Different indices describe current drought or wetness at selected sites of MeteoSwiss’ observation network. Indices are derived from primary meteorological measurements, and precipitation is the most important input. Some of the indices also consider evapotranspiration, which is depending on temperature, humidity, radiation and wind speed.
Drought indices
Category
The standardized precipitation index (SPI) describes the deviation of the precipitation amount in the past month or several months from the long term mean. Negative SPI values indicate a precipitation deficit relative to the long term median, positive values indicate higher than median precipitation. SPI is by definition a relative measure of precipitation anomaly, both with respect to location and time. The SPI directly indicates the frequency of the current deficit (or surplus) for the location of interest and the current season. By computing the SPI over different time periods (1, 3, 6, and 12 months) the water deficit over different timescales can be described with the same index. This is useful as drought impacts will typically depend on its duration. Vegetation and hence agriculture is sensible to droughts at shorter timescales of 1-3 months, whereas hydrological impacts tend to be stronger related to droughts at longer timescales.
The figures for individual observation sites show the course of the SPI over the past 12 months, the values of the index are colored according to following scale:
Evapotranspiration und potential evapotranspiration
Evapotranspiration includes the water release by plants (transpiration) and evaporation from any kind of surfaces. Transpiration depends on the state and activity of the plants. It varies with seasons and depends on the density of the plant canopy as well as on the water supply, as plants will reduce water release in case of water scarcity by reducing the opening of their stomata.
Potential evapotranspiration is the evapotranspiration of a reference grass surface with optimal water supply. As the vegetation state is exactly defined in this case, potential evapotranspiration solely depends on the state of the atmosphere. It is determined by temperature, radiation, humidity and wind speed.
Soil water model
This simplified model treats the soil as a water bucket, why this type of model is also referred to as „bucket model“: Precipitation fills the bucket, evapotranspiration removes water. Using the purely meteorological measures precipitation and potential evapotranspiration, the water balance of the soil is evaluated day to day. In case the bucket is full (the soil being saturated with water), precipitation no longer enters the soil, but surface runoff occurs. Furthermore the model considers that vegetation will transpire less in case soil moisture falls below a certain threshold. The actual evapotranspiration then depends on soil moisture and will is less than potential evapotranspiration. The model assumes a soil with average properties and a grass surface, these parameters determine the water storage capacity of the soil and the vegetations’ water release.
Different useful indices can be extracted from this simple model
- soil moisture index: soil moisture in this model in vol.%
- soil water deficit: required precipitation amount in mm to saturate the soil.
- vegetation water deficit: required precipitation amount in mm to optimally supply vegetation with water
- ARID: index measuring the deviation between actual and potential evapotranspiration, thus indicating the vegetation’s water deficit. It is defined as 1 minus the ratio of actual to potential evapotranspiration. It takes on values between 0 (optimal water supply) and 1 (maximum drought stress).
A simple soil water model is applied to compute soil moisture for an idealized soil. It uses the meteorological measures precipitation and potential evapotranspiration [interner Link: Weiterführende Informationen: evapotranspiration] to compute this theoretical soil moisture. Based on this soil moisture index, further drought indices can be derived. The figures for individual observation sites show the values of the indices over the past 12 months.
Soil moisture (in percent by volume) for an idealized soil at the respective location.
The brown bars show the soil water deficit with respect to water saturation (= field capacity). The deficit is given in mm, indicating directly what amount of precipitation would be necessary to saturate the soil, or, in other terms, the precipitation amount that can be absorbed by the soil.
The brown bars show the soil moisture deficit with respect to the water demand of the vegetation. It is given in mm, indicating the required precipitation amount to optimally supply the plants with water.
ARID (Agricultural Reference Index for Drought):This index measures the vegetation water deficit, derived from the ratio of actual to potential evapotranspiration [interner Link: Weiterführende Informationen: evapotranspiration]. It is computed as 1 minus the ratio of actual to potential evapotranspiration. It takes on values between 0 (optimal water supply) and 1 (maximum drought stress).
Evapotranspiration und potential evapotranspiration
Evapotranspiration includes the water release by plants (transpiration) and evaporation from any kind of surfaces. Transpiration depends on the state and activity of the plants. It varies with seasons and depends on the density of the plant canopy as well as on the water supply, as plants will reduce water release in case of water scarcity by reducing the opening of their stomata.
Potential evapotranspiration is the evapotranspiration of a reference grass surface with optimal water supply. As the vegetation state is exactly defined in this case, potential evapotranspiration solely depends on the state of the atmosphere. It is determined by temperature, radiation, humidity and wind speed.
Soil water model
This simplified model treats the soil as a water bucket, why this type of model is also referred to as „bucket model“: Precipitation fills the bucket, evapotranspiration removes water. Using the purely meteorological measures precipitation and potential evapotranspiration, the water balance of the soil is evaluated day to day. In case the bucket is full (the soil being saturated with water), precipitation no longer enters the soil, but surface runoff occurs. Furthermore the model considers that vegetation will transpire less in case soil moisture falls below a certain threshold. The actual evapotranspiration then depends on soil moisture and will is less than potential evapotranspiration. The model assumes a soil with average properties and a grass surface, these parameters determine the water storage capacity of the soil and the vegetations’ water release.
Different useful indices can be extracted from this simple model
- soil moisture index: soil moisture in this model in vol.%
- soil water deficit: required precipitation amount in mm to saturate the soil.
- vegetation water deficit: required precipitation amount in mm to optimally supply vegetation with water
- ARID: index measuring the deviation between actual and potential evapotranspiration, thus indicating the vegetation’s water deficit. It is defined as 1 minus the ratio of actual to potential evapotranspiration. It takes on values between 0 (optimal water supply) and 1 (maximum drought stress).
The standardized precipitation index (SPEI) describes the deviation of the water balance in the past month or several months from the long term mean. Negative SPEI values indicate a water balance deficit relative to the long term median, positive values indicate higher than median water balance. SPEI is by definition a relative measure of water balance anomaly, both with respect to location and time. The SPEI directly indicates the frequency of the current deficit (or surplus) for the location of interest and the current season. As for the SPI, the SPEI is computed over different time scales.
The figures for individual observation sites show the course of the SPEI over the past 12 months, the values of the index are colored according to following scale:
Evapotranspiration und potential evapotranspiration
Evapotranspiration includes the water release by plants (transpiration) and evaporation from any kind of surfaces. Transpiration depends on the state and activity of the plants. It varies with seasons and depends on the density of the plant canopy as well as on the water supply, as plants will reduce water release in case of water scarcity by reducing the opening of their stomata.
Potential evapotranspiration is the evapotranspiration of a reference grass surface with optimal water supply. As the vegetation state is exactly defined in this case, potential evapotranspiration solely depends on the state of the atmosphere. It is determined by temperature, radiation, humidity and wind speed.
Soil water model
This simplified model treats the soil as a water bucket, why this type of model is also referred to as „bucket model“: Precipitation fills the bucket, evapotranspiration removes water. Using the purely meteorological measures precipitation and potential evapotranspiration, the water balance of the soil is evaluated day to day. In case the bucket is full (the soil being saturated with water), precipitation no longer enters the soil, but surface runoff occurs. Furthermore the model considers that vegetation will transpire less in case soil moisture falls below a certain threshold. The actual evapotranspiration then depends on soil moisture and will is less than potential evapotranspiration. The model assumes a soil with average properties and a grass surface, these parameters determine the water storage capacity of the soil and the vegetations’ water release.
Different useful indices can be extracted from this simple model
- soil moisture index: soil moisture in this model in vol.%
- soil water deficit: required precipitation amount in mm to saturate the soil.
- vegetation water deficit: required precipitation amount in mm to optimally supply vegetation with water
- ARID: index measuring the deviation between actual and potential evapotranspiration, thus indicating the vegetation’s water deficit. It is defined as 1 minus the ratio of actual to potential evapotranspiration. It takes on values between 0 (optimal water supply) and 1 (maximum drought stress).
Water balance is defined as the difference between precipitation and potential evapotranspiration. In this case evapotranspiration is solely determined by meteorological quantities (temperature, humidity, radiation and wind speed), why this water balance definition is referred to as meteorological water balance. The water balance is computed over one to several months. Positive values indicate that more precipitation has occurred over the period of interest than the amount of water that has evaporated into the atmosphere. It is hence an absolute measure, in contrast to the SPEI (see below) describing the water balance in a relative sense, both with respect to location and time. The figures for individual observation sites show the course of the water balance in the past 12 months. The following colors are used to distinguish positive and negative values of the water balance:
Evapotranspiration und potential evapotranspiration
Evapotranspiration includes the water release by plants (transpiration) and evaporation from any kind of surfaces. Transpiration depends on the state and activity of the plants. It varies with seasons and depends on the density of the plant canopy as well as on the water supply, as plants will reduce water release in case of water scarcity by reducing the opening of their stomata.
Potential evapotranspiration is the evapotranspiration of a reference grass surface with optimal water supply. As the vegetation state is exactly defined in this case, potential evapotranspiration solely depends on the state of the atmosphere. It is determined by temperature, radiation, humidity and wind speed.
Soil water model
This simplified model treats the soil as a water bucket, why this type of model is also referred to as „bucket model“: Precipitation fills the bucket, evapotranspiration removes water. Using the purely meteorological measures precipitation and potential evapotranspiration, the water balance of the soil is evaluated day to day. In case the bucket is full (the soil being saturated with water), precipitation no longer enters the soil, but surface runoff occurs. Furthermore the model considers that vegetation will transpire less in case soil moisture falls below a certain threshold. The actual evapotranspiration then depends on soil moisture and will is less than potential evapotranspiration. The model assumes a soil with average properties and a grass surface, these parameters determine the water storage capacity of the soil and the vegetations’ water release.
Different useful indices can be extracted from this simple model
- soil moisture index: soil moisture in this model in vol.%
- soil water deficit: required precipitation amount in mm to saturate the soil.
- vegetation water deficit: required precipitation amount in mm to optimally supply vegetation with water
- ARID: index measuring the deviation between actual and potential evapotranspiration, thus indicating the vegetation’s water deficit. It is defined as 1 minus the ratio of actual to potential evapotranspiration. It takes on values between 0 (optimal water supply) and 1 (maximum drought stress).