Shallow Rocky Soil Above Hardpan

Greetings permies,

I am trying to help a friend with a site design. I have slowly infected him with permaculture over the years, and he bought 4 acres in 2018 and now he is letting me assist on some design aspects. He is excited and doesn't know much about any of this, and I don't have hardly any experience, but a lot of information. As far as zones, and sectors, and species, we will figure out something for that separately. I have run into a road block which I have never heard of being addressed in the literature/videos I have as yet discovered. Anyway, about the property and the problem I have:

This is a 2:1 rectangular property around 400m or 1300ft altitude with almost no variation. This property is in southern Oregon, in White City. This property is situated in between the two places discussed in the following excerpts:

Much of the DWA is a grass covered plain known locally as the Agate Desert. The desert landform is described as mounded prairie, forming a pattern of low mounds and depressions. The desert is underlain by a layer of cemented gravel, or hardpan, which causes shallow pools, called vernal pools, to form in the depressions during the rainy season.

The Agate Desert is an alluvial fan created by glacial deposits of coarse debris. Agates reach the surface by action of physical forces and pocket gophers. A shallow layer of clay loam overlies cemented hard pan creating patterned ground with mounds and vernal pools. Pools fill in winter and spring from 48 cm (~19") of annual rainfall. Summers are hot and dry.

When my friend built his house, he had to mound dirt on top of his water lines to pass inspection (24" requirement), because the hardpan started around 18". In digging the poles for a barn, he hit hardpan at 24". I had noticed when he moved to the property all the trees, which looked to be 20-30 years old, were dead standing and pretty knarly. This is a very windy area because there are almost no trees, because they can't get deep roots. I've heard the power goes out in this city very often because larger trees fall over during every windstorm.

Looking at well logs in the area, there doesn't seem to be any soil deeper than 2', and the hardpan goes down to over 320' (the deepest well I saw). Well loggers call it hard or fractured claystone, sandstone, or cobblestone. The most water from these wells was 5gpm.

Speaking of water, as is mentioned in the above excerpts from those websites, this property will have a large "vernal pool" aka no drainage from later Fall through early Summer.

The soil itself is ~30% clay, ~5-10% organic matter, humus etc. and close to 60% stone of every size from fishtank sized pebbles to 5" stones.

The storm winds come from the Pacific Coast (west) and the seasonal wind is from the north.

The original plan was to have a berm along the north, and bamboo on the west (east/west sides being the twice as long sides). Through sheer happenstance, that will probably be a good start. I think that bamboo roots will be able to grow wide enough to allow the 40-50' bamboo to be a valid windscreen and not blow over.

1) What are some issues I might run into with the hardpan?
2) How would you store water?
3)Is a pond off the docket?
4) Is it at all necessary to remove rocks from the current soil?
5) Is the best strategy going to be importing soil and making berms and hugelkultur?
6)Are swales an option because of the poor drainage?

Thanks for any and all ideas.

Kevin,

Welcome to Permies.  You have quite a list of concerns for your project.  I don’t think I can hit every single one of them, but I will see what I can offer.

I would think that the bamboo would work just fine.  Do you have plans for running or clumping bamboo?  The reason I ask is that around me, running bamboo gets out of control very easily.  Running bamboo can work just fine, but generally needs a root barrier to keep it under control.

But clearly, your biggest concern is the soil, or lack there of.  You mentioned importing soil.  You could do this, but my recommendation would be to bring in wood and chip it up.  Alternatively you could just bring in lots of woodchips.  Rotting woodchips make a wonderful, fertile bedding material.  If you can get some wood rotting down you can grow just about any crop you desire.  On top, the rotting wood will really soak up a lot of water during your wet period.

I am just speculating, but could you plant some type of cover crop that will drill a hole into your layer of pan?  I have worked with pan before and I know how frustrating it can be, but usually there is at least some plant that can get a taproot into the layer.  If you can find such a cover crop you can actually build up some soil.

I will have to keep the water storage on the back burner for now, but I think that you can at least start some soil building right away.

Good Luck and please keep us updated.

Eric

Eric,

Thank you for the welcome and the quick response. There is a strong sense in my friend of wanting as little to do as possible of daily chores with as little encroachment on the neighbors as possible, so clumping bamboo is necessary.

I had not considered just bringing in woodchips, and no soil at all. I realize there might be concerns in the sourcing of the soil/compost. Wouldn't we want soil,compost and woodchips? We are intending to turn about 1000 sq. ft. into garden this coming season. A large part of that will be hugelkultur on the north border, but I can't put the soil from the property on that, let alone the areas near the ground we want to plant veggies. Also, can I really plant into woodchips? I understand you said rotten woodchips make a fertile bedding material ... won't that take a few seasons to even begin? There are a few dead trees on the property which will work for the hugelbeds we make this year, but in coming years, there are not really any trees in the vicinity which can be utilized. Even that will have to be imported. If we want to plant something in the next couple months, how could we not import soil? I think maybe the area we want to use next year could use a few loads of chips this year, so we don't have to work through this again next winter.

As far as something to break up the hardpan, I would defer to anyone else to suggest what kind of plant might be able to do that (literally think like sanstone at the beach).

1) What benefit could I or a plant gain from drilling into hardpan?
2) Are there any tree species anyone might be able to suggest that would like to have wet feet 6 months of the year in zone 7, and spread roots wide enough and grow short enough to work in this soil?
3) How many feet of soil do I want under fruit trees to keep them happy?

I think I have an idea what we could do in an ideal situation, but this isn't my property, and I am not very willing to suggest drastic delays for the sake of principle. I am having a hard time seeing how importing gobs of soil isn't better than gobs of woodchips. Perhaps you are thinking of a specific reason on which you could elaborate? I may just be at a lower level and not understand an implied nuance in your suggestion. I've not taken a PDC, and have never met a real live permie. Thanks again, and looking forward to the conversations to be had.

Kevin,

So in this post here I am going to focus on the woodchip part of the equation and I will address the cover crop separately.

The reason I recommend the wood chips is because of the enormous fertility of properly composted woodchips.  I will detail what I have done with my garden (which is in an entirely different part of the country from you, so keep this in mind).  I have recently discovered wine cap mushrooms.  What I do is cut up a bunch of brush on my property and rent a chipper and make mountains of chips that then get spread into beds about 12” deep.  The first year I like to dig fertile holes and backfill with bagged manure, plant tomatoes (but you can certainly use other crops) and then Inoculate with wine caps.  The manure and tomato roots really help the mushrooms grow and you get to use the bed the first year.

By the second year, the woodchips are substantially decomposed and are incredibly fertile.  You very likely will have to top off the chips the second year.

By all means, you can bring in additional soil if you want and hugel mounds can be great as well.  I have had great luck with the woodchip and wine caps and I think that this can be helpful to others as well.

I will try to detail more later, but if you are still interested, the woodchips and wine caps can actually build extremely fertile garden bedding.

Eric

I am all about the mushrooms, so I would love more info on that. Are winecaps better or easier than some other species in woodchips? I have intermediate level experience with mushrooms. I have easy access to Douglas fir chips mostly. Not sure if any mills around here produce anything else. Also, fir shreddings are a lot cheaper, $15/yd vs $35/yd, and they would decompose far quicker. I assume that wouldn't be a bad thing right?

Digging fertile holes and backfilling with manure looks like what, precisely?

Since this was posted in earthworks because I thought that was my main problem, I wanted to discuss a pond idea. We have access to a skidsteer with tracks. If I just scraped with a skidsteer to push dirt to the sides of a theoretically 2000sqft area to a potential depth of 2', what would be the best way to seal the side sloping parts? How big would the sides be ideally? 6-8' high, and 6-8' wide? The bottom would potentially be sealed, and if not I understand some techniques to achieve that, but would I need to import non rocky soil that is high in clay to keep it from leaking? Since the soil is fairly high in clay, would it be viable to just compact it as much as possible and hope for the best? I.e. at that point can gleying or pigs or whatever else be utilized since any potential leaks will be small and the structure will be firm enough? Wish I knew how to ask this better.

Kevin,

So regarding wine caps, these mushrooms are sort of a perfect starter mushroom for mushroom newbies which you are not,  Wine Caps are aggressive colonizers and decomposers of wood.  The sow easily, grow fairly rapidly, and best of all are not terribly picky about their location.  What I do with wine caps (and just because this worked for me, please don’t assume that you cannot make alterations or adjustments) is I made raised beds about 8’ wide by 16’ long.  These were filled about 1’ deep with woodchips.  

I then excavated out 8 fertile holes which is simply digging out chips about 1’ wide at the top to a depth of 8”-12” deep.  Save those excavated chips for later use which I will get to.  Now fill the fertile holes with bagged topsoil, manure, or your own good topsoil if you have it.  The purpose is multi-fold.  Wine Caps like to have a soil interface and the fertile hole will provide that interface.  The fertile hole will provide a good nutrients for plants grown in them, I personally prefer tomatoes for a variety of reasons so for this example we will use tomatoes as the example.  Thirdly, the tomatoes will grow and really get big during the heat of summer. This is ideal because the tomatoes will provide dappled shade for the wine caps which actually prefer a bit of sunlight for growth.  Finally, the tomato roots will eventually exit the fertile hole and encounter the Wine Cap mycelium which will wrap itself around the roots of the plants.  Wine cap mushrooms grow best when they can get some intersection with a plant root—and the plant will benefit as well!

At any rate, once the fertile holes are dug and backfilled, mark where they are at with a stake or just something.  Then dig a series of little holes or pits around 6” deep around the bed and sprinkle spawn into the holes and cover.  Take the chips that you were saving from the fertile holes and sprinkle them back on top.  Now thoroughly water, and if you really want to take the extra step, cover with a layer 4” of straw and water again.  The extra straw will help protect from evaporation.  Finally sit back and wait.  It might well take a year to get actual mushrooms, but the fungi should be well at work in the meantime.

A couple of complications for you first.  Personally I would not scrape the soil down to bedrock/pan.  Wine Caps like to interact with soil so I would pile on top.  You can use any sort of edging you like—cinder blocks are cheap, sturdy and will last forever.  I am using 2x10 lumber painted with a masonry sealer.  

A bigger issue is the wood you have available.  Wine Caps love all sorts of hardwoods, especially the softer hardwood.  They will devour straw.  They tend to have some difficulty with pined and conifers.  I have heard of people doing this, but it is not their preferred substrate.

But you have experience with mushrooms so you might be just the person for this project.  Everything I have detailed here pertains to wine caps, but other mushrooms can work as well.  Oyster mushrooms are almost as user friendly as wine caps and are even more aggressive so they might stand a chance.  Perhaps you know of a more pine friendly mushroom?  I grow wine caps because I am an enthusiastic amateur who wants compost more than the actual mushrooms,  opinions vary on the quality of wine cap flavor.  My limited experience is that the optimal flavor is derived from when the mushrooms are rather small which can be difficult as the mushroom itself grows incredibly quickly so look early and often to get mushrooms at their peak flavor (I think it tastes like a woodier/nuttier portobello).

At any rate, these are all just introductions and you can make adjustments as necessary.

Good Luck and if you have any questions, please don’t hesitate to ask.

Eric

Kevin,

I forgot to add, I have a running thread detailing my journey from a complete fungal neophyte to having a degree of fungal competence today.  You can find it HERE:

https://permies.com/t/40/82798/composting-wood-chips-chicken-litter

hope this is helpful,

Eric

I’m not trying to be a downer here, but I don’t understand how you’re going to make enough soil for trees when you’re starting at 18”-24” deep.

We deal with caliche out here in AZ at those depths and it requires excavators, augers, jack hammers, and stuff like that. Soil is trucked in afterwards.

Even after all of that, drainage can still be an issue.

Eric, I will pursue the fungal questions on your other thread, thanks for the link. I wouldn't want mods feeling like this post needs to be moved.

Wayne, that is more what I was expecting people to say... Not a downer, I think a reality check here is appropriate. Is this something you personally dealt with? Your caliche is essentially the same as what we have, from what I can tell. How much soil did you end up having under your trees? Did you have some fail due to drainage? There ARE trees that grow in this soil, but the most successful, and only ones that get over 20', seem to be evergreen. Anything that broadens out becomes a sail. Well, if you have a suggestion on a range of how much soil would be needed for adequate drainage of fruit and nut trees, that would be the specific info I am most interested in. Or, just knowing that, in this kind of situation, such and such a number is definitely NOT enough, depending on soil type etc.

I can't help but think of bonsai trees, and potted trees in like 20 gal containers. They can fruit if their size is kept proportional. Obviously we want better quality food than that ... Maybe we will just experiment and find the answers if they aren't forthcoming. Seeds are cheap

I see situations like this all the time. I personally dealt with it in Phoenix and also fight with it to a lesser degree out here on a property I work on.

You must find out how thick it is. Sometimes it’s a foot, sometimes it’s 10’. I’ve seen acreage where it varied that much. Sometimes a jackhammer will be able to punch a few holes through it to allow drainage. This is a common practice for citrus trees. In your friend’s case, the hole will obviously be too shallow to accommodate most trees root mass. You would still need to remove the caliche.

I have a friend W. of Tucson who’s property sits on this stuff. We recently watched his neighbor have an average size septic system installed. It took 2 weeks to just get the hole for the tank excavated using a huge jackhammer mounted on a large skid steer and other tools.

Soil depth is something I see people shrug off. It’s an absolute killer, even if you have a yard full of heavy equipment. Having said that, I know Geoff Lawton deals with rock in the ME, so anything is possible...I guess?

Good luck sir.

Wayne,

As was mentioned in the first post, this layer of hardpan is several hundred feet deep. I checked well logs in the area, of which there are precious few because of the situation; none of the logs got past the rock layer (320' was the deepest I saw). This area was largely pasture land and marsh until city sewer and water came along. I would guess basically no homes 50 years ago (in this specific 1-2 sq. mi. area).

I feel like I should know what the ME is, but I'd love some help on that.

I am taking soil depth seriously, if for no other reason, all the bigger deciduous trees in the area are dead, dying, or have already fallen over.

Unless someone suggests otherwise, I think we will make large berms and hugelkultur beds for most if not all our trees. It is going to end up being WAY more earthworks up front and sooner than we would have liked, unfortunately, but alas, I don't want to delay too much.

Planting a nitrogen fixing tree/shrub as a pioneer would be a good thing for this situation if I never let it get very big ... right? I could use those for poles and coppice...

ME = Middle East.

1) What are some issues I might run into with the hardpan?
2) How would you store water?
3)Is a pond off the docket?
4) Is it at all necessary to remove rocks from the current soil?
5) Is the best strategy going to be importing soil and making berms and hugelkultur?
6)Are swales an option because of the poor drainage?

Kevin, I love the fact that you are trying, and your buddy will not fall into the trap that Gert just throws some seeds out and lives easy. It just isn't so. First what is the low winter temp? It will help for species suggestions.

1) You already have identified it. It is worse than rock because rock is a good source of minerals while hardpan is mostly clay. I lived in Tucson as well and never could see it getting to awesome, it is a long road but possible. I would say the good thing is that you have a huge amount of clay available, and if you can find a time in the late spring when the caliche (not sure htis is calcitic caliche but maybe)  is workable you might be able to move it to help your cause. the bottom will remain sealed. Thats a potential upside. So the only direction you are losing water is through evaporation and transpiration. Evaporation is bad, transpiration is OK because it means you have plants growing. If your skid steer is high flow as mentioned you can break caliche up and use it as a material elsewhere. Low slope is not a bad thing as making earthworks is very intensive, and they won't need to be huge. Check out the Soil Data and see what you can learn, it is pretty local so you will need a specific address. True caliche growing guide. Not sure if you are calcitic.
2) In the sonoran desert they made huge earthworks and planted shade to protect from evaporation. This seems like a good way to store water. As long as there is continuous collection of leaves you can build soil to heal the defects you make. It isn't fast. I would think an arid climate a pond will lose a lot of water, and in that soil type it will draw water from the surrounding area and lose that too. Any water would best be stored where it is not evaporated, i.e. underground in organic matter.
3) I think it would work against you, would be very shallow and not store much through the dry season.
4) Rocks are a mineral source, they are probably mostly small given the geology you referenced. I like stones smaller than my hand. I wish I had more stones! They are also good if you can break up your hardpan mechanically, you could separate them with a screen maybe and use them as rock mulch and air wells.
5) I think depends on the price. Sounds very expensive and energy intensive. I would think the bamboo as mentioned (I strongly recommend clumping but it runner might be a fine option there). Runner bamboo is being used as a forage in some places, but it can quickly get out of hand and become a nuisance. Other shallow rooted legumes interplanted will make a nice in-place compost mix and heal up the defects that are frank pools in spring. Species dependent on local climate and soil chemistry when you know those. Figs did really well in tucson they (along with bamboo) have no probelm with shallow soils. They might need to be on mounds they hate wet cold feet. Good success with pomegranate as well and olives if is doesn't get too cold.
6) The swales they had in Arizona were off contour because of the monsoons, you shouldn't have that issue. I think they would be a very important aspect of improvement- basically a dam to collect organics behind. The bigger they are the more water/organics can be stored uphill. Just need to dump in lots of organics which means leaf litter. I think shading is probably critical too- keep the soil cool as possible.

TJ,

That was a super helpful post, thanks. It got me thinking about maybe being able to attack the hardpan more effectively in late Spring once/if it has softened. We will definitely make an attempt. I really enjoyed rereading that Gert and Ferd article.

I had looked at the Soil Data study a couple times, I would have put some of that info in the original post but I couldn't select the text in the pdf and I didn't want to type it out.

The caliche info was helpful. We don't have that. In some ways, it seems what we have is worse. I will get in there periodically over the next 6 months and see if it has a soft season.

I will try to change my perspective on the stones. I know the minerals are helpful, and I want to work with them to make airwells and herb spirals and maybe even terrace the inside of a hugelbed sun scoop. If they were in soil and not clay, it would be a whole lot easier to separate them. I feel like a lot of water will be required, so maybe winter is the time to separate out the rocks.

My friend is a contractor so he will be bringing in the money and is going to rather spend to buy resources most of the time. If it takes 10 hours to shred 2 yards of wood, and he can buy 2 yards for 40 bucks (incl. delivery fee), no brainer; 10x more efficient. The shredder is using fuel also, and the lumber yard shredder is way more efficient than my 8hp one.

Figs grow here no problem, pomegranates too. Olives are another story, and I don't think he would want an olive. Bottom line, typical low is 25 deg, 19 deg is the 10 year low. Record low is 8 deg.

We will be hoarding organic matter, no question, and water is an absolute scarcity in summer, so non exposed water is a great tip. A duck pond is a strong desire however. We will shade it and protect it from wind to minimalize evaporation. Just gotta figure out the site work knowing we can only go down 2 feet.

Just found out today we have access to his in-laws 15 year old piles of cow and rabbit manure! That will make for a good start!

I am going to link to a new thread once we start actually working on stuff tomorrow. This one will stay for my earthworks questions, but the overall project I will put pics and stuff in a separate thread.

Thanks again for all the encouragement and tips so far everyone. I don't know why I waited so long to be an active member of these forums. I guess I didn't want to just ask theoretical questions with no where to try it out on.

Olives in my opinion are the ultimate resource. I have two that survived those temperatures- mission and arbequina. They produce an oil, which can be obtained pretty easily. In my opinion it’s way way better than oilseeds. Not as easy as rendering animal fat. But they make a durable leaf litter as well, and in mild winter will hold most leaves. You probably need some windbreak along with organic and they can be both. One issue I foresee is that you have a lot of competition in the same soil level since you only have a little, but you have a nice topography to make underground storage. I think making chinapa would potentially work for plants that can’t have wet feet ( most desert plants) and use the depressions as compost bins. Just make sure they are on contour so you save as much as possible- I’m assuming you don’t have torrential potential. Other plants that might work would be date palms we had them in Arizona.

I think with heavy equipment you might be able to dig a cystern at the top with or without other earthworks and use it for drip irrigation. If the root zone stays moist the roots can gradually deepen, but it’s tricky because only moisture tolerant roots will survive. I’m thinking something like elderberry. Other plants will access the cysterns available so you just seed one in the root diameter of each tree. I’ve dug a couple figs out and they will find water 5x the crown diameter. Olives are pretty good at it too. I suspect palms are not.

One other great biomass producer is pampas grass. Less invasive than bamboo but an amazing mulch plant. It makes an impressive root system, I use it like most tropical people use vetiver.

I agree that some type of olive would be a good idea, at least as a nutrient accumulator, so thank you for the suggestion. I don't know if animals will eat raw olive i.e. if it can be used as a food source, but I have always wondered if olives will fall off the tree and chickens or pigs will eat them and get a high dose of fat. Can you dry olives? I've only seen them from the freeway driving through Northern California. Having oil from on site would be pretty amazing. So, there was this farmer from Cali that moved to southern Oregon about 10 years ago. He reportedly spent well over $1mil on a massive olive orchard. They were the cold hardiest variety he could get, supposedly to 5 deg F. Well, the first winter was one of the coldest we've had, and it got down to 10 degrees. His elevation was about 1600, so a little ways up off the valley floor (1200), and being they were more sensitive the first year, he lost every single tree. Sold the property and moved away. No one around here will buy olive trees, or carry olive trees at a nursery, as far as I have seen. In theory, they would do fine, though. I have seen a few people with outdoor pomegranates, though. People around here just feel like, this is Oregon, we aren't subtropical, so none of that stuff should grow around here. It doesn't fit the decor. Heh, I'd love to have a "tropical" area, and fully intend to do that once we get a little ways in and can confirm where the hot pockets actually are.

I expect that we will have enough pond and swale or hugel that there won't really be any standing water eventually. I think we will be making raised beds to plant in, but I don't know that it would really be anything like chinampas. Yeah, not that much torrential water; it is usually pretty spread out. 1" in a day is almost unheard of. It flooded a little in late 1996/7 and people still talk about it. Much worse in other parts of the PNW.

I think with heavy equipment you might be able to dig a cystern at the top with or without other earthworks and use it for drip irrigation.

Can you expound on that? I don't quite follow how I would utilize a cistern to drip irrigate. I like the concept of having 5-10 gallon clay pots with a small hole in the base (I seem to recall unglazed don't need a hole...) that can be filled once a week or so to "drip" irrigate trees. Are you thinking something like that? Is a cistern a better use of energy/resources than a clay pot? My first thought is, I can move/replace a clay pot. Eventually, I won't really want to be watering the trees, right?

I love the idea of Pampas grass, was planning to put that between trees on the northern berm. Possibly it will work its way to other places around the farm later, as I have read it can be a forage plant. Any idea what types of animals prefer to eat pampas? Also, I read that vetiver is hardy to zone 7, so I wonder why you don't use it in Arizona? I am tempted to get some to try out because I like vetiver. I didn't even consider growing it until you mentioned it, heh.

We got 4 yards of fir wood shreddings and chopped up a cottonwood and started carrying it over to the hugel area. Right when we were digging an outline, a neighbor came over and started asking what we were up to. I let my buddy explain that he was indeed a "hippy" (this is a hotspot for rednecks) and was wanting to grow some of his own food to gain some freedom back. Well, after a few minutes he was on the phone with his brother ready to bring over an excavator to make short work of our digging project. He just needed my buddy to hang some doors when he has time. Good trade. Neighbors are a very valuable resource.

I’m probably the hippy around here and I agree, most “rednecks” are totally fine with self-sufficiency hippies. Foxfire types. They can’t stand trust fund hippies or sanctimonious people and I can understand it- it was really tough in the country in the south up until a generation ago and the regulations made it much tougher, including those that caused the economy of scale requirements in industrial ag. Yeah they fixed some issues with groundwater pollution (better to have 1000 hogs on 100 acres with adequate water control than 50 two acre lots where each had 6 hogs and no water control) but it did destroy the culture. If you understand and can sympathize with that, they are in my experience VERY interested in constructive ways to return to the old ways with improved sanitation and that is EXACTLY what I think regenerative agriculture and permaculture offer.

Olives are easy to cure. I did salt cure and brine cure, and I reused the salt (sea-90) for application on the plants. I prefer the brine cure but salt cure in tubs is much easier. I think you could reuse the salt several times, the brine only once. Olives are nasty unless cured, but they can be freshly pressed. If I can get them to do well here (in Virginia haven't lived in AZ for a decade) my preference is oil over olives. But you need many trees to make it worthwhile based on what I have seen. I can expound but essentially the small scale presses are a pain in th butt. I don't know if pigs will eat them fresh.

Vetiver in my understanding wants deep roots, which pampas grass is not as picky. they both have very fibrous root systems. I started with four clumps about 2' in diameter and every year I move root sections where I want them. The initial clumps fill back in in one year but I have many more starter clumps maybe 20 now. I don't think its either are a good forage, which is why I am trying to establish gama grass (a palatable clumping grass) this year. Gama is also good in those areas that are inundated part of the year, but like vetiver it wants very deep roots. It may tolerate your soils and if it does it is a topline addition.

I was suggesting you dig a pit, which would become a cystern in that geology. No need to seal the bottom, its naturally sealed. Using gravity to feed lower drip irrigation passively (via siphon) through the dry season. If you had a giant underground storage at the highest point it would be opimal. Keeping it from evaporating is the challenge. Tree cover or even a sandy fill would limit your loss.