The Importance of High Resolution Digital Elevation Models for Improved Hydrological Simulations of a Mediterranean Forested Catchment

Eco-hydrological models can be used to support effective land management and planning of forest resources. These models require a Digital Elevation Model (DEM), in order to accurately represent the morphological surface and to simulate catchment responses. This is particularly relevant on low altimetry catchments, where a high resolution DEM can result in a more accurate representation of terrain morphology (e.g., slope, flow direction), and therefore a better prediction of hydrological responses. This work intended to use Soil and Water Assessment Tool (SWAT) to assess the influence of DEM resolutions (1 m, 10 m and 30 m) on the accuracy of catchment representations and hydrological responses on a low relief forest catchment with a dry and hot summer Mediterranean climate. The catchment responses were simulated using independent SWAT models built up using three DEMs. These resolutions resulted in marked differences regarding the total number of channels, their length as well as the hierarchy. Model performance was increasingly improved using fine resolutions DEM, revealing a bR2 (0.87, 0.85 and 0.85), NSE (0.84, 0.67 and 0.60) and Pbias (−14.1, −27.0 and −38.7), respectively, for 1 m, 10 m and 30 m resolutions. This translates into a better timing of the flow, improved volume simulation and significantly less underestimation of the flow.

A history of the concept of time of concentration

The concept of time of concentration in the analysis of catchment responses dates back over 150 years to the introduction of the rational method. Since then it has been used in a variety of ways in the formulation of both unit hydrograph and distributed catchment models. It is normally discussed in terms of the velocity of flow of a water particle from the furthest part of a catchment to the outlet. This is also the basis for the definition in the International Glossary of Hydrology. While conceptually simple, this definition is, however, wrong when applied to catchment responses where, in terms of how surface and subsurface flows produce hydrographs, it is more correct to discuss and teach the concept based on celerities and time to equilibrium. While this has been recognized since the 1960s, some recent papers and texts remain confused over the definition and use of the time of concentration concept. The paper sets out the history of its use and clarifies its relationship with time to equilibrium but suggests that both terms are not really useful in explaining hydrological responses. An Appendix is included that quantifies the differences between the definitions of response times for subsurface and surface flows under simple assumptions that might be useful in teaching.

Random forest algorithm as a regionalization model of flood-mechanisms

<p>Hydrologists are challenged to estimate extreme discharges from catchments with data of poor temporal and spatial resolution. Floods are complex processes derived from catchment responses to various meteorological inputs, commonly summarized under one distribution function, representing the cumulative effect of all triggering events (Merz & Blöschl, 2003). A better understanding of driving precipitation inputs, catchment properties and a-priori conditions are required to characterize flood mechanisms and to determine shape, volume and peak of the extreme discharges. This research focuses on the estimation of floods. The study area is the northwestern Switzerland with small to medium catchments (0.5 to 200 km2), with low concentration times and a highly variable response to the meteorological input in terms of associated peak discharges and volumes.</p><p>We use a random forest algorithm to evaluate similar catchment reactions at the occurrence of a flood. We consider catchment descriptors and event specific characteristics for the training of the model. The flood hydrograph serves as the training target variable in order to describe the catchment response. Our regionalization method suggest that the meteorological input of a catchment, specifically the temporal entropy of precipitation, is the most significant parameter for clustering catchment reactions and should, therefore, be consider for such a task. This model has the potential of identifying donor catchments for estimating extreme discharge at the ungauged catchments, using the floods similarities derived by the random forest.</p><p><strong>References:</strong></p><p>Merz, R., and G. Blöschl, A process typology of regional floods, Water Resour. Res., 39(12), 1340, doi:10.1029/2002WR001952, 2003.</p>

A history of the concept of time of concentration

The concept of time of concentration in the analysis of catchment responses dates back over 150 years to the introduction of the Rational Method. Since then it has been used in a variety of ways in the formulation of both unit hydrograph and distributed catchment models. It is normally discussed The concept of time of concentration in the analysis of catchment responses dates back over 150 years to the introduction of the Rational Method. Since then it has been used in a variety of ways in the formulation of both unit hydrograph and distributed catchment models. It is normally discussed in terms of the velocity of flow of a water particle from the furthest part of a catchment to the outlet. This is also the basis for the definition in the International Glossary of Hydrology. While conceptually simple, this definition is, however, wrong when applied to catchment responses where, in terms of how surface and subsurface flows produce hydrographs, it is more correct to discuss and teach the concept based on celerities and time to equilibrium. While this has been recognized since the 1960s, some recent papers and text remain confused over the definition and use of time of concentration. The paper sets out the history of its use and clarifies its relationship to time to equilibrium but suggests that both terms are not really useful in explaining hydrological responses. An appendix is included that quantifies the differences between the definitions of response times for subsurface and surface flows under simple assumptions that might be useful in teaching.

Global Analysis of the Role of Terrestrial Water Storage in the Evapotranspiration Estimated from the Budyko Framework at Annual to Monthly Time Scales

Recent studies have extended the applicability of the Budyko framework from the long-term mean to annual or shorter time scales. However, the effects of water storage change ΔS on the overall water balance estimated from the Budyko models (BM) at annual-to-monthly time scales were less investigated, particularly at the continental or global scales, due to the lack of large-scale ΔS data. Here, based on a 25-yr (1984–2008) global gridded terrestrial water budget dataset and by using an analytical error-decomposition framework, we analyzed the effects of ΔS in evapotranspiration (ET) predicted from BM at both grid and basin scales under diverse climates for the annual, wet-seasonal, dry-seasonal, and monthly time scales. Results indicated that the BM underperforms in the short dry (wet) seasons of predominantly humid (dry) basins, with lower accuracy under more humid climates (at annual, dry-seasonal, and monthly scales) and under more arid climates (at wet-seasonal scale). When the effects of ΔS are incorporated into BM, improvements can be found mostly at annual and dry-seasonal scales, but not notable at wet-seasonal and monthly scales. The magnitudes of ΔS are positively correlated with the errors in BM-predicted ET for most global regions at annual and monthly scales, especially under arid climates. Under arid climates, the variability of ET prediction errors is controlled mainly by the ΔS variability at annual and monthly time scales. In contrast, under humid climates the effect of ΔS on ET prediction errors is generally limited, particularly at the wet-seasonal scale due to the more dominant influences of other climatic factors (precipitation and potential ET) and catchment responses (runoff).

Controls on spatial and temporal variability in streamflow and hydrochemistry in a glacierized catchment

Understanding the hydrological and hydrochemical functioning of glacierized catchments requires the knowledge of the different controlling factors and their mutual interplay. For this purpose, the present study was carried out in two sub-catchments of the glacierized Sulden River catchment (130 km2; eastern Italian Alps) in 2014 and 2015, characterized by a similarly sized but contrasting geological setting. Samples were taken at different space and timescales for analysis of stable isotopes in water, electrical conductivity, and major, minor and trace elements. At the monthly sampling scale, complex spatial and temporal dynamics for different spatial scales (0.05–130 km2) were found, such as contrasting electrical conductivity gradients in both sub-catchments. For the entire Sulden catchment, the relationship between discharge and electrical conductivity showed a monthly hysteretic pattern. Hydrometric and geochemical dynamics were controlled by interplay of meteorological conditions, topography and geological heterogeneity. A principal component analysis revealed that the largest variance (36.3 %) was explained by heavy metal concentrations (such as Al, V, Cr, Ni, Zn, Cd and Pb) during the melting period, while the remaining variance (16.3 %) resulted from the bedrock type in the upper Sulden sub-catchment (inferred from electrical conductivity, Ca, K, As and Sr concentrations). Thus, high concentrations of As and Sr in rock glacier outflow may more likely result from bedrock weathering. Furthermore, nivo-meteorological indicators such as daily maximum air temperature and daily maximum global solar radiation represented important meteorological controls, with a significant snowmelt contribution when exceeding 5 ∘C or 1000 W m−2, respectively. These insights may help in better understanding and predicting hydrochemical catchment responses linked to meteorological and geological controls and in guiding future classifications of glacierized catchments according to their hydrochemical characteristics.

Evaluating catchment response to artificial rainfall from four weather generators for present and future climate

The technical lifetime of urban water infrastructure has a duration where climate change has to be considered when alterations to the system are planned. Also, models for urban water management are reaching a very high complexity level with, for example, decentralized stormwater control measures being included. These systems have to be evaluated under as close-to-real conditions as possible. Long term statistics (LTS) modelling with observational data is the most close-to-real solution for present climate conditions, but for future climate conditions artificial rainfall time series from weather generators (WGs) have to be used. In this study, we ran LTS simulations with four different WG products for both present and future conditions on two different catchments. For the present conditions, all WG products result in realistic catchment responses when it comes to the number of full flowing pipes and the number and volume of combined sewer overflows (CSOs). For future conditions, the differences in the WGs representation of the expectations to climate change is evident. Nonetheless, all future results indicate that the catchments will have to handle more events that utilize the full capacity of the drainage systems. Generally, WG products are relevant to use in planning of future changes to sewer systems.

Controls on spatial and temporal variability of streamflow and hydrochemistry in a glacierized catchment

The understanding of the hydrological and hydrochemical functioning of glacierized catchments require the knowledge of the different controlling factors and their mutual interplay. For this purpose, the present study was carried out in two sub-catchments of the Sulden River catchment (130 km2, Eastern Italian Alps) in 2014 and 2015, characterized by similar size but contrasting geological setting. Samples were taken at different space and time scales for analysis of stable isotopes of water, electrical conductivity, major, minor and trace elements. At the monthly sampling scale for different spatial scales (0.05–130 km2), complex spatial and temporal dynamics such as contrasting EC gradients in both sub-catchments were found. At the daily scale, for the entire Sulden catchment the relationship between discharge and electrical conductivity showed a monthly hysteretic pattern. Hydrometric and geochemical dynamics were controlled by an interplay of meteorological conditions and geological heterogeneity. After conducting a PCA analysis, the largest share of variance (36.3 %) was explained by heavy metal concentrations (such as Al, V, Cr, Ni, Zn, Cd, Pb) during the melting period while the remaining variance (16.3 %) resulted from the bedrock type in the upper Sulden sub-catchment (inferred from EC, Ca, K, As and Sr concentrations). Thus, high concentrations of As and Sr in rock glacier outflow may more likely result from bedrock weathering. Furthermore, nivo-meteorological indicators such as maximum daily global solar radiation, three day maximum air temperature, and 15 day snow depth differences could explain the monthly conductivity and isotopic dynamics best. The decrease of snow depth calculated for different time lengths prior to the sampling day showed best agreements with conductivity and isotopic dynamics when time lengths varied. These insights may help to better predict hydrochemical catchment responses linked to meteorological and geological controls and to guide future classifications of glacierized catchments according to their hydrochemical characteristics.