Although drought characteristics, such as drought duration and drought onset, are expected to be different for different drought types this has never been systematically investigated. Therefore in this study the drought characteristics of agricultural drought and water resources drought were compared for different Köppen-Geiger subclimates and for different soil types and groundwater systems.
Comparison of agricultural and water resources drought characteristics across the world
Drought, which is defined as below normal water availability, is a recurring phenomenon that affects people across the world. Depending on the nature of the water deficit droughts can be categorized as meteorological drought, agricultural drought or water resources drought. A variety of drought indicators can be used to identify the different types of drought and study their characteristics and impacts. Some of the indicators use the characteristics of more than one drought type to estimate the impact of drought. However, drought characteristics such as the number of droughts, drought duration and drought onset are expected to be different for each drought type. To test this hypothesis in this study agricultural drought characteristics were compared with water resources drought characteristics for different Köppen-Geiger subclimates and for different soil types and different groundwater systems. To identify agricultural drought events a time series of soil moisture storage was used with a fixed threshold being equal to critical soil moisture storage. For identifying water resources drought events a time series of groundwater discharge was used with a variable threshold level being equal to the 80th percentile of monthly groundwater discharge. Comparison of average drought duration showed that for most equatorial and temperate climates and for the polar climate average agricultural drought duration was approximately twice as long as average water resources drought duration. For the cold subclimates average agricultural drought duration was between two and five times longer than average water resources drought duration. To describe the difference between the drought onset distribution of the two drought types over the year the distributions were divided into four categories. The first category contained subclimates for which both agricultural and water resources drought had a similar onset distribution over the year. The second category contained subclimates for which the onset distribution of agricultural and water resources drought coincided for part of the year. The subclimates for which only agricultural drought showed an onset distribution with a pronounced peak were defined as a third category. The fourth category contained subclimates for which agricultural drought and water resources drought showed a comparable onset distribution, but had a different timing of the peak onset probability. A change of soil type mainly affected the agricultural drought characteristics and only had minor effects on water resources drought characteristics. Changes of groundwater system had a larger effect on water resources drought characteristics than a change of soil type. This study demonstrates that a clear distinction needs to be made between the different drought types, because drought characteristics (onset, duration) can be very different and this will affect drought impacts.