Irrigation

Irrigation is what makes agriculture possible.

The history of irrigation

Humans started irrigating crops (on purpose) as far back as 6,000 B.C. Those early efforts were nothing more than redirecting the flows of the Nile and Tigris Rivers in Egypt and Mesopotamia. Twenty-five hundred years later, irrigation science led to the invention of the Nilometer. The Nilometer was generally a stick, or some stairs, that went down into the river to measure its changing depth. Priests would use that information to ‘magically’ predict the next flood. Another 400 years would pass before anyone thought to build actual canals and dams to redirect water into agricultural fields. It would take another thousand years before the Romans figured out concrete pipes. In fact, we owe a debt of gratitude to those same Romans for inventing indoor plumbing - yay! Two hundred and fifty years later, Hammurabi (the man responsible for the first set of written laws) instituted water regulations. Some of Hammurabi’s water regulations include:

55. If any one open his ditches to water his crop, but is careless, and the water flood the field of his neighbor, then he shall pay his neighbor corn for his loss.
56. If a man let in the water, and the water overflow the plantation of his neighbor, he shall pay ten gur of corn for every ten gan of land.

I have no idea what ‘gurs’ or ‘gans’ are, but you get the idea - we’ve been fighting over water rights for a really long time!

Good water management

Many years ago, while living in an RV and traveling the country, I stopped at a midwestern diner in the wee hours of the morning for some breakfast. The old men who filled the booths and sat at the counter had but one topic of conversation: rain. They were comparing notes on how many hundredths of an inch fell on their respective properties. It was not what I expected, but it made me realize how much science there is behind irrigating crops properly. Good water management means understanding the necessary timing, volume, and application method best suited to your garden and landscape plants.

Farmers deal with irrigation water in terms of acre-feet. One acre-foot refers to how much water is needed to cover one acre of land (43,560 sq. ft.) to a depth of one cubic foot. If you do the math, that ends up being 325,851 gallons of water! Per acre. Each week. You see why drought is such a Big Deal in the world of agriculture! Now, as home gardeners, we will never be using water at those rates. Instead, we have to look at our garden spaces as micro versions of the same calculations. Instead of acre-feet, we need to look at square feet.

How much water does my plant need?

One of the most common gardening questions asked is: How much water does my favorite garden plant need? And the answer is always: It depends. [This invariably irritates some people, but it’s the truth.] Water needs are a function of too many variables to give a pat answer. Soil structure and makeup, plant variety, microclimate, stage of development, life stage process, plant size, root depth*, and exposure to heat islands are just a few of those variables. In California, UC Davis recommends one inch of water each week during the peak of summer. To fill a one foot by one foot area with water to a depth of 1 inch uses 0.623 gallons.

For example, let’s say you have a 4’ by 6’ raised bed:
    
    4 * 6 = 24 sq. ft.
    24 * 0.623 = 14.952 gallons

Your raised bed should receive 15 gallons of water during the peak of summer. In spring and fall, half that amount should suffice. Now, if you dump 15 gallons of water into that raised bed all at once, most of it will percolate down through the soil and disappear from the plants’ root zone*. It is far better to water every day during summer. This would mean adding a little more than 2 gallons to our example bed each day of summer.

* How deep do garden plant roots go?

Rooting depth changes, depending on the variables mentioned earlier, but you can use the information below from UC Davis for a general idea for mature, full-sized plants:

Crop	Depth
​broccoli, cabbage, cauliflower, corn, lettuce, radish, garlic, onion, celery, potpotatoes.htmlato	​1-1/2 to 2 feet
bush and pole beans, cantaloupe, carrot, cucumber, eggplant, beets, peas, peppers, summer squash, transplanted tomato or melon	2 to 4 feet
​asparagus, lima beans, pumpkin, seeded tomato, seeded watermelon, sweet potato, winter squash	​more than 4 feet

How to irrigate

​You have several options when it comes to how you get the water to your plants. You can spray with a hose, use sprinklers, fill furrows, use soaker hoses, or install drip emitters. Each method has its pros and cons:

• Garden hose - inexpensive, simple, difficult to measure water output, promotes fungal diseases
• Sprinklers - uniform water, can be used on slopes, leaches out excess salts, reduces crusting, high initial cost, encourages weed growth, may promote some diseases
• Furrows - lowest initial cost, less even water distribution, may result in pooling, requires level land, can cause crusting problems
• Drip emitters - uses less water, delivers water directly to individual plants, high initial cost, can be used on slopes, requires some technical expertise, emitters can clog due to high mineral content of local water supply
• Soaker hoses - uses less water, relatively inexpensive, they tend to break if not handled gently [My method of choice.]

Measuring water

In each case, you will need to know how much water is being delivered. If your bed needs 2 gallons of water a day and you have a 2-gallon watering can, well, there you go. If you are using any other method, you will need to know how much water is coming out of the spigot. You can buy a gadget that is installed between the spigot and the hose for the easiest calculation, just be sure to avoid the cheap, plastic models. They are inaccurate and break very quickly.  Also, you need to know that just because you applied the water does not mean it went where you intended. Soil can do funny things to water underground. Take the time to gently dig around plant roots after irrigating to see that the water actually saturated the root zone, rather than running off someplace else.

Irrigating compacted clay soil

Here in the Bay Area, we have heavy clay soil that is often compacted. That means we must be careful when irrigating. Irrigating compacted or clay soil too heavily simply compounds existing problems. Water slowly and gently, and do not dig or till wet soil. The smooth edges left behind can harden into an impenetrable barrier for tender roots.

Benefits of proper irrigation

Not only will plants receive the correct amount of water as they grow, but less water is wasted, pests and diseases are minimized, and plants will be more likely to reach their full potential. You can simplify irrigation by grouping plants with similar water needs.

When it comes to container plants, you generally need to give them more water than in-ground plants. Self-watering containers are an easy way to make sure that your container plants have the water they need.