why does the author say that rain water is chief article of diet at supper

The rain water has soaked all of the food stores, and thus, becomes the main article of diet at supper.

Rainwater is the chief article of diet at supper. The bread is two-thirds rainwater, the beefsteak-pie is exceedingly rich in it, and the jam, and the butter, and the salt, and the coffee have all combined with it to make soup.