Irrigation impacts on the severe summer 2003 drought and heat wave event in Central Europe

Irrigation is triggered through climatic conditions, but reversely affects the climate itself. A model sensitivity analysis of the irrigation impacts on the severe summer 2003 drought and heat wave event in Central Europe is carried out here. For this purpose, the Weather Research and Forecasting (WRF) model is employed with a newly developed and modified irrigation scheme. A two-domain nested setup with 12 km horizontal grid resolution in the outer domain and convection-resolving 3 km in the inner domain is selected. Two ensembles, one with and one without irrigation, are initialized to assess the irrigation impacts with greater security. Four subregions are defined: a region containing all of Germany, two small regions with locally higher irrigation amounts within Germany and an area in the Po Valley, the region with highest irrigation quantities in Central Europe. This way, the influence of different irrigation amounts is investigated. Impacts on the following variables are examined in different temporal scales: air temperature, soil moisture, planetary boundary layer height (PBLH), sensible and latent heat flux, moisture flux divergence, convective available potential energy (CAPE), and convective inhibition (CIN). The results indicate that the overall influence of irrigation during the extreme event is rather small. This is related to the comparatively low irrigation amounts and the extreme conditions. A partially significant increase in soil moisture in the topsoil layer occurs in the Po Valley. Generally, irrigation is found to reduce PBLH and sensible heat flux as well as increasing the latent heat flux. In addition, a cooling effect is partly found in the daily mean cycle of temperature. Furthermore, there are visible effects on moisture flux divergence (tendency to decrease or convergence), on CAPE (increase) and on CIN (less increase). These effects are most pronounced in the Po Valley due to the higher irrigation amounts.