3.1. Formulation of the PDSI
[19] Here I briefly describe the relevant aspects of the
PDSI and sc_PDSI formulations. More details can be found
in the studies by Palmer [1965], Alley [1984], Karl [1986],
and Wells et al. [2004].
[20] Besides P, Palmer [1965] considered four other surface
where the overbar indicates averaging over the calibration
period. Thus, these coefficients represent the ratio of the longterm
mean values between a water flux and its potential value.
The CAFEC values are simply the product of the potential
value of a water flux times its coefficient, e.g., ai PE for
CAFEC evapotranspiration. In particular, the CAFEC precipitation
(^P), which represents the amount of precipitation
needed to maintain a normal soil moisture level for a given
time, is defined as