Characterizing the age distribution of catchment evaporative losses

Copyright © 2015 John Wiley & Sons, Ltd. The ‘Missing’ Exit age of Evaporation The past decade has seen significant advances in the development of tools to estimate the non-stationary age of streamwater (see review by Rinaldo et al., 2015). These approaches usually involve the use of direct measurements of inputs and outputs of various tracers and their integration into different types of catchment-scale hydrological models (Birkel and Soulsby, 2015). However, the ages of water fluxes from different internal stores are less well constrained, and mostly restricted to soils and groundwaters (Tetzlaff et al., 2014). But these flux ages can show marked spatial and (in the case of soil waters) temporal variation (Benettin et al., 2015). Importantly, estimates of the ages of water fluxes to the atmosphere from evaporation and transpiration have been somewhat ignored by comparison, even though Botter (2012) presented an analytical expression for the age distribution of evapotranspiration fluxes. Yet, evaporative losses are a crucial piece of the jigsaw in quantifying the total exit age of catchment waters (i.e. in both discharge and evaporation). Furthermore, we understand little about how evaporation affects the ages of terrestrial fluxes and, ultimately, those of stream water (Hrachowitz et al., 2013). Fortunately, methodologies are becoming available to advance this research agenda (e.g. Kool et al., 2014). The isotopic composition of vapour fluxes can be measured in real time (e.g. Garvelmann et al., 2012), which allows the differentiation of evaporation and transpiration fluxes (Zhang et al., 2010; Jasechko et al., 2013). Coupled model and laboratory experiments have proven useful in this regard (e.g. Sutanto et al., 2012). As there are promising approaches for relating these fluxes isotopically to water sources in soil and groundwaters that can be aged directly (Good et al., 2015), estimates of the age of evaporated waters are becoming feasible although significant methodological issues remain (Dubbert et al., 2014; Sprenger et al., 2015). The age distribution of evaporated waters is likely to be a fundamental signature of catchment function, which will bear the imprint of geographical differences in hydroclimate and catchment properties. Perhaps more importantly, it is also likely to be sensitive to the effects of catchment disturbance and land use management. Crucially, it may also contribute insights into the sustainability of vegetation water use in managed areas. Notably in water-demanding food and energy crops, the partitioning of blue and green water is likely reflected in their interrelated age distributions (Falkenmark and Rockstrom, 2005). For example, in many catchments, especially with irrigation and water-demanding crops, evaporation is likely to preferentially draw from near surface waters that are likely to be relatively young. In contrast, it has been suggested that transpired water in natural mature forest stands in highly seasonal climates may be drawn from fine pores containing older waters that have been in storage for some time (Brooks et al., 2010). In each case, the age of evaporated water has the