The Lago de las Monjas dried out a few decades ago. The state of Coahuila receives little more than 300 millimeters of rain annually (12 inches). So much water is being pumped in two farm regions near Cuatro Ciénegas to irrigate crops and care for livestock that not early enough is left to supply the pools and marshes. With every passing year, the desert claims more land, more ponds, and more streams that used to be wet.