Drought indices

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.

Evapotranspiration and potential evapotranspiration

EN (cq) Kategorie

SPI: Precipitation anomaly

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:

Values of the index
Values of the index

Soil moisture indices

A simple soil water model is applied to compute soil moisture for an idealized soil. It uses the meteorological measures precipitation and potential 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.

Figure soil moisture index

Soil moisture (in percent by volume) for an idealized soil at the respective location.

Values of the index
Values of the index

Figure soil water deficit

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.

Values of the index
Values of the index

Figure vegetation water deficit

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.

Values of the index
Values of the index

ARID (Agricultural Reference Index for Drought)

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).

Values of the index
Values of the index

Soil water model

SPEI: Anomaly of water balance

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:

Values of the index
Values of the index

Water balance

Water balance is defined as the difference between precipitation and potential evapotranspiration [internal Link: Weiterführende Informationen: 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:

Values of the index
Values of the index

Evapotranspiration and potential evapotranspiration