harvest
茬 centers on the act of gathering crops: harvesting. It is a specific agricultural term for reaping what has been grown.
茬 combines the grass radical 艹 with 在, which likely contributes a phonetic element. The exact historical development is uncertain, but the character is associated with plants and harvesting.
The grass top 艹 sits above 在, which can suggest 'to exist' or 'be present'. Picture crops existing in the field, ready to be harvested.
For シ, imagine a sheaf of harvested grain: the word 'sheaf' sounds like シーフ, and the first syllable シ cues the reading while the scene shows the harvest.