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Fast draining chalk subsoil/geology - how to harvest water and is it relevant?  RSS feed

 
Michael Cox
Posts: 1667
Location: Kent, UK - Zone 8
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I've been mulling this for a while and not really got anywhere, so I'm throwing this out there.

Here in the south east of the UK we get plenty of autumn and winter rainfall, but it is a little scarce sometimes in the growing season. We end up irrigating the formal garden and veggies to keep them thriving, for which we have a bore hole that supplies 60 litres per minute or more.

The area is a fairly thin layer of top soil with a reasonable amount of clay, but beneath is a few hundred meters deep of chalk. All the local valleys are dry, with rivers that occassionally flow when the aquifers are charged (averaging every 7 years, and flows for about 3 months of that). We never really see surface water or have run off to intercept because the water drains through the soil so quickly that it is just gone.

I can't see what benefit earthworks would give us - swales etc... - as all water in this region infiltrates quickly. The only places where we occassionally see erosion are in the surrounding ploughed fields if there is a major storm.

The house is at the lowest point in the grounds and currently sends roof runoff to storm water drains. It would be very difficult to redirect that water to somewhere more useful on the land because of the limitations of living in a listed building (we can't mess around with the structure). Over the length of the gardens we have maybe 6m drop, across 200m.

What benefits, if any, would we actually see from water catchment earth works in a situation like this?

Mike
 
Jennifer Wadsworth
Posts: 2679
Location: Phoenix, AZ (9b)
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Hi Michael - interesting question indeed.

I have a few questions of my own.
--How close are your veg beds and formal gardens from the house? What I'm getting at is would it be easy to catch water in tanks/barrels from the house roof and use that to irrigate the gardens?
--Swales/earthworks are tree growing systems and work to rehydrate landscapes with the swales slowing and spreading the water and the trees uptaking the extra moisture and evapotranspiring to aid the hydrology system. Over time (geoff lawton claims 3-7 years across climates), the swales will cause more and more moisture to be held in the soils (while simultaneously building more soil with chop and drop) and in many areas you may see seasonal or even continuous springs form. This rehydration serves to stabilize flood/drought cycles and minimize damage by either.

I guess it depends on what your overall plan is for the land and how sustainable you want to be and what kind of land stewardship works for you with your given time, budget and local laws being the restricting factors.

As far as the chalk being fast draining, perhaps a pond/dam lined with clay high up in your system (or a series of ponds/dams) may work, if, like you say you have a fair bit of clay onsite. The ponds could feed swales to either side if they should get over full. Geoff did a whole earthworks section in his online PDC on this and then another bonus section on top of that. Cool stuff.

 
Michael Cox
Posts: 1667
Location: Kent, UK - Zone 8
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Location of the beds + gardens.
The gardens are close to the house, but the intensive veggie beds are over 200m away and uphill. Both veggies and the formal garden get irrigated in summer (usually a weekly deep soak with sprinklers). There is no convenient location for a tank that wouldn't be smack in the middle of the formal garden. That is an absolute non-starter. There might be a spot on the downslope boundary of the land, but it would definitely need pumping to be used on site.

Regarding goals:
I'm pretty limited by what I can do on site with permaculture to just the main veggie area (the land is my parents and they need convincing that it won't jeopardise the formal gardens). The total area is about 3 acres, but I've got about 1 acre of that to work with - half an acre of fruit and veggie as an inherited raised bed intensive allotment, a quarter of an acre of wildflower meadow/orchard, quarter of an acre of degraded ex farm land with poor soil. This has a bunch of selfseeded willow coppice, but I'd like to transition to a small food forest.

It might be possible and beneficial to put some swales in the strip of old farm land as part of establishing a food forest - I need to build and improve the soil here as it was ploughed for decades and lost all organic matter.

Regarding the chalk being free draining - the borehole hits water at 40m below ground level. All the water in this area is supplied from boreholes so replenishing the aquifers should be a priority. Diverting water from storm drains to soakaways might be away forward? I'm aware that the water table used to be a lot higher - there are many hand dug old wells in the village at a depth of around 20m.

If I had more land (for example the 20 acre field uphill of the house!) in this area under intensive cultivation I'd be looking at keyline/swales more seriously.
 
Jennifer Wadsworth
Posts: 2679
Location: Phoenix, AZ (9b)
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A dilemma!

My initial thoughts are to "start small" and "observe".

Seeing as how you are not dealing with a huge amount of land, you could (stealthily) install some micro swales or DIY keylines. You could, say, hand dig a small swale (ditch) on contour above the veggies or main crop and then back fill with some mulch and plant out the berm so it would blend in with the rest of the landscape. Perhaps even grow a nectary on the berm for beneficials.

As for DIY keyline. I often take a straight shovel and "rip" a line across my flat property perpendicular to the flow of water just to slow and sink water. I do this in the bottoms of infiltration pits (I live in a desert on flat land so infiltration pits work best for me). Just stick the shovel in, give it a wiggle back and forth, pull it out and repeat right next to it until you have a DIY "keyline". I put mine about 18 inches apart. Over time, they both sink water and collect nutrients, building the soil which then holds more water.

I've also tried vertical French drains - essentially deep holes I've dug at various points around the property nearby to more water-needy plants and backfilled with woodchips or woodchips and shredded paper. Again these tend to attract water and nutrients and build soil over time. They won't last forever but they serve their purpose and are fairly invisible in the greater scheme of things (or when you're trying to hide a permaculture project).

As for the cistern. There are also "wet installed" cisterns where the downspout from the gutter is run underground to a cistern some ways away, even up hill. The cistern is fed from pipes up through the base. The top of the water in the cistern will never be higher than the level (the roof gutters) where the water originates. But cisterns are expensive to install and sometimes problematic to make "pretty". I'd try some stealth-earthworks first.

It would be fantastic if over time you were able to raise the water table bit by bit! I, myself, am trying to make a river run again
 
Michael Cox
Posts: 1667
Location: Kent, UK - Zone 8
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I know, I saw your thread and other videos I've seen are part of my inspiration for trying to get water sunk. I just wonder if I'm on to a hiding to nothing in this particular area. After all, all the rainfall in this region is soaking in and recharging the aquifers and we still only get flow every 7 years or so.

As far as making the existing rainfall more useful goes, we need a better way of holding it in the surface soil so that it doesn't sink beyond reach of the roots. I've dug a few big pits and seen root balls of fallen trees around here. They rarely break more than a few inches into the chalk so anything that has left the top meter or so of soil is essentially lost.
 
Jennifer Wadsworth
Posts: 2679
Location: Phoenix, AZ (9b)
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First of all, your current efforts are probably making more difference than you realize.

Some random thoughts:
--you could line the swales with a layer of clay (you'd probably have to experiment with how much), thus slowing the drainage into the chalk layer.
--biomass holds moisture, therefore woody mulch on the surface would help retain moisture in the top portion of the soil for longer.
--the plants themselves are holding moisture in those upper soil layers. I would be tempted to plant a "pretty" polyculture (so as not to offend the parents/neighbors) with many layers including natives accustomed to mining the chalk layer for water/nutrients. The water holding power of leaves alone is HUGE and the evapotranspiration aides the hydrology cycle of the region as a whole.
 
Michael Cox
Posts: 1667
Location: Kent, UK - Zone 8
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Your 'Swales are tree growing tools' comment sparked a little light bulb for me. We planted a few new trees last winter on some very gently sloping grassland. Watering them during the establishment phase was a real pain - we wanted to be quick and efficient, dumping a bucket or two on them then going, but water dumped that quickly was running off the planting spot.

I actually made a few tiny berms to hold back water by each tree.

I guess I'm so used to seeing photos of big deep swale ditches that I hadn't considered another scale for my situation. I could hand dig a few 6" deep Swales on contour and establish my future fruit trees by them. Water doesn't sit around long enough to need more than that depth but it will help the trees establish. Over time the shallow Swales will probably level our again with the surrounding soil, but they will have done their job by then.

Mike
 
S Bengi
Posts: 1359
Location: Massachusetts, Zone:6/7, AHS:4, Rainfall:48in even distribution
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I am going to assume that in the summer, your driest month gets over 2inches of rain aka 50mm.
If this is true then you are in luck esp with 3ft of soil aka 1meter.

1)Start planting "drought" hardy cultivars/species of veggies including perennial ones, by buying them,
or saving the seed of the hardiest plants and thus creating your own cultivar...this could be done on experimental garden.

2)Mulching is really your biggest friend, get some straw/hay/wood-chip to cut down on soil evaporation

3)To give your plants a boost plant 25% to 50% of your plot as Nitrogen fixers, they will look "healthy" without the extra water.

4)Improve the minerals available to your plants by amending your soil, with compost/rock dust/micro mineral

5)Recruit the right bacteria and fungi to work for you to improve the health of your plant.

6)Please use sacrificial dynamic accumulator as living mulch daikon radish will go down 2m to get minerals and water.

Let your current gardens stay as they are and do your trial on the ex-farm portion of the land.
 
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