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Water returned to the atmosphere as evapotranspiration (ET) is approximately 1.6x global river discharge and has wide‐reaching impacts on groundwater and streamflow. In the U.S. Midwest, widespread land conversion from prairie to pasture to cropland has altered spatiotemporal patterns of ET, yet there is not consensus on the direction of change or the mechanisms controlling changes. We measured ET at three locations within the Long‐Term Agroecosystem Research network along a latitudinal gradient with paired rainfed cropland and prairie sites at each location. At the northern locations, the Upper Mississippi River Basin (UMRB) and Kellogg Biological Station (KBS), the cropland has annual ET that is 84 and 29 mm/year (22% and 5%) higher, respectively, caused primarily by higher ET during springtime when fields are fallow. At the southern location, the Central Mississippi River Basin (CMRB), the prairie has 69 mm/year (11%) higher ET, primarily due to a longer growing season. Differences in climate and that the CMRB prairie is remnant native prairie, while the UMRB and KBS prairies are restored, make it challenging to attribute differences to specific mechanisms. To accomplish this, we examine the energy balance using the Two‐Resistance Method (TRM). Results from the TRM demonstrate that higher surface conductance in croplands is the primary factor leading to higher springtime ET from croplands, relative to prairies. Results from this study provide insight into impacts of warm season grasses on the hydrology of the U.S. Corn Belt by providing a mechanistic understanding of how land use change affects the water budget. Plain Language Summary: Evapotranspiration (ET) consists of evaporation from bare soil and plant leaves. ET is ∼1.6x greater than global river flow and has wide‐reaching impacts on groundwater and streamflow. In the U.S. Midwest, widespread land conversion from prairies to croplands has altered patterns of ET, yet there is no consensus on the direction of this change or the mechanisms controlling changes. In this study we use measurements of ET at three locations within the Long‐Term Agroecosystem Research (LTAR) network that have paired cropland and prairie sites. Surprisingly, we found that in the two northern sites, the croplands had higher ET than the prairies, particularly during springtime when the croplands are fallow. We used mathematical analysis of the energy budget to show that a parameter called the surface conductance controls the differences in ET between the croplands and prairies. During springtime in prairies, the standing, dormant vegetation blocks transfer of water vapor from the land surface, reducing the surface conductance, and limits the ET. Results from this study provide insight into the impact of land conversion from prairies to croplands on the hydrology of the U.S. Corn Belt by providing a mechanistic understanding of how land use change affects the water budget. Key Points: Differences in evapotranspiration between croplands and prairies was quantified by a mechanistic Two Resistance MethodBowen ratio during springtime is higher in prairies than croplandsSurface resistance is the primary factor causing springtime evapotranspiration differences between croplands and prairies [ABSTRACT FROM AUTHOR]

Lakes provide enormous economic, recreational, and aesthetic benefits to citizens. These ecosystem services may be adversely impacted by climate change. In the Twin Cities Metropolitan Area of Minnesota, USA, many lakes have been at historic low levels and water augmentation strategies have been proposed to alleviate the problem. White Bear Lake (WBL) is a notable example. Its water level declined 1.5 m during 2003–2013 for reasons that are not fully understood. This study examined current, past, and future lake evaporation to better understand how climate will impact the water balance of lakes within this region. Evaporation from WBL was measured from July 2014 to February 2017 using two eddy covariance (EC) systems to provide better constraints on the water budget and to investigate the impact of evaporation on lake level. The estimated annual evaporation losses for years 2014 through 2016 were 559 ± 22 mm, 779 ± 81 mm, and 766 ± 11 mm, respectively. The higher evaporation in 2015 and 2016 was caused by the combined effects of larger average daily evaporation and a longer ice-free season. The EC measurements were used to tune the Community Land Model 4 – Lake, Ice, Snow and Sediment Simulator (CLM4-LISSS) to estimate lake evaporation over the period 1979–2016. Retrospective analyses indicate that WBL evaporation increased during this time by about 3.8 mm year −1 , which was driven by increased wind speed and lake-surface vapor pressure gradient. Using a business-as-usual greenhouse gas emission scenario (RCP8.5), lake evaporation was modeled forward in time from 2017 to 2100. Annual evaporation is expected to increase by 1.4 mm year −1 over this century, largely driven by lengthening ice-free periods. These changes in ice phenology and evaporation will have important implications for the regional water balance, and water management and water augmentation strategies that are being proposed for these Metropolitan lakes. [ABSTRACT FROM AUTHOR]

Abstract: Development of optical isotope techniques over the last several years has provided scientists a set of tools for tracing the transport and cycling of CO2 and water vapor between the biosphere and atmosphere. Here, I take a micrometeorological perspective and review these technological advances, assess key instrument performance characteristics, examine how these techniques have been used in the field to improve our understanding of the processes governing the exchange of CO2 and water vapor, and discuss future research directions. Review of the recent literature indicates that: (1) optical techniques have been used to quantify the isotope composition of biosphere–atmosphere exchange using the traditional Keeling mixing line, flux–gradient, eddy covariance, and chamber approaches under a variety of field conditions with near-continuous data records now extending to more than 5-years; (2) high frequency and near continuous isotope measurements at the canopy scale have demonstrated important new insights regarding the behaviour of kinetic fractionation at the leaf versus canopy scales and the controls on ecosystem respiration that could not have been observed previously using traditional methods; (3) based on the assessment of instrument performance, carbon isotope disequilibrium (the difference between the isotope composition of photosynthesis and respiration), carbon turnover rates, and measurement uncertainties, 13C-CO2 investigations are best suited for examining the contributions and changes to ecosystem respiration with a need for more innovative 13C-isotope labeling experiments and compound specific isotope analyses under field conditions; (4) significant progress has been made in measuring the oxygen isotope composition of water vapor fluxes and canopy leaf water enrichment. These new data have provided an opportunity to evaluate models of leaf water enrichment and their application to the canopy scale; and (5) the use of 18O-H2O and 18O-CO2 as tracers of the coupled carbon–water cycle has matured significantly in recent years. Evidence from a range of ecosystems indicates that 18O-CO2 disequilibrium (ranging from 0 to 17‰) is much larger than for 13C-CO2 (typical less than 3‰ in natural C3 ecosystems) making it a useful tracer of coupled carbon and water cycle processes. However, a better understanding of the role of carbonic anhydrase in photosynthetic and respiratory processes under field conditions is now needed in order to make further progress. Finally, the increasing use and development of isotope-enabled land surface schemes along with the acquisition of high temporal resolution isotope data is providing a new opportunity to constrain the carbon and water cycle processes represented in these models. [Copyright &y& Elsevier]

Abstract: The stable carbon isotope ratio, , is a valuable tracer for studying the processes controlling the autotrophic (F Ra ) and heterotrophic (F Rh ) contributions to ecosystem respiration (F R ) and the influence of photosynthesis on F R . There is increasing interest in quantifying the temporal variability of the carbon isotope composition of ecosystem respiration (δ R ) because it contains information about the sources contributing to respiration and is an important parameter used for partitioning net ecosystem CO2 exchange using stable isotope methods. In this study, eddy covariance, flux gradient, automated chambers, and stable carbon isotope techniques were used to quantify and improve our understanding of the temporal variability in F R and δ R in a C3/C4 agricultural ecosystem. Six years (2004–2009) of isotope flux-gradient measurements indicated that δ R had a very consistent annual pattern during both C3 (soybean) and C4 (corn) growing seasons due to significant contributions from F Ra , which was strongly influenced by the isotope composition of the recent photosynthate. However, in the spring, δ R exhibited a C3 signal regardless of the crop grown in the previous season. One hypothesis for this anomaly is that at these low soil temperatures microbial activity relied predominantly on C3 substrates. Automated chamber measurements of soil respiration () and its isotope composition () were initiated in the early corn growing season of 2009 to help interpret the variability in δ R . These measurements showed good agreement with EC measurements of F R (within 0.5μmolm2 s−1) and isotope flux gradient measurements of δ R (within 2‰) at nighttime for near-bare soil conditions (LAI<0.1). At peak growth, nighttime δ R above the corn canopy was consistently 1–6‰ more enriched than . The relatively enriched signal above the canopy indicates that δ R was strongly influenced by aboveground plant respiration (F R,ag ), which accounted for about 40% of F R . The automated chamber data and analyses also revealed a strong diel pattern in . In the early growth period, showed a sharp morning enrichment of up to 4‰ followed by a gradual depletion throughout the afternoon and evening. Daytime enrichment in was most pronounced during dry conditions and was not observed when the upper soil was near saturation. We provide anecdotal evidence that the diel variability during early growth may have been influenced by turbulence (advection/non-diffusive transport), which reduced the kinetic fractionation effect. At peak growth, there is evidence that the sheltering effect of the corn plants diminished the influence of turbulence on the chamber measurement of . Further research is needed to evaluate and separate the contributions of biotic and abiotic (advection and non-steady state effects) influences on chamber observations. [Copyright &y& Elsevier]

• 3.5 year-long study with simultaneous open- and closed path CH4 flux measurements. • Quantification of systematic, random and gap-filling errors and resulting uncertainty. • Average annual emissions of 18 g−CH4 m-2 yr-1 agreed within 0.6 g−CH4 m-2 yr-1 between analyzers. • Total flux uncertainty on annual fluxes was estimated to range from 7% to 17%. Wetlands represent the dominant natural source of methane (CH 4) to the atmosphere. Thus, substantial effort has been spent examining the CH 4 budgets of global wetlands via continuous ecosystem-scale measurements using the eddy covariance (EC) technique. Robust error characterization for such measurements, however, remains a major challenge. Here, we quantify systematic, random and gap-filling errors and the resulting uncertainty in CH 4 fluxes using a 3.5 year time series of simultaneous open- and closed path CH 4 flux measurements over a sub-boreal wetland. After correcting for high- and low frequency flux attenuation, the magnitude of systematic frequency response errors were negligible relative to other uncertainties. Based on three different random flux error estimations, we found that errors of the CH 4 flux measurement systems were smaller in magnitude than errors associated with the turbulent transport and flux footprint heterogeneity. Errors on individual half-hourly CH 4 fluxes were typically 6%–41%, but not normally distributed (leptokurtic), and thus need to be appropriately characterized when fluxes are compared to chamber-derived or modeled CH 4 fluxes. Integrated annual fluxes were only moderately sensitive to gap-filling, based on an evaluation of 4 different methods. Calculated budgets agreed on average to within 7% (≤1.5 g−CH 4 m−2 yr−1). Marginal distribution sampling using open source code was among the best-performing of all the evaluated gap-filling approaches and it is therefore recommended given its transparency and reproducibility. Overall, estimates of annual CH 4 emissions for both EC systems were in excellent agreement (within 0.6 g−CH 4 m−2 yr−1) and averaged 18 g−CH 4 m−2 yr−1. Total uncertainties on the annual fluxes were larger than the uncertainty of the flux measurement systems and estimated between 7–17%. Identifying trends and differences among sites or site years requires that the observed variability exceeds these uncertainties. [ABSTRACT FROM AUTHOR]

The annual net ecosystem productivity (FNEP) of a second-growth Douglas-fir stand established in 1949 on the Canadian West Coast varied considerably over the 4-year period between 1998 and 2001. This period included the El Niño/La Niña cycle during the northern hemispheric winters of 1997/1998 and 1998/1999, offering a unique opportunity to study how a typical forest ecosystem in the Pacific Northwest reacts to interannual climate variability. This was possible even though annual FNEP values calculated from eddy covariance (EC) measurements of CO2 fluxes were subject to biases. These were largely due to the failure of the EC method to accurately measure losses of CO2 under low turbulence conditions at night, which caused FNEP overestimates of as much as 90 g C m-2 per year. As these biases were largely unaffected by interannual climate variability, it was possible to reliably quantify interannual differences in FNEP estimates if they were larger than random variability, which was estimated to be ±30 g C m-2 per year at most. Interannual differences were mainly due to differences in ecosystem respiration (R) between the 4 years. In the year following the 1997/1998 El Niño, high air temperatures led to the highest annual R of the 4 years, while annual gross ecosystem photosynthesis (P) was only slightly higher than normal. This resulted in 1998 having the lowest FNEP (270 g C m-2 per year) of the 4 years. For 1999, a cool and cloudy La Niña year, FNEP was 360 g C m-2 per year, much higher than 1998, but somewhat lower than the last 2 years, for which FNEP values were 390 and 420 g C m-2 per year, respectively. [Copyright &y& Elsevier]