What is the life-cycle of agricultural water used to grow crops/animals?

Question

A là this Slate article:

• 1 gallon of water is required to grow a single almond
• 220 gallons to grow a large avocado
• 5,000 gallons to grow 1 lb of beef

None of these things weighs anywhere near their water weight. So what happened to the water -- was it recycled to ecosystem?

Secondary question: because of the severe drought, California has imposed restrictions on water usage by agriculture. I'm wondering if most of this water ends up as ground water or if it vaporizes up into the atmosphere and blows with the winds... or there's no telling really where it goes?

Answer 1

The linked article talks about irrigated agriculture so I'll focus on that, but things are generally the same for rain-fed agriculture, except you have less control on the water application and there is no loss in getting the rain to where it hits the ground (yes some rain evaporates as it falls but we measure rain at the ground).

For irrigation water, though some is lost between the water intake and the field. We don't usually consider evaporation along rivers but when you take water out of a river some evaporates as it flows through canals to where it is used. Some water is also lost through leakage since many canals are just excavated in the soil. Relatively few irrigation water systems are piped but that is one way to make water use more efficient. Lining canals with cement or plastic can help prevent leakage (a number of channels are lined with plastic near where I live but animals like koalas can get trapped as they try to cross).

The next place water is lost is in evaporation from the irrigation spray or ground surface. That is usually considered in calculating water use for growing crops. Hot dry weather will increase the loss and for some soils a dry surface can inhibit infiltration of the water. Some water usually ends up back in the streams or wetlands from surface runoff or from returns to the canal systems.

Another loss is water that flows through the root zone before plants can use it. That flow can be important in keeping salts from building up to the point where the crop yield declines or plants can't survive. On the other hand it can leach metals from the soil down into the groundwater.

As you probably know, plants take up water from their roots and use it in photosynthesis to grow. They also respire at night when there is no photosynthesis. That water is the major part of the water use by the plants - very little is left in the biomass. There are some ways to breed plants or engineer your crop (like pruning trees) to make the plant water use more efficient in terms of how much product yield you get, but these are somewhat limited. And, of course, water is used by weeds.

Bottom line, most water in agriculture ends up as evapotranspiration (evaporation from the surface plus transpiration from plants). Some is lost to the groundwater or returned to surface water from the irrigation systems.

There are further inefficiencies in meat and dairy production. Some water is lost in metabolism, urine, feces, etc. And animals have to eat a lot of food to put on weight.

Answer 2

The water cycle isn't geology, but it's important to pedology, which is an earth science.

Agricultural water can be converted (by photosynthesis) directly into simple carbohydrates that we eat; or converted into carbs and then used as structure or energy that keeps the organism alive and growing, but isn't the mass we finally eat; or used to build more complicated parts of the organism (fats, protein) that we finally eat; or transferred to the atmosphere (mostly) because none of these biological processes are leak-proof; or transferred to the atmosphere or groundwater because we don't get all the water to the plant roots exactly when the plants can use it.

If we eat meat, there's all of the above in making the plants the animal eats, and then even greater inefficiency in turning water-in-plants into animal flesh, and the greater structure and energy costs to keep the animal alive and growing.

The crops listed are not simple carbs, or even self-fertilizing legumes. It takes a lot of tree to make an almond or an avocado. It takes a lot of water to grow the alfalfa and grains that we fatten cows on. There's also water in the processing, the cold chain, often in generating the power that makes nitrogen fertilizer or moves food from California (CA) to the markets. CA's main ag land is dry and warm and often windy in the summer, so it can lose water really quickly (either through the plants or from flood or overhead irrigation, and even a little bit through mulched soil).

I don't know about all of CA (it's a big place with a lot of different climates), but at least some -- the west side of the Central Valley -- is having soil trouble because they rarely get enough irrigation or rain to flush salts and excess fertilizer out of the root zone, so they at least aren't sending excess water to groundwater. One could check stream gauges to guess whether excess water is making it into surface water -- my guess, not.