WEBVTT 1 00:00:01.797 --> 00:00:02.630 Hello all. 2 00:00:02.630 --> 00:00:04.101 This is Dr. Megan Sindelar 3 00:00:04.101 --> 00:00:05.658 and we are going to continue 4 00:00:05.658 --> 00:00:07.861 our sample calculations here 5 00:00:07.861 --> 00:00:09.864 in soil physical properties. 6 00:00:09.864 --> 00:00:13.061 This one for water content calculations. 7 00:00:13.061 --> 00:00:16.069 So this video assumes you have a general idea 8 00:00:16.069 --> 00:00:19.494 of the concept of soil water content 9 00:00:19.494 --> 00:00:23.477 and just want to see a worked problem. 10 00:00:23.477 --> 00:00:26.231 Here is the data set that we have been working with 11 00:00:26.231 --> 00:00:29.398 so a sample was collected in the field 12 00:00:30.379 --> 00:00:32.421 and there is our wet weight, 13 00:00:32.421 --> 00:00:35.269 goes in the oven at 105 degrees celsius 14 00:00:35.269 --> 00:00:38.426 for 24 hours to drive off the water 15 00:00:38.426 --> 00:00:40.677 and so we have a dry weight. 16 00:00:40.677 --> 00:00:42.517 So the difference between these two then 17 00:00:42.517 --> 00:00:46.166 would be considered to be the weight of water. 18 00:00:46.166 --> 00:00:47.863 So there are two ways that you can calculate 19 00:00:47.863 --> 00:00:49.036 water content. 20 00:00:49.036 --> 00:00:52.246 The first is water content by mass. 21 00:00:52.246 --> 00:00:54.917 So this is generally represented in textbooks 22 00:00:54.917 --> 00:00:59.733 with a lower case "w" or an italic lowercase "w". 23 00:00:59.733 --> 00:01:02.816 You may also see a sort of a percent, 24 00:01:04.478 --> 00:01:07.395 water by weight type of demarcation 25 00:01:08.802 --> 00:01:12.885 so this is mass of water over the mass of solids. 26 00:01:13.951 --> 00:01:16.800 Simply put a wet sample in the oven, dry it; 27 00:01:16.800 --> 00:01:17.806 you've got the weight of water, 28 00:01:17.806 --> 00:01:19.973 the weight of just solids. 29 00:01:20.927 --> 00:01:22.767 So that is water content by mass, 30 00:01:22.767 --> 00:01:25.425 sometimes called gravimetric water content. 31 00:01:25.425 --> 00:01:28.159 Then we've got water content by volume, 32 00:01:28.159 --> 00:01:31.619 sometimes called volumetric water content, 33 00:01:31.619 --> 00:01:33.311 generally represented in textbooks 34 00:01:33.311 --> 00:01:35.394 by the Greek letter theta 35 00:01:37.019 --> 00:01:39.186 or you might say theta "v" 36 00:01:40.207 --> 00:01:44.249 or you might see percent water by volume. 37 00:01:44.249 --> 00:01:47.577 All of those are for volumetric water content. 38 00:01:47.577 --> 00:01:51.339 So the mass water content is really easy to measure 39 00:01:51.339 --> 00:01:54.089 but it's not necessarily the most useful 40 00:01:54.089 --> 00:01:56.672 because as we look at the soil, 41 00:01:58.009 --> 00:02:00.442 mass water content doesn't really tell us 42 00:02:00.442 --> 00:02:02.525 relative amount of water. 43 00:02:04.392 --> 00:02:06.072 You need to know a volumetric water for that 44 00:02:06.072 --> 00:02:09.848 so our soil is 50% solids and then of our pores, 45 00:02:09.848 --> 00:02:11.736 what percent is water, what percent is air. 46 00:02:11.736 --> 00:02:13.331 That's what we really want to know. 47 00:02:13.331 --> 00:02:16.808 So volumetric water content is more useful. 48 00:02:16.808 --> 00:02:20.696 However, how do you measure volume of water? 49 00:02:20.696 --> 00:02:21.828 You really can't. 50 00:02:21.828 --> 00:02:24.660 So instead we have to get the mass of water 51 00:02:24.660 --> 00:02:26.308 and then use the density of water 52 00:02:26.308 --> 00:02:28.682 to convert that to a volume of water. 53 00:02:28.682 --> 00:02:29.694 So the density of water 54 00:02:29.694 --> 00:02:32.619 is one gram per cubic centimeter 55 00:02:32.619 --> 00:02:35.369 so it's a very simple conversion. 56 00:02:36.474 --> 00:02:39.578 Likewise, if we don't have volume total 57 00:02:39.578 --> 00:02:43.328 we could use bulk density in order to convert 58 00:02:45.882 --> 00:02:49.067 from our mass of solids to our volume total. 59 00:02:49.067 --> 00:02:50.954 So therefore if we have a mass of water, 60 00:02:50.954 --> 00:02:54.086 we have a mass of solids and we know the density 61 00:02:54.086 --> 00:02:55.907 of our water and our solids, 62 00:02:55.907 --> 00:02:58.214 we could calculate volumetric water content 63 00:02:58.214 --> 00:03:00.275 this way so water content by mass 64 00:03:00.275 --> 00:03:02.163 times the bulk density 65 00:03:02.163 --> 00:03:05.300 divided by the density of water. 66 00:03:05.300 --> 00:03:09.383 In this case I am using ROE to represent density. 67 00:03:13.153 --> 00:03:15.486 Alright let's do an example. 68 00:03:16.977 --> 00:03:20.034 So here's the formula, mass of water. 69 00:03:20.034 --> 00:03:21.784 Here then is our data 70 00:03:23.622 --> 00:03:25.289 so our mass of water 71 00:03:26.822 --> 00:03:28.572 would be the wet mass 72 00:03:31.758 --> 00:03:34.067 minus the dry mass 73 00:03:34.067 --> 00:03:37.734 and our mass of solids would be the dry mass 74 00:03:40.290 --> 00:03:41.457 minus the tin. 75 00:03:45.870 --> 00:03:48.287 So 215 minus 182 is 32 grams, 76 00:03:51.473 --> 00:03:53.556 183 minus 32 is 151 grams 77 00:03:59.918 --> 00:04:02.381 so 32 divided by 151, 78 00:04:02.381 --> 00:04:05.964 we've got zero point two one gram per gram. 79 00:04:08.141 --> 00:04:08.974 That's it. 80 00:04:08.974 --> 00:04:10.669 That's our gravimetric water content. 81 00:04:10.669 --> 00:04:12.912 Now even though you might think the grams cancel, 82 00:04:12.912 --> 00:04:14.544 please keep them. 83 00:04:14.544 --> 00:04:16.095 Write gram per gram so that 84 00:04:16.095 --> 00:04:17.041 when we look at this number, 85 00:04:17.041 --> 00:04:19.917 we know it is the water content by mass 86 00:04:19.917 --> 00:04:23.586 and not the water content by volume. 87 00:04:23.586 --> 00:04:26.215 Another thing to remember is that you need to use 88 00:04:26.215 --> 00:04:29.511 just the mass of solids here on the bottom. 89 00:04:29.511 --> 00:04:32.183 Most of our other calculations have mass total, 90 00:04:32.183 --> 00:04:35.000 volume total, total on the bottom. 91 00:04:35.000 --> 00:04:37.917 But the mass of solids is constant. 92 00:04:39.384 --> 00:04:41.543 The mass total would change as the mass of water 93 00:04:41.543 --> 00:04:42.989 change and it gets kind of messy 94 00:04:42.989 --> 00:04:44.301 if you have an equation where both the top 95 00:04:44.301 --> 00:04:45.977 and bottom are changing. 96 00:04:45.977 --> 00:04:47.703 So we use mass of solids on the bottom, 97 00:04:47.703 --> 00:04:50.536 please make sure that you do that. 98 00:04:51.735 --> 00:04:54.359 Okay so that's water content by mass. 99 00:04:54.359 --> 00:04:57.165 Like I said, it's easy to do. 100 00:04:57.165 --> 00:04:59.165 Water content by volume. 101 00:05:00.055 --> 00:05:03.722 So in this case, we do have the total volume 102 00:05:05.751 --> 00:05:09.363 of the soil so we can go ahead and use that. 103 00:05:09.363 --> 00:05:11.280 So our volume of water, 104 00:05:13.807 --> 00:05:17.155 we still need to come up with our mass of water. 105 00:05:17.155 --> 00:05:21.071 215 minus 183 and then it's basically 106 00:05:21.071 --> 00:05:23.238 divide this number by one. 107 00:05:24.883 --> 00:05:27.829 So one gram per centimeters cubed. 108 00:05:27.829 --> 00:05:31.666 Okay then on the bottom we've got 118. 109 00:05:31.666 --> 00:05:33.583 Okay so that's 32 grams 110 00:05:34.709 --> 00:05:38.524 divided by one gram per cubic centimeter 111 00:05:38.524 --> 00:05:42.691 over 118 cubic centimeters is 32 cubic centimeters. 112 00:05:44.598 --> 00:05:47.252 See, divide bottom by the bottom loops 113 00:05:47.252 --> 00:05:48.169 to the top? 114 00:05:49.574 --> 00:05:52.519 Over one 118 cubic centimeters 115 00:05:52.519 --> 00:05:55.852 is zero point two seven cubic centimeter 116 00:05:59.645 --> 00:06:01.404 per cubic centimeter 117 00:06:01.404 --> 00:06:04.154 for the volumetric water content. 118 00:06:05.196 --> 00:06:06.863 So again, you might think 119 00:06:06.863 --> 00:06:08.588 "Well these units cancel, I don't need em" 120 00:06:08.588 --> 00:06:11.820 but no, please keep these so that we know 121 00:06:11.820 --> 00:06:14.092 we're talking about water content by volume 122 00:06:14.092 --> 00:06:16.335 since we've got units of volume. 123 00:06:16.335 --> 00:06:17.964 Cause as you can see, this is a different number 124 00:06:17.964 --> 00:06:20.881 than our water content by mass was. 125 00:06:23.729 --> 00:06:26.948 Okay so this could be converted into a percent. 126 00:06:26.948 --> 00:06:29.281 This soil is 27% water. 127 00:06:30.372 --> 00:06:35.076 As you look over here, that seems pretty typical 128 00:06:35.076 --> 00:06:38.132 so we've got half solids, half pores 129 00:06:38.132 --> 00:06:40.262 that could be either water or air. 130 00:06:40.262 --> 00:06:44.345 In this one, slightly more than half of the pores 131 00:06:45.228 --> 00:06:46.836 is water. 132 00:06:46.836 --> 00:06:47.919 Seems normal. 133 00:06:49.924 --> 00:06:52.916 Okay but if we didn't have our volume total 134 00:06:52.916 --> 00:06:53.844 for this particular soil 135 00:06:53.844 --> 00:06:55.237 but we did have a bulk density 136 00:06:55.237 --> 00:06:56.277 then we could calculate 137 00:06:56.277 --> 00:06:58.790 our volumetric water content this way. 138 00:06:58.790 --> 00:07:01.202 So our water content by mass that we calculated 139 00:07:01.202 --> 00:07:04.202 was zero point two one gram per gram 140 00:07:05.532 --> 00:07:08.594 times our bulk density, previously calculated 141 00:07:08.594 --> 00:07:12.677 be one point two eight grams per cubic centimeter 142 00:07:14.018 --> 00:07:15.261 and then divide all of that 143 00:07:15.261 --> 00:07:18.973 by the density of water which is one gram 144 00:07:18.973 --> 00:07:20.723 per cubic centimeter. 145 00:07:22.189 --> 00:07:24.557 Okay so these grams are gonna cancel out 146 00:07:24.557 --> 00:07:27.117 with this and then what's on the bottom 147 00:07:27.117 --> 00:07:29.295 of the bottom will loop around to the top 148 00:07:29.295 --> 00:07:31.797 and so we'll be left with the answer in units 149 00:07:31.797 --> 00:07:35.077 of cubic centimeter per cubic centimeter 150 00:07:35.077 --> 00:07:38.289 which is appropriate for a volumetric water content 151 00:07:38.289 --> 00:07:42.915 and zero point two one times one point two eight 152 00:07:42.915 --> 00:07:46.165 divided by one is zero point two seven. 153 00:07:47.267 --> 00:07:49.523 The exact same volumetric water content 154 00:07:49.523 --> 00:07:52.633 that we got from calculating it the other way. 155 00:07:52.633 --> 00:07:55.203 So got a little self test there, 156 00:07:55.203 --> 00:07:57.370 we have done it correctly. 157 00:07:58.723 --> 00:08:01.795 Incidentally, what is the maximum 158 00:08:01.795 --> 00:08:05.628 volumetric water content that a soil can have? 159 00:08:11.199 --> 00:08:13.838 Yeah look at this right here. 160 00:08:13.838 --> 00:08:16.504 The water content at saturation 161 00:08:16.504 --> 00:08:19.321 would be equal to the porosity. 162 00:08:19.321 --> 00:08:21.258 You cannot have greater volume of water 163 00:08:21.258 --> 00:08:23.722 than you have volume of pores. 164 00:08:23.722 --> 00:08:25.789 So if we had a compacted soil 165 00:08:25.789 --> 00:08:28.199 with a low amount of porosity 166 00:08:28.199 --> 00:08:30.407 then our maximum water content would be 167 00:08:30.407 --> 00:08:34.058 a small number and if we had a really porous soil 168 00:08:34.058 --> 00:08:35.771 then our saturated water content could be 169 00:08:35.771 --> 00:08:37.780 a quite high number but generally, 170 00:08:37.780 --> 00:08:40.129 we're looking for something around 50% 171 00:08:40.129 --> 00:08:42.296 in an ideal, healthy soil. 172 00:08:43.585 --> 00:08:45.027 So those are some examples 173 00:08:45.027 --> 00:08:47.873 of calculating water content 174 00:08:47.873 --> 00:08:50.123 both by mass and by volume.