Search

Transpiration (T) is perhaps the largest fluxes of water from the land surface to the atmosphere and is susceptible to changes in climate, land use and vegetation structure. However, predictions of future transpiration fluxes vary widely and are poorly constrained. Stable water isotopes can help expand our understanding of land–atmosphere water fluxes but are limited by a lack of observations and a poor understanding of how the isotopic composition of transpired vapour (δT) varies. Here, we present isotopic data of water vapour, terrestrial water and plant water from a deciduous forest to understand how vegetation affects water budgets and land–atmosphere water fluxes. We measured subdiurnal variations of δ18OT from three tree species and used water isotopes to partition T from evapotranspiration (ET) to quantify the role of vegetation in the local water cycle. We find that δ18OT deviated from isotopic steady‐state during the day but find no species‐specific patterns. The ratio of T to ET varied from 53% to 61% and was generally invariant during the day, indicating that diurnal evaporation and transpiration fluxes respond to similar atmospheric and micrometeorological conditions at this site. Finally, we compared the isotope‐inferred ratio of T to ET with results from another ET partitioning approach that uses eddy covariance and sap flux data. We find broad midday agreement between these two partitioning techniques, in particular, the absence of a diurnal cycle, which should encourage future ecohydrological isotope studies. Isotope‐inferred estimates of transpiration can inform land surface models and improve our understanding of land–atmosphere water fluxes. [ABSTRACT FROM AUTHOR]

Forests play an integral role in the terrestrial water cycle and link exchanges of water between the land surface and the atmosphere. To examine the effects of an intermediate disturbance on forest water cycling, we compared vertical profiles of stable water vapor isotopes in two closely located forest sites in northern lower Michigan. At one site, all canopy‐dominant early successional species were stem girdled to induce mortality and accelerate senescence. At both sites, we measured the isotopic composition of atmospheric water vapor at six heights during three seasons (spring, summer, and fall) and paired vertical isotope profiles with local meteorology and sap flux. Disturbance had a substantial impact on local water cycling. The undisturbed canopy was moister, retained more transpired vapor, and at times was poorly mixed with the free atmosphere above the canopy. Differences between the disturbed and undisturbed sites were most pronounced in the summer when transpiration was high. Differences in forest structure at the two sites also led to more isotopically stratified vapor within the undisturbed canopy. Our findings suggest that intermediate disturbance may increase mixing between the surface layer and above‐canopy atmosphere and alter ecosystem‐atmosphere gas exchange. Plain Language Summary: Forests play an important role in the climate system and link water fluxes between the land surface and the atmosphere. Here we compare water vapor isotopes in two adjacent forest sites in the northern lower peninsula of Michigan to understand the effects of intermediate disturbance and canopy structure on forest water cycling. One site is dominated by aspen and birch and has a thick, closed canopy. All of the aspen and birch were killed at the second site. As a result, the disturbed site has a more open‐canopy structure. From our comparison, we found that both the species of tree and the spacing around trees are important controls on forest water cycling. With more space between trees, air mixes more freely into the canopy, which dries the forest air. Alternatively, air may be poorly mixed within and above thick, closed canopies. Key Points: Intermediate disturbance can change the contribution of entrained, evaporated, and transpired water vapor to forest canopiesCanopy gaps increase hydrologic mixing between the surface layer and the free atmosphereThe assumption of a well‐mixed canopy atmosphere may be violated in the case of thick, homogeneous forest canopies [ABSTRACT FROM AUTHOR]

Water resources management in Maui, Hawaii, remains a challenge due to uneven distribution of rainfall, water scarcity in populated areas, and an ill-defined hydrological system. Based on data collected in June 2014, Niu et al. (2017) suggested that noble gases in this rapid groundwater infiltration system record seasonality and may shed light on water sources, recharge locations, and groundwater flow paths. This study goes one step further by analyzing rainwater, basal and perched (springs) aquifer data collected in June 2014 and February 2016. It shows for the first time that both rain and spring water display sharp temporal variations which are related to either seasonal (summer/winter) variations or to the strong 2015–2016 El Niño year. These spring water temporal variations also suggest that perched aquifer groundwater is extremely young and likely unreliable as a long-term water supplier. In contrast, noble gas temporal variations are absent in the basal aquifer. This, in turn, is consistent with tritium derived groundwater ages on the order of a few decades for the basal aquifer. This study further demonstrates that Ne is an excellent candidate to estimate the source elevation of rain and thus, to distinguish between orographic and synoptic-scale rain. Water stable isotope data are also presented for both years with 2016 samples exhibiting greater deuterium excesses than those of 2014, an observation consistent with the drier 2015–2016 El Niño climate. Estimated water source elevations based on δ18O for rainwater display a positive correlation with those based on Ne concentrations. • Both rain and spring water display sharp noble gas temporal variations. • Perched aquifers are very young and likely unreliable as a long-term water supplier. • Ne is an excellent candidate to distinguish orographic rain from synoptic-scale rain. [ABSTRACT FROM AUTHOR]