Off is a preposition:
moving away and often down from
Flow refers to something in motion, and many times takes the preposition of after it:
the flow of the river
the flow of water
the flow of electricity
the flow of words
Overland flow is a compound noun. It refers to water that flows over land down towards a body of water.
It's natural to think that of is the intended word in your sentence, giving:
Water enters a lake from overland flow of the surrounding land
However, due to context and, in particular, the lack of the definite article before overland flow, we can correctly read off as a preposition.
Water enters a lake from overland flow off the surrounding land
overland flow is running or flowing off the surrounding land into the lake.