Identification and Spatiotemporal Characteristic Analysis of Compound Weather and Climate Extremes for Maize in Different Climate Zones of the Songliao Plain

Table 4 Drought and waterlogging grades based on the standardized precipitation requirement index (SPRI)

Drought grades	SPRI	Waterlogging grades	SPRI
Extreme drought	SPRI ≤ − 2.0	Extreme waterlogging	SPRI ≥ 2.0
Severe drought	−2.0 < SPRI ≤ −1.5	Severe waterlogging	1.5 ≤ SPRI < 2.0
Moderate drought	−1.5 < SPRI ≤ –1.0	Moderate waterlogging	1.0 ≤ SPRI < 1.5
Mild drought	−1.0 < SPRI ≤ −0.5	Mild waterlogging	0.5 ≤ SPRI < 1.0