Based on the watershed experiment, the stained water represents the residue of the food coloring.
What is Residue?
This refers to the remainder of a mixture after it has been taken, separated, or designated in the process of a reaction.
Hence, we can note that when a watershed experiment is made and the runoff mixes with food coloring and stains the water at the bottom, this stained water represents the residue
Read more about residue here:
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