Let up and Run low

Let up verb - To come to an end.

Run low and let up are semantically related In some cases you can use "Run low" instead a verb "Let up".

Run low verb - To be spent or finished.

Let up and run low are semantically related in decrease topic. Sometimes you can use "Let up" instead a verb phrase "Run low", if it concerns topics such as wither, grow less or make less.