Quantification of dew and fog water inputs to temperate grasslands

Climate change scenarios for Switzerland predict a decline in summer precipitation by the end of the century. Furthermore, increasing temperatures are projected, which could lower water availability due to a higher evapotranspiration. Hence, plants could become more water stressed which in turn could lower productivity and thereby pose a major threat for food security. Grasslands are a widespread agricultural ecosystem in Switzerland, providing the basis for livestock production and provide an important habitat for many animal and plant species. These agricultural ecosystems could suffer greatly from prolonged dry periods. Non-rainfall water (NRW) inputs such as dew, hoar frost, fog, rime and water vapour adsorption could reduce this water stress and thus avoid productivity losses. Hence, the goal of this thesis was to assess the importance of NRW inputs to Swiss grasslands. In Chapter 2, we developed and tested an instrument based on micro-lysimeter (ML) with ancillary sensors to quantify the amount and frequency of NRW inputs of grasslands. The ML had an unprecedented high accuracy of ±0.005 mm, which allowed to measure even small NRW events. Furthermore, the combination between micro-lysimeters and ancillary sensors allowed to differentiate among different types of NRW inputs, i.e., dew, hoar frost, fog, rime, and the combinations thereof, and also potential water vapour adsorption events could be measured. The ML system could simulate natural conditions in terms of canopy temperatures very well. However, soil temperatures in the ML pots were elevated compared to the control. Thus, the ML system provides rather conservative estimates for dew formation on soils. In Chapter 2, we also show the application of the ML system at a field site. Over 12 months, we measured 127 NRW events, which correspond to a NRW input ca. every third day, summing up to 15.9 mm. This suggests a high ecohydrological relevance of NRW inputs for this temperate grassland ecosystem. In Chapter 3, we present NRW measurements at nine sites over a spatial and elevational gradient. On average, the water input of a single NRW event was 0.14±0.10 mm. NRW inputs were highly variable across sites. During a comparison period of ca. two months, the frequency of NRW events ranged between sites from 5 to 29 events. In analogy, the cumulative NRW input ranged from 0.12 to 7 mm. Furthermore, we present measurements of night-time evapotranspiration (ETnight). II Compared to the amounts of average NRW inputs, the average water loss by ETnight (0.24±0.16 mm) was 0.1 mm period-1 higher than the average NRW input. During the comparison period, the average water loss per ETnight period was even double than that of the average NRW input. However, at five out of nine sites, cumulative NRW inputs were higher than cumulative water losses by ETnight periods. NRW inputs and ETnight were highly site dependent, and were not influenced by elevation. Prevailing meteorological conditions at a site were decisive in determining if water is more likely gained or lost during a night. NRW inputs and ETnight, furthermore, were shown to influence net ecosystem exchange (NEE) in the early morning hours after sunrise. After ETnight periods, respiration was on average higher than after NRW events. This suggests a beneficial effect of NRW inputs by lowering respiratory losses. In general, NRW inputs and its counterpart ETnight were found to occur at all sites, suggesting a high relevance for water availability of temperate grassland ecosystems during no-rainfall periods. NRW inputs of dew and radiation fog often occur in combination during clear and calm nights. The input pathways for these events can be (1) a downward pathway, during which atmospheric water vapour is condensing, or (2) an upward pathway, during which soil water is evaporated, and afterwards condensing on the plant surface (termed as distillation). These input pathways have hardly been studied, especially during combined dew and radiation fog events. In Chapter 4, we use stable isotopes of water to investigate the two different input pathways into a temperate grassland ecosystem. Our results showed that distillation contributed 9 %–42 % of the water that condensed on plant surfaces. The moisture transfer during a distillation process can make water available to plants from parts of the soil that otherwise would have remained inaccessible. Our results provide an insight in the highly dynamic water fluxes during NRW inputs and show the ecohydrological relevance of the water redistribution process of distillation. In conclusion, we developed, tested, and deployed a micro-lysimeter system with ancillary sensors that was able to determine different NRW inputs with high accuracy. However, further research should focus to improve such an instrument in terms of simulating natural soil temperature and moisture conditions. NRW inputs were a frequent water input to temperate grassland ecosystems. ETnight resulted in water losses and partly counteracted NRW inputs. The frequency and amount of NRW input events and ETnight periods were highly site dependent. The contribution of distillation in the NRW inputs was found to be up to 42%, depending on atmospheric saturation conditions. In brief, this thesis has shown that NRW inputs can be important ecohydrological factors for temperate grassland ecosystems