Combined Drought Index
The combined drought index (CDI) shows the water deficit in a particular region. It is based on measured and predicted values of precipitation, river discharge levels and lake levels as well as soil moisture. CDI forecasts provide a basis for drought warnings.
Purpose
The CDI summarises in one value the various factors affecting drought. Meteorological, agricultural and hydrological aspects of drought are included in the calculation of this value.
The CDI provides an overview of the drought situation throughout Switzerland and thus simplifies cooperation between the various stakeholders in this domain.
Explanation
The CDI can have a value from 1 (not dry) to 5 (extremely dry). The CDI value primarily reflects the frequency of an event and not any actual lack of water. For example, it is common for an Alpine river such as the Lonza (VS) to carry very little water in winter, as a lot of water is stored in the snow at this time of year. In summer, however, such low discharge values are very rare. In other words, the CDI takes account of seasonal variations; the threshold values that determine the drought level depend on the time of year.
The extent to which drought affects the various sectors is also strongly dependent on the time of year. For example, drought has a greater impact on agriculture in early summer than in winter. Each sector has its particular sensitivity, which cannot be directly derived from the CDI.
In general, a high CDI value indicates an unusual or even extreme water deficit for the season. The nature and intensity of the impact also depends on the time of year and the seasonal water requirements.
Calculation
The CDI is calculated automatically and takes account of the following variables:
Each variable is categorised according to its current value. The CDI is updated daily based on the latest data and model runs. The CDI forms the basis for the weekly assessment of the situation and, if necessary, warnings.
Drought levels
To determine drought levels (1–5), percentiles are first calculated for each variable that could affect the drought (e.g. groundwater level). The percentile indicates the proportion of comparative values that are lower than the current value. The region and season are also taken into account. For example, the percentile relating to the groundwater level answers the question: "In what percentage of previously measured cases was the groundwater level in this region lower than the currently measured value at a comparable time?" The same method is applied to the other variables.
The drought level is defined as the deviation from the statistically expected values for the measurement period 1991–2020 (reference or standard period). The current, regionally averaged, calendar weekly average of a given variable is compared with the corresponding value of the standard period and then allocated to a drought level.
The values are calculated over a period of +/- 15 days. This takes into account the seasonal nature of the variable, and drought is seen as a rare event compared to the usual values for the time of year. Drought levels are set at the 2nd, 5th, 10th and 15th percentiles.
Example:
If more than 15% of all weekly averages in the years 1991–2020 are lower than the current weekly average of a region, level 1 is assigned. This is the lowest drought level.
If a value is below the 15% percentile, this indicates drought. The key shows the gradations of the different drought levels.
If a region is assigned the highest drought level 5, less than 2% of all weekly averages for the years 1991–2020 were below the current weekly average. In other words, during these years it was drier in only 2% of all measurements in a given comparison period. This indicates an extreme drought event.
Determination of the CDI
The CDI is calculated by first taking all individual variables (e.g. total precipitation over the last 30 days) and determining their respective drought level. The second-highest value of all categorised variables considered then corresponds to the total value of the CDI. The highest value is not considered, as analyses have shown that this reflects the general situation less well and is susceptible to measurement errors and outliers.
Example:
|
Variable |
Drought level |
|
Total precipitation in the past 30 days |
5 (extremely dry) |
|
Total precipitation in the past 90 days |
4 (very dry) |
|
Total precipitation in the past 730 days |
1 (not dry) |
|
Modelled soil moisture |
3 (dry) |
|
Discharge (m3/s) and water level (m) |
3 (dry) |
|
CDI |
4 (very dry) |
Level 4 is the second highest value that occurs for a single variable (here: Total precipitation in the past 90 days). So the CDI here is also 4 – very dry. This calculation method was chosen for its simplicity and robustness.
The CDI values roughly correspond to the following frequencies (standard period 1991– 2020):
The variables considered depend on the data currently available. Additional variables (e.g. snow cover, groundwater, measured soil moisture) are integrated at a later stage if required.