WUR Colloquium Niko Wanders
Datum publicatie 28-05-2010
Titel: Indicators for drought characterization on a global scale
Datum: Thursday, 3 June 2010
Tijd: 12.30 - 13.15 uur
Plaats: Atlas 1, Atlas Gebouw Droevendaalsesteeg 4, Wageningen
Begeleiders: Henny A.J. van Lanen en Anne F. van Loon
Abstract :
Droughts are caused by a situation with less than normal water availability due to climate variability. They occur in every hydroclimatic region and in all components of the hydrological cycle. Droughts can be classified either as meteorological, soil moisture or hydrological drought. Identification of droughts on a global scale has been done in recent studies. Different drought indicators were used for the identification of droughts on a global scale. However, the effect of the choice for a certain indicator on the drought characterization (e.g. severity, frequency and duration of droughts) is not fully understood, as is the impact of hydroclimate and physical catchment structure. The objective of this study is to examine the characterization of drought with different drought indicators across the world. It also includes the impact of climate and physical catchment structure on the performance of the drought indicators. Time series of meteorological variables were retrieved from or calculated with the WATCH Forcing Data (WFD) at a daily time step, for cells with a resolution of 0.5◦ by 0.5◦. The NUT DAY model was applied to generate time series of hydrological variables (e.g. soil moisture storage, discharge). NUT DAY is a synthetic rainfall-runoff model which uses precipitation, temperature and potential evapotranspiration time series from WFD as input. Three different soil types (low, medium and high soil moisture storage capacity) and groundwater systems types (fast, medium and slow responding) have been used in the model simulations, to explore the effect of changes in the physical catchment structure. Hydroclimatic regions were defined with the Köppen-Geiger classification. For all hydroclimatic regions drought analyses were done. Per cell, drought events for each drought indicator were identified by applying the threshold method to the time series of meteorological and hydrological variables. The threshold is either variable or fixed, depending on the indicator. In this study 14 indicators were selected, of which 2 were newly developed (Moving Average Precipitation, Standardized Streamflow Index). All 14 indicators were applied to the 5 major hydroclimates; performances were tested and evaluated with expert knowledge mainly from literature. From this 14 indicators finally 6 have been selected, for a more detailed analysis. In total 961 cells were randomly selected for that purpose ensuring that all K¨open-Geiger climate regions are adequately represented. The 6 selected indicators are: the Standardized Precipitation Index (SPI), Effective Drought Index (EDI), Total Storage Deficit Index (TSDI), Moving Average Precipitation with Variable Threshold (MAPVT), Soil moisture with Variable Threshold (SVT), Discharge with Variable Threshold (QVT). These indicators were used to study the effects of hydroclimate and physical catchment structure on drought characterization and subsequently to assess their performance. It was found that the hydroclimate has a profound impact on the average drought durations and deficit volumes as identified by all indicators. The SPI, EDI and MAPVT are not influenced by the physical catchment structure, because they only depend on precipitation. Average drought durations and deficit volumes determined by the TSDI and QVT increase for slower responding groundwater systems. In general, a higher soil water storage capacity increases the average drought durations as identified by the TSDI, SVT and QVT. Overall, the effects of hydroclimate and of properties of the groundwater system are more profound than changes in soil type. The MAPVT and QVT seem to be the most promising indicators for drought analysis on a global scale. Both indicators had a very constant performance for different hydroclimates and physical catchment structures and are rather straight forward to calculate.
Abstract :
Droughts are caused by a situation with less than normal water availability due to climate variability. They occur in every hydroclimatic region and in all components of the hydrological cycle. Droughts can be classified either as meteorological, soil moisture or hydrological drought. Identification of droughts on a global scale has been done in recent studies. Different drought indicators were used for the identification of droughts on a global scale. However, the effect of the choice for a certain indicator on the drought characterization (e.g. severity, frequency and duration of droughts) is not fully understood, as is the impact of hydroclimate and physical catchment structure. The objective of this study is to examine the characterization of drought with different drought indicators across the world. It also includes the impact of climate and physical catchment structure on the performance of the drought indicators. Time series of meteorological variables were retrieved from or calculated with the WATCH Forcing Data (WFD) at a daily time step, for cells with a resolution of 0.5◦ by 0.5◦. The NUT DAY model was applied to generate time series of hydrological variables (e.g. soil moisture storage, discharge). NUT DAY is a synthetic rainfall-runoff model which uses precipitation, temperature and potential evapotranspiration time series from WFD as input. Three different soil types (low, medium and high soil moisture storage capacity) and groundwater systems types (fast, medium and slow responding) have been used in the model simulations, to explore the effect of changes in the physical catchment structure. Hydroclimatic regions were defined with the Köppen-Geiger classification. For all hydroclimatic regions drought analyses were done. Per cell, drought events for each drought indicator were identified by applying the threshold method to the time series of meteorological and hydrological variables. The threshold is either variable or fixed, depending on the indicator. In this study 14 indicators were selected, of which 2 were newly developed (Moving Average Precipitation, Standardized Streamflow Index). All 14 indicators were applied to the 5 major hydroclimates; performances were tested and evaluated with expert knowledge mainly from literature. From this 14 indicators finally 6 have been selected, for a more detailed analysis. In total 961 cells were randomly selected for that purpose ensuring that all K¨open-Geiger climate regions are adequately represented. The 6 selected indicators are: the Standardized Precipitation Index (SPI), Effective Drought Index (EDI), Total Storage Deficit Index (TSDI), Moving Average Precipitation with Variable Threshold (MAPVT), Soil moisture with Variable Threshold (SVT), Discharge with Variable Threshold (QVT). These indicators were used to study the effects of hydroclimate and physical catchment structure on drought characterization and subsequently to assess their performance. It was found that the hydroclimate has a profound impact on the average drought durations and deficit volumes as identified by all indicators. The SPI, EDI and MAPVT are not influenced by the physical catchment structure, because they only depend on precipitation. Average drought durations and deficit volumes determined by the TSDI and QVT increase for slower responding groundwater systems. In general, a higher soil water storage capacity increases the average drought durations as identified by the TSDI, SVT and QVT. Overall, the effects of hydroclimate and of properties of the groundwater system are more profound than changes in soil type. The MAPVT and QVT seem to be the most promising indicators for drought analysis on a global scale. Both indicators had a very constant performance for different hydroclimates and physical catchment structures and are rather straight forward to calculate.
Keywords: Drought, Hydrology, Drought indicators, Global scale, Hydroclimate, Physical catchment
structure
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