Evidence of Self‐Organized Criticality in riverbank mass failures: a matter of perspective?

A growing body of field, theoretical and numerical modelling studies suggests that predicting river response to even major changes in input variables is difficult. Rivers are seen to adjust rapidly and variably through time and space as well as changing independently of major driving variables. Concepts such as Self‐Organized Criticality (SOC) are considered to better reflect the complex interactions and adjustments occurring in systems than traditional approaches of cause and effect. This study tests the hypothesis that riverbank mass failures which occurred both prior to, and during, an extreme flood event in southeast Queensland (SEQ) in 2011 are a manifestation of SOC. Each wet‐flow failure is somewhat analogous to the ‘avalanche’ described in the initial sand‐pile experiments of Bak et al. (Physical Review Letters, 1987, 59(4), 381–384) and, due to the use of multitemporal LiDAR, the time period of instability can be effectively constrained to that surrounding the flood event. The data is examined with respect to the key factors thought to be significant in evaluating the existence of SOC including; non‐linear temporal dynamics in the occurrence of disturbance events within the system; an inverse power‐law relation between the magnitude and frequency of the events; the existence of a critical state to which the system readjusts after a disturbance; the existence of a cascading processes mechanism by which the same process can initiate both low‐magnitude and high‐magnitude events. While there was a significant change in the frequency of mass failures pre‐ and post‐flood, suggesting non‐linear temporal dynamics in the occurrence of disturbance events, the data did not fit an inverse power‐law within acceptable probability and other models were found to fit the data better. Likewise, determining a single ‘critical’ state is problematic when a variety of feedbacks and multiple modes of adjustment are likely to have operated throughout this high magnitude event. Overall, the extent to which the data supports a self‐organized critical state is variable and highly dependent upon inferential arguments. Investigating the existence of SOC, however, provided results and insights that are useful to the management and future prediction of these features. Copyright © 2014 John Wiley & Sons, Ltd.     

Analysis of hydrological seasonality across northern catchments using monthly precipitation–runoff polygon metrics

Abstract

Seasonality is an important hydrological signature for catchment comparison. Here, the relevance of monthly precipitation–runoff polygons (defined as scatter points of 12 monthly average precipitation–runoff value pairs connected in the chronological monthly sequence) for characterizing seasonality patterns was investigated to describe the hydrological behaviour of 10 catchments spanning a climatic gradient across the northern temperate region. Specifically, the research objectives were to: (a) discuss the extent to which monthly precipitation–runoff polygons can be used to infer active hydrological processes in contrasting catchments; (b) test the ability of quantitative metrics describing the shape, orientation and surface area of monthly precipitation–runoff polygons to discriminate between different seasonality patterns; and (c) examine the value of precipitation–runoff polygons as a basis for catchment grouping and comparison. This study showed that some polygon metrics were as effective as monthly average runoff coefficients for illustrating differences between the 10 catchments. The use of precipitation–runoff polygons was especially helpful to look at the dynamics prevailing in specific months and better assess the coupling between precipitation and runoff and their relative degree of seasonality. This polygon methodology, linked with a range of quantitative metrics, could therefore provide a new simple tool for understanding and comparing seasonality among catchments.

Editor Z.W. Kundzewicz; Associate editor K. Heal

Citation Ali, G., Tetzlaff, D., Kruitbos, L., Soulsby, C., Carey, S., McDonnell, J., Buttle, J., Laudon, H., Seibert, J., McGuire, K., and Shanley, J., 2013. Analysis of hydrological seasonality across northern catchments using monthly precipitation–runoff polygon metrics. Hydrological Sciences Journal, 59 (1), 56–72.     

Spatio‐temporal tracer variability in the glacier melt end‐member — How does it affect hydrograph separation results?

Geochemical and isotopic tracers were often used in mixing models to estimate glacier melt contributions to streamflow, whereas the spatio‐temporal variability in the glacier melt tracer signature and its influence on tracer‐based hydrograph separation results received less attention. We present novel tracer data from a high‐elevation catchment (17 km², glacierized area: 34%) in the Oetztal Alps (Austria) and investigated the spatial, as well as the subdaily to monthly tracer variability of supraglacial meltwater and the temporal tracer variability of winter baseflow to infer groundwater dynamics. The streamflow tracer variability during winter baseflow conditions was small, and the glacier melt tracer variation was higher, especially at the end of the ablation period. We applied a three‐component mixing model with electrical conductivity and oxygen‐18. Hydrograph separation (groundwater, glacier melt, and rain) was performed for 6 single glacier melt‐induced days (i.e., 6 events) during the ablation period 2016 (July to September). Median fractions (±uncertainty) of groundwater, glacier melt, and rain for the events were estimated at 49±2%, 35±11%, and 16±11%, respectively. Minimum and maximum glacier melt fractions at the subdaily scale ranged between 2±5% and 76±11%, respectively. A sensitivity analysis showed that the intraseasonal glacier melt tracer variability had a marked effect on the estimated glacier melt contribution during events with large glacier melt fractions of streamflow. Intra‐daily and spatial variation of the glacier melt tracer signature played a negligible role in applying the mixing model. The results of this study (a) show the necessity to apply a multiple sampling approach in order to characterize the glacier melt end‐member and (b) reveal the importance of groundwater and rainfall–runoff dynamics in catchments with a glacial flow regime.     

Is ‘Centre of Volume’ a robust indicator of changes in snowmelt timing?

The centre of volume (COV), or the hydrograph centroid, is a measure of streamflow timing that is a widely used indicator of the effects of warmer temperatures on the hydrology of snowmelt streams. The COV was originally developed as a measure of land‐use effects, and its response is affected by several factors other than temperature, particularly total run‐off. A ‘toy’ model is used to demonstrate some of these effects, and these effects are also shown for streamflow data from Canada's Reference Hydrologic Basin Network. These deficiencies indicate that COV is neither specific nor robust as an indicator. Although these effects might be overcome by streamflow decomposition, the use of COV as an indicator of snowmelt timing should be avoided. Copyright © 2013 John Wiley & Sons, Ltd.     

Using hysteretic behaviour and hydrograph classification to identify hydrological function across the “hillslope–depression–stream” continuum in a karst catchment

In cockpit karst landscapes, fluxes from upland areas contribute large volumes of water to low-lying depressions and stream flow. Hydrograph hysteresis and similarity between monitoring sites is important for understanding the space–time variability of hydrologic responses across the “hillslope–depression–stream” continuum. In this study, the hysteretic feature of hydrographs was assessed by characterizing the loop-like relationships between responses at upstream sites relative to subsurface discharge at the outlet of a small karst catchment. A classification of hydrograph responses based on the multi-scale smoothing Kernel -derived distance classifies the hydrograph responses on the basis of similarities between hillslope and depression sites, and those at the catchment outlet. Results demonstrate that the temporal and spatial variability of hydrograph hysteresis and similarity between hillslope flow and outlet stream flow can be explained by the local heterogeneity of depression aquifer. Large depression storage deficits emerging in the highly heterogeneous aquifer produce strong hysteresis and multiple relationships of upstream hydrographs relative to the outlet subsurface discharge. In contrast, when depression storage deficits are filled during consecutive rainfall events, depression hydrographs at the high permeability sites are almost synchronous or exhibit a monotonous function with the hydrographs at the outlet. This reduced hydrograph hysteresis enhances preferential flow paths in fractured rocks and conduits that can accelerate the hillslope flow to the outlet. Therefore, classification of hydrograph similarities between any upstream sites and the catchment outlet can help to identify the dominant hydrological functions in the heterogeneous karst catchment.     

The wetting ability of Si-bearing liquid Fe-alloys in a solid silicate matrix—percolation during core formation under reducing conditions?

The wetting characteristics of liquid Fe–Si alloys in a matrix of the respective predominating stable silicate mantle mineral (forsterite or silicate perovskite) at pressures of 2–5 and 25 GPa and temperatures of 1600–2000 °C were studied by determining the liquid metal–solid silicate contact angles. The median angle values from texturally equilibrated samples were found to be independent of pressure, temperature, silicate mineralogy and the Si content in the metal fraction and range between 130° and 140° which is far above the critical wetting boundary of 60°. This shows that within the studied range of conditions dissolved Si does not lower the surface energies between Fe-rich liquids and silicate mantle grains. As a consequence, under reducing conditions the presence of Si in the metal phase of planetary bodies would not have enhanced percolative flow as an effective metal–silicate separation process.     

Increasing annual runoff—broadleaf or coniferous forests?

According to widely held belief, annual evapotranspiration (ET) for broadleaf forests is less than that for coniferous forests, resulting in higher annual runoff for broadleaf forests. We processed 82 catchment runoff and 126 interception loss data from temperate regions and found that although the belief is valid under conditions of broadleaf deciduous forests and high winter precipitation (e.g. the United States), it is invalid under conditions of broadleaf evergreen forests (e.g. New Zealand) or low winter precipitation (e.g. Japan). Thus, forest management policies based on this belief should be reconsidered on the basis of our results for regions with broadleaf evergreen forests or low winter precipitation. Copyright © 2010 John Wiley & Sons, Ltd.     

Discussion: Misrepresentation of the USLE in ‘Is sediment delivery a fallacy?’

The assertion that the application of the USLE to predicting soil losses within a catchment or watershed is not sound because the USLE provides an estimate of erosion that would be measured if the entire area were divided up into 22·1 m long plots, and the output from them all added together, is incorrect. The slope length factor was derived from data obtained using a wide range of plot lengths and included the 22·1 m length simply to force it to take on a value of 1·0 when the slope length is 22·1 m. The 22·1 m length has no physical significance but the USLE slope length factor has a physical basis when applied to planar and convex hillslopes. The use of sediment delivery ratios when the USLE is applied to concave areas attempts to correct for applying the USLE beyond its design criteria. It fails because, in using the sediment delivery ratios in the prediction sediment delivery, it is incorrectly assumed that sediment delivery ratios de not vary with the amount of sediment entering a zone of deposition.     

Mangrove‐forest evolution in a sediment‐rich estuarine system: opportunists or agents of geomorphic change?

The majority of the world's mangrove forests occur on mostly mineral sediments of fluvial origin. Two perspectives exist on the biogeomorphic development of these forests, i.e. that mangroves are opportunistic, with forest development primarily driven by physical processes, or alternatively that biophysical feedbacks strongly influence sedimentation and resulting geomorphology. On the Firth of Thames coast, New Zealand, we evaluate these two possible scenarios for sediment accumulation and forest development using high‐resolution sedimentary records and a detailed chronology of mangrove‐forest (Avicennia marina) development since the 1950s. Cores were collected along a shore‐normal transect of known elevation relative to mean sea level (MSL). Activities for lead‐210 (²¹⁰Pb), caesium‐137 (¹³⁷Cs) and beryllium‐7 (⁷Be), and sediment properties were analysed, with ²¹⁰Pb sediment accumulation rates (SARs), compensated for deep subsidence (~8 mm yr^?1) used as a proxy for elevation gain. At least four phases of forest development since the 1950s are recognized. An old‐growth forest developed by the late‐1970s with more recent seaward forest expansion thereafter. Excess ²¹⁰Pb profiles from the old‐growth forest exhibit relatively low SARs near the top (7–12 mm yr^?1) and bottom (10–22 mm yr^?1) of cores, separated by an interval of higher SARs (33–100 mm yr^?1). A general trend of increasing SAR over time characterizes the recent forest. Biogeomorphic evolution of the system is more complex than simple mudflat accretion/progradation and mangrove‐forest expansion. Surface‐elevation gain in the old‐growth forest displays an asymptotic trajectory, with a secondary depocentre developing on the seaward mudflat from the mid‐1970s. Two‐ to ten‐fold increases in ²¹⁰Pb SARs are unambiguously large and occurred years to decades before seedling recruitment, demonstrating that mangroves do not measurably enhance sedimentation over annual to decadal timescales. This suggests that mangrove‐forest development is largely dependent on physical processes, with forests occupying mudflats once they reach a suitable elevation in the intertidal. Copyright © 2015 John Wiley & Sons, Ltd.     

Catchment transit times and landscape controls—does scale matter?

Mean transit times (MTTs) can give useful insights into the internal processes of hydrological systems. However, our understanding of how they vary and scale remains unclear. We used MTT estimates obtained from δ¹⁸O data from 20, mostly nested, contrasting catchments in North East Scotland, ranging from 1 to 1700 km². The estimated MTTs ranged between 270 and 1170 days and were used to test a previously developed multiple linear regression (MLR) model for MTT prediction based on metrics of soil cover, landscape organization and climate. We show that the controls on MTT identified by the MLR model hold with the independent data from these 20 sites and that the MLR can be used to predict MTT in ungauged montane catchments. The dominant controls also remain unchanged over four orders of magnitude of catchment size, suggesting no major change of dominant flow paths and mixing processes at larger scales. This is consistent with the fact that only the variance of MTT, rather than MTT, showed a scaling relationship. MTTs appeared to converge with increasing catchment scale, apparently due to the integration of heterogeneous headwater responses in larger downstream catchments. Copyright © 2009 John Wiley & Sons, Ltd.     

Are spatial patterns of soil moisture at plot scales generalisable across catchments,climates, and other characteristics? A synthesis of synoptic soil moisture across the Mid-Atlantic

The spatial variation of soil moisture over very small areas (<100 m²) can have nonlinear impacts on cycling and flux rates resulting in bias if it is not considered, but measuring this variation is difficult over extensive temporal and spatial scales. Most studies examining spatial variation of soil moisture were conducted at hillslope (0.01 km²) to multi-catchment spatial scales (1000 km²). They found the greatest variation at mid wetness levels and the smallest variation at wet and dry wetness levels forming a concave down relationship. There is growing evidence that concave down relationships formed between spatial variation of soil moisture and average soil moisture are consistent across spatial scales spanning several orders of magnitude, but more research is needed at very small, plot scales (<100 m²). The goal of this study was to characterise spatial variation in shallow soil moisture at the plot scale by relating the mean of measurements collected in a plot to the standard deviation (SD). We combined data from a previous study with thousands of new soil moisture measurements from 212 plots in eight catchments distributed across the US Mid-Atlantic Region to (1) test for a generalisable mean–SD relationship at plot scales, (2) characterise how landcover, land use, season, and hillslope position contribute to differences in mean–SD relationships, and (3) use these generalised mean–SD relationships to quantify their impacts on catchment scale nitrification and denitrification potential. Our study found that 98% of all measurements formed a generalised mean–SD relationship like those observed at hillslope and catchment spatial scales. The remaining 2% of data comprised a mean–SD relationship with greater spatial variation that originated from two riparian plots reported in a previous study. Incorporating the generalised mean–SD relationship into estimates of nitrification and denitrification potential revealed strong bias that was even greater when incorporating mean–SD observations from the two riparian plots with significantly greater spatial variation.     

Assessing the status of an artisanal shrimp fishery in a Ramsar wetland in Jamaica: The effects of seasonality,extreme La Niña episodes and elevated temperature on landings

An invasive crayfish, Cherax quadricarinatus, and several native shrimp species (Macrobrachium acanthurus, Macrobrachium faustinum, Macrobrachium carcinus, Xiphocaris elongata, and Atyidae sp.) found in the Black River Lower Morass (BRLM), a Ramsar Wetland in Jamaica, support subsistence and artisanal fisheries. Management of this fishery requires information on factors that influence their abundance. Consequently, we assessed the effects of seasonality, extreme and double La Niña episodes (in 2011) and elevated atmospheric temperature (in 2011 and 2012) on weekly Decapoda landing data, collected over a period of two years (2010–2012). The catch of native species showed a cyclical trend every 6 months, coinciding with the dry and wet seasons. The invasive crayfish landing showed a reverse trend during the first year, after which no pattern could be discerned. A dynamic factor analysis (DFA) model with two common trends and four explanatory variables (conductivity, mean weekly number of traps hauled, weekly mean water level, and mean dissolved oxygen) was the optimal model to characterize the variation in wet-weight landings. A generalized additive mixed model with an auto-regressive moving average (ARMA) error structure was used to show that the extreme and double La Niñas were associated with lower monthly atmospheric temperature. Sea surface temperature anomaly in region 3 (a proxy to ENSO) and the trend in temperature were then used to predict the wet weight of native shrimp (U-shaped relationship) and the invasive crayfish (reverse-j shaped relationship), respectively. The native shrimp (mean sustainable yield, MSY = 3469 kg and mean catch per fisher = 2.67 kg) and invasive crayfish (MSY = 11 kg and mean catch per fisher = 0.67 kg) are under fished, although populations of the native shrimp are possibly declining, whereas that of the invasive crayfish may be growing. The declining trend may have adverse implications for the stock of the native shrimp species, which has a higher economic value, if fishing pressure is not reduced/restricted or increased on the invasive crayfish, especially during seasonal and/or ENSO related declines in native shrimp stocks.     

Vertical migration of Daphnia galeata – Evidence for the use of an alternative resource from a lethal environment

Metazoans normally avoid anaerobic environments, at least when they are combined with toxic stress due to hydrogen sulfide. In Lake Speldrop, a small but deep gravel pit lake at the Lower Rhine, Daphnia galeata was found not only to dominate the zooplankton community, but was also regularly found in anoxic and even sulfidic layers during summer. We conducted field experiments with a newly developed “Zooplankton In-situ Incubator”, simulating vertical migrations of D. galeata. When daphnids were exposed to sulfidic conditions, mortality increased with exposure time, revealing LT₅₀-values between 129 and 42 min in relation to increasing concentrations of sulfide. Additionally, those experimental individuals originating from 12.5 m depth showed significantly higher mortality rates than those from 7.5 m depth. Further migration experiments showed that an interruption period of sulfidic exposition in less stressful environments reduced mortality rates significantly. Daphnids found in the hypolimnion belonged partly to moribund parts of the population; however, the majority of daphnids showed regular mowing activity and was able to withstand sulfidic conditions in the hypolimnion for a limited time. It is so far unclear what maybe the major ultimate factor for this type of short term migratory behavior, to seek for shelter or to use high amounts of sulfur bacteria as an alternative food resource.     

Does Aqueous‐Phase Oxidation of HCHO Opens a Pathway to Formic Acids in Atmosphere?

Formic acid is the major contributor to acid rain in some regions but its sources are not fully understood. We investigated the aqueous‐phase reactions of HCHO (aq) and OH ^. radicals at enlarged rainwater pH values (2.49–5.89) in Guiyang, China from May 2006 to April 2007. Our results show that there were no significant correlation between the [HCOOH]_t/[HCHO] (aq) and the rainwater pH. The ratio did not appear to vary consistently as a function of rainwater pH as predicted by theoretical model. In addition, we saw no clear evidence that oxidation of HCHO (aq) would produce significant HCOOH (aq) which indicates this reaction may be only a minor contribution to the budget of HCOOH (g) in atmosphere. Further investigation is strongly suggested to be carried out in field cloud water, fog water, or rainwater because the ratios would be diverged from equilibrium value as a result of other chemical or physical processes.     

Continuous measurement of nitrate concentration in a highly event‐responsive agricultural catchment in south‐west of France: is the gain of information useful?

A nitrate sensor has been set up to measure every 10 min the nitrate signal in a stream draining a small agricultural catchment dominated by fertilized crops during a 2‐year study period (2006–2008) in the south‐west of France. An in situ sampling protocol using automatic sampler to monitor flood events have been used to assume a point‐to‐point calibration of the sensor values. The nitrate concentration exhibits nonsystematic concentration and dilution effects during flood events. We demonstrate that the calibrated nitrate sensor signal gathered from the outlet is considered to be a continuous signal using the Nyquist–Shannon sampling theorem. The objectives of this study are to quantify the errors generated by a typical infrequent sampling protocol and to design appropriate sampling strategy according to the sampling objectives. Nitrate concentration signal and flow data are numerically sampled to simulate common sampling frequencies. The total fluxes calculated from the simulated samples are compared with the reference value computed on the continuous signal. Uncertainties are increasing as sampling intervals increase; the method that is not using continuous discharge to compute nitrate fluxes bring larger uncertainty. The dispersion and bias computed for each sampling interval are used to evaluate the uncertainty during each hydrological period. High underestimation is made during flood periods when high‐concentration period is overlooked. On the contrary, high sampling frequencies (from 3 h to 1 day) lead to a systematic overestimation (bias around 3%): highest concentrations are overweighted by the interpolation of the concentration in such case. The in situ sampling protocol generates less than 1% of load estimation error and sample highest concentration peaks. We consider useful such newly emerging field technologies to assess short‐term variations of water quality parameters, to minimize the number of samples to be analysed and to assess the quality state of the stream at any time. Copyright © 2012 John Wiley & Sons, Ltd.