It is commonly accepted that drought is a multi-scalar phenomenon. McKee et al. (1993) clearly illustrated this essential characteristic of droughts through consideration of usable water resources including soil moisture, ground water, snowpack, river discharges, and reservoir storages. The time period from the arrival of water inputs to availability of a given usable resource differs considerably. Thus, the time scale over which water deficits accumulate becomes extremely important, and functionally separates hydrological, environmental, agricultural and other droughts. For this reason, drought indices must be associated with a specific timescale to be useful for monitoring and management of different usable water resources. This explains the wide acceptance of the SPI, which is comparable in time and space (Hayes et al., 1999), and can be calculated at different time scales to monitor droughts with respect to different usable water resources.
References
(1) McKee, T.B.N., J. Doesken, and J. Kleist, 1993: The relationship of drought frecuency and duration to time scales. Eight Conf. On Applied Climatology. Anaheim, CA, Amer. Meteor. Soc. 179-184.
(2) Hayes, M., D.A. Wilhite, M. Svoboda, and O. Vanyarkho, 1999: Monitoring the 1996 drought using the Standardized Precipitation Index. Bulletin of the American Meteorological Society 80, 429-438.
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