Irrigation Water Requirement

Meghan Sindelar Author

08/09/2016 Added

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This recording captures the author working calculations to determine irrigation requirement from water content data.

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[00:00:01.147]Hello, this is Doctor Meghan Sindelar
[00:00:03.132]and continuing with Soil Physical Property calculations.
[00:00:07.291]This video is for those looking to do
[00:00:10.075]some irrigation requirement calculations.
[00:00:14.649]So, for this particular calculation using input data
[00:00:17.378]of some volumetric water contents,
[00:00:20.315]current water content,
[00:00:22.094]and the water content at field capacity.
[00:00:25.586]Some of you may have water content by mass data
[00:00:29.347]and then you would need to use your bulk density
[00:00:31.077]to convert that to volumetric water content.
[00:00:35.431]Some of you may be working with irrometers,
[00:00:38.101]or tensiometers, and, therefore,
[00:00:40.493]your water data will be in water potential,
[00:00:43.442]or water tension, just the inverse of water potential.
[00:00:47.320]And so, you would need to convert your water tension
[00:00:51.887]to a water content before stepping into
[00:00:54.994]where I'm starting with this calculation.
[00:00:57.966]So, let's assume we're doing our calculations
[00:01:00.242]for a rooting depth of 18 inches.
[00:01:04.061]And so, our water content is currently somewhere
[00:01:07.998]here between the wilting point and field capacity.
[00:01:11.840]So, we haven't let our plants die,
[00:01:14.650]but we could fit more water in.
[00:01:17.865]Remember, you wouldn't want to irrigate past field capacity
[00:01:20.849]because that water just freely drains away
[00:01:23.241]and it would be wasted.
[00:01:28.222]All right, pretty simple stuff, here.
[00:01:30.973]We'll just use the depth of water
[00:01:33.690]is equal to the water content times the soil depth.
[00:01:37.173]So, in this case, we're interested in the 18 inch
[00:01:41.202]rooting depth.
[00:01:42.769]Our current water content
[00:01:44.619]is 0.27
[00:01:47.861]cubic centimeters per cubic centimeter.
[00:01:51.358]So, that means
[00:01:52.423]we have 4.86 inches.
[00:01:56.880]So, your depth of water will always be in the same units
[00:02:01.084]as the soil depth
[00:02:04.276]that you started with.
[00:02:07.011]So, that's our current inches of water
[00:02:09.646]in the rooting depth.
[00:02:12.304]Some of that is unavailable.
[00:02:17.550]Okay.
[00:02:19.006]The water content, at field capacity, is
[00:02:25.270]0.4 cubic centimeter per cubic centimeters.
[00:02:28.431]So, take that times 18 inches
[00:02:31.558]and you get an answer that is
[00:02:33.533]in inches.
[00:02:36.210]So, this soil can store
[00:02:39.193]7.2 inches.
[00:02:43.152]So, figure out, then, how much to add
[00:02:45.846]to bring up to field capacity
[00:02:47.884]is just a matter of subtraction.
[00:02:51.646]So, the depth of water that the soil can hold
[00:02:55.542]minus the depth of water it currently has.
[00:03:00.783]So, 3.4 inches
[00:03:04.150]would be the irrigation to add.
[00:03:07.737]So, adding any more than that
[00:03:08.991]would be going past field capacity
[00:03:10.733]and some of the water would freely drain away.
[00:03:13.182]Now, that's just the straightforward calculation.
[00:03:16.131]Of course, you need to also consider
[00:03:17.908]if it is actually dry enough to warrant irrigation,
[00:03:21.449]if you have subsoil moisture that roots might be accessing,
[00:03:27.068]this number may exceed your infiltration rate,
[00:03:29.669]so you have to think about how you're going to
[00:03:31.194]add two inches.
[00:03:32.352]You may have to make adjustments for salinity, and so on.
[00:03:35.450]There is a lot that goes into the irrigation decision,
[00:03:37.784]but here is just a short version of a calculation
[00:03:40.689]that you might do.

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Irrigation Water Requirement

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