Calculating water "banked" for future pumping in the desert landscape?

I would like some guidance in calculating the quantity of ground water that is “fair” to pump for my residential and permaculture landscape use. Rather than say, “use as little as possible,” I’d like to give myself a water ration or allotment based on some rationale.

As a permaculture challenge, I am limiting the quantity of water used on this property to the average annual rainfall. Since I have a well, I have decided to use the aquifer as my “holding tank” instead of buying giant plastic tanks to hold rainwater that runs off the building roofs. Any water that I pump has to be offset by rainwater percolating into the ground. How do I measure this extra runoff?

Some site info: 1 acre property formerly in the Rio Grande flood plain. The native soil includes layers of boulders, gravel, sand, silt and caliche. Drainage is good below 6 feet where the subsoil is boulders, gravel and sand. The first 4 feet is mostly silt and caliche. The ground water starts at 50 feet and the sand is barely damp at 4 feet.

From what I can gather, the caliche requires 25% water to reach saturation. So 1 foot of water will saturate 4 feet of caliche. Based on the fact all 12 inches of annual rainwater would be used to saturate the caliche, if this land was 100% exposed, no rainwater returns to the aquifer below this one acre site.

For water to actually enter the aquifer from my property (assuming no runoff), some of the land must be covered with impermeable surfaces to create a bankable surplus. The square footage of impermeable surfaces on this property (pavement, roof coverings and garden walls) prevent evaporation and increase the quantity cubic feet of water seeping into the aquifer by a number equal to the square footage of the impermeable surface. For example, if my home and out-building roofs + walls + paved surfaces = 5,000 square feet, I could pump 5,000 x 12 inches per year to get the total cubic feet of water that I can redirect: (7.48 gallons per cubic feet) or 37,400 gallons per year (excluding what filters through the leach field).

Anyone have any improvements to this method of measuring impermeable surfaces as a means to calculate extractable water harvesting rations?

I think other variables will make it impossible to calculate anything meaningful, it sounds like an entire Phd to work it out!

Here where we have 32inches of rain a year we are told that none of that reaches the aquifers after the trees have leafed out, so only water that falls during the winter here gets down to the water table. everything else either leaves as surface runoff if the rain is heavy, is used by plants, or evaporates directly back into the air.  Plants are very good at drying out soil so the more plants you have the less rain will be getting down to the subsoil never mind the aquifer.

How the rain falls will also impact it, a short shower on a hot day will just 100% evaporate off, heavy rain might run off the property as well, but unless that makes it to the river you can probably count it as absorbed. Heavy dew will increase the amount of water you are actually getting if it manages to collect somewhere and not evaporate.

You are probably right, Skandi, that there is no answer that could account for all the underground mysteries. Dr. Seuss wrote a most memorable book on the subject called, McElligot’s Pool, which I highly recommend. In any event, how does one determine how much ground water is “fair” to pump if we make an effort to optimize seepage back into the land using methods such as dry ponds, covering soil, creating shade structures, "building up" with adobe to cut low gardens, using large stones as landscape features, using berms to create runoff and water oozing opportunities, building French drains, covering land with local flagstone, making naturally repellant adobe risers for outdoor living spaces and so on. These projects take tremendous effort. How much groundwater can I use as pay-back? Most of our water comes one time per year as a monsoon. Rather than store the deluge for a year, is there any way to translate our ingenuity and effort into gallons so we can avoid using precious land for plastic water tanks?

I can't answer your original question, but there are steps you can take to store seasonal water in the landscape that are cheap and highly effective. A hydrated landscape both replenishes aquifers and needs less irrigation through the dry period.

Thanks for the link Michael. I actually live on a "sand dam" system. Instead of gathering water by hand as in the video, locals here pump the water out of the sand-gravel reservoir. The neighbor (a corporation) about 3 miles from me pumps over 7 million gallons per day. As you can imagine, the effort of collecting water by hand and storing it in tanks seems really absurd when the community lacks a shared sense of responsibility to replenish the resource. The natural sand dam system here has a tremendous amount of water and some pump and others try to create a micro-strategy for replenishing an increasingly scarce resource.  I would like to pump some water from the communal sand dam instead of moving buckets around. I'd like to prove, at least in a broad sense, that I put back what I take out.

Amy, I really like what you are trying to do.

I found this Thesis put out by the state of South Dakota.

On page 6 it explains Groundwater Recharge.

Groundwater recharge happens when a part of precipitation on the ground surface infiltrates through the soil and reaches the water table.

The paper goes into how to calculate groundwater recharge.

2.4 Estimating the groundwater recharge is one of the most difficult measures regarding groundwater resources.

2.4.2, 2.4.3, and 2.4.4 show methods to calculate the groundwater recharge.

https://openprairie.sdstate.edu/cgi/viewcontent.cgi?article=1957&context=etd

I hope this will be of some value to you.  I found what I read of the paper very informative.

Amy Gardener wrote:Thanks for the link Michael. I actually live on a "sand dam" system. Instead of gathering water by hand as in the video, locals here pump the water out of the sand-gravel reservoir. The neighbor (a corporation) about 3 miles from me pumps over 7 million gallons per day. As you can imagine, the effort of collecting water by hand and storing it in tanks seems really absurd when the community lacks a shared sense of responsibility to replenish the resource. The natural sand dam system here has a tremendous amount of water and some pump and others try to create a micro-strategy for replenishing an increasingly scarce resource.  I would like to pump some water from the communal sand dam instead of moving buckets around. I'd like to prove, at least in a broad sense, that I put back what I take out.

I didn't realise there were any sand dam systems in the US.  That's great to hear. Do you have a link to any details about the project? I'd love to read more about it.

The corporation use of a share resource like that seems absurd at that scale - hard to see how it can be justified in a region where water is scarce. Just how big is this sand dam system? Is the corporation's water extraction new? Has it impacted the viability of the project as a whole?

What happens to their waste water when they are done?

The Rio Grande and its many sand filled tributaries is a natural "sand dam system" that is enhanced by dredging and removing infill in stepped check-dam fashion. Due to deforestation and misapplied agricultural methods, the largest pollutants of the Rio Grande system are topsoil and sand runoff. Dredging the natural runoff systems to create large seasonal seepage ponds is the current practice to protect villages from seasonal flooding. Due to the fact that the sand filling the Rio Grande has raised the surface water level, creating holding stations for the  are designed to protect communities from flooding. Dams up and down the river system hold water and the desiccated environment blows into the runoff channels. Ground water is well below the layers of blowing sand particulates. You can investigate the Rio Grande, its dams and tributaries on Wikipedia https://en.wikipedia.org/wiki/Rio_Grande
Following Paul Wheaton's path of "Building a Better World Instead of Being Angry...," I'll focus on my efforts on measuring recharge for future pumping.

Got you now. You are talking about the overall river system.

On your own property, what happens with the water that does fall? How much of it leaves your land, vrs sinking into the soil? I don't think you have described the actual landscape you are personally dealing with - do you, for example, have smaller channelled flow on your property where you could install you own water catchment/infiltration systems? Do you have swales or similar on contour? All of those features will contribute to sinking your water.

Unfortunately it sounds like the overall water catchment area is simply massively over exploited though; the wikipedia article I saw suggested that the amount of rainfall that eventually reaches the sea via the Rio Grande is way down on historic levels, because of the amount of extraction upstream. It isn't really analogous to the smaller sand dam systems being installed in communities in Africa etc... where the seasonal rainfall typically flows to the sea and is lost. What ever justification you use for your own use, it seems like increasing extraction of water in your circumstances is adding pressure to the existing stressed system.

Anne, I will read Badr Qablawi's thesis tonight - thank you very much for locating this research.

Michael, thank you for your interest in the details of this exploration. We scale our applications, such as dams, to models that we observe in nature. I have never been to Kenya but I see the small dam systems built centuries ago on the Rio Grande still in use today walking distance from my home. I can only draw on my experience here in New Mexico but on the scale of harvesting seasonal water along small tributaries leading to the Rio Grande, sand dams seem analogous. Yes, the system is incredibly stressed.
Here are the answers to your questions:
 100% of rainwater stays on the property via swales, infiltration pond, permeable berms:
 The infiltration pond is 100 sq ft, 4 feet below ground level and below the nearly-impermeable caliche layer
 The berms surrounding two sides of the pond are 12 feet above pond floor
 Other swales and drainage trenches on the property make up 15,000 sq ft
Impermeable constructed surfaces on the acre (43,560 sq ft) equal about 6,100 sq ft (14%)

How would you determine how much water I could pump?

In conclusion, after reading the thesis linked by Anne Miller, and listening to the comments on this thread, I agree that groundwater recharge happens over a very large geological area and one acre cannot be realistically evaluated or calculated.

Teasing apart the four calculations in Badr Qablawi’s, A Comparison of Four Methods to Estimate Groundwater Recharge for Northeastern South Dakota, reaffirms four permaculture practices that one can do on their property to help with groundwater recharge.

1. Prevent Runoff
2. Prevent Evaporation
3. Create systems to support infiltration
4. Build soil water storage capacity

Regarding my personal allotment of ground water, I have used the USGS data on
https://www.usgs.gov/special-topic/water-science-school/science/water-qa-how-much-water-do-i-use-home-each-day?qt-science_center_objects=0#qt-science_center_objects
This says that the average person in the US uses 80-100 gallons of water per day.

By keeping water use at the low end of this average for two people, I could pump 2.5 gallons per minute 64 minutes per day. To the best of my ability, this water will remain in or below this landscape. Given that I’ve done my best to fulfill the 4 practices noted above, I’ll give the acre the 389 annual pumping hours. That's a shocking 58,400 gallons per year.