This package adopts a quasi-weekly time step, dividing each month into four fixed periods:
days 1–7, 8–14, 15–21, and 22 through the end of the month. This approach ensures a
consistent total of 48 quasi-weekly intervals per year, regardless of the selected
aggregation time scale (TS).
For instance, when TS = 4 (representing a one-month backward-looking window), the
function computes cumulative precipitation totals over four consecutive quasi-weekly
periods. This aggregation is repeated across all 48 periods of each year, generating
a time series of aggregated precipitation values suitable for subsequent SPI estimation.