4. Results
Differences in the behavior of the SPI and SRI are illustrated
using areal averages of observed precipitation and
simulated runoff in the Feather River basin of California.
Figure 2 shows monthly time series of the indices for four
accumulation periods (1-, 3-, 6- and 12-month) for the period
1975-1995. The 12-month SPI and SRI are very similar,
which is to be expected given that annual precipitation and
temperature in the basin are highly correlated (r=0.9). The
differences between the indices increase as the accumulation
period decreases, with correlations between the SPI and SRI
dropping from 0.88 for the 12-month period to 0.82, 0.69 and
0.01 for the 6-, 3-, and 1-month period indices. For shorter
period accumulations, the SRI is less variable from month
to month than the SPI due to the inertia of the land surface
moisture, which has a major influence on runoff generation.
For both SPI and SRI, the 12-month duration indices convey
one drawback of using metrics with long temporal integration:
as an extreme event enters and leaves the accumulation
period, it creates the table-top form of index such as
in 1986. As a result, at the onset of the 6-year drought, the
12-month indices remain above 1.0 for 2 months after the
SHUKLA AND WOOD: VALUE OF SRI X - 3
1- and 3-month SRIs fall below values of -1.0, in December,
1986.