0:00:02.381,0:00:05.382
Okay now that you know a little bit about[br]ground water systems.

0:00:05.382,0:00:07.430
Some of the vocabulary associated with[br]them.

0:00:07.430,0:00:10.154
Let's talk about groundwater flow.

0:00:10.154,0:00:14.291
Groundwater is usually not stagnant,[br]okay, it's usually moving.

0:00:14.291,0:00:17.395
It isn't moving fast like a stream but it [br]is usually moving,

0:00:17.395,0:00:21.716
and typical flow rates would be on the [br]order of about a half a meter per day.

0:00:21.716,0:00:26.388
It flows away from areas where it enters[br]aquifers.

0:00:26.388,0:00:31.743
Two places where water exits aquifers.

0:00:31.743,0:00:37.879
We call places where water enters an aquifer[br]recharge areas and places where water

0:00:37.879,0:00:41.883
exits aquifers are referred to [br]as discharge areas.

0:00:41.883,0:00:46.764
So you can say that water flows from[br]recharge areas to discharge areas,

0:00:46.991,0:00:49.099
and that's what's shown on the slide.

0:00:49.393,0:00:55.530
The blue lines depict groundwater flow[br]paths and you can see that some of these

0:00:55.530,0:00:57.404
flow paths are fairly short.

0:00:57.404,0:01:02.809
The shortest ones, groundwater can flow[br]along in a matter of days.

0:01:02.809,0:01:09.247
Others, however, are quite long, uh, [br]water can be isolated from the, um,

0:01:09.247,0:01:13.402
from the surface and aquitards and[br]aquifers for thousands of years,

0:01:13.402,0:01:15.253
or even more.

0:01:15.253,0:01:20.508
Some research, um, recently identified [br]some groundwater that was at least as old

0:01:20.508,0:01:28.484
as 1.5 billion years old, okay, and been[br]isolated in the subsurface for that long,

0:01:28.484,0:01:31.037
which is incredible.

0:01:31.037,0:01:34.139
Okay so that-that would definitely be[br]exceptional.

0:01:34.139,0:01:40.546
So what controls the movement of rocks and[br]sediment, well the equation that we use to

0:01:40.546,0:01:45.719
describe the flow of water through coarse[br]materials like sand was, uh, defined by this

0:01:45.719,0:01:48.037
guy, Henry Darcy.

0:01:48.037,0:01:53.209
And the equation that he used, or that he[br]defined, is known as Darcy's Law.

0:01:53.209,0:01:56.246
So at some point he must have asked a [br]question, what the heck controls the

0:01:56.246,0:01:59.298
movement of water through sand?

0:01:59.298,0:02:03.752
And I imagine that we've all asked that[br]question time or two.

0:02:03.752,0:02:07.856
Although, he probably would've said it[br]in French, cause he was, um, he was

0:02:07.856,0:02:13.322
actually a French engineer, uh, just to[br]give you a little historical context on

0:02:13.481,0:02:15.056
this topic.

0:02:15.165,0:02:18.837
Uh, he was an engineer and he lived 'bout[br]the same period of time as

0:02:18.837,0:02:20.720
Abraham Lincoln.

0:02:20.720,0:02:24.158
During his life he was very famous for[br]bringing a water distribution system to

0:02:24.158,0:02:28.111
Dijon in 1840.

0:02:28.111,0:02:31.799
Okay this was a big deal because very[br]few places at the time had water

0:02:31.799,0:02:33.318
distribution systems.

0:02:33.318,0:02:38.389
Wasn't, it was also at this time that few[br]places had sewer systems, okay.

0:02:38.389,0:02:42.542
So if you were living in a city, a good [br]reliable source of clean water was

0:02:42.542,0:02:44.628
really important.

0:02:44.628,0:02:49.534
Um, for reference, Paris didn't have,[br]oops, Paris didn't have a water

0:02:49.534,0:02:55.690
distribution system until 1865, a full[br]20 years after the little city of Dijon

0:02:55.690,0:03:00.178
um, got it's water distribution system.

0:03:00.178,0:03:04.298
It wasn't until very late into his life, 2[br]years before he died, that he carried

0:03:04.298,0:03:08.669
out the experiments that perhaps he is[br]best known for today.

0:03:08.669,0:03:13.191
He performed the experiments in hospital,[br]that might seem like, y'know a really

0:03:13.191,0:03:17.345
odd choice, at the time there probably[br]weren't as many places where, uh, that

0:03:17.345,0:03:21.081
were, y'know, convenient for setting[br]up experiments.

0:03:21.081,0:03:27.557
The experiment that he performed, uh, he[br]published results in his report in 1856,

0:03:27.557,0:03:32.061
and uh, they were just described in a[br]couple of pages in the very back of the

0:03:32.061,0:03:38.205
report, I think one of the appendices [br]for the, um, for the report.

0:03:38.205,0:03:41.493
Uh, so it was very much just an [br]afterthought, but it turned out to be a

0:03:41.493,0:03:45.111
very, uh, a really useful important piece [br]of work.

0:03:45.111,0:03:49.765
So what we're gonna do now is walk through[br]some aspects of Darcy's experiment.

0:03:49.765,0:03:53.757
This will help illustrate some basic controls on[br]the movement of water through porous

0:03:53.757,0:03:57.594
medium, and also the process of science,[br]how you can apply some basic reasoning,

0:03:57.594,0:04:03.016
collect data, and then figure out[br]relationships.

0:04:03.016,0:04:08.821
So what Darcy did was, uh, set up a tube[br]that contained some sand, and that

0:04:08.821,0:04:13.827
tube had some smaller tubes in it called[br]manometers poking into the, near the in

0:04:13.827,0:04:16.146
flow and out flow end,

0:04:16.146,0:04:22.161
and he'd float water through the sand tube[br]and, uh, observed how different variables

0:04:22.161,0:04:24.889
affected flow.

0:04:26.249,0:04:31.955
Hydraulic head is the name given to the[br]height at water rises and the manometers

0:04:31.955,0:04:35.726
relative to some arbitrary data.

0:04:35.726,0:04:39.845
In natural environments we would typically[br]pick sea level to be the datum.

0:04:39.845,0:04:43.366
In a lab experiment you might just pick[br]the bench top where you're carrying out

0:04:43.366,0:04:46.752
the experiment, because it's more[br]convenient.

0:04:46.752,0:04:52.707
Hydraulic head is the sum of two[br]components, the height that water rises in

0:04:52.707,0:04:58.430
the tube as a result of water pressure, we[br]call that pressure head, and then the

0:04:58.430,0:05:04.368
height that the water has because of how[br]far the tube is above the datum.

0:05:04.368,0:05:08.226
Okay, so that would be elevation head.

0:05:10.897,0:05:19.138
What Darcy observed is that water always[br]flows from high head to low head, okay,

0:05:19.138,0:05:22.708
and that the rate of flow through that[br]sand tube is proportional to the

0:05:22.708,0:05:26.461
difference in hydraulic head measured[br]between the manometers,

0:05:26.461,0:05:31.617
in other words, Delta H, uh, the[br]difference between this-this hydraulic

0:05:31.617,0:05:35.970
head and this hydraulic head,[br]is Delta H right here.

0:05:35.970,0:05:38.689
Okay, how about column area?

0:05:38.689,0:05:40.724
How would this affect flow?

0:05:40.724,0:05:43.628
Well this is really-really straight[br]forward, basically if you were to put a

0:05:43.628,0:05:50.811
partition in the column such that the area[br]available to flow, uh, was divided into

0:05:50.811,0:05:56.899
the flow rate, uh, through each half of[br]the sand tube, would be equal to one half

0:05:56.899,0:06:03.823
of the total flow, okay, so from this, [br]Darcy concluded that the rate of flow is

0:06:03.823,0:06:08.980
proportional to the area of sand [br]available.

0:06:10.630,0:06:14.313
Lastly, what about the length of the[br]tube.

0:06:14.313,0:06:19.768
Well Darcy figured out that if you keep[br]the head difference between each end of

0:06:19.768,0:06:25.958
the tubes the same but you lengthen the[br]tube, it effects the flow rate, okay.

0:06:25.958,0:06:33.025
Uh, so imagine, in fact it causes flow[br]rate to decrease, imagine that this sand

0:06:33.025,0:06:38.982
tube is one foot long, and that the[br]difference in head is 6 inches.

0:06:38.982,0:06:40.931
Okay, that's a pretty steep change in

0:06:41.018,0:06:44.547
hydraulic head over the-over the length of[br]that tube, right.

0:06:44.818,0:06:48.239
And so water would want to flow through [br]that fairly quickly.

0:06:48.239,0:06:53.594
But if the tube were 10 feet long that 6[br]inch drop over 10 feet isn't nearly

0:06:53.594,0:06:55.313
as steep.

0:06:55.313,0:06:58.615
In that case the water wouldn't move[br]through as fast, okay.

0:06:58.615,0:07:05.317
So from this we can conclude that, um, the[br]length, the flow through the tube is

0:07:05.317,0:07:08.087
inversely proportional to length.

0:07:08.087,0:07:15.272
In other words, if all else is the same,[br]as length increases, flow decreases.

0:07:18.897,0:07:24.018
Okay, so these 3 lines just summarized[br]what we just discussed flow is directly

0:07:24.018,0:07:28.540
proportional to the, uh, difference in[br]hydraulic head between each end of the

0:07:28.540,0:07:33.411
tube, it's proportional to the area of the[br]column, and it's inversely proportional

0:07:33.411,0:07:35.514
to the length of the column.

0:07:35.514,0:07:40.000
Okay, this symbol right here is, uh,[br]means proportional, right.

0:07:40.000,0:07:43.764
So this means inversely proportional,[br]because 1 over L.

0:07:43.835,0:07:48.806
So if we put this together, okay, this is[br]what we get.

0:07:48.806,0:07:55.564
Now we can replace this proportional sign,[br]with a sign, with an equal sign, by adding

0:07:55.564,0:08:02.754
a constant of proportionality, 'K',[br]and when we do that, this is what we

0:08:02.754,0:08:05.041
get, okay.

0:08:05.041,0:08:09.428
And if we just rearrange this a little[br]bit, we end up with this form of the

0:08:09.428,0:08:13.649
equation, and that is Darcy's Law.

0:08:13.649,0:08:16.384
So it really is not that complicated.

0:08:16.384,0:08:19.939
If you understood each part, each of the[br]parts that we went through in the previous

0:08:19.939,0:08:24.335
slides, this just combines them all into[br]one equation.

0:08:24.335,0:08:28.156
Um, one of the things that might seem a [br]little mysterious is this 'K'.

0:08:28.156,0:08:30.442
What the heck is 'K'?

0:08:30.442,0:08:34.930
Well, 'K' is hydraulic conductivity.

0:08:34.930,0:08:37.124
So let me explain this a little bit.

0:08:37.124,0:08:42.391
Um, basically you can think of it as,[br]y'know, basically the same thing as

0:08:42.391,0:08:47.607
permeability, it's a little bit different,[br]but think about it as permeability.

0:08:47.607,0:08:54.865
Aquifers have high hydraulic conductivity,[br]um, because they can transmit water

0:08:54.865,0:09:00.621
relatively easily, water can flow through[br]them, they're fairly permeable, right.

0:09:00.621,0:09:06.126
Aquitards have low hydraulic conductivity[br]they're relatively impermeable.

0:09:06.126,0:09:09.947
Water does not flow through them very[br]easily.

0:09:09.947,0:09:14.842
In reality, hydraulic conductivity is a[br]little bit more then permeability.

0:09:14.842,0:09:18.129
It combines permeability with properties[br]of the fluid.

0:09:18.129,0:09:22.532
But in any case just think about it as a[br]measure of how easily water can flow

0:09:22.532,0:09:26.220
through a porous meeting.

0:09:26.220,0:09:30.256
Okay, so how do geoscientists use Darcy's[br]Law and this information that I just

0:09:30.256,0:09:33.545
gave you?

0:09:33.545,0:09:38.151
Essentially you can think of wells, uh, as[br]the same thing as those manometers.

0:09:38.151,0:09:42.672
They fill the same roll as the manometers[br]I showed you in Darcy's experiment.

0:09:42.672,0:09:47.793
So we can go out and measure the elevation[br]of water in a well, and that tells us the

0:09:47.793,0:09:54.566
hydraulic head, where that water, where [br]that well is open and an aquifer.

0:09:54.566,0:10:01.189
Okay, so, so we can use these measurements[br]to help determine what direction ground

0:10:01.189,0:10:06.978
water is flowing in an aquifer, okay, so[br]in the, uh, in the illustration shown here,

0:10:06.978,0:10:11.094
this simple illustration, we would conclude[br]that ground water is flowing from the

0:10:11.094,0:10:12.645
left to the right.

0:10:12.645,0:10:14.214
Towards well B.

0:10:14.214,0:10:18.418
Because hydraulic head, uh, decreases[br]in that direction.

0:10:18.418,0:10:27.008
Remember ground water always flows from[br]high head to low head, okay.

0:10:27.008,0:10:31.948
And we can use this to help understand[br]flow rates, by assessing ground water

0:10:31.948,0:10:36.319
flow and combining it with, uh, [br]knowledge of geological properties of

0:10:36.319,0:10:40.640
aquifers we can evaluate how much water is[br]available in aquifers and how that

0:10:40.640,0:10:46.163
changes over time, okay.

0:10:47.143,0:10:48.561
How do we use this?

0:10:48.561,0:10:49.829
How is this useful?

0:10:49.829,0:10:53.967
Well given the extent to which we rely[br]on ground water as a source of drinking

0:10:53.967,0:10:58.580
water and irrigation water, this [br]information is extremely important,

0:10:58.787,0:11:03.244
because it helps us manage critical[br]water sources more effectively.

0:11:04.161,0:11:07.485
This is a huge, uh, this is a big deal.

0:11:07.835,0:11:11.855
It's really important, these water sources[br]are extremely important so we need to be

0:11:11.855,0:11:14.058
able to manage them effectively.

0:11:14.058,0:11:17.845
One of the examples, um, that I'm[br]going to focus on in the le-in the next

0:11:17.845,0:11:19.897
lecture, is shown here.

0:11:19.897,0:11:22.516
It is the high planes aquifer.

0:11:22.516,0:11:26.670
Okay, an aquifer that plays a vital role [br]in sustaining human populations in the

0:11:26.670,0:11:30.441
Great Plains, and an aquifer that's up [br]against some serious challenges

0:11:30.441,0:11:32.642
in the future.

0:11:32.642,0:11:35.630
So I look forward to talking about that[br]with you in more detail in the

0:11:35.630,0:11:37.230
next lecture.

0:11:37.230,0:11:39.790
Thank you for your attention.