ALBERT

Description: In this article, we present a Bayesian geostatistical framework that is particularly suitable for interpolation of hydrological data when the available dataset is sparse and includes missing values and short records of data. A key feature of the proposed framework is that several years of runoff is modeled simultaneously with two Gaussian random fields (GRFs): One that is common for all years under study and represents the runoff generation due to long-term climatic conditions, and one that is year specific. The climatic GRF learns how short records of runoff from partially gauged catchments vary relatively to longer time series from other catchments, and transfers this information across years. Another property, is that the model takes the nested structure of catchments into account such that the water balance is preserved for any point in the landscape. The framework is demonstrated by interpolation of annual and monthly runoff from around 200 catchments in Norway, and we compare it to Top-Kriging (interpolation method) and simple linear regression (method for exploiting short records). The results show that if the correlation between neighboring catchments is high, a model that considers several years of runoff simultaneously is considerably better at capturing large spatial variability than a model that treats each year of data separately.

Description: Sustainable irrigation with treated wastewater (TWW) is a promising solution for water scarcity in arid and semi-arid regions. Soil aquifer treatment (SAT) provides a solution for both the need for tertiary treatment and seasonal storage of wastewater. Stresses over land use and the need to control the obtained water quality makes the optimization of SAT of great importance. This study looks into the influence of SAT systems' operational dynamics (i.e. flooding and drying periods) as well as some aspects of the inflow biochemical composition on their bio-geo-chemical state and the ultimate outflow quality. A series of four long-column experiments was conducted, aiming to examine the effect of different flooding/drying period ratios on dissolved oxygen (DO) concentrations, oxidation-reduction potential (ORP) and outflow composition. Flooding periods were kept constant at 60 minutes for all experiments while drying periods (DP) were 2.5 and 4 times the duration of the flooding periods. Our results show that the longer DP had a significant advantage over the shorter periods in terms of DO concentrations and ORP in the upper parts of the column as well as in the deeper parts, which indicates that larger volumes of the profile were able to maintain aerobic conditions. This advantage was evident also in outflow composition analyses that showed significantly lower concentrations of DOC, TKN and ammonium in the outflow for the longer DP. Comparing experimental ORP values in response to different DP to field measurements obtained in one of the SAT ponds of the SHAFDAN, Israel, we found that despite the major scale differences between the experimental 1D system and the field 3D conditions, ORP trends in response to changes in DP, qualitatively match. We conclude that longer DP not only ensure oxidizing conditions close to the surface, but also enlarge the active (oxidizing) region of the SAT. While those results still need to be verified in full scale, they suggest that SAT can be treated as a pseudo-reactor that to a great extent could be manipulated hydraulically to achieve the desired water quality while increasing the recharge volumes.

Description: Between 1992 and 2012, concentrations of annual mean suspended sediment decreased at over half (58 %) of the 137 stream sites assessed across the contiguous United States (US). Increases occurred at 17 % of the sites and the direction of change was uncertain at the remaining 25 %. Sediment trends were characterized using the Weighted Regressions on Time, Discharge, and Season model, and decreases in sediment ranged from −95 % to −8.5 % of the 1992 concentration. To explore potential drivers of these changes, the sediment trends were (1) parsed into two broad contributors of change, changes in land management versus changes in the streamflow regime, and (2) grouped by land use of the watershed and correlated to concurrent changes in land use/cover, hydrology and climate and static/long-term watershed characteristics. At 83 % of the sites, changes in land management (captured by changes in the concentration–streamflow relationship over time) contributed more to the change in the sediment trend than changes in the streamflow regime alone (i.e. any systematic change in the magnitude, frequency or timing of flows). However, at 〉 60 % of the sites, changes in the streamflow regime contributed at least a 5 % change in sediment and at 10 sites changes in the streamflow regime contributed over half the change in sediment, indicating that at many sites changes in streamflow were not the main driver of changes in sediment but was often an important supporting factor. Correlations between sediment trends and concurrent changes in land use/cover, hydrology and climate were often stronger at sites draining watersheds with more homogenous, human-related land uses (i.e. agricultural and urban lands) compared to mixed-use or undeveloped lands. At many sites, decreases in sediment occurred despite small to moderate increases in the amount of urban or agricultural land in the watershed, suggesting conservation efforts to reduce sediment runoff to streams may be successful, up to a point, even as lands are converted to urban and agricultural uses.

Description: Seasonality is ubiquitous in nature, and it is closely linked to water quality, ecology, hydrological extremes, and water resources management. Hydrological signatures aim at extracting relevant information about hydrological behaviour, and they can be used to better understand hydrological processes and to evaluate hydrological models. Commonly used seasonal hydro-climatological signatures consider climate or streamflow seasonality, but not how climate seasonality translates into streamflow seasonality. We propose and test hydrological signatures based on the attenuation of the seasonal climate input by a catchment. We approximate the seasonality in the input (precipitation minus potential evapotranspiration) and the output (streamflow) by sine waves. A catchment alters the input sine wave by reducing its amplitude and by shifting its phase. We use these quantities, the amplitude ratio and the phase shift, as seasonal hydrological signatures. We present analytical solutions describing the response of linear reservoirs to periodic forcing to interpret the seasonal signatures in terms of configurations of linear reservoirs. Using data from the UK and the US, we show that the seasonal signatures exhibit hydrologically interpretable patterns and that they are a function of both climate and catchment attributes. Wet, rather impermeable catchments hardly attenuate the seasonal climate input. Drier catchments, especially if underlain by a productive aquifer, strongly attenuate the input sine wave leading to phase shifts up to several months. Finally, we test whether two commonly used hydrological models (IHACRES, GR4J) can reproduce the observed ranges of seasonal signatures in the UK. The results show that the seasonal signatures can aid model building and evaluation.

Description: Lakes provide many important benefits to society including drinking water, flood attenuation, nutrition, and recreation. Anthropogenic environmental changes may affect these benefits by altering lake water levels. However, background climate oscillations such as the El Nino Southern Oscillation, and the North Atlantic Oscillation can obscure long-term trends in water levels, creating uncertainty over the strength and ubiquity of anthropogenic effects on lakes. Here we account for the effects of background climate variation and test for long-term (1992–2019) trends in water levels in 117 globally-distributed large lakes using satellite altimetry data. On average, 27 % of water level variation in individual lakes was associated with background climate variation. The relative influence of specific axes of background climate variation on water levels varied substantially across and within regions. After removing the effects of background climate variation on water levels, long-term water level trend estimates were lower (+1.0 cm year−1) than calculated from raw water level data (+1.4 cm year−1). However, the trends became more statistically significant in 76 % of lakes after removing the effects of background climate variation (the median p-value of trends changed from 0.12 to 0.02). Thus, robust tests for long-term trends in lake water levels which may or may not be anthropogenic will require prior isolation and removal of the effects of background climate variation. Our findings suggest that background climate variation often masks long-term trends in environmental variables, but can be accounted for through more comprehensive statistical analyses.

Description: The studies conducted in the second half of the Sixteenth Century were crucial both for the birth of the modern hydrological science and for the modern epistemology. Thanks to quantitative observations and to the new experiment–based scientific approach, the Sun was about to be fully recognized as the engine of the hydrological cycle, instead of an endogenous engine within the Earth, originally conjectured by Aristotle to explain the water supply of great rivers. In this context of great vitality and rapid cultural changes, two works were published on the origin of springs. In 1663 Gaspar Schott published the Anatomia physico–hydrostatica fontium ac fluminum (Physical–hydrostatic anatomy of springs and rivers), deeply rooted in the Aristotelian epistemology and based on the ancient humanistic and dialectical method. In this book, Schott cited some observations recently made about the capability of the water to rise within a soil column. Pierre Perrault referred to these observations as a starting point to design the experiments reported in his classical opus De l'origine des fontaines (On the origin of springs), published in 1674. With this approach Perrault places himself in the perspective of a deeply renewed epistemology: only a decade passed between the publication of Schott's opus and Perrault's one, but their perspective is radically different. At the same time, the questions posed on the hydrological cycle and on the soil hydrology, which are hardly reproducible by means of a controlled laboratory model, severely tested the modern scientific approach at its beginning. Perrault seems to be aware of such difficulties and, after discussing the results of the experiments, he moves the point of view from the scale of the laboratory model, to that of the catchment. This choice makes him a progenitor of the contemporary epistemology of complexity, which is framed both on laboratory analysis and on cases study. Thus even if Perrault's conclusions went in the direction of the ancient opinion, his work is not only seminal for hydrology, but also it helps to enlighten some intricate features of scientific revolution. Aiming at contributing to understand the importance of Perrault's opus, we will discuss his epistemological relevance through the lens of the repeatability of the experiments, of the intriguing didactic aspects which, starting from his experiments, arise for modern teaching of hydrology, and of his attitude to face the complexity of the hydrological processes.

Description: Spatial downscaling of rainfall fields is a challenging mathematical problem for which many different types of methods have been proposed. One popular solution consists in redistributing rainfall amounts over smaller and smaller scales by means of a discrete multiplicative random cascade (DMRC). This works well for slowly varying, homogeneous rainfall fields but often fails in the presence of intermittency (i.e., large amounts of zero rainfall values). The most common workaround in this case is to use two separate cascade models, one for the occurrence and another for the intensity. In this paper, a new and simpler approach based on the notion of equal-volume areas (EVAs) is proposed. Unlike classical cascades where rainfall amounts are redistributed over grid cells of equal size, the EVA cascade splits grid cells into areas of different sizes, each of them containing exactly half of the original amount of water. The relative areas of the sub-grid cells are determined by drawing random values from a logit-normal cascade generator model with scale and intensity dependent standard deviation. The process ends when the amount of water in each sub-grid cell is smaller than a fixed bucket capacity, at which point the output of the cascade can be re-sampled over a regular Cartesian mesh. The present paper describes the implementation of the EVA cascade model and gives some first results for 100 selected events in the Netherlands. Performance is assessed by comparing the outputs of the EVA model to bilinear interpolation and to a classical DMRC model based on fixed grid cell sizes. Results show that on average, the EVA cascade outperforms the classical method, producing fields with more realistic distributions, small-scale extremes and spatial structures. Improvements are mostly credited to the higher robustness of the EVA model to the presence of intermittency and to the lower variance of its generator. However, improvements are not systematic and both approaches have their advantages and weaknesses. For example, while the classical cascade tends to overestimate small-scale extremes and variability, the EVA model tends to produce fields that are slightly too smooth and blocky compared with observations.

Description: It has been advocated that history-matching numerical models to a diverse range of observation data types, particularly including environmental tracer concentrations and their interpretations/derivatives (e.g., mean age), constitutes an effective and appropriate means to improve model forecast reliability. This study presents two regional-scale modeling case studies that directly and rigorously assess the value of discrete tritium concentration observations and tritium-derived mean residence time (MRT) estimates in two decision-support contexts; value herein is measured as the improvement (or otherwise) in the reliability of forecasts through uncertainty variance reduction and bias minimization as a result of assimilating tritium or tritium-derived MRT observations. The first case study (Heretaunga Plains, New Zealand) utilizes a suite of steady-state and transient flow models and an advection-only particle-tracking model to evaluate the worth of tritium-derived MRT estimates relative to hydraulic potential, spring discharge and river/aquifer exchange flux observations. The worth of MRT observations is quantified in terms of the change in the uncertainty surrounding ecologically-sensitive spring discharge forecasts via first-order second-moment analyses. The second case study (Hauraki Plains, New Zealand) employs paired simple/complex transient flow and transport models to evaluate the potential for assimilation-induced bias in simulated surface-water nitrate discharge to an ecologically-sensitive estuary system; formal data assimilation of tritium observations is undertaken using an iterative ensemble smoother. The results of these case studies indicate that, for the decision-relevant forecasts considered, tritium observations are of variable benefit and may induce damaging bias in forecasts; these biases are a result of an imperfect model's inability to properly and directly assimilate the rich information content of the tritium observations. The findings of this study challenge the unqualified advocacy of the increasing use of tracers, and diverse data types more generally, whenever environmental model data assimilation is undertaken with imperfect models. This study also highlights the need for improved imperfect-model data assimilation strategies. While these strategies will likely require increased model complexity (including advanced discretization, processes and parameterization) to allow for appropriate assimilation of rich and diverse data types that operate across a range of spatial and temporal scales commensurate with a forecast of management interest, it is critical that increased model complexity does not preclude the application of formal data assimilation and uncertainty quantification techniques due to model instability and excessive run times.

Description: Identification and understanding of the dominant control mechanisms of hydrological extremes has drawn worldwide attention in recent decades. However, detailed understanding of drought and wet spells within semi-arid regions has been hampered by the fact that identification is difficult for no flow conditions. Classification methods that have been developed for regions with perennial flow, do not work for ephemeral semi-arid rivers, while approaches for arid environments have difficulties to deal with seasonal runoff. Recently, a method was presented to identify hydrological extremes within semi-arid regions, by combining approaches developed for perennial flow and arid environments. However, this combined approach shows difficulties to identify drought and wet spells within semi-arid domains with a yearly precipitation cycle (e.g. monsoon). The current paper proposes to modify the combined method and make it suitable for these domains. The modified combined approach presented here to identify hydrological extremes was applied to decade-long discharge observations from 12 different locations within the San Pedro basin in southeastern Arizona. These locations correspond to catchments covering multiple elevation bands and runoff characteristics. Southern Arizona receives the majority of its rainfall from the summertime North American Monsoon (NAM), with frontal systems providing additional precipitation in winter. Using the modified method, the identified droughts and wet spells last longer compared to the previously defined combined procedure, and drought generally does not only start in spring at the end of the dry season. Furthermore, results show that if a drought or wet spell starts during the NAM or post-NAM season, it will generally last longer as compared to one that starts in winter or spring. This specifically holds for catchments with no perennial flow. By increasing the flow averaging interval, the new method also enables to observe multi-year drought and wet spells patterns. For the precipitation limited semi-arid San Pedro basin results show that multi-year wet spell and drought are rare. This is caused by the strong impact of the summertime NAM that generally acts both as a start and reset button for both types of hydrological extremes.

Description: Large parts of Europe have faced extreme low river flows in recent summers (2003, 2011, 2015, 2018) with major economic and environmental consequences. Understanding the origins of extremes like these is important for water resources management. To reveal how weather drives low flows, we explore how deviations from mean seasonal climatic conditions (i.e. climatic anomalies) of precipitation and potential evapotranspiration shaped the occurrence and magnitude of the annual 7-day lowest flows (Qmin) across 380 Swiss catchments from 2000 through 2018. Most annual low flows followed periods of below average precipitation and above average potential evapotranspiration, and the most extreme low flows resulted from both of these drivers acting together. Extremely dry years saw simultaneous drought conditions across large parts of Europe, but low flow timing during these years was still spatially variable across Switzerland. Longer climatic anomalies led to lower low flows. Most low flows were typically preceded by climatic anomalies lasting up to two months, whereas low flows in the extreme years (2003, 2011, 2015, 2018) were associated with much longer-lasting climatic anomalies. Weather conditions on even longer time scales have been reported to sometimes affect river flow. However, across Switzerland, we found that precipitation totals in winter only slightly influenced the magnitude and timing of summer and autumn low flows. Our results provide insight into how precipitation and potential evapotranspiration jointly shape summer and winter low flows across Switzerland, and could potentially aid in assessing low-flow risks in similar mountain regions using seasonal weather forecasts.