Help me design a water infiltration experiment

Growing in a dryland, any bit of rain is precious. However, all too often, light misty rains or fast moving thunderstorms don't actually wet the soil; the mulch surface gets damp, but it evaporates away almost at once. Bare soil also has this problem, but smaller rain events will be useful on bare soil then will be in a heavily mulched system.

Next spring, I want to do a large sized experiment on the right kind of mulch to maximize infiltration. But before I go to a whole lot of trouble, I'd like to run a pilot experiment in buckets.

Here is my idea for the setup:

Fill 5 gallon buckets with equal amounts of soil, half way up. The upper half of the buckets should be cut so as to be removable.

Put the different mulches to be tested on top of the buckets.

Weight the buckets, and record the weights. (This will correct for differences in soil density.)

Using a light mist nozzle on the hose, apply a quarter inch of "rain", determined by first running the hose over an empty bucket, and timing the filling to a quarter inch. Make sure there are no winds to blow away "rain," which would mess up results. The half filled bucket would mean that the hose nozzle could be below the rim, thus helping accuracy.

Remove upper half of buckets.

Put all buckets in a dry, sunny place for a few days.

Reweigh all buckets to determine which retained most water.

Mulches to be tested:

Wood chips, 3 inches.
Straw, 3 inches
River pebbles, 3 inches
Chopped sticks, 3 inches
Small planks laid on soil
Bricks laid on soil


What changes should I make to this in the interests of accuracy?

Great experiment!  I'm interested to see the results.  My prediction is that the "rain" may not even moisten the soil beneath the organic mulches, and that the bricks and stones will be the most successful at capturing and holding moisture. But I could be way off!

TO:  Gilbert Fritz
FROM:  Eric Koperek = erickoperek@gmail.com
SUBJECT:  Water Infiltration
DATE:  PM 8:27 Wednesday 7 September 2016
TEXT:

(1)  I have re-vegetated about 1,000 miles of wadis (in Morocco) and hundreds of strip mines around the planet.  1/4 inch of rain in an arid or semi-arid environment is a bonanza.  Any mulch will work but in desert areas rocks are cheap and plentiful.  Use a thin layer of pebbles or any other rocks close to hand.  the surface of a rock may be burning hot, but the soil underneath will stay cool and moist.  Rocks make good mulch.  Apply fertilizers, manure, and other organic materials UNDERNEATH rock mulches.  For best results build micro-catchments for each tree, bush, or plant.  Micro-catchment size = 2 times plant height.  Erect small bunds = curbs = dams of earth about 6 to 8 inches high to prevent water from washing over land.  Mulch each micro-catchment with rocks to prevent evaporation.  Concentrate all available manure and other soil amendments in planting hole in center of each micro-catchment.  Plants thrive from extra water and concentrated nutrients close to plant roots.  In this way you can plant trees where rainfall is scant.

(2)  Protect trees or other plants with "nurse stones" = flat, upright rocks that shield plants from drying winds and abrading sand.  Plants grow better in more humid micro-climate created by nurse stones.

ERIC KOPEREK = erickoperek@gmail.com

end comment

 

I'd attempt to apply the mist to each bucket at the same rate as your normal rain events.  IE spread the 1/4" of mist water application over 30 minutes if that's how fast it comes down from mother nature.  

For that matter, when you get your misty rain, is the ground hot?  That may be a variable to simulate in your experiment.  Sun-baked mulch may evaporate the first bit of mist even as the rain is falling.  And that effect may be different for different mulches.

Lastly, I'd suggest you keep the mist from hitting the sides of the bucket.  Otherwise it may run down between the bucket plastic and the mulch and infiltrate better than in the middle of the bucket.  

The more surface area you have in your buckets the better it will be for the experiment (less likely for edge effects to affect the overall data).  

Would you consider adding one more type of mulch?  Broken glass.  

Seriously.  There is zero water absorption by the glass so once there was enough rain to overcome the surface tension between the water and the glass, all of the water should make it to the underlying soil and stay there.  Evaporation should be minimal as the glass would not allow water to make it back to the air above easily.  I guess the best option would be sea glass so as not to have too many sharp edges.  The color of the glass could have an effect on soil surface temperature one way or the other.  Clear glass that is sanded turns almost white which will reflect sunlight to some degree.  On the other hand darker glass will have the opposite effect. (i think)

The other thing that I think would have an impact is the size and shape of the glass pieces.  I would imagine that something along the size of gravel would be the best size to maximize infiltration.  

I can't recall if you mentioned it or not but don't forget a control group with no mulch so that you know where your baseline is.

Keep enough space between buckets so that they don't shade each other and cause differences in your evaporation rates as well.  

Other than that I say you've got a reasonable set up there.

good luck

eric koperek wrote:

(1)  I have re-vegetated about 1,000 miles of wadis (in Morocco) and hundreds of strip mines around the planet.  

I'm sure we'd all really appreciate any photos or links you might have so we can see those projects!

Mike;

Good points! Especially the one about hot mulch. I will let all buckets sit in the sun for a day before applying, because yes, the thundershowers come in the afternoon, generally after hot sunny days.

I could make this even more realistic by just waiting for a rain, and using a rain gauge to measure it! But that might not happen for a while.

And the point about surface area is also a good one; maybe I should use larger, flatter containers.

Craig;

I will definitely include some glass if I can find any! Do you suppose a cement mixer could turn unwanted glass bottles into mulch with at least somewhat rounded edges?

And I will add a control bucket with bare soil.

Or could I use whole glass bottles?! That would be interesting, anyway! Maybe stuffed full of an insulating material they would function as dew catchers?

Tyler, I will certainly record results! I'm leaning toward the gravel, too, but Mike has a point that HOT rocks may evaporate as much rain as the organic mulches hold. In which case I'd vote for the tilted planks.

I'm imagining that a good material would not get blazing hot in the sun, wouldn't absorb the water and would let it run off and down into the soil quickly.  Bone dry wood or planks can soak up a modest amount of water before it runs off.  I like the idea of tilting the planks though.  This might be silly but what if you put an oil finish on the planks so the water beads up and rolls off of them?

I wonder about white rocks.  They hopefully wouldn't build up as much heat as normal pebbles and may not burn off as much water when the mist starts?

Cement mixer glass could be a good option.  I wouldn't want sharp or possibly sharp glass in my landscape though.

If you have access to grass clippings or leaves, that could be another mulch material to test out.

Good luck, this sounds like a great experiment!

So, some experiment modifications;

I think I will just leave the buckets out in the weather, tracking weight on a regular basis and recording rainfalls and daily temperature/ evapotranspiration rates. I will also use a soil thermometer to measure soil surface temperature.

Should we make the mulch layers removable so that just the soil can be weighed? That way water stuck in the top layers of mulch, where it will be useless to plants, would not be counted. Or would that water quickly evaporate anyway? I could put the mulch into cut-off buckets with screen bottoms, and set those on the dirt buckets.

I'm going to add tumbled glass, whole glass bottles, and sand, to the list.

I might also add one bucket with a small foil dew catcher on top, for comparison; are rocks, etc. dew traps?

Musing beeswax or linseed oil on the planks sounds like a great idea.

Gravel (and crushed glass) have always seemed problematic in the garden because they are hard to move when organic matter and fertilizer need to be added. However, I think I might have come up with a solution.

There are mesh net sand bags that are supposed to last at least 5 years. If the pebbles or glass were put in these bags, the bags could be moved back and forth as needed. They could be formed into flat pancake type shapes.

I've seen some videos on youtube about tumbing glass in a cement mixer.  It's loud as hell and takes a long time to do but it does look pretty cool as landscaping mulch.

Now I'm thinking that quartz would also be a good stone to use because it's very hard and not at all porous.  

good luck

A couple of comments to add:  about the depth, since soil and subsoil are infinitely deep, I don't see how making shallower test buckets improves the experiment or makes it more like the natural conditions the experiment is trying to emulate.

And, about moisture being "wasted" when absorbed by bark, chips  and other organic type mulches, I wonder about the idea that it is wasted when absorbed.  When it vaporizes, I imagine the water vapor moving all directions, just through the process of diffusion, where substances move from higher to lower concentrations.   If the air is very dry, a lot of the vapor may go into the air, but that will decrease a plant's loss through evapotranspiration.  Also, if moisture is being lost from the surface of the mulch, it is protecting from water loss at deeper levels.

If it is humid, much of the water vapor will go down into the soil.

I guess what I am saying is that even though more water may seep down and through gravel, doesn't mean water on organic mulches is "lost".  And when mist falls on hot gravel, the first moisture is probably going to evaporate, cooling the rocks.  Where do you get the best capture of the precious moisture?

The idea of this experiment delights me, because it is going to test all the conjecture we are capable of, and find out what really happens under the conditions of the experiment.  It may lead to another experiment to further test the findings.  

Way to go Gilbert!

Hello Thekla,

About the shallower containers; the idea was to make them wider, minimizing the edge effects, while making them shallower was to keep wider containers reasonable in weight.

This will be a good preliminary test, but next year we will have to do testing with the various mulches and live plants. You might be right about higher humidity, etc. That also brings up the point that we may not be able to replicate whatever results I get in a different climate.

Hi Gilbert,

It's a great beginning, and sure to inspire many.  

Thanks for explaining the objective with the shallower wider container.  It makes sense, and there are just so many variables to speculate about, and which elements play which roles in water capture and retention, and which are not active at all.  

I'm glad you know it might be a mistake to assume your results would hold true for all climates, soils, conditions, and circumstances.  However if you tinker with this, and optimize YOUR plot, you will be developing a test model for others to use to determine what works best in THEIR conditions, and you'll be modeling the strategy of testing assumptions.  

Again, Bravo!

About the broken glass and sharp edges - My son played strong man in the circus his gradeschool put on every year. One of his tricks was to strip to the waist, lie down on broken glass and let one of the smaller kids stand on his chest. He carefully explained the trick to his half hysterical mother: They boiled the broken glass pieces, so the edges wouldn't cut him, but the pieces still looked like raw broken glass. Might be easier and probably more effective than a cement mixer?

I plant most of my tender plants in containers. I use a hollow water reservoir in them so water wicks back up through the soil.  The most effective long lasting mulch on these containers is chunks of biochar [actually chunks of charcoal left in my wood stove when the fire goes out]  It is light and insinuative. The black surface absorbs the light but radiates it back at a longer wave length which I understand is more usable by the plants. If it later gets crushed and mixed in the soil it is beneficial to the soil.

If you have adobe type soil I would shape it like a funnel with a small clay pot at the bottom and a larger clay pot under it upside down. Any run off from the rain event then would go through the drain hole in the top pot into the reservoir pot to slowly infiltrate into the soil.

For a test directly in the ground: dig a trench, place a corrugated drain tile down the center and place growing soil on both sides then slope the adobe soil on top on both sides to the top of the drain tile and place the charcoal on top as a filter. Cut through the clay at plant spacing to transplant into the good soil underneath.