S. R. Faruqi:

To speak of dry-lipped people as causing streams to flow is very fine. Since weeping doesn't assuage thirst, he has called the desert of passion 'waterless', and he has also produced a proof [daliil] of their remaining dry-lipped. In juu))e;N bah chalii;N there's also the point that these channels will have flowed off somewhere else, and they will probably have made some other desert verdant. He's composed a verse of considerable 'mood', and has also constructed it well.

The interpretation of the desert of passion as being waterless can also be that in it no flower bloomed, no harvest of hope became verdant-- that is, the desire of the heart was not obtained. We wept, and wept to such an extent that streams flowed off, but the work of streams-- that is, to make crops verdant-- was not obtained.

FWP:

SETS
MOTIFS == DESERT
NAMES
TERMS == MOOD; PROOF

The lovers' lips were dry because they were thirsty, yet they kept on weeping away whole fountains and streams of water. The first line invites us to imagine the clever uses of the tear-water that could be made in the second line. Do they drink it, or not? Does it quench their thirst, or not? Do they wither away from sheer desiccation, or not?

But the second line (when we're finally allowed, under mushairah performande conditions, to hear it) takes an entirely different tack. What's really going on in the verse is an affirmation of the essential, irrevocable dryness of the desert of passion. No amoun of lovers' tears can irrigate it, or even create the occasional oasis to assuage their thirst. What is the tone of the second line? Is it melancholy, grim, ironic, matter-of-fact, amused, philosophical, fatalistic? Thanks to the idiomatic effect of thaa so thaa , the 'so what?' effect can also be defiant ('So what if the desert is waterless, we don't care, so you non-lovers just leave us alone!').

Compare Ghalib's take on tears and desertification:

G{31,1}.