The rain is plentious but, by God's decree, Only a third is...

The rain is plentious but, by God's decree, Only a third is meant for you and me; Two-thirds are taken by the growing things Or vanish Heavenward on vapour's wings: Nor does it mathematically fall With social equity on one and all. The population's habit is to grow In every region where the water's low: Nature is blamed for failings that are Man's, And well-run rivers have to change their plans.