Long-term daily hydrometeorological drought indices, soil moisture, and evapotranspiration for ICOS sites

Pohl, Felix; Rakovec, Oldřich; Rebmann, Corinna; Hildebrandt, Anke; Boeing, Friedrich; Hermanns, Floris; Attinger, Sabine; Samaniego, Luis; Kumar, Rohini

doi:10.21203/rs.3.rs-2516603/v1

Cited by 4 publications

(5 citation statements)

References 71 publications

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“…Such dry conditions during annually more than 2 weeks have been recorded at this site only five times since 1950. However, out of these five times, three occurred after 2010 (2011,2018,2020) (Pohl et al, 2022). The results from the temperate ecosystem confirm statements from the classic literature that SVA is strongest in central-European climate conditions in late summer when the soil is dry and uncovered (Blume et al, 2016b).…”

Section: Seasonal and Diel Meteorologysupporting

confidence: 79%

“…In this time period (20 July 2018 until 22 August 2018) the Standardized Precipitation Evaporation Index (aggregated over 30 d; SPEI_30) at DE-RuS indicates extreme drought (Appendix Fig. E1; Svoboda et al, 2002) (Pohl et al, 2023(Pohl et al, , 2022. Such dry conditions during annually more than 2 weeks have been recorded at this site only five times since 1950.…”

Section: Seasonal and Diel Meteorologymentioning

confidence: 83%

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Interpretability of negative latent heat fluxes from eddy covariance measurements in dry conditions

Paulus,

Orth,

Lee

et al. 2024

Biogeosciences

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Abstract. It is known from arid and semi-arid ecosystems that atmospheric water vapor can directly be adsorbed by the soil matrix. Soil water vapor adsorption was typically neglected and only recently received attention because of improvements in measurement techniques. One technique rarely explored for the measurement of soil water vapor adsorption is eddy covariance (EC). Soil water vapor adsorption may be detectable as downwardly directed (i.e., negative) EC latent heat (λE) flux measurements under dry conditions, but a systematic assessment of the use of negative λE fluxes from EC flux stations to characterize adsorption is missing. We propose a classification method to characterize soil water vapor adsorption, excluding conditions of dew and fog when λE derived from EC is not trustworthy due to stable atmospheric conditions. We compare downwardly directed λE fluxes from EC with measurements from weighing lysimeters for 4 years in a Mediterranean savanna ecosystem and 3 years in a temperate agricultural site. Our aim is to assess if overnight water inputs from soil water vapor adsorption differ between ecosystems and how well they are detectable by EC. At the Mediterranean site, the lysimeters measured soil water vapor adsorption each summer, whereas at the temperate site, soil water vapor adsorption was much rarer and was measured predominantly under an extreme drought event in 2018. During 30 % of nights in the 4-year measurement period at the Mediterranean site, the EC technique detected downwardly directed λE fluxes of which 88.8 % were confirmed to be soil water vapor adsorption by at least one lysimeter. At the temperate site, downwardly directed λE fluxes were only recorded during 15 % of the nights, with only 36.8 % of half hours matching simultaneous lysimeter measurement of soil water vapor adsorption. This relationship slightly improved to 61 % under bare-soil conditions and extreme droughts. This underlines that soil water vapor adsorption is likely a much more relevant process in arid ecosystems compared to temperate ones and that the EC method was able to capture this difference. The comparisons of the amounts of soil water vapor adsorption between the two methods revealed a substantial underestimation of the EC compared to the lysimeters. This underestimation was, however, comparable with the underestimation in evaporation by the eddy covariance and improved in conditions of higher turbulence. Based on a random-forest-based feature selection, we found the mismatch between the methods being dominantly related to the site's inherent variability in soil conditions, namely soil water status, and soil (surface) temperature. We further demonstrate that although the water flux is very small with mean values of 0.04 or 0.06 mm per night for EC or lysimeter, respectively, it can be a substantial fraction of the diel soil water balance under dry conditions. Although the two instruments substantially differ with regard to the measured ratio of adsorption to evaporation over 24 h with 64 % and 25 % for the lysimeter and EC methods, they are in either case substantial. Given the usefulness of EC for detecting soil water vapor adsorption as demonstrated here, there is potential for investigating adsorption in more climate regions thanks to the greater abundance of EC measurements compared to lysimeter observations.

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Section: Seasonal and Diel Meteorologysupporting

confidence: 79%

Section: Seasonal and Diel Meteorologymentioning

confidence: 83%

Interpretability of negative latent heat fluxes from eddy covariance measurements in dry conditions

Paulus,

Orth,

Lee

et al. 2024

Biogeosciences

Self Cite

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Section: Seasonal and Diel Meteorologymentioning

confidence: 83%

Interpretability of negative latent heat fluxes from Eddy Covariance measurements during dry conditions

Paulus,

Orth,

Lee

et al. 2023

Preprint

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Abstract. It is known from arid and semi-arid ecosystems that atmospheric water vapor is directly adsorbed by the soil matrix during the night. Soil water vapor adsorption was typically neglected and only recently got attention because of improvements in measurement techniques. One technique rarely explored is eddy covariance (EC). EC nighttime measurements are usually discarded, but soil water vapor adsorption may be detectable as downwards-directed EC latent heat (λE) flux measurements under dry conditions. We propose a classification method to exclude conditions of dew and fog when λE derived from EC is not trustworthy due to stable atmospheric conditions. We compare downwards-directed λE fluxes from EC with measurements from weighable lysimeters for four years in a Mediterranean Savannah ecosystem and three years in a temperate agricultural site. Our aim is to assess if overnight water inputs from soil water vapor adsorption differ between ecosystems and how well they are detectable by EC. At the Mediterranean site, the lysimeters measured soil water vapor adsorption each summer whereas at the temperate site soil water vapor adsorption was much rarer, and measured predominantly under extreme drought. In 30 % of nights in the four-year measurement period at the Mediterranean site, the EC technique detected downward-directed λE fluxes of which 88.8 % were confirmed to be soil water vapor adsorption by at least one lysimeter. At the temperate site, downward-directed λE fluxes were only recorded during 15 % of the nights, with only 36.8 % of half-hours matching simultaneous lysimeter measurement of soil water vapor adsorption. Although this relationship slightly improved to 60% under bare soil conditions and extreme droughts, this underlines that soil water vapor adsorption is likely a much more relevant process in arid ecosystems compared to temperate ones and that the EC method was able to capture this difference. The comparisons of the magnitudes between the two methods revealed a substantial underestimation of soil water vapor adsorption with EC. This underestimation was, however, on par with the underestimation in evaporation. Based on a random forest-based feature selection we found the mismatch between the techniques being dominantly related to the site's inherent spatiotemporal variations in soil conditions, namely soil water status, and soil (surface) temperature. We further demonstrate that although the water flux is very small with mean values of 0.04 or 0.06 mm per night depending on either EC or lysimeter detection it can be a substantial fraction of the diel soil water balance under dry conditions. Although the two instruments substantially differ with regard to the evaporative fraction with 64% and 25% for the lysimeter and EC methods, they are in either case substantial. Given the usefulness of EC for detecting soil water vapor adsorption as demonstrated here, there is potential for investigating adsorption in more climate regions at longer timescales thanks to the greater abundance of EC measurements compared to lysimeter observations.

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“…Reference ET, estimated based on the Hargreaves–Samani model using ERA‐5 air temperature data (daily minimum, maximum, and mean) as input (Hargreaves & Samani, 1985). The Hargreaves‐Samani model is a parsimonious option with low data demand and reasonable accuracy (Pohl et al., 2023) and was therefore chosen in this study based on previous experiments showing that different formulations of PET/ET ref could be applied with no significant impact on the simulations regarding capturing the inter‐annual variability across different European regions (Rakovec, Kumar, Mai, et al., 2016).…”

Section: Methodsmentioning

confidence: 99%

Tradeoffs Between Temporal and Spatial Pattern Calibration and Their Impacts on Robustness and Transferability of Hydrologic Model Parameters to Ungauged Basins

Demirel,

Koch,

Rakovec

et al. 2024

Water Resources Research

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Optimization of spatially consistent parameter fields is believed to increase the robustness of parameter estimation and its transferability to ungauged basins. The current paper extends previous multi‐objective and transferability studies by exploring the value of both multi‐basin and spatial pattern calibration of distributed hydrologic models as compared to single‐basin and single‐objective model calibrations, with respect to tradeoffs, performance and transferability. The mesoscale Hydrological Model (mHM) is used across six large central European basins. Model simulations are evaluated against streamflow observations at the basin outlets and remotely sensed evapotranspiration patterns. Several model validation experiments are performed through combinations of single‐ (temporal evaluation through discharge) and multi‐objective (temporal and spatial evaluation through discharge and spatial evapotranspiration patterns) calibrations with holdout experiments saving alternating basins for model evaluation. The study shows that there are very minimal tradeoffs between spatial and temporal performance objectives and that a joint calibration of multiple basins using multiple objective functions provides the most robust estimations of parameter fields that perform better when transferred to ungauged basins. The study indicates that particularly the multi‐basin calibration approach is key for robust parametrizations, and that the addition of an objective function tailored for matching spatial patterns of ET fields alters the spatial parameter fields while significantly improving the spatial pattern performance without any tradeoffs with discharge performance. In light of model equifinality, the minimal tradeoff between spatial and temporal performance shows that adding spatial pattern evaluation to the traditional temporal evaluation of hydrological models can assist in identifying optimal parameter sets.

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Long-term daily hydrometeorological drought indices, soil moisture, and evapotranspiration for ICOS sites

Cited by 4 publications

References 71 publications

Interpretability of negative latent heat fluxes from eddy covariance measurements in dry conditions

Interpretability of negative latent heat fluxes from eddy covariance measurements in dry conditions

Interpretability of negative latent heat fluxes from Eddy Covariance measurements during dry conditions

Tradeoffs Between Temporal and Spatial Pattern Calibration and Their Impacts on Robustness and Transferability of Hydrologic Model Parameters to Ungauged Basins

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