Last week I found out it takes more than three and a half gallons of water to grow one head of lettuce. Maybe you saw this article too. I wasn’t sure if that was a lot of water or not - plants do need water to grow. But how much do they need?
Digging further into the source of the article, I found a concept that I hadn’t heard about - the water footprint of a crop.
The water footprint of a crop is a measurement of both how much water is needed to produce a crop and where that water comes from. The water footprint idea was introduced by Hoekstra and Hung in 2002 and has been built upon since then. It takes into consideration direct and indirect water usage defined by three groups: green water, blue water and grey water.
Green water is the rainwater used by a crop plant (that doesn’t run off into a lake or stream).
Blue water consumed by a plant is more complex. Ground water is included, as is irrigation water and water assimilated into the plant as it grows. Water used during production and transport is also included as blue water.
Grey water is the water needed to offset the load of pollutants used to grow the crop. A way to think of this is how much water is needed to dilute pollutants in the soil to levels that are within the region’s water quality standards. Pollutants largely refer to the fertilizer and pesticide residues that remain, unused, in the soil.
Green, blue and grey water combined gives the total amount of water needed to produce the crop. Mekonnen and Hoekstra (2011) have listed 200 common food crops and the water needed to grow them. Their models to calculate green, blue and grey water are based on global data and are listed as cubic meters of water for every ton of crop produced.
Cubic meters and tons?
These are not measurements that I can easily relate to. Assisted with a scale and a unit converter, I did some good old-fashioned dimensional analysis to figure out how much water it takes to produce the food I eat on an average day.
I require at least 800 gallons of water for the food I eat each day.
That’s a lot of water. I could take a 90-minute shower everyday with that much water.
(There are a few disclaimers. These are very simple meals that don’t include tasty things like herbs, spices, or animal products. These all take a lot of water to produce, but I was trying to keep things simple for this experiment.)
In each meal one ingredient stands out as a real water hog. During breakfast it’s almonds; at lunch, chocolate; and during dinner wheat pasta takes more than a few bucketfuls of water to produce. By weight, none of these foods are the most plentiful in the meal, which means weight is not an indicator of water use.
Almonds were the headline that started me down the rabbit hole of water use. The 1.4 ounces of almonds I eat in my oatmeal use a whopping 186 gallons of water to grow. The same weight of sunflower seeds uses a moderate 39 gallons by comparison. Why does one seed take way more water to produce than another? My guess is that they are from completely different kinds of plants. Almond trees are in the prunus family, closely related to peaches. If you cracked open any almonds this Christmas, you know they have a tough shell. A tree is very different from a sunflower. At maturity, sunflowers are rather woody, but they still are an annual crop that dies each fall. So, a tree (like the almond tree) needs more energy to produce seeds than an herbaceous flower (sunflower plant). At least that’s my thinking.
The chocolate I eat for lunch accounted for more than half of the water used to produce the meal - 213 gallons for a mere handful of chocolate chips. Cocoa beans go through a fair amount of processing before they become chocolate. The beans are made into paste, the paste is separated into butter and powder, and at some point sugar is added. Each of these steps adds additional gallons of water to the final product. In contrast, apples or carrots can be eaten straight from the tree or ground (depending on how much dirt you mind). So, foods that require more processing, like chocolate, wheat flour or wine, will take more water to produce than those that can be consumed raw.
Going through this exercise made me start to feel badly about the foods I eat - my favorite foods use so much water! Plants need water to grow; it’s one of the things that make them so interesting. Furthermore, for food there is a larger picture to consider that involves nutrition and taste. Nonetheless, major droughts now plague areas like California, where most of US food is grown. So, it’s important to be aware of the gallons of water needed to grow our food. If you want to calculate your yearly water footprint you can do so through waterfootprint.org.
How much water do you use?
Water is continually a topic of interest and concern and is often a topic here on FTDM. Here are more posts we’ve written on water: