Projected impact of climate change on irrigation needs and groundwater resources in the metropolitan area of Hamburg (Germany)

Irrigated agriculture is an important economic factor in the rural parts of the metropolitan area of Hamburg. It is commonly expected that climate change will reduce the groundwater quantities available for field irrigation. Against this background, the ratio of irrigation need and groundwater recharge (IGR-ratio) is suggested as an indicator to assess climate change impacts on the vulnerability of groundwater resources towards overexploitation by agricultural irrigation. The IGR-ratio has been assessed based on the distributed water balance model mGROWA, i.e. under consideration of the simulated groundwater recharge levels and the field crop-specific irrigation need of the commonly cultivated field crops. The spatial IGR-ratio distribution determined for the observed reference period 1971–2000 has shown that the delineated vulnerable areas coincide with the regions for which high irrigation quantities have been documented at present. Additionally, the IGR-ratio depicts the areas in which irrigation is currently still negligible, but in which the introduction of irrigation into agricultural practice would lead to an immediate overexploitation of the sustainably available groundwater budget. The possible impact of future climate on IGR-ratios was determined by using a model chain of mGROWA and the regional climate models REMO and WETTREG2010. The related ensemble simulations did not provide a uniform tendency of possible future IGR-ratio changes. Whereas the mGROWA–WETTREG2010 realisations projected a very high increase in the IGR-ratios, the mGROWA–REMO realisations did not show a pronounced trend of increasing IGR-ratios. Therefore, considerable uncertainties remain regarding the future bandwidth of IGR-ratio changes.