I could not find a definition for this word.
"When Burrows reached the end of the strip of prairie he pulled his sorrel to the right and let him pick his way daintily across a sacuista flat through which ran the ragged, dry bed of an arroyo. "


P.S. I found a site that says "sacuista" is a kind of grass.

I checked google and apparently sacuista grass grows in clumps.