Researchers at the University of Florida have developed a new index called the Agricultural Reference Index for Drought or ARID. ARID is a simple and reliable index to monitor and predict agricultural drought. It is based on a reference crop (grass) and takes into account the soil-plant-atmosphere relationships.
This 5-page fact sheet describes the nuts and bolts of ARID and how to monitor the index on Agroclimate.org. Written by Clyde W. Fraisse, Eduardo M. Gelcer, and Prem Woli, and published by the UF Department of Agricultural and Biological Engineering, February 2011.
http://edis.ifas.ufl.edu/ae469