Berechnung der Bewässerungsbedürfnisse unter aktuellen und zukünftigen Bedingungen in der Schweiz

With climate change, rising temperatures and decreasing summer precipitation are likely to induce increasing water demands for irrigation in the future. Although Switzerland as a whole is a country with abundant water supplies, temporary regional water shortages are already occurring under current climatic conditions. Hydrological projections suggest that the availability of water from rivers for irrigation will decrease even further as climate change progresses. Information on expected changes in irrigation needs is of great importance for the long-term planning of irrigation infrastructure projects.

Against this background, this paper utilizes the FAO method for estimating crop-specific irrigation needs and applies it on the basis of climate projection data until the end of the century for potato, carrot, sweetcorn, apple, lettuce, strawberry, clover, ryegrass and grazing crops at selected sites on the Swiss Central Plateau (Aigle, Basel, Bern, Changins, Güttingen, Payerne, Reckenholz) and for different soil types (sand, sandy loam, loam, silt, silty clay loam, silty loam, loamy sand, silty clay, clay).

The projected changes are analysed by emission scenario, site, and projection horizon. The changes for a field crop (potatoes), a vegetable crop with multiple harvests (lettuce) and ryegrass are discussed as examples. The results for all other crops are given in the Appendix.

Estimates of the changes in irrigation-water needs differ substantially, depending on the assumptions regarding fu-ture global emissions of greenhouse gases (Emission path RCP2.6: Effective climate-change mitigation measures gain traction and lead to a reduction in CO2 concentration in the atmosphere to around 400 ppm by the end of the century; Emission path RCP8.5: Without climate-protection measures, the CO2 content in the atmosphere rises con-tinuously to almost 1000 ppm by the year 2100): with RCP2.6, a slight increase in irrigation for all crops is projected up to the middle of the century, followed by a slight decrease; with RCP8.5, irrigation needs for all crops rise steadily up to the end of the century.

The percentage changes in irrigation needs differ only slightly between the sites on the Central Plateau and according to soil type. In addition, only minor differences in terms of the change signals were noted between the crop types studied with regard to the reference timeframe of 1981–2010: by the time horizon of 2045−2074, RCP8.5 projections (i.e. without climate-change mitigation) suggest an average increase in irrigation needs of 26% for potatoes and lettuce and 28% for ryegrass. According to this emission pathway, increases of 48% (potatoes), 49% (lettuce) and 55% (ryegrass) would be expected by the end of the century (2070−2099).

The different seasonal changes in temperature and precipitation levels also have a varying impact on the additional irrigation needs according to growing season. Major changes in irrigation levels over the century are to be expected in summer and autumn, regardless of site and crop. Minimal changes in irrigation are to be expected in spring. With all these estimates, however, the climate-projection uncertainties that continue to increase along with the advancing time horizon must be taken into account.

The method operationalised here provides a basis for estimating the irrigation needs of different crops on the Swiss Central Plateau under current and future climatic conditions, depending on important soil parameters. Reviews of the model parameters and validations of estimated values for different climate regions, soils and crops are of crucial importance for the future, in order to reduce existing model insecurities to the greatest extent possible.