One liter of water is 1 millimeter in depth across 1 square meter, and will increase soil moisture in the top 10 centimeters of 1 square meter by 1%. The golf course shown here is Salobre Golf Resort in Maspalomas, Gran Canaria, Canary Islands. Photo by Micah Woods
Growing up and first working in the turfgrass industry in the United States, I naturally used U.S. customary units: inches and feet, pounds and ounces, fluid ounces and gallons, and so on. I started to use metric units when I went to work as a golf course superintendent in China and Japan, and I have been using these units ever since.
I like to use 1 square meter (1 m2) as the base unit. There are a few reasons for this. First, I can see 1 square meter, I can imagine it, and I can think of how that base unit will be managed. Then, it’s simply a matter of considering how many of those base units of 1 square meter are to be managed.
Second, the numbers work out in a convenient way in three-dimensional space. I’ll elaborate on the convenience of this below.
Third, I like to work with numbers from 1 to 100 as much as possible, and working with 1 square meter as the base unit is handy in this way. Nitrogen might be applied at about 3 grams per square meter per month; in Thailand, one might apply 30 to 50 grams of nitrogen per square meter in a year. Wetting agents will be applied at about 2 milliliters per square meter. Spray volumes will usually be from 40 to 80 milliliters per square meter. These numbers fall in a range that is easy to work with and easy to think about. There are no 100s, no 1,000s, no 10,000s, and few numbers less than 1.
Now, for the three-dimensional space of the root zone. For managed turfgrass, the root system, averaged over the course of a year, can be considered 10 centimeters deep. This is where most of the nutrients will be taken up, where the grass will obtain water, and where coring and cultivation practices will be done.
Maybe you would like to use a root-zone depth of 7.5 centimeters, or 15 centimeters, or 30 centimeters. That’s fine, and can be done, but using 10 centimeters has some attractive properties. One square meter to a depth of 10 centimeters has a volume of 100 liters. If the volumetric water content (VWC) of the soil is 18%, that means there are 18 liters of water in 1 square meter to a 10-centimeter depth. Want to increase the soil moisture to 22%? That will require 4 liters per square meter.
This is also convenient in two-dimensional space. One liter of water spread over 1 square meter has a depth of 1 millimeter. If it rains 6 millimeters, that’s 6 liters of water per square meter. And if the soil VWC drops from 20% to 16% from morning to evening, that’s a water loss of 4 liters, equivalent to 4 millimeters at the surface. That’s the evapotranspiration (ET). Not some estimate from a computer, but the real consumptive water use.
Want to compare the ET of a full-sun area with the ET of a shaded area? Measure the difference in VWC from morning to evening at both sites. Now you’ve got the answer.
Thinking of water application in millimeters and liters translates directly to the VWC in the soil if one assumes the root zone has a depth of 10 centimeters. Of course, a soil moisture meter may have rods at a 6- or 7.5- or 12-centimeter depth; one can make appropriate adjustments in assumptions and estimates.
Being able to make important calculations on the fly without much effort makes it easy to start a relationship with the metric system, but be careful — once you see how easy this is, you may start to use the metric system for fertilizer, topdressing and spray applications too.
Micah Woods is chief scientist at the Asian Turfgrass Center and an assistant adjunct professor at the University of Tennessee.