Abstract
Meteorological drought indices are commonly calculated using data from weather stations and then interpolated to create a map of moisture conditions. These maps are used to communicate drought information to decision makers and the general public. This study analyzes five of the factors (drought index, interpolation method, seasonality, climate region, and station density) that influence the accuracy of these maps. This study compared the Standardized Precipitation Index (SPI) and Standardized Precipitation Evapotranspiration Index (SPEI) using data from the Cooperative Observer Network (COOP) and United States Historical Climatology Network (USHCN). The accuracy of the drought maps varied significantly over time and space. The most significant factor affecting the accuracy of the meteorological drought maps was seasonality. Errors were higher in regions (e.g., southeastern U.S.), and months (e.g., summer), dominated by convective precipitation. The choice of interpolation method also had an influence. We found that Ordinary Kriging (OK) performed better than Inverse Distance Weighting (IDW) in all cases and therefore it was recommended for interpolating drought indices. Not surprisingly, maps that were created using more stations (COOP) were more accurate. The normalized errors of SPI and SPEI were very similar and so the choice of drought index had little impact on the accuracy of the drought maps.
Highlights
Drought is a naturally recurring feature of the climate system that is characterized by a prolonged deficiency of precipitation (Dai, 2011)
1-month Standardized Precipitation Index (SPI) and 1-month SPEI based on Cooperative Observer Network (COOP) stations and United States Historical Climatology Network (USHCN) stations for continental United States (CONUS)
The differences between interpolation methods are small, but statistically significant between Ordinary Kriging (OK) and Inverse Distance Weighting (IDW) based on a paired t-test (90% confidence level)
Summary
Drought is a naturally recurring feature of the climate system that is characterized by a prolonged deficiency of precipitation (Dai, 2011). Quantitative information on the duration, severity and spatial extent of drought events are used to help monitor conditions and make decisions (Rhee et al, 2008). There is no uniform method to characterize drought conditions and many different drought indices have been used to monitor meteorological drought (Heim, 2002; Quiring, 2009). Some of the drought indices that are commonly used to monitor drought conditions include: Palmer Drought Severity Index (PDSI) introduced by Palmer (1965), Standardized Precipitation Index (SPI) introduced by McKee et al (1993), Standardized
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