The Rio Grande de Nasca drainage basin is located (approximate latitude 14° and longitude 74°) in the southwestern portion of Peru (Figs. 1 and 2) and receives less than 50 mm of rainfall per year (Aravena et ah, 1999). Inhabitants have occupied this area for over 2000 years (Schreiber and Lancho, 1995), despite the aridity and chronic water shortages. These shortages have led researchers to focus on obtaining data to better understand the water resources of the Rio Grande de Nasca drainage basin (Schreiber and Lancho, 1995; Johnson, 1998, 1999).