Sforno on Numbers 21:18/ספורנו על במדבר כ״א:י״ח

1באר חפרוה שרים, this well was not full to its brim such as wells which are filled by waters emptying into it from above.

2וממדבר מתנה, this phenomenon demonstrated that its waters originated in the miraculous waters flowing from the rock at Massah and Merivah.