Water year types (or
indices) are used to present the historical hydrological data in a simplified form. These indices help to categorize similar water years for the planning of the rule-based water operations. A typical set includes:
very dry year,
dry year, normal year,
wet year,
very wet year. The years are characterized through setting numerical thresholds for the
water runoff in the water year. The methods of calculation (and the set of types) naturally vary by the region, therefore many indices exists, for example: •
Palmer Drought Severity Index (PDSI). Proposed by W. C. Palmer in 1965, PDSI is extensively used in the US since then; • Standardized Precipitation Index (SPI) was proposed by McKee et al. in 1993; • Reclamation Drought Index; •
deciles. Many practically used indices were created
ad-hoc. For example, California River Indices are
weighted averages of the estimates of
spring melt, runoff for the rest of the year, and the result for the previous year, calculated for few
river basins separately to classify the water year as a
wet,
above normal,
below normal,
dry, and
critical ("normal" years in California are extremely rare). These California indices were not created "through a systematic statistical analysis of historic basin conditions and river flows". All indices by nature reflect the historic values and therefore cannot capture the variations in climate that are known to cause the distribution of water year types to be non-stationary in time. ==Uses==