Here in the Washington DC area, we’ve been flirting with drought conditions all spring. By mid-April, we were at the center of a little isolated area of drought. Now, we sit at the southern edge of an area of moderate drought extending from Canada southward. This, per the National Drought Monitor:
Source: U.S. Drought Monitor. The tan color corresponds to moderate drought.
It’s not a crisis. Nobody is talking about water restrictions. Our main source of water (the Potomac) is only now getting low enough to fall into drought territory (reference). Ironically enough, we’re getting a little much-needed rainfall as I’m writing this.
And it’s not as if this has never happened before. We seem to have some weeks to months of drought every few years, regardless. Here’s a graph of the past couple of decades. Darker = worse.
Source: U.S. Drought Monitor.
Still, panic early and often, that’s my motto. So, with my rain barrels just barely beginning to refill, after having been mostly empty for the past month, I’m going to take a look at my options, in case this gets worse.
Near as I can tell those are, in order of expense:
- AC condensate
- Municipal (tap) water.
- Rain barrels and other storage media
- Greywater
- Atmospheric water harvesting
The bottom line is that, right now, the only real option I have in a drought is to turn on the tap, and use municipal water. If we ever have a severe enough drought that we get a ban on outdoor watering, my only option will be to let my garden shrivel.
You don’t know what drought really is.
No, seriously. You almost certainly don’t know what drought actually means.
I mean, think about it. It’s a rather loosey-goosey term.
You’re thinking, lack of rainfall? Check out the Arizona desert in the map at the top of the pages. No drought there.
Drought is defined by an area being a lot drier than it normally is. So, sure, it’s dry in Arizona. But it’s always dry in Arizona. So that’s normal. Everything there is adapted to it. And that’s not drought.
So, weirdly enough, my drought is not the same as your drought, for purposes of monitoring drought. Or maybe, all drought is local. It’s all defined with respect to the climate norms for an area.
The details of how the U.S. Drought Monitor actually defines drought are beyond me. A brief description is on this web page. They integrate a lot of information, and a lot of judgment, to arrive at the map above.
For the moment, just assume that the drought status of an area is simply a function of annual rainfall, compared to the average for that area. Then if you took 100 years of rainfall data for an area, the four levels of drought severity on the map above correspond to how often you’d expect to see that little rainfall.
Source: https://droughtmonitor.unl.edu/About/AbouttheData/DroughtClassification.aspx
Now eyeball the second graph above, and, yeah, that seems to fit. Nothing shows, about 70% of the time. Some form of dryness appears about 30% of the time. But a significant area of red (extreme drought) only appeared once in the past couple of decades.
This is complicated by the fact that the data aren’t annual, but reflect different time periods for short-term and long-term drought. And the drought monitor uses a lot of other inputs. But that’s the gist of it. Drought occurs when rain (or soil moisture, or some other measure) gets below the 20% percentile of all observations for an area.
Putting that another way, in most parts of the country, you should expect to see some drought on a regular basis. That’s how it’s defined. The beginnings of drought (D1) are defined as being in the bottom 20% of (e.g.) the annual or seasonal rainfall for an area. You’re going to see that every five years or so, by definition.
So the mere fact of drought is no cause for panic. Not as drought is defined for the U.S. Drought Monitor.
Alternatives to rainfall for watering a garden.
Obvious, around here, just turn on the tap. But this section looks at some frequently-mentioned alternatives that might, in theory, be available to the average suburban homeowner. (E.g., I’m not considering drilling a well, but I am considering capturing condensate from my AC units.)
Just to put a benchmark on the amount needed, I’m trying to find enough water to put 2″ of water, per week, on about 300 square feet of garden beds. I’m going to round up and call that 400 gallons of water per week. Or some source yielding about 55 gallons of water per day, suitable for the garden.
Where can I find that, if the rain isn’t falling?
(N.B., American households use just over 100 gallons of fresh water per capita per day, or an average of 300 gallons per day per household (per the EPA). This includes, I think, outdoor uses. I find it hard to imagine how they could use that much other than by retaining inefficient fixtures, or maybe by extensive outdoor watering. Averaged over the past three years, our three-person household used about 115 gallons a day, on average. That includes outdoor watering. I attribute that to low-flow fixtures, rather than to any behavioral differences. With modern fixtures, for a household of three, you can do 21 toilet flushes, 3 10-minute showers, a load of dishes, and a load of laundry, and still be under 100 gallons for the day.)
AC condensate: Maybe 3 gallons a day
Bottom line: Fine for gardening, but not nearly enough of it. See air harvesting below.
Near as I can tell, there’s basically nothing wrong with watering your garden with the condensate from your HVAC system, except that there’s too little of it. A common rule-of-thumb (from some typical U.S. climate, I guess), is that an AC unit should put out about a gallon of condensate per ton of cooling per day.
The long and the short of it is that there’s just not enough to matter. Enough to water a few potted plants. Not really enough to water a vegetable garden.
Plus, modern window units no longer produce condensate. The sling any condensed water onto the hot coils of the unit, and so gain some extra cooling from that.
Municipal water: As much as you want, I guess. Watch out for chloramines, cost, and water restrictions.
Chloramines. As written up in Post G22-026, almost all municipal water supplies now use chloramine, not chlorine, for disinfection. Unlike chlorine, chloramine is persistent, and some plants are quite sensitive to it. (For example, my peas bleach out if I water them straight from the tap.) A simple activated-charcoal filter seems adequate to pull the chloramine out of thousands of gallons of water, for a cost of about $3 per 1000 gallons.
Cost: My municipal water now costs me a bit over $20 per 1000 gallons, combined water and sewer rate, thanks in large part to my Town using the water system as a source of general revenues. (And yes, I have to pay the sewer fee, even for water used for irrigation.) Still, a weekly 2″ of water, on 300 square feet of vegetable beds, works out to be just $7.50. Not a big expense for a hobby gardener.
Water restrictions. But the use of tap water pretty much begs the question of what to do in a severe drought. Plausibly, local governments would issue water use restrictions. And even though none of that would ever be enforced in this area, in theory, I might lose the right to use tap water to water my garden.
Rain barrels and other storage: No good in a drought.
I did a complete writeup of this issue, along with a numerical simulation, in Post G21-043. Bottom line is that a reasonable number of rain barrels will get you through a short time period with no rain. You’d need an impractical number of them to get you through a drought of any length.
Right now, I have eight rain barrels connected to my downspouts. And even in an average year, I’d expect them to run dry for some period of time.
I also tried a “core bed” or self-watering garden bed (Post G21-008). That’s a way of storing water inside a raised bed, by replacing the soil at the center of it with straw or hay. Near as I could tell, that simply does not work in my climate (Post G21-052).
Bottom line is that, for storing rainwater, you’d have to have something the size of a small pond or an in-ground cistern, in order to get a modest-sized garden through a prolonged drought. It’s just not practical to store enough rain water in barrels, of you have a garden of any size.
Greywater: 30 gallons a day, but many significant restrictions.
Bottom line: Not a short-term fix, and comes with lots of restrictions.
Greywater refers to household waste water that isn’t too dirty. Typically, its defined as water draining from showers, sinks, and washing machines. Any household waste water not infused with food or human waste.
Without breaking into my house plumbing, the only greywater I can tap is from my washing machine. Because we’re not in a water-short area, and I hang my clothes to dry, I use a traditional water-intensive top-loading Speed Queen washer. And it’s conveniently located right next to a ground-floor window, where I could direct the discharge to a barrel outside as needed.
One load per day (with optional second rinse) would put out about 30 gallons of water per day. This is a reasonable match with California-based estimates of about 125 gallons per household per week (presumably, with a washing machine that is more water-sparing than mine).
But you need to look to California for guidelines on how to use this in the garden. They are the pioneers in this area, and that’s one of the few places in the U.S. where greywater is used extensively for landscaping.
Turns out, the instructions are clear, and quite restrictive.
First, graywater should not be used for surface watering. I assume that’s to prevent runoff of detergents directly into streams. Instead, California guidelines call for it to be piped to wherever it is to be used, and released at least two inches underground. That’s per this extensive newspaper writeup. It’s adequate to lay a trench, put in the pipe, and mulch over it. Just as long as the water doesn’t reach the surface.
Second, you can’t store it. It should be used as soon as its produced, because if it stands, it’ll start to stink. (Same source.)
Third, greywater is not suitable for use on root crops, or on any crops where the produce comes into contact with the ground. No potatoes, no sweet potatoes, and so on. Its not suitable for some crops that are sensitive to components of detergents, such as fruit trees.
You have to take some care about the detergents you use, or you risk adding significant alkalinity to the soil. In our area, with the native soils tending to be acidic, I don’t think that’s much of a problem for me. But the writeups of that tend to be pretty vague.
Finally, not explicitly mentioned, but clearly an issue, you’d have to filter it before running it through a drip irrigation system. This isn’t a big issue — you have to filter rain barrel water as well. You’d just have to make sure to put a filter in-line with this system.
Atmospheric water harvesting: Not enough, not yet.
A final way to get water for gardening is to condense humidity out of the air.
Using the condensate from your AC system is the classic form of atmospheric water harvesting.
And you can, in fact, buy machines that amount to great big outdoor de-humidifiers. Where you use massive amounts of electricity to condense water out of the outdoor air.
But there are “passive” systems that don’t use, in effect, a refrigeration system to condense water.
The most promising of these, from a home-use standpoint, are those that use materials that will absorb moisture out of the air at a relatively cool temperature, then release it as they warm up.
An example familiar to most Americans is silica gel. That’s the stuff in those little packets inside (e.g.) boxes for new electronics, or packages of vitamins. The silica gel absorbs humidity out of the air, keeping everything else in the package dry. Some silica gels are “rechargeable”, that is, you can put them in the oven, and that will drive the absorbed moisture out of the silica gel.
For most of these systems, something akin to a silica gel is spread on a flat surface, then covered with a clear plate of some sort. At night, the silica gel (or equivalent) absorbs moisture from the air. During the day, sunlight heats the gel enough to make it release water, which then condenses and runs off the inside surface of the cover plate.
Effectively, it’s a solar still. Only, instead of distilling sea water, you distill water from some damp material. The dampness coming from exposing that material to atmospheric humidity. There are other systems that are far more cosmic than that. But one way to do it is to distill in the day, whatever water you capture at night, from some hygroscopic material.
Near as I can tell, no passive system currently available can produce nearly enough water to be practical for the home garden. Typical values seem to run around a liter of fresh water per kilogram of the sorbent. Roughly speaking, you’d need to have one of these the size of a tennis court to provide the 55 gallons per day that I’m seeking.
Although the science is progressing, it’s not clear to me that passive atmospheric water harvesting will ever be a practical solution for the home gardener. Atmospheric humidity just isn’t dense enough. Roughly speaking, at 60% relative humidity and moderate (20 C) temperature, air contains about 12 grams of water per cubic meter. To get my 55 gallons a day, I’d need to extract 100% of the water from about 17,000 cubic meters of air. Assuming I did the math right, that’s about 21x the volume of a 3,000 square foot house.
I just can’t envision some unpowered little box, sitting in the sunshine, capable of doing that. No matter what miracles of modern science ensue in atmospheric water harvesting.
Bottom line.
My only practical option is to use tap water until this current drought breaks. All other options are either impractical, restricted, or produce too little usable water. If we ever have a drought that progresses to the point of a ban on outdoor water use, my garden is toast.
And now the sun’s back out. Damnit.
