Meteorological Drought, Hydrological Drought, and NDVI in the Heihe River Basin, Northwest China: Evolution and Propagation

Table 3

Classification of SPI, SDI, and VCI.

Value	Drought grades
SPI/SDI ≤ −2.0, VCI < 15	Extreme drought
−2.0 < SPI/SDI ≤ −1.5, 15 ≤ VCI < 30	Severe drought
−1.5 < SPI/SDI ≤ −1.0, 30 ≤ VCI < 45	Moderate drought
−1.0 < SPI/SDI ≤ 0, 45 ≤ VCI < 60	Mild drought