Andreas Krüger

Groundwater recharge in Northrhine-Westfalia predicted by a statistical model for greenhouse gas scenarios

A statistical model that generates regional climate scenarios on the basis of observed time series is applied for precipitation calculations for Northrhine-Westfalia (NRW, Germany). The method consists of three steps and can be used for any climate station. In the first step, 30–50 years observed daily values of Tmax, Tmin, Tmean, spread between Tmin and Tmax and the duration of daylight are clustered by a non hierarchical cluster analysis. The resulting clusters are considered to be typical weather situations with respect to these five “relevant parameters”. Secondly, artificial time series for a future climate state are generated. The time series are designed to reproduce the observed statistical characteristics for the “relevant parameters”. Additionally, a transient increase of the mean temperature of 1.5 K / 50 years is prescribed, which is chosen as an estimate for temperature change by doubling the CO2. In step three every element of the generated time series can be assigned to one of the clusters estimated in step one. Thus it is possible to associate an observed day to each artificially generated situation. Other parameters like precipitation can then be taken from the observed day. In this way scenario temperature and precipitation values are produced for each of the 48 stations. Annual groundwater recharge is calculated from these values employing the Turc formula. Finally, a kriging interpolation estimates the distribution of groundwater recharge for the complete area. With the prescribed rise in mean temperature the statistical model simulates decreasing precipitation. Both effects contribute to a reduction of groundwater recharge. While in the mountainous parts of NRW the groundwater recharge change is small, in the planes a reduction of up to 30% of the current groundwater recharge is predicted. A comparison to direct model output from GCM integrations for a standard greenhouse gas scenario produces a similar reduction for that region.