etc. I lost them all during my last HDD crash. I want to complete that part first. So excuse me even if I don’t come back for some 3–4 weeks or more for now. I will try to post a brief note or two even if not a blog post, but no promises. [And, yes, I have now begun my weekly backups, and am strictly following the policy—the notifications from the operating system.]

Bye for now.


[May be one more editing pass, later today or tomorrow… Done.]

[E&OE]

Micro-level water-resources engineering—5

Important Update added on 2015.04.22!

See near the end of this post.

* * * * *   * * * * *   * * * * *

Today is (Sanskrit) “akshay tritiya.” In the Khandesh area of North Maharashtra (in which the Shirpur town of the famous Shirpur pattern falls), in the local lingo, the day is known as “aakhaaji.” In Khandesh, it is a festival of women visiting their (Marathi) “maaher” (i.e. their parents’ home, or the home before marriage). There are traditional songs in the local “AhiraNi” dialect, and dance of the Gujarathi garbaa style, but played only by women. When I was a school-boy in that region, the dance would be done with “Tipri”s, but as far as I remember, without any dhols (i.e. drums), loud-speakers, or any large-scale organization, even if all the streets of the town would overflow with women playing “Tipri”s. I don’t know what the situation is like today.

Festivals mark days of relief. But otherwise, summer is a time of gruelling hard work for rural women. Maharashtra’s population is already 11.4 crores. Even if you take only 20% population as drought-affected, that makes it about 2 crores. (In the 2013 drought, the estimates were about 3 crores.) That means about 1 crore drought-affected women. So, as a rough estimate, there must be at least about 50–75 lakh women facing water-scarcity in a normal year—e.g., right now! Imagine, some tens of lakhs of women having to daily carry pots on their heads for several kilometers a day, just for fetching water—often only poor quality and soily/brackish water, but something that allows them at least cooking food for their families for their barest sustenance. What can we do to spare them the hardship? Obvious.

Find out cost-effective water resources strategies and solutions that can be within their means, without remaining dependent either on government or even on charity.

* * * * *   * * * * *   * * * * *

I have been browsing a lot of material, but have found that in this area, even some of the simplest questions are so hard to answer.

For instance, the data about the local geology. Or, even the data about local morphology—I mean, maps with contour lines having 1, 2, or even 5 m resolution, and not 20 or 30 m resolution. Digital DEMs with say 5 m resolution are impossible to find, and so, some creative solutions have to be found out. Here, I first thought of an idea, and later on found a part of it mentioned in a Marathi official document for irrigation department that my father gave me. I will write about it, and address many such questions some time later in this series.

In this post, instead, let me touch upon another simple question.

* * * * *   * * * * *   * * * * *

How much of the fallen rainwater really runs off the ground to the rivers? Why don’t some of the dams fill up every monsoon?

In Maharashtra, Marathwada is the region of the lowest rainfall (about 65 cm/annum). The river Godavari flows through this region. There is a big dam called the Jayakwadi project [^]; the reservoir is called “Naath Saagar;” it is named after the great Marathi poet-saint Eknath Maharaj.

The simple question is this: Why does the Naath Saagar reservoir does not fill up to its full capacity every rainy season?

An easy answer would be to say: “Because the rainfall is deficient.”

Let us see whether, starting from some simple basic assumptions, this answer turns out to make any sense or not. Let’s try to do a quick, back-of-the-envelop estimate for how much water should flow into the Jayakwadi dam every year. (My objective behind working out this simple exercise was to get some kind of a datum for the small check dams.)

The basic source of water is the rainfall. To estimate the total water received via rainfall, we have to know the watershed area of the dam. The Wiki page on the Jayakwadi dam [^] notes the catchment area as 21,750 square km. Referring to the rainfall for the upstream catchment area of this dam [I used the map in the book River Basins of India (p. 66)], I estimate the average rainfall for this region as about 65 cm (which also is the figure quoted by P. R. Pisharoty as noted in an earlier post in this series). So, the total annual water received via rainfall in the Jayakawadi watershed is about 1.41375E10 cubic m. Raghunath’s text on hydrology [^] on page 5 notes that for India’s total annual rainfall water of 370 million ha-m, the total runoff into all the rivers is 167 million ha-m. Thus, the runoff to the biggest river in a basin should be about 45% of the total water received from the skies. Assuming that a similar figure applies here, I get the runoff calculations as the following Python code shows:

# Assumed data
dRainfall = 0.65 # meter, assumed
dRunoffCoefficient = 0.45 # for all India
dCatchmentArea = 21750.0*1000*1000 # meters
# Calculations
dRainwaterVolume = dRainfall*dCatchmentArea
dEvaporationLoss = dRainwaterVolume/3.0
dRunoff = dRunoffCoefficient*dRainwaterVolume
dSeepageIntoSubsoil = dRainwaterVolume - dEvaporationLoss - dRunoff
print ("Total Volume: %E, Runoff: %E, Evaporation Loss: %E Seepage: %E" % (dRainwaterVolume, dRunoff, dEvaporationLoss, dSeepageIntoSubsoil))

On Ubuntu, open a terminal, type “python” in it, and at the Python prompt (say >>>) copy-paste the above lines, and hit enter. (On Windows, you will have to install a suitable Python environment first.) Or, use the online interactive Python terminal here [^].

Thus, the total quantity of water rushing into the Jayakawadi dam every year should be 6.36E9 m^3 (i.e. cubic meters). The Wiki page on the Jayakwadi dam notes that the total capacity of the dam is just 2.91E9 m^3.

Thus, the total quantity of water flowing into the dam should be about 2.1 times the dam reservoir capacity. The dam should more than overflow every year.

Yet, the dam doesn’t even fully fill up every year—it does so only about 2–3 times in a decade.

Even if we take a lower runoff coefficient, say as low as 0.35 (e.g., to factor in the presence of the relatively smaller dams upstream, and also an increased forest cover—which must be a very unrealistic assumption), and even if we take the annual rainfall in the watershed region to be as drastically low as just 40 cm (i.e. the worst drought situation, because they declare a drought if the rainfall is 20% lower [^], and the average for the catchment area of Jayakawadi is above 65 cm), you should still get some 3.045E9 m^3 of water into the Jayakawadi reservoir—more than enough to fill it up fully.

Indeed, there is a further irony. This dam has already gathered a lot of silt (because it is situated in a relatively flatter region, with more loose soil), and the live storage has gone down by 14% (as per Wiki).

[Incidentally, there has been some criticism that the project was moved 100 km upstream. Chances are, the reservoir then would have covered even a flatter area of loose topsoil.]

All of which means that the Naath Saagar reservoir should fill up and overflow even in the worst drought-hit years.

In practice, it doesn’t.

What gives? Any idea? I have no clue.

There must be something the wrong with the way the run-off calculations are performed—they are going off the mark by some 250%. That is too big an error. Has anyone looked into this aspect more carefully?

Exercise: Undertake the same exercise for the Ujjani dam. It is supposed to supply water to another severely drought-prone region in Maharashtra, viz. that of the Solapur district and the nearby areas. It too doesn’t fill up every year.

Realize that big dams like Jayakwadi and Ujjani have comparatively huge catchment areas. For a check dam, the catchment area (or the watershed area) is very small—and therefore, subject to even wider fluctuations from year to year. This is one sobering point we must keep in mind in our enthusiasm for the micro-level projects.

* * * * *   * * * * *   * * * * *

The Evaporation Loss, the High Intensity Rains, and Seepage vs. Storage:

As Raghunath’s book [^] shows on page 5, the water balance equation is:

total rainfall = evaporation loss + runoff + seepage into sub-soil.

The item of seepage into subsoil is further split into two parts: (i) a slightly bigger part (53%) of the contribution to moisture (which is the part that is used by the shrubs and the trees in their life-processes; this is the part that is ultimately released to the atmosphere via transpiration), and (ii) a smaller part (47%) of the recharge into groundwater.

Thus, the biggest items here are (1) runoff and (2) evaporation.

This primary importance of the runoff quantity in the overall quantitative scheme was the reason why I decided to check whether the simple runoff factors or calculations are realistic or not. If they were to be OK, I could use them as they are, in my detailed calculations concerning computational watershed modelling.  But as the above example of the Jayakwadi dam showed, at least at a gross scale, the runoff factors are not reliable. Now, even if there are some more detailed (say GIS based) models and micro-models for computing runoffs, I am not sure how reliable they would be. And, if the runoff factor itself is wrong, we are basically regressing back to the stage of empirical data collecting and validation, and so, coarsening out of the micro-models to macro-models (even if using GIS) would be an even more distant a step … Anyway, to proceed further….

For India, the runoff figure is several times higher than the total groundwater recharge.

Further, in the Deccan trap basalt region of Maharashtra, the seepage mechanism is not as efficient as it is in the alluvial soils (for instance, of Aravali region of Rajasthan, or in the Tapi and Purna basins of Maharashtra (the area of the Shirpur pattern)).

Recall Pisharoty’s paper I quoted in my last post in this series [^]. In Maharashtra, half of the annual rainfall occurs within only 15 to 20 high-intensity hours, which occur sometime over only 35–45 days of any actual rainfall.

Thus, in the Deccan trap region of Maharashtra, first there is an overall inefficiency of seepage. It is further compounded worse due to the infrequent high intensity bursts of rains. If the same amount of rainfall were to occur as a slow drizzle over a long period of time, say a continuous stretch of weeks, then such a rainfall pattern would be better conducive to seepage. On the other hand, a sporadic high intensity pattern implies less seepage and more runoff.

Hence, in the Deccan trap regions of Maharashtra, it is the surface storage strategy which is likely to prove better than the underground seepage strategy.

For the above reasons, our solutions should be able to handle arresting the high intensity runoff, for storage.

Next, in India in general, evaporation also is several times higher than the groundwater recharge.

Evaporation is a really bad factor in hot climates like India. At the level of large-scale dams and even for check dams, there is precious little that can be done about it. (Solutions have been sought, e.g., spreading some chemicals on top of the water, but their side effects is a worry.)

Realize that evaporation is a surface phenomenon. Dams with a shallow and wide-spread reservoir (e.g. Jayakwadi) tend to have a relatively higher percentage of evaporation losses, as compared to the deeper dams. Ditto, also for check dams. (That is the reason why the nallah-deepening aspect of the Shirpur pattern makes good sense—provided the cost of excavation is low enough. For the time being, let’s focus on evaporation.)

Evaporation maps for Maharashtra show losses as high as 1.5 m to even 2.5 m per year. Thus, if you build a check-dam with a 3 m high wall, expect to lose more than half of the water to evaporation alone. (The famous bund of the bund-garden in the Pune city used to be actually shallow; it would hold a nice expanse of water simply because the bigger Khadakvasla dam upstream would periodically release water into the river.)

For the same reason of evaporation, most nallah-bunding and contour-trenching works, such as those by Anna Hazaare in Ralegan Siddhi, or those typically undertaken under the socialist programs like MNREGA, don’t translate to anything at all for storage, or for that matter even seepage. Typically, the bunds are less than 1 m tall, and theoretically, water in them is expected to plain evaporate out right before December. Practically, that anyway is the observation! No matter what Anna Hazare might tell you, it is a waste of money and effort.

* * * * *   * * * * *   * * * * *

Farm Ponds:

Before closing, let me mention farm ponds as an effective storage strategy, apart from check-dams.

One big advantage with check dams as compared to conventional dams is that there is no cost incurred in the acquisition of land, and for rehabilitation of the displaced people. Speaking purely costs-wise, I read somewhere (or heard in Suresh Khanapurkar’s interviews) that about half of the cost of the conventional dams is on just these two counts.

However, in the case of check dams, if the natural depth of the river gorge is not sufficient (try the small software I wrote earlier in this series), then in order to reduce the percentage loss due to evaporation and thus to make the project economical, some extra expenditure may become necessary in deepening the nallahs.

Now, in the Deccan trap region of Maharashtra, since small rivers and nallahs tend to run through regions of hard rocks (the top soil layer can be as thin as just 0.5 to 1 m or even zero, in them), the excavation work to deepen the nallahs can easily prove to be too costly—this is a factor going against the Shirpur pattern. (More on the economics of excavation in check dams, and of farm ponds, in a later post.)

An advantage with the farm ponds, when compared to check dams, is that since they are built in the field, i.e. in a deeper layer of soft soil, deepening the ditch turns out to be much more economical.

On the downside, as compared to check dams, the storage capacity of farm ponds is much smaller. They also eat up what otherwise could have been a productive farm land.

But, as we saw, a deeper water body means lower percentage of evaporation losses.

Thus, overall, costs-wise, there are many oppositely directed factors, and it’s not possible to draw general conclusions. It’s the cost balance that really determines whether a farm pond makes sense in a given location or not, or is it a check dam for storage, or a check dam for seepage. (I will write another post covering the economics of check dams vs. farm ponds vs. conventional dams).

Second, as far as evaporation is concerned, there is an incredibly creative solution which I ran into only today. It suggests that if you cover a farm pond with a floating layer of the used plastic bottles, then the evaporation loss (at least for very small experimental ponds) can be cut by up to 40%! [(1.7 MB .PDF file) ^]. The work has been done at the well-known Vigyaan Aashram at PaabaL near Pune; guess they also have some kind of collaboration with COEP and MIT (USA). Anyway, coming back to evaporation losses, this is a huge, huge advantage at a throw-away price! The suggestion right now is only at a preliminary stage. I think it should be seriously taken up for studies on a larger scale of a realistic pond. But yes, as an engineer, I simply marvel at this idea—it again takes a perceived problem (“Gee, even Indians have started buying bottled water, huh?” and “Now how do we deal with this mess of all this plastic waste!”), turns it around on its head, and provides a cost-effective solution to another pressing problem. Neat!

BTW, the farm ponds need not always get only“naturally” filled, i.e., with the surface-running rain-water falling from the sky on the same field. Sometimes a combination of a lift-irrigation scheme + a farm pond can also be cost-effective.

Further, as has been practically demonstrated at many sites (e.g. in the Ahmednagar district, by actually practising farmers) a farm pond can also be very easily used for farming fish, as a side business (read side income)! Farming for fish requires relatively little labour—mostly, only some aeration (if at all required) and throwing food into the pond regularly (like twice/thrice a week or so), that’s all. Fish is not only very tasty food, it also is a very high quality and easily digested protein.

The side-walls of the farm-ponds can be also be planted with the kind of trees that give lush green shadow while making do with less water demand. A welcome sight, and a welcome spot for rest, on a hot summer afternoon.

Finally, talking of water bodies, trees, and how beautiful and (literally) cool a surroundings they can go on to make, and of side-businesses, and of creativity, here is yet another creative side-business that has come out of a bigger farm pond; check out the Saguna Baug in Neral near Mumbai [^].

* * * * *   * * * * *   * * * * *

Important Update added on 2015.04.22:

Python scripts for predicting the extent to which the Jayakwadi and Ujjani dams should fill, at a given rainfall level. Also, analysis of results, and some comments:

Since writing this post yesterday, I studied more closely the topic of the rainfall–runoff relationships in standard textbooks on hydrology.

There are quite a few models that allow you to calculate the expected runoff volume from the rainfall extent. These are specific to soil types, gradients, type of surface, etc. For Maharashtra, there is a well-known model by Inglis and De Souza (1929); e.g. see p. 180 here [^]. This is an empirical (curve-fitted) model that gives you two separate equations.

For the ghat region:
R = 0.85P - 30.5

and for the Deccan plateau:
R = \dfrac{1}{254} P \left( P - 17.8\right)

where R is the annual runoff and P is the annual rainfall, both in cm.

Since the formulae seemed to give far lower values for the runoff coefficient, and hence the runoff volume even for the average rains, I decided to write a Python script to find out the extent to which the Jayakwadi (and later, also Ujjani) dam would fill up, given different levels of rainfall.

Here is a link to a zip containing the Python script files and their outputs, in the CSV format [^]. In my code, I calculate the R parameter using both the above equations and take their average. Effectively, I assume that half of the watershed region is of the ghat region, and the other half is of the Deccan plateau. Though the zip file doesn’t show it, there aren’t very glaring differences in the values of R as estimated via the two equations.

The files make it clear that at the average rainfall level of 65 cm in the watershed (i.e. the catchment) region, the Jayakwadi dam is expected to fill only to 89.6%. If there is a drought year and so the watershed region receives some 45 cm rains, this dam would fill only to 21%!! For this dam to fill 100%, an above average rainfall of 67–68 cm would be necessary. Little wonder that the dam has filled only rarely.

The figures for the Ujjani dam are not much different, only slightly different. For instance, it would take 77–78 cm rain in its catchment area for it to fill up. The catchment area of the Ujjani dam does receive a somewhat higher rain, and so, let’s say the normal rainfall there is 70 cm. At this level, the Ujjani dam is expected to fill only to 73% of its full capacity!

There are other methods to estimate runoff too. But given the informal general knowledge about these dams, Inglis and De Souza’s correlations would seem to hold well.

I am no expert in dams design. To my lay engineer’s eyes, these are decidedly wasteful designs. After all, bigger-than-necessary designs imply greater-than-necessary expenditures, too. The Wiki page informs us that Jayakwadi project has cost more than Rs. 10,000 crores by now. … What portion of this huge amount are wasteful?

Among the Maharashtrian “intellectual,” “chattering” etc. classes  (and also among the NRI community, esp. that settled in the USA, esp. in California), it has become a fashion to blame politicians for every conceivable ill concerning water scarcity in Maharashtra.

Do you think that these civil engineering designs were done by politicians? Do you think that, for instance, say Vasantrao Naik, would have knowingly ordered a deliberately bigger and costlier dam, just to score some political high point for himself or his party? (I mention him because the Wiki informs us that the Jayakwadi dam design was finalized when he was the CM. Similarly, the Ujjani dam got designed when he was the CM.)

If you think such things are possible—things like increasing a dam size just to score some political high point for oneself or one’s party—then I would say that you just don’t know the way a typical politician thinks and operates. Yes, even in a mixed economy.

Yes, it’s important for a typical politician in a mixed economy (regardless of the party to which he belongs) to show that he is doing something meaningful, whether something actually meaningful gets done or not; yes, it is possible that given a choice, he would always pick up that choice which benefits his own constituency; yes, it is possible that he might even bring a bit of pressure on the officers to tweak solutions so that his constituency benefits. (They even publicly admit if not boast about such things.)

But a deliberate increase of size of a dam for absolutely no conceivable benefit to anyone? No way. No politician even in a mixed economy—and esp. in reference to a country like India—ever operates that way.

Here, if you say that an increased dam size means an increased budget, and therefore greater bribes to him/his party-men, then I would say, you just don’t understand the system. Even if I grant you the premise that every politician is always on principle on the look-out for kickbacks and bribes, granted that, a typical politician still wouldn’t want to increase a dam’s size, because—and get this right—he doesn’t have to. He can easily use the same amount of the additional (wasteful) budget on some other project without ever affecting the total quantum of his bribe, so why should he insist on making just one dam/project bigger than necessary, at the expense of all other possible dams/projects? If he can build two dams in the same budget—note, the amount of bribe has stayed the same—since he stands to get double the publicity with the same budget-money, he would rather go in for that.

Thus, all this “logic” is simply the smart “white-collared” Indian’s way of saving the “behind” of his own—or of groups of people he regards similar to him—nothing else.

Are bureaucrats and even engineers ever going to admit there might have been some mistakes here? More importantly, how many Indians of the “intellectual” kind are going to be willing to even think of such a possibility? (Even Anna Hazaare’s all-water-vaporizing nallah-bunds seem almost innocuous by comparison; they must have involved relatively smaller amounts, say of a mere few hundreds of crores. In contrast, wasteful big projects like these would involve thousands of crores.)

And, even more importantly, is there anyone willing to have an honest second look about the socialistic nature of the system that produces costly errors of this kind? (Not just costs. Since a wrong datum for the capacity of the dam has been established, now there also are fights: Marathwada people think that if the Jayakwadi dam doesn’t fill up fully, the reason must be that the smaller dams upstream have been criminally diverting water that is rightfully theirs… Think of the bigger and bigger mess it all gets into.)

…Anyway, since I can’t do anything about it, let me wind down on that topic, and instead, let me focus on what I can do.

What this exercise means is that I can use some of these even simple text-book methods to build computational models for the check dams/farm ponds with acceptable enough accuracy; they would be accurate at least to a first-order. (After all, Inglis and De Souza’s second equation shows that a quadratic dependence of the runoff on the rainfall. Also, the other rainfall-runoff models show different kinds of nonlinear relations. So, they should be accurate at least to the first order in further usage.)

One more point before we close. Let me note a couple of good links on the topic of farm ponds. Here they are:

“Farm Ponds: A Climate Resilient Technology for Rainfed Agriculture: Planning, Design and Construction” [(13.1 MB PDF) ^]

“Rainwater Harvesting and Reuse through Farm Ponds: Experiences, Issues and Strategies” [(5.4 MB PDF) ^]

Also, a useful reference on the topic of evaporation: “Potential Evapotranspiration Estimation for Indian Conditions: Improving accuracy through calibration coefficients” [(3.8 MB PDF) ^]

* * * * *   * * * * *   * * * * *

A Song I Like:

[I recently randomly stumbled on a listing of this song, and then listened to it after a long, long time—long enough that reading only through the words, I was not able to place the song; I had to listen to it to “get” it back…  A relaxed tempo, and a beautiful melody by SD. … I don’t know if RD assisted SD for this song or not (this one is from 1963), but going by the orchestration, there is at least a hint of the young RD here, esp. in those interludes of the sax and guitar. I doubt if Dada Burman, in 1963, could have given that much of a freedom to Manohari Singh (an assistant and the probable sax player) sans RD’s presence. The tune, of course, is unmistakably SD’s own. … It’s the kind of a tune which you inadvertently catch yourself humming aloud sometime later, perhaps even a few days later, and it still surprises you, and it still makes you want to re-listen to the song…  Shailendra is at his lyrical best… And yes, it’s Suman Kalyanpur, not Lata.  Enjoy the brilliant simplicity of SD (possibly with a small assistance coming from RD)…]

(Hindi) “ye kis ne geet chheDaa…”
Music: S. D. Burman
Singers: Mukesh, Suman Kalyanpur
Lyrics: Shailendra

 

[PS: Though the content will not change much, tomorrow, I might come back and add just a few links to some good documents on design/experience/economics of farm ponds that I have downloaded. Done. Also added Python scripts for computing the percentage of dam capacity to which the Jayakwadi and Ujjani dams would fill up, at various levels of rainfall in their respective catchment areas.]