HESS Opinions "Catchments as meta-organisms – a new blueprint for hydrological modelling"

Catchment-scale hydrological models frequently miss essential characteristics of what determines the functioning of catchments. The most important active agent in catchments is the ecosystem. It manipulates and partitions moisture in a way that supports the essential functions of survival and productivity: infiltration of water, retention of moisture, mobilization and retention of nutrients, and drainage. Ecosystems do this in the most efficient way, establishing a continuous, ever-evolving feedback loop with the landscape and climatic drivers. In brief, hydrological systems are alive and have a strong capacity to adjust themselves to prevailing and changing environmental conditions. Although most models take Newtonian theory at heart, as best they can, what they generally miss is Darwinian theory on how an ecosystem evolves and adjusts its environment to maintain crucial hydrological functions. In addition, catchments, such as many other natural systems, do not only evolve over time, but develop features of spatial organization, including surface or sub-surface drainage patterns, as a by-product of this evolution. Models that fail to account for patterns and the associated feedbacks miss a critical element of how systems at the interface of atmosphere, biosphere and pedosphere function.In contrast to what is widely believed, relatively simple, semi-distributed conceptual models have the potential to accommodate organizational features and their temporal evolution in an efficient way, a reason for that being that because their parameters (and their evolution over time) are effective at the modelling scale, and thus integrate natural heterogeneity within the system, they may be directly inferred from observations at the same scale, reducing the need for calibration and related problems. In particular, the emergence of new and more detailed observation systems from space will lead towards a more robust understanding of spatial organization and its evolution. This will further permit the development of relatively simple time-dynamic functional relationships that can meaningfully represent spatial patterns and their evolution over time, even in poorly gauged environments.Water Resource

Migration as flow: Using hydrological concepts to estimate the residence time of migrating birds from the daily counts

Estimating the length-of-stay, the number of days a bird can be expected to stay at a site, at stopover sites is critical to understanding the migration ecology and estimating the population sizes of birds as they move between breeding and non-breeding sites. Estimating the length-of-stay of migrating animals at stopover sites has an analogue in the hydrological concept of transit time, the amount of time that water spends in a reservoir, which can be calculated as a numerical integration of inflow and outflow rates with an underlying Storage Age Selection function. We used this approach to estimate the lengths-of-stay of migrating Western Sandpiper (Calidris mauri) and Dunlin (Calidris alpina) based on the time series of daily counts at two sites in British Columbia, Canada. The approach yielded mean transit times for Western Sandpiper during southward migration at Sidney Island that ranged between 9·6 days and 3·8 days, and showed a significant decline over time, 1992-2001, and is consistent with the estimates obtained from the capture-mark-resight studies. Transit times during northward migration at Roberts Bank, Fraser River Delta, based on the best available information ranged from 1·8 to 3·2 days for Western Sandpiper, and had a median value of 2·0 days for Dunlin, which is consistent with the estimates obtained from the radio-telemetry studies. These results indicate that the hydrological flow models may offer a means to estimate the length-of-stay from the daily counts of birds during migration. The models present an opportunity for testing the alternate hypotheses concerning the roles of behavioural- vs. habitat-related mechanisms driving shorebird population sizes.Water Resource

Flood of July 13-15 2021: a new type of floods in Western Europe?

Sanitary EngineeringWater ResourcesRivers, Ports, Waterways and Dredging Engineerin

Integrated glacier and snow hydrological modelling in the Urumqi No.1 glacier catchment

Water ManagementCivil Engineering and Geoscience

Modelling landuse change with dynamic moisture storage capacities

Water ManagementCivil Engineering and Geoscience

Satellite-based drought analysis in the Zambezi River Basin: Was the 2019 drought the most extreme in several decades as locally perceived?

Study region: The study area is the river basin upstream of the Kariba dam located in the Zambezi River at the border of Zambia and Zimbabwe. Study focus: During the dry season of 2019 in Sub-Saharan Africa, extremely low water levels occurred in the Zambezi. According to news media, locals perceived this drought as the worst in several decades. We analyzed the 2019 drought in the Zambezi River Basin upstream of the Kariba dam to determine whether it indeed was the longest, most intense, and severe drought, in terms of precipitation, total water storage and reservoir water level observations over recent decades. New hydrological insights for the region: Data analysis indicates that the 2019 drought indeed had the lowest basin-averaged annual rainfall, most severe local rainfall deficit in the north of the basin, and lowest reservoir level since 1995. However, the rainfall deficit was more severe in 2002, both basin-wide and locally in the south of the basin. The total storage deficit was more severe in 2004, both basin-wide and locally in the central part of the basin. However, as the available storage data did not cover the entire deficit for 2019, its final duration and severity remain unknown. Therefore, it depends on the drought characteristic, hydrological variable, and location within the basin, whether the 2019 drought was indeed the most extreme over recent decades.Water Resource

Integration of observed and model-derived groundwater levels in landslide threshold models in Rwanda

The incorporation of specific regional hydrological characteristics in empirical statistical landslide threshold models has considerable potential to improve the quality of landslide predictions towards reliable early warning systems. The objective of this research was to test the value of regional groundwater level information, as a proxy for water storage fluctuations, to improve regional landslide predictions with empirical models based on the concept of threshold levels. Specifically, we investigated (i) the use of a data-driven time series approach to model the regional groundwater levels based on short duration monitoring observations and (ii) the predictive power of single variable and bilinear threshold landslide prediction models derived from groundwater levels and precipitation. Based on statistical measures of the model fit (R2 and RMSE), the groundwater level dynamics estimated by the transfer function noise time series model are broadly consistent with the observed groundwater levels. The single variable threshold models derived from groundwater levels exhibited the highest landslide prediction power with 82 %–93 % of true positive alarms despite the quite high rate of false alarms with about 26 %–38 %. The further combination as bilinear threshold models reduced the rate of false alarms by about 18 %–28 % at the expense of reduced true alarms by about 9 %–29 % and is thus less advantageous than single variable threshold models. In contrast to precipitation-based thresholds, relying on threshold models exclusively defined using hydrological variables such as groundwater can lead to improved landslide predictions due to their implicit consideration of long-term antecedent conditions until the day of landslide occurrence.Water Resource

HESS Opinions: The complementary merits of competing modelling philosophies in hydrology

In hydrology, two somewhat competing philosophies form the basis of most process-based models. At one endpoint of this continuum are detailed, high-resolution descriptions of small-scale processes that are numerically integrated to larger scales (e.g. catchments). At the other endpoint of the continuum are spatially lumped representations of the system that express the hydrological response via, in the extreme case, a single linear transfer function. Many other models, developed starting from these two contrasting endpoints, plot along this continuum with different degrees of spatial resolutions and process complexities. A better understanding of the respective basis as well as the respective shortcomings of different modelling philosophies has the potential to improve our models. In this paper we analyse several frequently communicated beliefs and assumptions to identify, discuss and emphasize the functional similarity of the seemingly competing modelling philosophies. We argue that deficiencies in model applications largely do not depend on the modelling philosophy, although some models may be more suitable for specific applications than others and vice versa, but rather on the way a model is implemented. Based on the premises that any model can be implemented at any desired degree of detail and that any type of model remains to some degree conceptual, we argue that a convergence of modelling strategies may hold some value for advancing the development of hydrological models.Water Resource

Landslide precipitation thresholds in Rwanda

Regional empirical-statistical thresholds indicating the precipitation conditions initiating landslides are of crucial importance for landslide early warning system development. The objectives of this research were to use landslide and precipitation data in an empirical-statistical approach to (1) identify precipitation-related variables with the highest explanatory power for landslide occurrence and (2) define both trigger and trigger-cause based thresholds for landslides in Rwanda, Central-East Africa. Receiver operating characteristics (ROC) and area under the curve (AUC) metrics were used to test the suitability of a suite of precipitation-related explanatory variables. A Bayesian probabilistic approach, maximum true skill statistics and the minimum radial distance were used to determine the most informative threshold levels above which landslide are high likely to occur. The results indicated that the event precipitation volumes E, cumulative 1-day rainfall (RD1) that coincide with the day of landslide occurrence and 10-day antecedent precipitation are variables with the highest discriminatory power to distinguish landslide from no landslide conditions. The highest landslide prediction capability in terms of true positive alarms was obtained from single rainfall variables based on trigger-based thresholds. However, that predictive capability was constrained by the high rate of false positive alarms and thus the elevated probability to neglect the contribution of additional causal factors that lead to the occurrence of landslides and which can partly be accounted for by the antecedent precipitation indices. Further combination of different variables into trigger-cause pairs and the use of suitable thresholds in bilinear format improved the prediction capacity of the real trigger-based thresholds.Water Resource

Trigger characteristics of torrential flows from high to low alpine regions in Austria

Torrential processes like fluvial flows (flash floods with or without intensive sediment transport) and debris flows can represent a threat to people and infrastructure in alpine domains. Up to now the hydro-meteorological trigger conditions and their connection with geomorphic watershed characteristics that favor the initiation of either process are largely unknown. Based on modeled wetness states we determined the trigger types (long-lasting rainfall (LLR), short-duration storm (SDS) and intense snow melt (SM)) of 360 observed debris flow and fluvial flood events in six climatically and geomorphologically contrasting watersheds in Austria. Results show that the watershed wetness states play very distinct roles for triggering torrential events across the study regions. Hydro-meteorological variables have little power to explain the occurrence of fluvial flows and debris flows in these regions. Nevertheless, trigger type separation highlighted some geomorphic influences. For example, intense SM triggered more events in sub-watersheds (torrential watersheds in the study region) that are characterized by significantly higher Melton ruggedness numbers than LLR does. In addition, the data show that events triggered by LLRs occur in sub-watersheds of similar exposures (aspects) other than SDS. The results suggest that the consideration of different trigger types provides valuable information for engineering risk assessment.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Water Resource