The Khmer Empire's decline is attributed in large part to a cycle of droughts and floods caused by El Nino and led to their abandonment of Angkor. The city complex and its economy depended on a complex canal and irrigation system. From both existing records and tree ring samples (see here, and here) we know that two long droughts occurred in the late 14th and early 15th centuries. To meet its demographic needs, they were forced to clear more land for wet rice agriculture and the loss of trees made the area prone to flooding when the droughts ended. This cycle of drought and flood led to the silting in of the city's vital canals. Of course, other factors like Khmer's wars with the neighboring Ayutthaya Kingdom also played a significant role.