How much water do you think could be contained in Elijah's trench around the altar in 1Kings 18:32? The NIV text says two seahs of seed or 15 litres according to the footnote but that doesn't seem like very much to me.
Swanson's Dictionary of Biblical Languages with Semantic Domains says that a seah is "variously reckoned at 7 to 12 liters". The Enhanced Brown-Driver-Briggs Hebrew and English Lexicon is more precise:
On size of seʾāh = 1/3 ephah, = 12.148 litres ( = 10.696 qts.)
The common Jewish interpretation here, though, seems to be that the text is not referring to the volume of water, but rather the surface area it would cover (see, e.g., Oxford Jewish Study Bible). This seems to be why seahs of seed rather than simply seahs are specified. The measure here is of the amount of water it would take to cover an area that could be sown by two seahs of seed. One might look here at:
If anyone dedicates to the LORD part of their family land, its value is to be set according to the amount of seed required for it—fifty shekels of silver to a homer of barley seed
A ten-acre vineyard will produce only a bath of wine; a homer of seed will yield only an ephah of grain