Drought Index Calculator

No -- it's a simplified single-period analog. The genuine PDSI, developed by Wayne Palmer in 1965, runs a continuous two-layer soil-moisture water balance calibrated with station-specific coefficients built from decades of climate normals, so a real PDSI value depends on the running history of prior months, not just one period's numbers. This tool borrows Palmer's -6 to +6 scale and his standard nine-category classification (Extreme Drought through Extremely Wet) so the output reads the same way, but it derives the number from a single period's precipitation and temperature departure instead of the full historical bookkeeping.

Because higher temperature raises Potential ET (Potential Evapotranspiration) -- the atmospheric demand for moisture that crops and soil have to satisfy before any water counts as "surplus." Once Average Temperature climbs above Normal Temperature, the excess is multiplied by 3 and subtracted from the moisture-departure term that feeds the PDSI Estimate, so the same rainfall total reads as more of a shortfall in a hotter-than-normal period than in a normal one -- exactly how real drought indices treat heat as a stressor, not just a lack of rain.

Only when the Precip Ratio (Total Precipitation divided by Normal Precipitation) has already dropped below 0.8, i.e. conditions are meaningfully drier than normal. In that specific case, a Soil Moisture Capacity above 150mm adds +0.5 to the PDSI Estimate (deeper soils buffer a dry spell longer) while shallower soil subtracts 0.5. Above a 0.8 Precip Ratio, Soil Moisture Capacity has no effect on PDSI Estimate at all -- it still feeds the separate Water Deficit and water-balance chart figures, just not the index itself.

They're the two one-sided halves of the same comparison: Potential ET (the temperature-driven moisture demand) against Total Precipitation (the actual supply). Water Deficit is how much demand exceeds supply, floored at zero, and Water Surplus is how much supply exceeds demand, also floored at zero -- so for any given period only one of the two can be non-zero, and together they describe whether the period ran a moisture shortfall or a moisture excess independent of the PDSI category itself.