The United States, where the most careful measurements have been done, discovered in 1982 that about one-fifth of its farmland was losing topsoil at a rate likely to diminish the soil’s productivity. Can “while” work here? No—not in this sentence structure. Here’s why: • “Where” refers to the place (the United States) and connects it to what was done there—measurements. • “While” introduces a time or contrast, but that doesn’t fit inside this clause. Short tip: Use “where” to describe place, especially when adding extra information with commas. Use “while” for contrast or time, like: While many countries ignore soil loss, the U.S. began measuring it.

With changes, you can use "while" but the meaning would be different. You could say "The U.S., while the most careful measurements WERE BEING DONE, discovered ..." "While the measurements were being done" describes a time interval. "While" can also mean "although" or "whereas": "While Shana has the ability to learn any language, she picked English because she likes it."