Evaluation of MODIS derived perpendicular drought index for estimation of surface dryness over northwestern China

In this paper, drought status of northwestern China is evaluated using the Terra–Moderate Resolution Imaging Spectroradiometer (MODIS) data with a newly developed method called perpendicular drought index (PDI), which is defined as a line segment that is parallel with the soil line and perpendicular to the normal line of soil line intersecting the coordinate origin in the two-dimensional scatterplot of red against near infrared (NIR) wavelength reflectance. To validate the PDI in macroscale applications, quantitative evaluation of drought conditions inNingxia, Northwestern China is carried out by comparing the PDI with one ofthe well-known drought indexes, namely, temperature-vegetation index (TVX).Linear regression between ground-measured soil moisture data and the PDI andthe TVX was made. Results show that satellite based PDI and TVX hassignificant correlation with 0–20 cm averaged soil moisture obtained over themeteorological observing stations across the whole study area. The highestcorrelation of R250.48 for the PDI and R250.40 for the TVX is obtained whencompared with average soil moisture from 0 to 20 cm soil depth. According to thedrought critical values defined by soil hydrologic parameters including soilmoisture, wilting coefficient and field moisture capacity, the PDI based droughtguidelines are established, and then the drought status in the study area isevaluated using the PDI. It is evident from the results showing the spatialdistribution of drought in northwestern China that the PDI is highly accordantwith field drought status.