One of the very first things we have to take into account in our
permaculture projects is
water. Water, sun, oxygen and minerals are the base for life. The more water, the more life. There's never too much water (ask the whales!), there's only species adapted to more or less water. When we have water, we can manage it, we can create different biomes adapted for the things we'd love to grow; while missing water limits our options. So, our goal is to gain access to as much water as possible. For doing it properly, we first need to understand Hydrology.
We can find water in the following forms:
- Seas and oceans, below the sea level
- Air humidity
- Rain / fog / snow
- Runoff water, when rainwater is not absorbed by the soil.
- Ground water, it is water that has been infiltrated in the soil.
- Aquifer, it is the water that is hold
underground by an impermeable layer.
- Water table, it is the layer of the aquifer where groundwater becomes saturated of water and depleted of oxygen.
- Surface water, it is water that we can see on
land, they are rivers, lakes, ponds... it may be sourced from an aquifer or from runoff water.
- Artesian water, it is ancient water that is trapped between two impermeable layers at high pressure.
Here is a nice picture showing these elements:
http://www.ecra-climate.eu/images/pictures/CP_CHC_Picture.png
The Water Cycle. Desert-Forest modes.
Water comes primarily from the evaporation of water bodies, seas and oceans, but also lakes and rivers. The air above these large water bodies is high on humidity. This humidity can go to the land in two modes.
In Desert mode (abiotic), land is so hot that it creates a barrier for the humid air, until it builds up so much pressure that large storm clouds form and rain falls heavy on the land, causing floods, losing much of the rainwater from runoff water and eroding soils. Eroded soils can't infiltrate water properly, preventing the replenishment of aquifers, therefore water table remains low. When the surface becomes completely dry, it is hydrophobic, meaning that it will take even longer to start infiltrating water. A tilled surface may become a hard pan, which both repels rainwater and allows groundwater evaporation. The effects are scarce vegetation, seasonal surface water, a flood-draugh cycle, extreme temperatures and in the long run, a desertic landscape with angular hills.
In Forest mode (biotic), the breath of the Forest refreshes the land so the humid air comes from the ocean unimpeded. Then, hydrophilic bacteria released by the
trees capture the air humidity and form rain clouds which make a soft and frequent rain that keeps the soil surface moist. The abundant vegetation helps rainwater to infiltrate, increasing aquifers and rising water tables. The effects are abundant vegetation,
perennial bodies of surface water, seasonal rains, moderate temperatures and in the long run, a humid landscape with rounded hills.
Here is a nice short film on the topic:
https://permies.com/t/218000/Full-Water-Cycle
Understanding water flows.
Once water touches the ground it is subject to two major forces: gravity and capilarity.
Gravity is easy to understand, it makes water to move downwards. As runoff water it takes the route of highest steepness. As seepage water, if the earth was homogeneous, it would go directly downwards (but it is not!). Once it touches the impermeable layer, it would follow again the route of the highest steepness of the layer.
Capilarity is best understood if we think of the earth as a sponge. When we put a dry sponge over a dish full of water, the water is absorbed by the sponge, going upwards, beating gravity: that's the capilarity force. When the sponge is too dense, water is absorbed slowly and released slowly, when the sponge is too light it can't absorb water and it dries fast. As you imagine, a sand layer is similar to the light sponge that can't absorb water easily, while a clay layer is like the sponge that is too dense and absorbs too slowly.
So, what happens if I have a sand layer over a clay layer in a slope? The water moves fast on the sand and goes directly downwards down to the clay layer. The clay layer can't absorb the water as fast as the sand layer is sending water. The water that can't be absorbed by the clay layer moves down the slope of the clay layer.
What happens if the clay layer is on top of the sand layer in the slope? The absorbed water in the clay layer is forced downwards by the gravity and everywhere by the capilarity, so it goes directly downwards at a slow pace. When it touches the sand layer, this absorbs the water faster than it is receiving water, so there's no runoff effect, and the water still goes directly downwards, now at a faster speed, until it touches a more dense layer.
Summarized:
- Water going from denser to lighter --> directly downwards.
- Water going from lighter to denser --> Absorbed water goes downwards, not absorbed water goes down the slope.
Now, geology can be weird, but if we can't dig and check, we may assume that we have a sedimentary soil, where the deeper layers are more compacted, and roughly follows the same slope as the surface. In these conditions, ground water will move in the same direction as runoff water, only at a slower speed.
It
should be clear by now that if we want to increase water retention we need to prevent runoff water and increase capilarity.
Moisture in the non-saturated part of the aquifer.
As more water is infiltrated in the ground, the water table rises, this is the part of the aquifer that is saturated with water. Most plants live on aerobic bacteria, but this bacteria can't live in water saturated soil. A tree may dig its
roots in the saturated layer, but it needs some unsaturated layer where the microbes it feeds on can live. So these plants need to be above the water table, or be adapted to live in water.
The ideal condition for the roots in aerobic plants is humid soil with pockets of air: The earth should not be too compacted, and the water table has to be below the
root zone.
When it rains frequently, the top layer is usually moist, plants take from seepage water and it rains again before the ground loses its moisture. When it doesn't rain as frequently, the soil may remain humid if the aquifer holds
enough water and the water table is close enough. Otherwise, the top layer will dry and kill any plant not adapted to draugh or with
deep roots. Remember that the earth behaves as a sponge: when it is too compacted, the grain is too fine, it absorbs slowly but given the time the humidity reaches high; when it is too loose, it absorbs fast but the moisture doesn't reach high.
In clayey soil, with low water table, it becomes a brick. Clayey soil with high water table may turn a swamp. Sand with low water table, a sand desert. Sand with high water table can be an oasis.
Organic matter in the soil
So far, I've discussed the way inert soil behaves, but there's a game changer: humus. Humus is a long lasting organic matter that results from decomposition. A great advantage is that it can absorb water and increase in size, thus changing the porosity of the soil. When humus is dry, it is compacted, behaves like clay: takes longer to absorb, but on the other hand, it is able to drive moisture from deeper ground. When humus is wet, it is looser, behaves like sand: absorbs and drains fast, preventing water saturation, while retaining some water inside its fibers.
A healthy soil full with life has the same physical properties than a sponge. It is hard like dirt when it is dry, and soft and fluffy when moist. Also, it is clumped together by the roots and fungi. A moist chunck of soil is able to keep its form and we need to make some effort to break it apart. As you may imagine, substrate for pots as we find in
gardening centers is not healthy soil, it is too loose.
While humus can be added to a soil as amendment, the best way to have it is by letting Life do its work. A soil full of life, lush with plants and animals, will create humus continuously on its own, until the point of balance where decay meets production.
If uncertain about the difference of dirt and soil, check this post:
https://permies.com/t/63914/Soil
Hydrology of pots
All the above also applies to pots: The water canning is the rainfall, the plate under the pot is the water table, and the substrate is the ground. We may use only humus, perfect for growing but the decay rate will be high require us to purchase more substrate every year, or a mix with a high percentage of organic matter that we can replace with homemade
compost. A cactus will prefer a sandy soil mixture, while tropical plants will prefer it more clayey. In any case, we must avoid water saturation for long periods, since this creates anaerobic bacteria that kills our plants (it's easy to recognize by the putrid smell).
In wicking pots, water saturation in the reservoir is not so critical because there's little nitrogen coming to the water, so bacteria can't proliferate. But it's worth to be vigilant.
If the substrate becomes completely dry, it will be hydrophobic, the water will drain down without really hydrating the plant. Ever noticed that the water reached too fast to the bottom of the pot? that's it. Using the finger 5 cm deep we can notice that the watering did nothing to hydrate the pot. In this case, we can fake a water table (puttin a deep dish under the pot), or we may water a little the pot every few minutes until it recovers. In extreme cases, the pot may be immersed in water until it rehydrates, then left to drain.
The effect of tilling
Tilling makes the land to degrade. By removing the living mulch it creates a hard pan that prevents rainwater to sink in and allows groundwater to evaporate. So tiller farmers follow two strategies: shallow tilling right after a hard pan is formed, so the ground water doesn't dry so fast, or deep tilling before planting, so the hard pan is formed below the root zone. Initially, deep tilling sounds like a good idea, the hard pan will hold some water close to the root zone where crops may benefit more, but in the long run it dries the aquifers and reduces the organic matter in the soil, losing the amazing properties of humus.
A note for hugelkulture and compost piles
The hill-alike
garden bed which is so popular among permaculturists, faces some issues when hydrology is not understood. For the decay of the woods it needs that some water seeps into it rather constantly, decomposition is made by fungal activity, and they hate being dry. When rain is frequent, the mound may still be humid and absorb all the water with little to no runoff water. But when rain is abiotic (desert mode), the surface dries and little water seeps into the hugelkulture. Moreover, the excesive rains when it is storming might erode the surface and ruin the growing.
The opposite may happen if the mound is too rich in woody material, making the substrate too porous and unable to hold water long enough.
The same goes for compost piles. They need to be hydrated for the fungal activity, but if it becomes too compact (or too hot!), it will promote anaerobic bacteria. Too loose, and it won't be able to retain water. If its surface dries out, it will be too difficult to hydrate it again.
