Issues to be considered for strategic adaptation to climate evolution – is atmospheric evaporative demand changing?

摘要

The predicted developments in climate are region-specific and adaptation can only be successful considering the regional characteristics with its diverse technical, environmental, economic and social implications. Beyond some obvious adaptation strategies in response to emerging environmental constraints for example there are many more “basic” challenges below “the surface”. One of the key concerns for many regions is the availability of water and how increasing temperature will drive the evaporative demand of the atmosphere. For this, individual regions need to be analysed to quantify possible associated risks. This paper will address differences in regional water relations of grape growing areas in different parts of the world as a basis to address the points listed above. Introduction Climate change effects on the terrestrial water cycle show regional differentiated patterns. While temperature is increasing in many world grape growing regions (Jones et al., 2005; Schultz and Jones, 2010; Webb et al., 2012; Hannah et al., 2013; Tóth and Végvári, 2016) precipitation patterns can vastly differ between regions and can show substantial temporal variations (between and within years) (IPCC, 2014). From rising temperatures it is mostly assumed that water holding capacity of the atmosphere will increase in the future as a function of the Clausius-Clapeyron law (Krysanova et al., 2008) which predicts an increase in the saturation vapour pressure of the atmosphere of 6-7% per degree Celsius. As a consequence, a simultaneous increase in potential evapotranspiration (the amount of water that could potentially be evaporated from soils and transpired by plants due to changes in climatic factors such as temperature, vapour pressure deficit, radiation and wind speed, ETp) is assumed in many cases, which would alter soil and plant water relations. However, the same underlying principles also predict an increase in precipitation by 1-2 % per degree warming (Farquhar and Roderick, 2007). Additionally, model predictions for many regions forecast altered precipitation patterns and thus in combination with the possibility of increased ETp, farmers around the world fear an increase in the likelyhood of water deficit and the availability of water for irrigation. However, the large spatial and temporal variability in precipitation patterns between regions preclude generalizations in predicted consequences with respect to soil and plant water status development. Especially the temporal variability may mask longer-term trends in the development of ETp and consequently soil and plant water status (van Leeuwen et al., 2010). Additionally, the focus on the developments within a growing season (spring-summer) in many studies may miss decisive effects occurring during the “off-season” (winter-early spring) but having substantial carry-over effects into the season. Evaporation is driven by changes in temperature, humidity, solar radiation and wind speed and contrary to expectations due to climatic changes, there have been reports on a reduction in evaporative demand worldwide (Farquhar and Roderick, 2007). In many cases this has been related to a decrease in solar radiation observed for many areas on earth including wine growing regions in Europe until the beginning of the 80th (global dimming, (Wild et al., 2005; Hofmann and Schultz, 2010)) of the last century. However, ETp in some areas has continuously increased which suggests that changes in the aerodynamic component must have more than offset the decrease in radiation over that part of the observed time span (Schultz and Hofmann, 2016). For some regions in Germany, wind speed and vapour pressure deficit (VPD) of the atmosphere have increased in the past and contributed to changes in evapotranspiration (Bormann, 2011) but this is not in agreement with a worldwide observed decrease in wind speed and pan evaporation (Farquhar and Roderick, 2007; McVicar et al., 2012). These conflicting observations depending on climate classification, country or region, make it necessary to analyze grape growing regions with respect to developments in ETp and precipitation patterns much more in detail in order to make predictions with respect to an increased risk in terms of water shortage. There is a general lack of studies analyzing the past development in ETp and precipitation for different wine growing regions across the planet in order to answer the question whether the threat for sustained drought will increase. When ETp was set to increase in a future climate scenario, substantial reductions in pre-dawn leaf water potential resulted when a dynamic physiological grapevine water model was used (Lebon et al., 2003) to estimate water consumption (Schultz and Lebon, 2005). However, the large spatial and temporal variability in precipitation patterns between regions preclude generalizations in predicted consequences with respect to soil and plant water status development. Water limited worlds versus