Soil Water Content Calculations

Meghan Sindelar Author

08/09/2016 Added

13401 Plays

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This recording captures the author working calculations to determine soil content by mass and by volume from collected data.

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[00:00:01.797]Hello all.
[00:00:02.630]This is Dr. Megan Sindelar
[00:00:04.101]and we are going to continue
[00:00:05.658]our sample calculations here
[00:00:07.861]in soil physical properties.
[00:00:09.864]This one for water content calculations.
[00:00:13.061]So this video assumes you have a general idea
[00:00:16.069]of the concept of soil water content
[00:00:19.494]and just want to see a worked problem.
[00:00:23.477]Here is the data set that we have been working with
[00:00:26.231]so a sample was collected in the field
[00:00:30.379]and there is our wet weight,
[00:00:32.421]goes in the oven at 105 degrees celsius
[00:00:35.269]for 24 hours to drive off the water
[00:00:38.426]and so we have a dry weight.
[00:00:40.677]So the difference between these two then
[00:00:42.517]would be considered to be the weight of water.
[00:00:46.166]So there are two ways that you can calculate
[00:00:47.863]water content.
[00:00:49.036]The first is water content by mass.
[00:00:52.246]So this is generally represented in textbooks
[00:00:54.917]with a lower case "w" or an italic lowercase "w".
[00:00:59.733]You may also see a sort of a percent,
[00:01:04.478]water by weight type of demarcation
[00:01:08.802]so this is mass of water over the mass of solids.
[00:01:13.951]Simply put a wet sample in the oven, dry it;
[00:01:16.800]you've got the weight of water,
[00:01:17.806]the weight of just solids.
[00:01:20.927]So that is water content by mass,
[00:01:22.767]sometimes called gravimetric water content.
[00:01:25.425]Then we've got water content by volume,
[00:01:28.159]sometimes called volumetric water content,
[00:01:31.619]generally represented in textbooks
[00:01:33.311]by the Greek letter theta
[00:01:37.019]or you might say theta "v"
[00:01:40.207]or you might see percent water by volume.
[00:01:44.249]All of those are for volumetric water content.
[00:01:47.577]So the mass water content is really easy to measure
[00:01:51.339]but it's not necessarily the most useful
[00:01:54.089]because as we look at the soil,
[00:01:58.009]mass water content doesn't really tell us
[00:02:00.442]relative amount of water.
[00:02:04.392]You need to know a volumetric water for that
[00:02:06.072]so our soil is 50% solids and then of our pores,
[00:02:09.848]what percent is water, what percent is air.
[00:02:11.736]That's what we really want to know.
[00:02:13.331]So volumetric water content is more useful.
[00:02:16.808]However, how do you measure volume of water?
[00:02:20.696]You really can't.
[00:02:21.828]So instead we have to get the mass of water
[00:02:24.660]and then use the density of water
[00:02:26.308]to convert that to a volume of water.
[00:02:28.682]So the density of water
[00:02:29.694]is one gram per cubic centimeter
[00:02:32.619]so it's a very simple conversion.
[00:02:36.474]Likewise, if we don't have volume total
[00:02:39.578]we could use bulk density in order to convert
[00:02:45.882]from our mass of solids to our volume total.
[00:02:49.067]So therefore if we have a mass of water,
[00:02:50.954]we have a mass of solids and we know the density
[00:02:54.086]of our water and our solids,
[00:02:55.907]we could calculate volumetric water content
[00:02:58.214]this way so water content by mass
[00:03:00.275]times the bulk density
[00:03:02.163]divided by the density of water.
[00:03:05.300]In this case I am using ROE to represent density.
[00:03:13.153]Alright let's do an example.
[00:03:16.977]So here's the formula, mass of water.
[00:03:20.034]Here then is our data
[00:03:23.622]so our mass of water
[00:03:26.822]would be the wet mass
[00:03:31.758]minus the dry mass
[00:03:34.067]and our mass of solids would be the dry mass
[00:03:40.290]minus the tin.
[00:03:45.870]So 215 minus 182 is 32 grams,
[00:03:51.473]183 minus 32 is 151 grams
[00:03:59.918]so 32 divided by 151,
[00:04:02.381]we've got zero point two one gram per gram.
[00:04:08.141]That's it.
[00:04:08.974]That's our gravimetric water content.
[00:04:10.669]Now even though you might think the grams cancel,
[00:04:12.912]please keep them.
[00:04:14.544]Write gram per gram so that
[00:04:16.095]when we look at this number,
[00:04:17.041]we know it is the water content by mass
[00:04:19.917]and not the water content by volume.
[00:04:23.586]Another thing to remember is that you need to use
[00:04:26.215]just the mass of solids here on the bottom.
[00:04:29.511]Most of our other calculations have mass total,
[00:04:32.183]volume total, total on the bottom.
[00:04:35.000]But the mass of solids is constant.
[00:04:39.384]The mass total would change as the mass of water
[00:04:41.543]change and it gets kind of messy
[00:04:42.989]if you have an equation where both the top
[00:04:44.301]and bottom are changing.
[00:04:45.977]So we use mass of solids on the bottom,
[00:04:47.703]please make sure that you do that.
[00:04:51.735]Okay so that's water content by mass.
[00:04:54.359]Like I said, it's easy to do.
[00:04:57.165]Water content by volume.
[00:05:00.055]So in this case, we do have the total volume
[00:05:05.751]of the soil so we can go ahead and use that.
[00:05:09.363]So our volume of water,
[00:05:13.807]we still need to come up with our mass of water.
[00:05:17.155]215 minus 183 and then it's basically
[00:05:21.071]divide this number by one.
[00:05:24.883]So one gram per centimeters cubed.
[00:05:27.829]Okay then on the bottom we've got 118.
[00:05:31.666]Okay so that's 32 grams
[00:05:34.709]divided by one gram per cubic centimeter
[00:05:38.524]over 118 cubic centimeters is 32 cubic centimeters.
[00:05:44.598]See, divide bottom by the bottom loops
[00:05:47.252]to the top?
[00:05:49.574]Over one 118 cubic centimeters
[00:05:52.519]is zero point two seven cubic centimeter
[00:05:59.645]per cubic centimeter
[00:06:01.404]for the volumetric water content.
[00:06:05.196]So again, you might think
[00:06:06.863]"Well these units cancel, I don't need em"
[00:06:08.588]but no, please keep these so that we know
[00:06:11.820]we're talking about water content by volume
[00:06:14.092]since we've got units of volume.
[00:06:16.335]Cause as you can see, this is a different number
[00:06:17.964]than our water content by mass was.
[00:06:23.729]Okay so this could be converted into a percent.
[00:06:26.948]This soil is 27% water.
[00:06:30.372]As you look over here, that seems pretty typical
[00:06:35.076]so we've got half solids, half pores
[00:06:38.132]that could be either water or air.
[00:06:40.262]In this one, slightly more than half of the pores
[00:06:45.228]is water.
[00:06:46.836]Seems normal.
[00:06:49.924]Okay but if we didn't have our volume total
[00:06:52.916]for this particular soil
[00:06:53.844]but we did have a bulk density
[00:06:55.237]then we could calculate
[00:06:56.277]our volumetric water content this way.
[00:06:58.790]So our water content by mass that we calculated
[00:07:01.202]was zero point two one gram per gram
[00:07:05.532]times our bulk density, previously calculated
[00:07:08.594]be one point two eight grams per cubic centimeter
[00:07:14.018]and then divide all of that
[00:07:15.261]by the density of water which is one gram
[00:07:18.973]per cubic centimeter.
[00:07:22.189]Okay so these grams are gonna cancel out
[00:07:24.557]with this and then what's on the bottom
[00:07:27.117]of the bottom will loop around to the top
[00:07:29.295]and so we'll be left with the answer in units
[00:07:31.797]of cubic centimeter per cubic centimeter
[00:07:35.077]which is appropriate for a volumetric water content
[00:07:38.289]and zero point two one times one point two eight
[00:07:42.915]divided by one is zero point two seven.
[00:07:47.267]The exact same volumetric water content
[00:07:49.523]that we got from calculating it the other way.
[00:07:52.633]So got a little self test there,
[00:07:55.203]we have done it correctly.
[00:07:58.723]Incidentally, what is the maximum
[00:08:01.795]volumetric water content that a soil can have?
[00:08:11.199]Yeah look at this right here.
[00:08:13.838]The water content at saturation
[00:08:16.504]would be equal to the porosity.
[00:08:19.321]You cannot have greater volume of water
[00:08:21.258]than you have volume of pores.
[00:08:23.722]So if we had a compacted soil
[00:08:25.789]with a low amount of porosity
[00:08:28.199]then our maximum water content would be
[00:08:30.407]a small number and if we had a really porous soil
[00:08:34.058]then our saturated water content could be
[00:08:35.771]a quite high number but generally,
[00:08:37.780]we're looking for something around 50%
[00:08:40.129]in an ideal, healthy soil.
[00:08:43.585]So those are some examples
[00:08:45.027]of calculating water content
[00:08:47.873]both by mass and by volume.

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Soil Water Content Calculations

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