Nocturnal surface fluxes of N2O and CH4 determined from atmospheric measurements at the Cabauw tall tower

<p>The agricultural emissions are the dominant sources of N<sub>2</sub>O and CH<sub>4</sub> in the Netherlands. In this study, we have estimated nocturnal surface fluxes of both N<sub>2</sub>O and CH<sub>4</sub> using atmospheric measurements at the Cabauw tall tower (4.927◦ E, 51.971◦ N, - 0.7 m a.s.l.). The nocturnal N<sub>2</sub>O and CH<sub>4</sub> surface fluxes were derived using two different methods, the vertical gradient method (VGM), i.e. the sum of the storage flux and the turbulent flux, and the radon-tracer method (RTM), for the period of March 2017-December 2018 and 2016-2018, respectively. For N<sub>2</sub>O, we show that a few events occurring between May 30 and June 4 in 2018 dominated the monthly means. Using the VGM, we have estimated the annual mean nocturnal surface flux to be 0.59 ± 0.38 g/m<sup>2</sup>/yr (1 σ, the same as below) and 0.53 ± 0.19 g/m<sup>2</sup>/yr with and without events, respectively. The fluxes are high in the summer and low in the winter, with a seasonal amplitude of around 1.0 g/m2/yr and 0.5 g/m<sup>2</sup>/yr, with and without events, respectively, which is likely caused by the seasonality of agricultural activities. For CH<sub>4, </sub>the annual mean nocturnal surface flux is 12.1 ± 3.3 g/m<sup>2</sup>/yr and the amplitude is around 9.9 g/m<sup>2</sup>/yr. Using the RTM, the mean fluxes of the whole period for N<sub>2</sub>O and CH<sub>4 </sub>are estimated to be 1.18 ± 2.25 (1.08 ± 1.29, without the events) g/m<sup>2</sup>/yr and 26.9 ± 24.8 g/m<sup>2</sup>/yr, respectively; in contrast to the VGM, no apparent seasonal pattern has been found. However, there is a good linear correlation between the estimated N<sub>2</sub>O fluxes from the two methods and the monthly means show a similar pattern when the same nights are considered; the R-squared value is around 0.9 with events and 0.6 without events, and the slope varies from 1.9 to 0.8 when different estimates of radon fluxes are used. Furthermore, we found that large N<sub>2</sub>O fluxes are related to the amount of rainfall occurring days before, with the correlation coefficient of around 0.6 (p value<0.01). For CH<sub>4</sub>, there is no correlation between the estimated CH<sub>4</sub> fluxes from the two methods. Our findings demonstrate that nocturnal N<sub>2</sub>O and CH<sub>4</sub> fluxes in the Cabauw area are highly variable and vary over different seasons, and that both VGM and RTM are useful to quantify regional N<sub>2</sub>O and CH<sub>4</sub> fluxes.</p>

Assessing the influence of soil freeze–thaw cycles on catchment water storage–flux–age interactions using a tracer-aided ecohydrological model

Ecohydrological models are powerful tools to quantify the effects that independent fluxes may have on catchment storage dynamics. Here, we adapted the tracer-aided ecohydrological model, EcH2O-iso, for cold regions with the explicit conceptualization of dynamic soil freeze–thaw processes. We tested the model at the data-rich Krycklan site in northern Sweden with multi-criterion calibration using discharge, stream isotopes and soil moisture in three nested catchments. We utilized the model's incorporation of ecohydrological partitioning to evaluate the effect of soil frost on evaporation and transpiration water ages, and thereby the age of source waters. The simulation of stream discharge, isotopes, and soil moisture variability captured the seasonal dynamics at all three stream sites and both soil sites, with notable reductions in discharge and soil moisture during the winter months due to the development of the frost front. Stream isotope simulations reproduced the response to the isotopically depleted pulse of spring snowmelt. The soil frost dynamics adequately captured the spatial differences in the freezing front throughout the winter period, despite no direct calibration of soil frost to measured soil temperature. The simulated soil frost indicated a maximum freeze depth of 0.25 m below forest vegetation. Water ages of evaporation and transpiration reflect the influence of snowmelt inputs, with a high proclivity of old water (pre-winter storage) at the beginning of the growing season and a mix of snowmelt and precipitation (young water) toward the end of the summer. Soil frost had an early season influence of the transpiration water ages, with water pre-dating the snowpack mainly sustaining vegetation at the start of the growing season. Given the long-term expected change in the energy balance of northern climates, the approach presented provides a framework for quantifying the interactions of ecohydrological fluxes and waters stored in the soil and understanding how these may be impacted in future.

Assessing the influence of soil freeze-thaw cycles on catchment water storage – flux – age interactions using a tracer-aided ecohydrological model

Ecohydrological models are powerful tools to quantify the effects that independent fluxes may have on catchment storage dynamics. Here, we adapted the tracer-aided ecohydrological model, EcH2O-iso, for cold regions with the explicit conceptualisation of dynamic soil freeze-thaw processes. We tested the model at the data-rich Krycklan site in northern Sweden with multi-criteria calibration using discharge, stream isotopes and soil moisture in 3 nested catchments. We utilized the model’s incorporation of ecohydrological partitioning to evaluate the effect of soil frost on evaporation and transpiration water ages, and thereby the age of source waters. The simulation of stream discharge, isotopes, and soil moisture variability captured the seasonal dynamics at all three stream sites and both soil sites, with notable reductions in discharge and soil moisture during the winter months due to the development of the frost front. Stream isotope simulations reproduced the response to the isotopically-depleted pulse of spring snowmelt. The soil frost dynamics adequately captured the spatial differences in the freezing-front throughout the winter period, despite no direct calibration of soil frost to measured soil temperature. The simulated soil frost indicated a maximum freeze-depth of 0.25 m below forest vegetation. Water ages of evaporation and transpiration reflect the influence of snowmelt-inputs, with a high proclivity of old water (pre-winter storage) at the beginning of the growing season and a mix of snowmelt and precipitation (young water) toward the end of the summer. Soil frost had an early season influence of the transpiration water ages, with water pre-dating the snowpack mainly sustaining vegetation at the start of the growing season. Given the long-term expected change in the energy-balance of northern climates, the approach presented provides a framework for quantifying the interactions of ecohydrological fluxes and waters stored in the soil and understanding how these may be impacted in future.