Off is a preposition:

moving away and often down from

Oxford dictionary

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.

You can have a sentence such as

The overland flow of the surrounding land causes the lake to rise in volume.

so 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.

Off is a preposition:

moving away and often down from

Oxford dictionary

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.

Off is a preposition:

moving away and often down from

Oxford dictionary

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.

The overland flow is running or flowing off the surrounding land into the lake.

But you can have a sentence such as

The overland flow of the surrounding lands causes the lake to rise in volume.

so it's natural to think that of should follow flow.

Off is a preposition:

moving away and often down from

Oxford dictionary

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.

You can have a sentence such as

The overland flow of the surrounding land causes the lake to rise in volume.

so 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.