Testing nonlinearity and nonstationarity of the connection between Palmer drought indices and Danube discharge in the lower basin

The aim of the study is to reduce the uncertainty of the influence of Palmer-type drought indices in estimating seasonal discharge in the lower Danube basin. For this, four indices were considered: Palmer Drought Severity Index (PDSI), Palmer Hydrological Drought Index (PHDI), Weighted PDSI (WPLM) and Palmer Z-index (ZIND). These indices were quantified by PC1 of EOF decomposition, obtained from 15 stations located along the Danube basin.

The influences of these indices on the Danube discharge were tested, both simultaneously and with certain lags, by linear and nonlinear methods applying the elements of information theory. Nonstationarity was tested by wavelet analysis. The results differ depending on the season and the Palmer index.

The linear connections are generally obtained for synchronous links, and the nonlinear and nonstationary ones for the predictors considered with certain lags (in advance) compared to the discharge predictand. This result is useful for estimating the discharge, as Palmer indices can be estimated from the simulated data by the General Circulation Models or Regional Climate Models.