Upscaling Point Measurements Research Articles

Spatial heterogeneity of ammonia fluxes in a deciduous forest and adjacent grassland

Gas-phase ammonia (NH3), emitted primarily from agriculture, contributes significantly to reactive nitrogen (Nr) deposition. Excess deposition of Nr to the environment causes acidification, eutrophication, and loss of biodiversity. The exchange of NH3 between land and atmosphere is bidirectional and can be highly heterogenous when underlying vegetation and soil characteristics differ. Direct measurements that assess the spatial heterogeneity of NH3 fluxes are lacking. To this end, we developed and deployed two fast-response, quantum cascade laser-based open-path NH3 sensors to quantify NH3 fluxes at a deciduous forest and an adjacent grassland separated by 700 m in North Carolina, United States from August to November, 2017. The sensors achieved 10 Hz precisions of 0.17 ppbv and 0.23 ppbv in the field, respectively. Eddy covariance calculations showed net deposition of NH3 (-7.3 ng NH3-N m−2 s−1) to the forest canopy and emission (3.2 ng NH3-N m−2 s−1) from the grassland. NH3 fluxes at both locations displayed diurnal patterns with absolute magnitudes largest midday and with smaller peaks in the afternoons. Concurrent biogeochemistry data showed over an order of magnitude higher NH3 emission potentials from green vegetation at the grassland compared to the forest, suggesting a possible explanation for the observed flux differences. Back trajectories originating from the site identified the upwind urban area as the main source region of NH3. Our work highlights the fact that adjacent natural ecosystems sharing the same airshed but different vegetation and biogeochemical conditions may differ remarkably in NH3 exchange. Such heterogeneities should be considered when upscaling point measurements, downscaling modeled fluxes, and evaluating Nr deposition for different natural land use types in the same landscape. Additional in-situ flux measurements accompanied by comprehensive biogeochemical and micrometeorological records over longer periods are needed to fully characterize the temporal variabilities and trends of NH3 fluxes and identify the underlying driving factors.

Streambed Flux Measurement Informed by Distributed Temperature Sensing Leads to a Significantly Different Characterization of Groundwater Discharge

Groundwater discharge though streambeds is often focused toward discrete zones, indicating that preliminary reconnaissance may be useful for capturing the full spectrum of groundwater discharge rates using point-scale quantitative methods. However, many direct-contact reconnaissance techniques can be time-consuming, and remote sensing (e.g., thermal infrared) typically does not penetrate the water column to locate submerged seepages. In this study, we tested whether dozens of groundwater discharge measurements made at “uninformed” (i.e., selected without knowledge on high-resolution temperature variations at the streambed) point locations along a reach would yield significantly different Darcy-based groundwater discharge rates when compared with “informed” measurements, focused at streambed thermal anomalies that were identified a priori using fiber-optic distributed temperature sensing (FO-DTS). A non-parametric U-test showed a significant difference between median discharge rates for uninformed (0.05 m·day−1; n = 30) and informed (0.17 m·day−1; n = 20) measurement locations. Mean values followed a similar pattern (0.12 versus 0.27 m·day−1), and frequency distributions for uninformed and informed measurements were also significantly different based on a Kolmogorov–Smirnov test. Results suggest that even using a quick “snapshot-in-time” field analysis of FO-DTS data can be useful in streambeds with groundwater discharge rates <0.2 m·day−1, a lower threshold than proposed in a previous study. Collectively, study results highlight that FO-DTS is a powerful technique for identifying higher-discharge zones in streambeds, but the pros and cons of informed and uninformed sampling depend in part on groundwater/surface water exchange study goals. For example, studies focused on measuring representative groundwater and solute fluxes may be biased if high-discharge locations are preferentially sampled. However, identification of high-discharge locations may complement more randomized sampling plans and lead to improvements in interpolating streambed fluxes and upscaling point measurements to the stream reach scale.

From Points to Patterns: Using Groundwater Time Series Clustering to Investigate Subsurface Hydrological Connectivity and Runoff Source Area Dynamics

AbstractGroundwater levels are typically measured at only a limited number of points in a catchment. Thus, upscaling these point measurements to the catchment scale is necessary to determine subsurface flow paths and runoff source areas. Here we present a data‐driven approach composed of time series clustering and topography‐based upscaling of shallow, perched groundwater dynamics using groundwater data from 51 monitoring sites in a 20‐ha prealpine headwater catchment in Switzerland. The agreement between the upscaled (modeled) and measured groundwater dynamics was strong for most of the 19‐month study period for the upslope and footslope locations but weaker at the beginning of events and for the midslope locations. However, these differences between measured and modeled groundwater levels did not significantly affect modeled groundwater activation, that is, the time when groundwater levels were within the more transmissive soil layers near the soil surface. The resulting groundwater activation maps represent the groundwater response across the catchment and highlight the dynamic expansion and contraction of the subsurface runoff source areas, particularly along the channel network. This is in agreement with the variable source area concept. However, there were also isolated active zones that did not get connected to the stream during rainfall events, highlighting the need to distinguish between variable active and variable stream‐connected runoff source areas. Our data‐driven approach to upscale point measurements of shallow groundwater levels appears useful for studying catchment‐scale variations in groundwater storage and connectivity and thus may help to better understand runoff generation in mountain catchments.

Quantifying Regional Fresh Submarine Groundwater Discharge With the Lumped Modeling Approach CoCa‐RFSGD

AbstractFresh Submarine Groundwater Discharge (FSGD) has been increasingly recognized as a fresh water resource and potential path for pollutants to the ocean. Since directly measuring FSGD requires great effort and is currently only feasible at local scale, we developed the regional transient water balance model Coastal Catchment Regional FSGD (CoCa‐RFSGD) to calculate daily FSGD at long stretches of coastlines. The conceptualization of the model takes into account the main processes between soil and aquifer, and, depending on the research question, in situ data, as well as anthropogenic processes, can be added to the water balance. It provides daily FSGD per coastal catchment. Within this model, FSGD is the precipitation surplus of the catchment area minus surface runoff, which highly correlates with the top soil water‐holding capacity and less with soil hydraulic conductivities. The model uses the island of Java, Indonesia, as an example. Java generates a total FSGD of 15.27 km3/a. Extrapolating the identified correlation of top soil water‐holding capacity and precipitation surplus on FSGD to the world highlights Indonesia, Vietnam, Nigeria, Cameroon, Equatorial Guinea, and Columbia as potential FSGD hot spots. Using mean global aquifer hydraulic conductivities might underestimate the seasonal variation of FSGD on Java. The model can be applied anywhere in the world to locate regional FSGD hot spots without obtaining in situ data, because it can run solely with global data sets. Hence, it is especially feasible for the locations mentioned. Furthermore, FSGD studies can be supplemented with this model by upscaling point measurements of FSGD to a full year.

Measurements of nitric oxide and ammonia soil fluxes from a wet savanna ecosystem site in West Africa during the DACCIWA field campaign

Abstract. Biogenic fluxes from soil at a local and regional scale are crucial to study air pollution and climate. Here we present field measurements of soil fluxes of nitric oxide (NO) and ammonia (NH3) observed over four different land cover types, i.e. bare soil, grassland, maize field, and forest, at an inland rural site in Benin, West Africa, during the DACCIWA field campaign in June and July 2016. At the regional scale, urbanization and a massive growth in population in West Africa have been causing a strong increase in anthropogenic emissions. Anthropogenic pollutants are transported inland and northward from the megacities located on the coast, where the reaction with biogenic emissions may lead to enhanced ozone production outside urban areas, as well as secondary organic aerosol formation, with detrimental effects on humans, animals, natural vegetation, and crops. We observe NO fluxes up to 48.05 ngN m−2 s−1. NO fluxes averaged over all land cover types are 4.79±5.59 ngN m−2 s−1, and maximum soil emissions of NO are recorded over bare soil. NH3 is dominated by deposition for all land cover types. NH3 fluxes range between −6.59 and 4.96 ngN m−2 s−1. NH3 fluxes averaged over all land cover types are -0.91±1.27 ngN m−2 s−1, and maximum NH3 deposition is measured over bare soil. The observations show high spatial variability even for the same soil type, same day, and same meteorological conditions. We compare point daytime average measurements of NO emissions recorded during the field campaign with those simulated by GEOS-Chem (Goddard Earth Observing System Chemistry Model) for the same site and find good agreement. In an attempt to quantify NO emissions at the regional and national scale, we also provide a tentative estimate of total NO emissions for the entire country of Benin for the month of July using two distinct methods: upscaling point measurements and using the GEOS-Chem model. The two methods give similar results: 1.17±0.6 and 1.44 GgN month−1, respectively. Total NH3 deposition estimated by upscaling point measurements for the month of July is 0.21 GgN month−1.

Upscaling point measurements of N2O emissions into the orchard scale under drip and microsprinkler irrigation

Agricultural activity is one of the major sources of nitrous oxide (N2O) emission into the natural environment. Yet, due to the soil’s spatial heterogeneities it is hard to accurately upscale point N2O emission measurements into the orchard scale. This study aims to introduce a simple, yet robust, way for upscaling point N2O emission measurements into the orchard scale, under drip and micro-sprinkler irrigation systems. Surface point measurements of N2O emissions were performed at five distances from drip and micro-sprinkler emitters in two almond orchards, following irrigation and fertigation events. Principal component analysis (PCA) and linear regression were used to study the correlations between the soil water filled pore-space (WFPS), and subsurface N2O, NO3− and NH4+ concentrations down to depth of 60 cm. The correlation tables indicated that most of the N2O emission resulted from microbial nitrification in the top soil (< 10 cm). However, in many cases the correlations did not provide meaningful explanations to the relations between the subsurface parameters and the surface N2O emission flux. It was suggested that the main limitation of the analysis results from the current soil sampling method, where soil samples are not taken from the profile inside the collar in-order to keep the soil profile undisturbed over long measuring periods. In the microsprinkler irrigation, relative water application depth of the emitter, showed strong positive linear correlation to the soil NH4+ concentration and surface N2O emission flux. These correlations could be used for upscaling the point measurements into the tree and orchard scale. In the drip irrigation, the N2O emission flux was found to follow the water and NH4+ distribution pattern, and could be upscaled to the tree and orchard level using a sinusoidal function using only measurement of the peak emission and the radius of the wetting pattern. These results improve current understanding on the dynamics of N2O production in orchards irrigated with micro-irrigation systems in arid and semi-arid ecosystems and might contribute to modeling of N2O emissions using less rigorous methods.

Water Balance Dynamics during Ten Years of Ecological Development at Chicken Creek Catchment

Core Ideas Water balance was calculated for the constructed catchment Chicken Creek. Influence of abiotic and biotic factors feedback mechanisms on the hydrology was determined. Different phases were derived during the development of the ecohydrological system. Difficulties in quantitatively closing the water balance of catchments arise when upscaling point measurements and from insufficient knowledge of the physical boundaries, inner structure, and storage volumes of natural catchments. In addition, there is a strong need for generalizing the relationship between catchment characteristics and hydrological response. Therefore, experimental catchments with well‐known boundaries and conditions could contribute valuable data to hydrological and critical zone research. One of the most well‐established and largest constructed catchments is the Chicken Creek catchment (6 ha including a pond, Brandenburg, Germany) representing an initial ecosystem undergoing highly dynamic ecological development starting from clearly defined starting conditions. Directly after completion of the construction, extensive monitoring equipment was installed to track the ecosystem development and to capture the spatiotemporal variability of meteorological, hydrological, ecological, and soil conditions and vegetation succession. In this study, we focused on the water balance dynamics of the Chicken Creek catchment for the period 2005 to 2015 as influenced by ecological development. Water storage in the catchment was calculated from a three‐dimensional model of groundwater volumes, soil moisture measurements, and water level recordings of the pond. The catchment water balance equation was resolved for evapotranspiration, the only part that was not measured directly. Time series of meteorological, hydrological, and ecological data for 10 yr enabled us to characterize the transient development of the catchment and to evaluate the effect of different feedback mechanisms on catchment hydrology.

Early‐Time GPR: A Method to Monitor Spatial Variations in Soil Water Content during Irrigation in Clay Soils

Core Ideas Early‐time analysis extends the use of GPR to attenuating, clay‐rich soils. Early‐time GPR can upscale point measurements of water content when calibrated. Ongoing work studies alternative statistics to represent the early‐time signal. We used a recent ground‐penetrating radar (GPR) methodology, early‐time amplitude analysis, with the goal of monitoring changes in soil water content (SWC) in response to irrigation in clayey soils. We hypothesized that early‐time analysis could be used to monitor changes in SWC in clay‐rich soil where ground wave and reflection‐based GPR methods traditionally fail. An overnight irrigation experiment was performed in a 20‐ by 14‐m section of natural grassland at the Samford Ecological Research Facility in southeastern Queensland, Australia. Both GPR reflection surveys and ground wave velocity analysis were ineffective at the site due to the signal attenuation associated with the clay‐rich soil. We collected daily GPR and time‐domain reflectometry (TDR) data sets during a 5‐d period in August 2014, with soil samples collected for gravimetric analysis at the conclusion of data collection. The GPR data display a clear response of the early‐time signal amplitude to changes in SWC. The GPR data sets exhibit a strong correlation with SWC, as measured by TDR and gravimetric analysis of soil cores, which is consistent with the dependence of GPR early‐time amplitude on relative permittivity. The results suggest that the early‐time method can be used to obtain spatially distributed information on subsurface moisture content in clay‐rich soils.

An Evaluation of Snow Initializations in NCEP Global and Regional Forecasting Models

Abstract Snow plays a major role in land–atmosphere interactions, but strong spatial heterogeneity in snow depth (SD) and snow water equivalent (SWE) makes it challenging to evaluate gridded snow quantities using in situ measurements. First, a new method is developed to upscale point measurements into gridded datasets that is superior to other tested methods. It is then utilized to generate daily SD and SWE datasets for water years 2012–14 using measurements from two networks (COOP and SNOTEL) in the United States. These datasets are used to evaluate daily SD and SWE initializations in NCEP global forecasting models (GFS and CFSv2, both on 0.5° × 0.5° grids) and regional models (NAM on 12 km × 12 km grids and RAP on 13 km × 13 km grids) across eight 2° × 2° boxes. Initialized SD from three models (GFS, CFSv2, and NAM) that utilize Air Force Weather Agency (AFWA) SD data for initialization is 77% below the area-averaged values, on average. RAP initializations, which cycle snow instead of using the AFWA SD, underestimate SD to a lesser degree. Compared with SD errors, SWE errors from GFS, CFSv2, and NAM are larger because of the application of unrealistically low and globally constant snow densities. Furthermore, the widely used daily gridded SD data produced by the Canadian Meteorological Centre (CMC) are also found to underestimate SD (similar to GFS, CFSv2, and NAM), but are worse than RAP. These results suggest an urgent need to improve SD and SWE initializations in these operational models.

Estimating heterotrophic respiration at large scales: challenges, approaches, and next steps

AbstractHeterotrophic respiration (HR), the aerobic and anaerobic processes mineralizing organic matter, is a key carbon flux but one impossible to measure at scales significantly larger than small experimental plots. This impedes our ability to understand carbon and nutrient cycles, benchmark models, or reliably upscale point measurements. Given that a new generation of highly mechanistic, genomic‐specific global models is not imminent, we suggest that a useful step to improve this situation would be the development of “Decomposition Functional Types” (DFTs). Analogous to plant functional types (PFTs), DFTs would abstract and capture important differences in HR metabolism and flux dynamics, allowing modelers and experimentalists to efficiently group and vary these characteristics across space and time. We argue that DFTs should be initially informed by top‐down expert opinion, but ultimately developed using bottom‐up, data‐driven analyses, and provide specific examples of potential dependent and independent variables that could be used. We present an example clustering analysis to show how annual HR can be broken into distinct groups associated with global variability in biotic and abiotic factors, and demonstrate that these groups are distinct from (but complementary to) already‐existing PFTs. A similar analysis incorporating observational data could form the basis for future DFTs. Finally, we suggest next steps and critical priorities: collection and synthesis of existing data; more in‐depth analyses combining open data with rigorous testing of analytical results; using point measurements and realistic forcing variables to constrain process‐based models; and planning by the global modeling community for decoupling decomposition from fixed site data. These are all critical steps to build a foundation for DFTs in global models, thus providing the ecological and climate change communities with robust, scalable estimates of HR.

Influence of spatial temperature estimation method in ecohydrologic modeling in the Western Oregon Cascades

Key Points Surface temperature interpolation method effects ecohydrologic modeling Temperature interpolation strategy influences soil parameter calibration Improved temperature lapse rates increase estimates of net primary productivity

Variability of the radial profile of sap velocity in Pinus patula from contrasting stands within the seasonal cloud forest zone of Veracruz, Mexico

Characterizing the variability of the radial profiles of sap velocity (vs) is a critical step to improve upscaling point measurements of vs to whole-tree sap flow. One promising approach is the use of a probability distribution function (pdf) to model radial profiles of vs, because shape parameters could potentially be generalized to trees of the same species based on the premise that the shape remain consistent regardless of the tree size and age, and over time, even though the magnitude of vs may vary with environmental conditions. The objective of this study was to characterize and assess the variability of the radial profile and to examine the validity of the premises underlying this approach by applying it to Pinus patula, one of the most widely planted tree species in the uplands of central-eastern Mexico. We measured vs with the Heat Ratio method at various sapwood depths in 18 P. patula trees with a dbh between 7.3 and 59.7cm and age between of 10 and 34 years, over a period of 1.5 years. Trees were growing in two stands: a mature forest stand and a young plantation. By fitting the Beta-pdf to hourly radial profiles of vs, we derived a lumped shape parameter (ρ) to denote the radial position relative to sapwood depth with average vs and a scaling parameter (cs). The typical radial profile was unimodal, asymmetrical and with peak vs generally within the outermost 20–33% of the sapwood depth. However, tree-to-tree variability in ρ was considerable among trees within the same stand and also across stands. Long-term and day-to-day variation of ρ was marginal. The hourly dynamics of the radial profile, characterized by cs, can be explained by a linear combination of incoming shortwave radiation, vapor pressure deficit, the hour of day and their interaction (r2=0.74). An independent field evaluation confirmed that a radial profile of fixed shape can be effectively used to estimate whole-tree sap flow with relatively low bias (4–26% underestimation) relative to cut-tree water uptake, particularly for trees for which vs observations covered at least 60% of the sapwood depth. Our findings emphasize the importance of conducting multiple vs point measurements covering most of the sapwood depth for accurate characterization of the radial profile, and demonstrate the utility of fitting a pdf to point vs measurements in order to assess the variability of vs radial profiles as well as to compute sap flow at the whole-tree level.

Application of observation operators for field scale soil moisture averages and variances in agricultural landscapes

Summary Scale difference between in situ and remotely sensed soil moisture observations and model grid size has been an issue for validation of remote sensing data, soil moisture data assimilation and calibration of hydrologic models. This study aims to link two different scales of soil moisture estimates by upscaling single point measurements to field averages for representing field-scale agricultural areas (∼2 ha) located within the Upper Cedar Creek Watershed in northeastern Indiana. Several statistical methods, mainly focusing on cumulative distribution function (CDF) matching, are tested to upscale point measurements to spatially representative soil moisture. These transforming equations are termed observation operators. The CDF matching is found to be the most successful upscaling method followed by the mean relative difference method using temporally stable point measurements. This study also tests the temporal and spatial (horizontal and vertical) transferability of the different observation operators. Results indicate that the two observation operators from the CDF matching approach and the mean relative difference method using a temporally stable location are transferable in space, but not in time. Rainfall characteristic is most likely the dominant factor affecting the success of observation operator transferability. In addition, the CDF matching approach is found to be an effective method to deduce spatial variability (standard deviation) of soil moisture from single point measurements.

Regional hydrochemical modelling by delineation of chemical hydrological response units (CHRUs) within a GIS: An approach of observing man-made impacts in the Broel River catchment (Germany)

Abstract Characterizing and modelling hydrochemical catchment dynamics implies dealing with the challenges of upscaling point measurements to a regional, spatial distribution and with the regional spatial dynamics of nonpoint sources and their relation to catchment management. These two challenges can be met by applying the concept of ‘chemical response units (CHRUs)’ and using GIS-analysis to delineate their spatial distribution. CHRUs can be defined as areas having similar hydrochemical dynamics dependent on their landuse management. This concept was applied to the mesoscale catchment of the Broel River (A = 216km 2 ) in the middle-mountain range of the Rheinische Schiefergebirge in Germany. The catchment is underlayn by impermeable Devonian shale where losses of water due to deep percolation are negligible. The climate is oceanic and mean annual rainfall ranges between 950 and 1100 mm. The landuse pattern of the catchment was mapped in the field and digitized with the aid of a GIS-system. About 34% is covered by coniferous and deciduous forest, 11% — impervious, 52% — range land and the remaining 3% — agriculture. The catchment was instrumented to capture atmospheric dry and wet deposition as bulk samples, and two river gauging stations were equipped with instruments for continuous logging of electrical conductivity EC (mS/m) and water temperature. Additionally, water samples were taken in two-week intervals for complete anorganic analysis. An intensive questionary of farmers regarding their fertilizing practices was carried out and trade figures of fertilizers, sold to the farming community, were supplied by local traders to estimate chemical-fertilizer input. Data from neighbouring forested test catchments were used to evaluate the salt contribution from forested areas. The database also included other physiographic factors like soils and topography and GIS-overlay analysis was used to delineate specific CHRUs. Their hydrochemical dynamics is strongly characterized by landuse and atmospheric deposition. The annual chemical balances of the Broel catchment were calculated using these CHRUs and the results compared with the measured output at the two gauging stations. First, results from these balance calculations showed deviations associated with undetected point sources but also good fits during periods with unique hydrological conditions. In summary, the concept of CHRUs proved to be a practical method for the evaluation of the chemical dynamics of the heterogeneous mesoscale Broel River catchment. Future research is directed at combining the CHRU-concept with a hydrological model like MMS/PRMS, WASNOD&STOMOD or APEX, and thereby interfacing the hydrochemical and the hydrological dynamics, on a basin-scale.