Martin Šanda

MR properties of water in saturated soils and resulting loss of MRI signal in water content detection at 2 tesla

This paper reports a systematic MRI study at 2 tesla of 23 soils, each separately saturated with a known amount of water. The percentage of that water which could be detected using various MR methods was determined by comparison with a liquid reference sample. A pulse-acquire sequence gave a bulk detection of between 47 and 94% of the known water content of saturated soil. Also for bulk measurements, the inversion-recovery sequence used for determining T1 values detected a range of 0.7–75% of the existing soil water. The CPMG sequence with an echo time (TE) of 1 ms used for determining the bulk T2 values gave lower values, in the range of 0.4–66% overall. A spin-echo MRI sequence with a TE of 2.9 ms gave an even lower bulk detection, ranging from 0.2 to 57%. These low values for the detectable water content of bulk saturated soil water reflect the loss of water magnetization which occurs even during short echo time MR sequence at 2 tesla field strength. The source of the above findings was investigated by measurements of the longitudinal (T1) and transverse (T2) relaxation times and spectral linewidths of the soil-water protons, and by conventional analysis of soil properties. The MR parameters of critical importance to water quantification are T2 and T2∗, shorter values of which lead to a progressively greater loss of signal intensity for all MR protocols. Those parameters are affected by the following soil chemical and physical features: soil magnetic susceptibility, and the content of free iron oxides, clay, sand, exchangeable cations (K, Na and Ca), and organic matter. The implication of this work is that the only soil water which can be detected quantitatively at 2 tesla using a conventional spin-echo MRI protocol with echo times of 3 ms or longer is that located in the relatively large soil pores. Using the protocols investigated in this work, water in smaller pores will only be detected accurately for soils which have relatively low paramagnetic-metal impurities and/or have low clay content. Future MR studies of soil water should consider the use of other MRI protocols (e.g. for solid state), and measurement at low magnetic fields.

A review of CO2 and associated carbon dynamics in headwater streams: A global perspective

Terrestrial carbon export via inland aquatic systems is a key process in the global carbon cycle. It includes loss of carbon to the atmosphere via outgassing from rivers, lakes or reservoirs and carbon fixation in the water column as well as in sediments. This review focuses on headwater streams that are important because their stream biogeochemistry directly reflects carbon input from soils and groundwaters that becomes superimposed by additional inputs further downstream. Major drivers of carbon dioxide partial pressures (pCO2) in streams and mechanisms of terrestrial dissolved inorganic, organic and particulate organic carbon (DIC, DOC, and POC) influxes are summarized in this work. Our analysis indicates that the global river average pCO2 of 3,100 ppmV is more often exceeded by contributions from small streams when compared to rivers with larger catchments (>500 km2). Because of their large proportion in global river networks (>96 % of the total number of streams), headwaters contribute large – but still poorly quantified – amounts of CO2 to the atmosphere. Conservative estimates imply that globally 36 % (i.e. 0.93 Pg C yr-1) of total CO2 outgassing from rivers and streams originate from headwaters. We also discuss challenges in determination of CO2 sources, concentrations and fluxes. To overcome uncertainties of CO2 sources and its outgassing from headwater streams on the global scale, new investigations are needed that should include groundwater data. Such studies would also benefit from applications of integral CO2 outgassing isotope approaches and multi-scale geophysical imaging techniques.

Transforming Hydrographs in the Hillslope Subsurface

Transforming Hydrographs in the Hillslope Subsurface To reveal and evaluate the mechanism of transforming rainfall into runoff in the region, where the subsurface flow plays a dominant role in the runoff formation, a continuous hydrological and climatic data monitoring has been set-up in the experimental catchment Uhlířská (the Jizera Mountains, CR). The soil profile (Dystric Cambisol), formed on the weathered granite bedrock, is shallow and highly heterogeneous. Beside a standard catchment data observation a hillslope transect was instrumented to control the flow dynamics in the soil profile. From three soil horizons, the subsurface outflow is recorded in the subsurface trench. Adjacent to the trench the soil water suction is scanned by triplets of automatic tensiometers. Within the soil profile the unsaturated regime prevails, nevertheless the soil keeps almost saturated. Nearly simultaneous reaction of suction on a rainfall in all soil horizons implies a rapid vertical flow. Local preferential flow paths are conducting infiltrating water at significantly variable rates when saturation is reached. Groundwater table, soil moisture and subsurface runoff measured at the hillslope transect and the total outflow from the catchment, are correlated. The outflow from the catchment is dominantly controlled by soil moisture however the mechanism of its generation is not yet fully understood. Transformace hydrogramu podpovrchového odtoku na svahu V oblasti s dominantním podpovrchovým odtokem bylo započato s kontinuálním hydrologickým a klimatickým monitoringem s cílem popsat a vyhodnotit transformaci srážky na odtok. Experimentální povodí Uhlířská se nachází v severní části České republiky v Jizerských horách. Půdní profil, klasifikovaný jako dystrická kambizem na zvětralém žulovém podloží, je mělký a velmi heterogenní. Svahový transekt byl vystrojen pro sledování dynamiky podpovrchového odtoku. Ve třech půdních horizontech je monitorován odtok a půdní sací tlak. V půdním profilu převládá nenasycený stav, ačkoliv je půdní vlhkost dlouhodobě blízko nasycení. Rychlé vertikální proudění je indikováno téměř současnou odezvou půdního sacího tlaku na srážku ve všech půdních horizontech. Po dosažení nasycení infiltrující voda protéká preferenčními cestami s výrazně odlišnými lokálními rychlostmi. Závislost hladiny podzemní vody, půdní vlhkosti, podpovrchového odtoku ve svahovém transektu na odtoku z povodí je významná. Odtok vody z povodí, které leží na zvětralém žulovém podloží, je dominantně určován půdní vlhkostí. Přes tato zjištění není mechanismus tvorby odtoku zatím jednoznačně popsatelný.

The relationships between MR parameters and the content of water in packed samples of two soils

Abstract The relationship between the known water content and that determined using Magnetic Resonance (MR) methods was studied for samples of two soils packed to various dry bulk densities; four basic MR protocols were employed (single 90° pulse, inversion-recovery, Carr–Purcell–Meiboom–Gill (CPMG), and spin-echo (SE) pulse sequences). The soil types were chosen from a previously studied range of soils to represent both an “easy” (fine sand) and a “difficult” (coarse sandy loam) system for MR application. Whereas for the “easy” soil, the MR-detected water relates linearly to the soil water content as expected, for “difficult” soil the observed relationships are non-linear and depend on MR properties of soil water and MR protocols used.

Uncertainty Analysis of a Dual-Continuum Model Used to Simulate Subsurface Hillslope Runoff Involving Oxygen-18 as Natural Tracer

Uncertainty Analysis of a Dual-Continuum Model Used to Simulate Subsurface Hillslope Runoff Involving Oxygen-18 as Natural Tracer A one-dimensional dual-continuum model (also known as dual-permeability model) was used to simulate the lateral component of subsurface runoff and variations in the natural 18O content in hillslope discharge. Model predictions were analyzed using the GLUE generalized likelihood uncertainty estimation procedure. Model sensitivity was evaluated by varying two separate triplets of parameters. The first triplet consisted of key parameters determining the preferential flow regime, i.e., the volumetric proportion of the preferential flow domain, a first-order transfer coefficient characterizing soil water exchange between the two flow domains of the dual-continuum system, and the saturated hydraulic conductivity of the preferential flow domain. The second triplet involved parameters controlling exclusively the soil hydraulic properties of the preferential flow domain, i.e., its retention curve and hydraulic conductivity function. Results of the analysis suggest high sensitivity to all parameters of the first triplet, and large differences in sensitivity to the parameters of the second triplet. The sensitivity analysis also confirmed a significant improvement in the identifiability of preferential flow parameters when 18O content was added to the objective function. Analýza Nejistot Při Modelování Podpovrchového Odtoku ze Svahu Metodou Duálního Kontinua s Využitím Izotopu Kyslíku 18O Jako Přirozeného Stopovače K simulacím laterální složky podpovrchového proudění a změn koncentrace izotopu kyslíku 18O ve vodě vytékající ze svahu byl použit jednorozměrný model využívající přístupu duálního kontinua. Nejistota modelových předpovědí byla odhadnuta s využitím metody zobecněné věrohodnosti (GLUE). Citlivost modelu byla zjišťována pomocí variací dvou samostatných trojic parametrů. První trojice sestávala z klíčových parametrů pro určení režimu preferenčního proudění, tj. objemového podílu preferenční domény proudění, přenosového koeficientu charakterizujícího výměnu vody mezi oběma doménami duálního systému a nasycené hydraulické vodivosti preferenční domény. Druhá trojice zahrnovala výhradně parametry určující hydraulické charakteristiky preferenční domény proudění, tj. retenční křivku a funkci hydraulické vodivosti. Z výsledků analýzy vyplývá vysoká citlivost modelu na všechny parametry z první trojice a velké rozdíly v citlivostech parametrů druhé trojice. Analýza dále potvrdila významné zlepšení zjistitelnosti parametrů preferenční domény v případě, kdy je do cílové funkce přidána koncentrace izotopu kyslíku 18O.

Isotopes of oxygen-18 and deuterium in precipitation in Slovakia / Izotopy kyslíka-18 A deutéria v zrážkach na Slovensku

The article synthesizes available information on isotopic composition of precipitation in Slovakia (the Western Carpathians). Monthly δ18O data from eleven stations and period 1988-1997 were used to investigate correlations among the stations, altitude, air temperature and precipitation amount effects. The mean annual altitude and air temperature gradients of δ18O in precipitation were 0.21‰/100 m and 0.36‰/1°C, respectively. Maps of spatial distribution of mean annual δ18O in precipitation based on both gradients were constructed. The two maps do not significantly differ for the majority of Slovakia. δ2H data were available for only three stations. Local meteoric water line derived for the station with the longest data series (δ2H = = 7.86δ18O + 6.99) was close to the Global Meteoric Water line. Its parameters in periods 1991-1993 and 1991-2008 did not change. The study indicates that a more detailed monitoring of isotopic composition of precipitation in mountains should be carried out in the future. The highest station exhibited very small seasonal variability of δ18O in precipitation compared to other Slovak stations. The second highest mountain station had significantly higher deuterium excess than the neighboring stations located in the valley. In some analyses the data from the nearest stations situated abroad (Vienna, Krakow) were used. Príspevok syntetizuje dostupné údaje o izotopickom zložení zrážok na území Slovenska (Západné Karpaty). Hodnotí koreláciu hodnôt δ18O v kumulatívnych mesačných zrážkach z rokov 1988-1997 medzi jednotlivými stanicami, vplyv nadmorskej výšky, teploty vzduchu a úhrnu zrážok. Priemerný ročný výškový gradient δ18O v zrážkach je 0,21‰/100 m, priemerný ročný teplotný gradient δ18O v zrážkach je 0,36‰/1°C. Oba gradienty boli použité na vytvorenie mapy priestorového rozdelenia δ18O v zrážkach na území Slovenska. Mapa vytvorená pomocou výškového gradientu δ18O v zrážkach sa pre väčšinu územia Slovenska významne nelíši od mapy vytvorenej pomocou teplotného gradientu. Hodnoty δ2H v zrážkach boli k dispozícii len pre tri stanice na Slovensku. Pre stanicu s najdlhším radom údajov (1991-2008) bola určená lokálna meteorická čiara (δ2H = 7.86δ18O + 6.99), ktorej priebeh je blízky globálnej meteorickej čiare. Parametre lokálnej meteorickej čiary sa počas skúmaného obdobia nezmenili. Merané mesačné údaje poukazujú na potrebu podrobnejšieho monitoringu izotopického zloženia zrážok v horách. Najvyššie ležiaca stanica mala v porovnaní s ostatnými stanicami veľmi malú sezónnu variabilitu δ18O v zrážkach. Druhá najvyššie položená stanica mala podstatne vyšší exces deutéria, ako susedné stanice ležiace v kotlinovej polohe. Pri niektorých analýzach boli údaje zo Slovenska porovnávané s najbližšie ležiacimi zahraničnými stanicami (Viedeň, Krakov).

Field leaching of pesticides at five test sites in Hawaii: study description and results

BACKGROUND Following the discovery of pesticides in wells, the Hawaii Department of Agriculture (HDOA) supported research to evaluate the likelihood of pesticide leaching to the groundwater in Hawaii. The aim of this study was to evaluate the relative leaching pattern of five pesticides at five different sites on three islands and to compare their leaching behavior with bromide and a reference chemical (atrazine) that is known to leach in Hawaiian conditions. Laboratory measurements of sorption and degradation of the pesticides were made. RESULTS Most of the applied mass of pesticides was still present in the top 80 cm after the 16 week study period. The aggregated oxisol at Kunia showed the most intensive leaching among the five sites. The revised attenuation factor screening approach used by the HDOA indicated that all chemicals, with the exception of trifloxystrobin, had the potential to leach. Similarly, the groundwater ubiquity score ranked trifloxystrobin as a non-leacher. The field leaching data, however, suggested that trifloxystrobin was the most mobile compound among the pesticides tested. CONCLUSION Although the results were variable among the sites, the field and laboratory experiments provided useful information for regulating use of these pesticides in Hawaii.

Application of two-component model of drainage discharge to nitrate contamination

A conceptual two-component model of drainage discharge based on delta(18)O, nitrate content and delta(15)N data was constructed. It comprises the infiltrated precipitation and the local groundwater, both discharging into the drainage system. The movement of the water via the unsaturated zone is described as a piston-like flow with a varying amount contributing to the total drainage. Two tile drainage systems were studied for nitrate loss. The transit time between the rainfall infiltration and the drainage into the tile system is estimated to be approximately one year. This process is strongly dependent on the duration of the infiltration and its magnitude, and thus on the discharge dynamics in general. The local groundwater contribution to the system formed a significant part of the drainage discharge (varying as 65-98% of the whole drained amount). Nitrate content and delta(15)N data were used for the specification of the nitrate flux and nitrate origin in the drainage discharge.

Acid rain footprint three decades after peak deposition: Long-term recovery from pollutant sulphate in the Uhlirska catchment (Czech Republic)

The granitic Uhlirska headwater catchment with a size of 1.78km2 is located in the Jizera Mountains in the northern Czech Republic and received among the highest inputs of anthropogenic acid depositions in Europe. An analysis of sulphate (SO42-) distribution in deposition, soil water, stream water and groundwater compartments allowed to establish a SO42- mass-balance (deposition input minus surface water export) and helped to evaluate which changes occurred since the last evaluation of the catchment in 1997. The determined SO42- concentrations decreased in the following order: wetland groundwater>groundwater from 20m below ground level (bgl)>groundwater from 30m bgl>stream water>groundwater from10m bgl>hillslope soil water>wetland soil water>bulk deposition with median values of 0.24, 0.21, 0.17, 0.15, 0.11, 0.07, 0.03 and 0.01mmolL-1, respectively. Our results show that average deposition reductions of 62% did not result in equal changes of the sulphate mass-balance, which changed by only 47%. This difference occurs because sulphate originates from internal sources such as the groundwater and soil water. The Uhlirska catchment is subject to delayed recovery from anthropogenic acid depositions and remains a net source of stored sulphur even after three decades of declining inputs. The wetland groundwater and soil water provide environmental memories of legacy pollutant sulphate. Elevated stream water sulphate concentrations after the unusually dry summer 2015 imply importance of weather and climate patterns for future recovery from acidification.

Pre-event water contributions and streamwater residence times in different land use settings of the transboundary mesoscale Lužická Nisa catchment

Abstract The objective of the study was to evaluate the spatial distribution of peakflow pre-event water contributions and streamwater residence times with emphasis on land use patterns in 38 subcatchments within the 687 km2 large mesoscale transboundary catchment Lužická Nisa. Mean residence times between 8 and 27 months and portions of pre-event water between 10 and 97% on a storm event peakflow were determined, using 18O data in precipitation and streamwater from a weekly monitoring of nearly two years. Only a small tracer variation buffering effect of the lowland tributaries on the main stem was observed, indicating the dominant impact on the mountainous headwaters on the runoff generation. Longest mean streamwater residence times of 27 months were identified in the nearly natural headwaters of the Jizera Mountains, revealing no ambiguous correlation between the catchment area and altitude and the mean residence time of streamwater. Land use control on the pre-event water portions were determined for three land use categories with percentage of urban areas from 0 to 10%, 10 to 20% and more than 20%. The fraction of pre-event water in the first category decreases from 97% to 65% with the increasing percentage of forest from 76% to 100%, revealing that forests may provide only a limited infiltration of precipitation due to leaf interception and soil water use for transpiration. Fractions of pre-event water of 39–87% in the second (agricultural catchments) and of 10–35% in the third (urbanized catchments) category increase with percentage of non-urban areas.