Runoff Generation Mechanisms Research Articles

Modelling the hydrological processes of Koupendri catchment Northwest, Benin

This study calibrated and validated rainfall-runoff model (WaSiM) to aid decisions on sustainable management of scarce water resources in Koupendri catchment. The model was successfully calibrated (NSE = 0.61; R2 = 0.61, RMSE = 0.63) and validated (NSE = 0.68; R2 = 0.78; RMSE = 0.57) using optimised soil and land use parameters. Though the model underpredicted some of the peak flows, the overall calibration result could be adjudged satisfactory with a p-factor of 0.94 and an r-factor of 0.93 which were all within the acceptable range of uncertainty. Evapotranspiration and discharge constituted 98%-99% of the total water balance of the catchment while the change in soil moisture storage constituted only < 2%. Surface runoff was the predominant streamflow component (86%-90%), suggesting infiltration excess overland flow as the main runoff generation mechanism for the catchment. The water balance reflected the hydro climatic conditions of the catchment with actual evapotranspiration ranging between 68% and 75% of the total annual rainfall.

Effects of soil and vegetation development on surface hydrological properties of moraines in the Swiss Alps

The near-surface saturated hydraulic conductivity (Ksat) is an important hydrological characteristic because it determines surface infiltration rates and the vertical and lateral redistribution of water in the soil. However, there is comparatively little knowledge about the changes in Ksat during landscape development and how the co-evolution of biological, pedological and hydrological characteristics affect the movement of water through the soil. On the one hand, increasing vegetation cover is expected to increase macroporosity and thus Ksat. On the other hand, clay formation is expected to decrease Ksat. To investigate how hillslope aging affects Ksat, we used a space-for-time approach and conducted comprehensive measurements of vegetation, soil and topography on a chronosequence of moraines in a proglacial area of the Swiss Alps. On four moraines, ranging from about ten thousand to ~30 years in age, we measured for three plots the near-surface soil characteristics. Surface and near-surface Ksat were high and decreased with depth on all moraines. Surface Ksat was highest on the youngest moraine (median: 4320 mm hr−1) and lowest (540 mm hr−1) on the oldest moraine. Ksat was significantly positive correlated with soil texture and the gravel content in the surface soil layer. The correlation analyses and Structural Equation Model suggested that the larger fraction of small particles for the older moraines had a bigger effect on Ksat than the denser root network. Even though the variability in measured Ksat-values within the moraines was high and water movement is thus likely very heterogeneous, the measured Ksat values suggest that infiltration-excess overland flow is very unlikely on these hillslopes but (lateral) near surface flow likely increases with the age of the hillslope. This information is important for understanding differences in runoff generation mechanisms in alpine areas with moraines of different ages, as well as landscape evolution models.

Evaluating the utility of remotely sensed soil moisture for the characterization of runoff response over Canadian watersheds

Remotely sensed soil moisture measurements from satellite platforms are increasingly reliable, cost-effective and widely available data sources where in situ measurements are unavailable. This research uses the Soil Moisture and Ocean Salinity mission (SMOS) satellite-derived soil moisture anomalies over a database of 65 watersheds across Canada from 2011 to 2014 to analyze the soil moisture-runoff relationship. A spatial analysis of the variability and influences on the strength of this relationship revealed that 32% of catchments showed significant (1 tailed, p < 0.05) correlations between the weekly antecedent soil moisture state of the catchment and the weekly runoff ratio. Regions of strongest correlation were related to the topographic variables of slope and elevation. These results support the use of coarse-scale satellite remote sensing as a valuable data source in hydrological studies, but recommend caution when applying the data to regions where the accuracy of satellite soil moisture data sets is less certain (such as wetlands and areas with high topography) or areas where the runoff generation mechanisms are complex (frozen soils, wetlands or prairie environments).

Mechanisms of surface and subsurface runoff generation in subtropical soil-epikarst systems: Implications of rainfall simulation experiments on karst slope

Southwest China receives abundant rainfall with a mean annual precipitation of 1450 mm (1960–2013) but surface runoff is small, whereas subsurface runoff is relatively large on karst hillslopes. However, not enough studies have been done to investigate the mechanisms of surface and subsurface runoff generation in subtropical karst landscapes. Here we report the dynamics of soil water content (SWC), instantaneous water levels at the soil-epikarst interface (SEI), and runoff characteristics related to the mechanisms of near-surface runoff generation at the slope scale (5 m × 20 m). Four field rainfall simulation experiments were conducted with rainfall intensities ranging from 35 to 136 mm h-1. Subsurface saturation started first at the relatively flat lower slope, and then extended up slope. Subsurface runoff began after subsurface saturated areas connected to each other, representing a “fill-and-spill” mechanism. Surface runoff, which mainly developed after instantaneous water levels reached near the surface, represents an “infiltration-excess and saturation-excess” runoff mechanism, where two thresholds must be attained: rainfall amount and intensity. The rainfall amount threshold is dependent on soil water deficit, water capacity of the epikarst-surface depression at the SEI, and deep percolation from SEI. The rainfall intensity threshold must be larger than the steady infiltration rate of SEI, which is the prerequisite for the saturation of the epikarst-surface depression and soil layer. Steady SEI infiltration rate was estimated (40 mm h-1) according to the surface runoff generation mechanism. This parameter is important as it represents the lower boundary condition in modeling hillslope hydrological processes. Rainfall-runoff thresholds for surface and subsurface runoff decrease with increasing rainfall intensity. Overall, our results show that epikarst permeability along karst hillslopes is relatively high, being the main factor controlling surface and subsurface runoff generation. Therefore, epikarst permeability significantly affects near-surface hydrological processes in karst landscapes. Our data contribute to a more comprehensive understanding of runoff generation processes and water cycle in the critical zone.

Field testing study on the rainfall thresholds and prone areas of sandstone slope erosion at Mogao Grottoes, Dunhuang.

The Dunhuang Mogao Grotto is a famous Buddhist monument and was inscribed in the list of world cultural heritage sites by UNESCO in 1987. Water poses a major threat to the preservation of this heritage even though it is located in an arid region. This study was conducted to investigate the effect of rainfall on rock erosion. Specifically, the formation mechanism of slope runoff and the erosion threshold of rainfall were analyzed, and erosion-prone areas of the site were identified. This was carried out using field artificial rainfall simulation testing, and the results inform methods of preventing rainfall-induced cliff erosion. In addition, the rainfall threshold and erosion-prone areas obtained from the experiment were further validated and optimized using monitoring data for natural rainfall and historic documentation. The threshold value of erosive rainfall obtained by empirical statistical analysis method was found to be similar to that obtained by the runoff generation mechanism. The areas identified as prone to erosion using field tests coincided with areas of historic erosion as recorded in site documentation. Furthermore, the forecast grade of cliff slope erosion and its erosion-prone areas are determined after comprehensive analysis of the results obtained by these two methods. The research results are critical for the monitoring, early warning, and prevention of cliff slope erosion. The research methods can also be used as reference in areas for which rainfall data are missing.

Occurrence of typical antibiotics in Nansi Lake's inflowing rivers and antibiotic source contribution to Nansi Lake based on principal component analysis-multiple linear regression model.

The occurrence and distribution of 14 antibiotics in the surface water and sediment of Nansi Lake's inflowing rivers were studied. The concentrations of the antibiotics in the sediment and water were not detected (ND)-193,440 ng kg-1 and ND-694 ng L-1, respectively, and ofloxacin was identified as the main antibiotic. The target antibiotics were identified at decreased levels in the study area compared with the inflowing rivers of other lakes. The decreased antibiotic concentrations resulted from the dilution effect, strong biodegradation, and rapid photolysis during the wet season. The spatial variations were due to the differences in regional contributions; the concentrations of antibiotics from Jining and Peixian were the highest. Antibiotic pollution in different seasons originated from different sources; pollution levels were determined by water levels and rainfall as well as complicated runoff generation and confluence mechanisms. Based on the risk quotients, ofloxacin, sulfamethoxazole and sulfadiazine were identified as the main antibiotics that contributed to high ecological risks. Algae and aquatic plants were the main model organisms exposed to these risks. This study has great significance for environmental prevention and the control of antibiotic contamination in Nansi Lake, which is an important water transport channel and the main impounded lake for the eastern route of the south-to-north water diversion project.

Comparison of discharge pulses in temperate and tropical rainforest headwater stream networks

We use sub-daily gage records from montane headwater channels in the Luquillo Experimental Forest of Puerto Rico (tropical rainforest) and the HJ Andrews Experimental Forest of Oregon (temperate rainforest) to characterize differences in discharge pulses (rapid, high-magnitude discharge fluctuations) and determine whether the characteristics of discharge pulses differ with respect to drainage area between these two regions. The study sites have different precipitation regimes and runoff generation mechanisms, and we quantify differences in discharge pulses between sites. The assessment of discharge pulses, which are defined as flows that are higher than one standard deviation above the historical mean flow at each site, represents a novel approach to characterizing flashy discharge events in headwater streams using high temporal resolution datasets. Our analyses indicate a clear difference between regions, with discharge in the Luquillo streams pulsing more frequently, for shorter periods of time, and at higher magnitudes than discharge in the Andrews streams. We also quantify how discharge pulses change with increasing drainage area at each of the sites. Differences in discharge pulse metrics with respect to drainage area include an increase in the number of pulses, an increase in the normalized magnitude of pulses, an increase in the standard deviation in normalized magnitude, and a decrease in kurtosis with increasing drainage area at the Luquillo site. These results indicate that there is no attenuation of discharge pulses with increasing drainage area at Luquillo. In contrast, there is a decrease in the normalized magnitude with increasing drainage area at the Andrews site, indicating some attenuation of discharge pulses with increasing drainage area. The characteristics of these pulsed events have implications for ecological processes by transporting sediment, biota, and organic matter, likely altering stream substrates, biotic communities, and organic matter retention times.

On the use of mean monthly runoff to predict the flow–duration curve in ungauged catchments

ABSTRACT We examine the applicability of predicting the daily flow–duration curve (FDC) using mean monthly runoff represented in its stochastic form (MM_FDC) to aid in predictions in ungauged basins, using long-term hydroclimatic data at 73 catchments of humid climate, in the eastern USA. The analysis uses soil hydrological properties, soil moisture storage capacity and the predominant runoff generation mechanism. The results show that MM_FDC did not distinguish the shapes of the upper and lower thirds of the FDC. The upper third is where the precipitation pattern and the antecedent moisture conditions are dominant, while the lower third is where drought-induced low flows and the evapotranspiration effect are prevalent. It is possible to use the MM_FDC to predict the middle third of the FDC (exceedence probabilities between 33% and 66%). The method is constrained by the catchment flow variability (slope of FDC), which changes in accordance with landscape properties and the predominant runoff generation mechanism.

A topographic index explaining hydrological similarity by accounting for the joint controls of runoff formation

Abstract. Surface topography is an important source of information about the functioning and form of a hydrological landscape. Because of its key role in explaining hydrological processes and structures, and also because of its wide availability at good resolution in the form of digital elevation models (DEMs), it is frequently used to inform hydrological analyses. Not surprisingly, several hydrological indices and models have been proposed for linking geomorphic properties of a landscape with its hydrological functioning; a widely used example is the “height above the nearest drainage” (HAND) index. From an energy-centered perspective HAND reflects the gravitational potential energy of a given unit mass of water located on a hillslope, with the reference level set to the elevation of the nearest corresponding river. Given that potential energy differences are the main drivers for runoff generation, HAND distributions provide important proxies to explain runoff generation in catchments. However, as expressed by the second law of thermodynamics, the driver of a flux explains only one aspect of the runoff generation mechanism, with the driving potential of every flux being depleted via entropy production and dissipative energy loss. In fact, such losses dominate when rainfall becomes runoff, and only a tiny portion of the driving potential energy is actually transformed into the kinetic energy of streamflow. In recognition of this, we derive a topographic index called reduced dissipation per unit length index (rDUNE) by reinterpreting and enhancing HAND following a straightforward thermodynamic argumentation. We compare rDUNE with HAND, and with the frequently used topographic wetness index (TWI), and show that rDUNE provides stronger discrimination of catchments into groups that are similar with respect to their dominant runoff processes. Our analysis indicates that accounting for both the driver and resistance aspects of flux generation provides a promising approach for linking the architecture of a system with its functioning and is hence an appropriate basis for developing similarity indices in hydrology.

Simulation of snowmelt runoff and sensitivity analysis in the Nyang River Basin, southeastern Qinghai-Tibetan Plateau, China

The water-runoff in the plateau mountainous areas is mainly contributed by precipitation, snowmelt and glacial meltwater; the different runoff components result from different mechanism of runoff generation. Plateau mountainous areas have not only a unique hydrological cycle mechanism but also are sensitive to climate change. Glacier and snow meltwater in the plateau mountainous areas have a large proportion in runoff and are a main water resources for industrial, agricultural and domestic water use in the basin. Two commonly used model, HBV and SRM, were selected for the quantitative analysis of snowmelt runoff contribution and the hydrological response to climate change scenarios in the Nyang River Basin in the southeastern part of the Qinghai-Tibet Plateau. Based on the characteristics of the models, the HBV model was used to analyze the runoff composition, while the SRM model was used to analyze the runoff in climate change scenarios. The results showed that both models have a good performance in modeling the hydrological processes in the basin. The snow melts mainly concentrate in May, in the average annual precipitation, rainfall and snowfall accounted for 85% and 15%, respectively. From the results of sensitivity analysis, the increase in temperature would accelerate the melting of snow in April and May and turns the snowfall into rainfall in October. However, the change in precipitation mainly affects the runoff in July, August and September, when precipitation is dominated by rain. The results indicate that the timing of the effects of temperature and precipitation on the runoff process is different.

Impact of soil properties on water and sediment transport: A case study at a small catchment in the Loess Plateau

The changes of soil properties caused by ecosystem restoration have played critical roles in controlling water and sediment loss in Loess Plateau. However, it is difficult to isolate and quantify the individual impact of each factor as the impact is hard to monitor separately. The objectives of this study are to explore how the changes in different soil properties influence the hydrologic response and sediment transport at catchment scale. We applied a coupled hydrologic-sediment transport model to a small catchment in the Loess Plateau. Sensitivity analyses were conducted on three influential soil properties: saturated hydraulic conductivity (Ks), Manning’s roughness coefficient (n) and the variations of erodibility coefficient (φ). Our results indicated that the increase in near surface saturated hydraulic conductivity could effectively reduce the peak and total flow (i.e. when Ks is smaller than rainfall intensity, 22.5% increase in Ks leads to over 70% reduction in total discharge), and could lead to transition of runoff generation mechanism from Horton Overland Flow to Dunne Overland Flow. The increase in Manning’s roughness coefficient would also reduce the discharge and sediment yield substantially by slowing down the flow velocity which leads to more infiltration along the flow path after the runoff is generated. The decrease in erodibility coefficient reduces the sediment rate and total sediment mass delivered (i.e. 20% decrease can reduce total sediment mass by 5%), alleviating the undercutting in upstream gullies and deposition at downstream, eventually slowing down landscape evolution. Our findings could be used to provide valuable guidance in the ecosystem restoration and land use management.

An approach to simulate the climate-driven streamflow in the data-scarce mountain basins of Northwest China

With global warming, the inland river basin in the arid region of Northwest China is facing a serious water supply situation. The headwater basin of the inland river is located in the high-altitude mountainous region, and there are few meteorological observation sites, so it is difficult to apply distributed hydrological models and other models based on the physical mechanism of runoff generation to evaluate climate change and its impact on streamflow. To simulate the climate-driven streamflow in data-scarce mountain basins of Northwest China, we developed an integrated approach by using downscaled reanalysis data, Mann–Kendall test, ensemble empirical mode decomposition and backpropagation artificial neural networks together with the weights connection method. We validated the approach in the Kaidu River basin located in the Tianshan mountains. The results showed that the streamflow increased 12.9% by $$2.5 \times 10^{8}\,\hbox {m}^{3}$$ per decade with the warm and wet climate, while the average annual temperature increased 5.2% at a rate of $$0.3{^{\circ }}\hbox {C}$$ per decade and the precipitation increased 37.3% at a rate of 16.4 mm per decade during the period from 1980 to 2015. The impact of temperature variability on streamflow was 44.21 ± 2.08% and the impact of precipitation variability on streamflow was 55.79 ± 2.08%.

Runoff Response to Soil Moisture and Micro-topographic Structure on the Plot Scale

Structural hydrological connectivity has been proposed to describe the geological structure of the landscape as well as to explain hydrological behaviors. Indices based on the topological or soil condition were developed to interpret their relationships. While previous studies mainly focused on well-instrumented catchments which are narrow in humidity or temperate zone, the hydrological responses to structural connectivity at the plot and hill slope scale as well as in arid or semi-arid climate conditions remain unclear. This study was conducted in the semi-arid mountainous region of northern China in Haihe Basin which is the source of water of about 350 million people. Experiments were conducted during the rainy season in 2012 and 2013 using four runoff plots. Two indices, flow path length (FL) based on topography and integral connectivity scale length (ICSL) based on soil moisture conditions, developed to represent hydrological connectivity structure and the runoff response to rainfall were analyzed. The results showed that the surface runoff coefficient was strongly and positively linearly correlated to FL, and the correlation between subsurface flow and ICSLs was quadratic. Plots with shorter FL required more rainfall to generate surface runoff. In the shallow soil layer, when the ICSLs are relatively low, the soil can store more water and less rainfall feeds subsurface runoff. Further analysis indicated that improved shallow soil connectivity conditions might enhance the water-holding capacity and lead to lower water yields for each event. This study demonstrated that hydrological structure connectivity could explain the mechanism of runoff generation in semi-arid areas while further experiments should be undertaken to find the threshold-like relationship between FL and surface runoff as well as the influence of plant cover on hydrological behaviors.

Applicability assessment of the CASCade Two Dimensional SEDiment (CASC2D‐SED) distributed hydrological model for flood forecasting across four typical medium and small watersheds in China

Hydrological modelling is a critical tool for preventing and mitigating severe flood disasters. This study aims to assess the applicability of a physically based distributed hydrological model, CASCade Two Dimensional SEDiment (CASC2D‐SED), for flood forecasting in four typical medium and small watersheds across three hydroclimatic zones (semi‐arid, semihumid, and humid) in China. CASC2D‐SED model has the best performance in Xianbeigou (semi‐arid), followed by Luanchuan (semihumid), and Shujia (humid), while has the relatively lowest performance in Chengcun (humid). It tends to underestimate a large portion of the discharges in the watersheds, indicating that the Green‐Ampt infiltration‐excess model, the only runoff generation scheme in CASC2D‐SED model, imposes restrictions on hydrological simulation without further accounting for subsurface and groundwater flows. This limitation is particularly obvious in the semihumid and humid regions. Our analysis suggests that combining infiltration‐excess and saturation‐excess runoff generation mechanisms and accounting for subsurface and groundwater flows in CASC2D‐SED will further enhance its capacity for flood forecasting and hydrological simulation for semihumid and humid hydroclimatic zones.

Rainfall-runoff simulations in the Lukovska River Basin with the HEC-HMS model

Hydrologic models are important for effective water resources management at a basin level. This paper describes an application of the HEC-HMS hydrologic model for simulations of flood hydrographs in the Lukovska River basin. Five flood events observed at the Mercez stream gauge were available for modelling purposes. These events are from two distinct periods and two seasons with different prevailing runoff generation mechanisms. Hence the events are assigned to either ?present? or ?past?, and ?spring? or ?summer? group. The optimal parameter sets of each group are obtained by averaging the optimal parameters for individual events within the group. To assess model transferability, its applicability for simulation of flood events which are not considered in the model calibration, a cross-validation is performed. The results indicate that model parameters vary across the events, and that parameter transfer generally leads to considerable errors in hydrograph peaks and volumes, with the exception of simulation of summer events with ?spring? parameters. Based on these results, recommendations for event-based modeling are given.

The worthiness of using information on land-use–land-cover in watershed models for Western Ghats: A case study

The variable source area (VSA) theory of runoff generation mechanisms has been proved to hold good in many wet mountainous areas, decades ago. According to this theory, infiltration-excess overland flow is limited to very small areas in mountainous and forested catchments. But, the perception that the land surface characteristics, including land-use–land-cover (LULC), form the major factors influencing the response of the catchment to rainfall has dominated the thought in hydrology to such an extent that models based on the overland flow theory continue to be used even in such areas. The present study was taken up in order to understand the worthiness of using parameters, including the curve number (CN), that are based on the physiographic characteristics of the catchment in a watershed model designed to estimate runoff in the wet mountainous areas of the Western Ghats in southern India, where the VSA theory has been proved to hold good. The study has been accomplished by applying the NITK model developed for estimating runoff using daily rainfall data. This model is believed to estimate reliably the streamflow in the region using parameter values that can be computed from catchment characteristics. In the present study, it is applied on three gauged streams in the region of Western Ghats in Karnataka. Initially, the performance of the model has been studied with the parameters fixed using the catchment characteristics. Later, the model has been used as a tool to test hypotheses concerning the catchment response, by varying the parameter values, adopting a trial and error procedure. Initial results showed that the model performance is poor as the coefficients of efficiency vary between –66.9 and 82%. The sensitivity analysis carried out subsequently showed that the model parameters are required to be altered greatly for good performance and that the model simulations are not sensitive to the parameter CN. Further, the performance of this model was compared with that of a VSA model, known to suit the region well. This showed that even after all the changes in the model parameters, the model results are not highly reliable. Hence, in order to understand the reasons for the poor performance of the model, a technique was developed to compute the CN values that would be actually necessary to simulate daily direct runoff (DRO) reliably in this method, the daily values of CN are computed by applying backwards the expression for runoff on the DRO estimated by the VSA model. The variations in the values of CN computed using this method are then studied. It is found that the variations in daily CN are high and highly random too, whereas the NITK model uses only three fixed values of CN. It is thus concluded that factors other than those on which the CN is popularly believed to depend control the runoff generation in the region and that influence of LULC on runoff is not discernible at all from the kind of data that is commonly available.

Improved Process Representation in the Simulation of the Hydrology of a Meso-Scale Semi-Arid Catchment

The variability of rainfall and climate, combined with land use and land cover changes, and variation in geology and soils makes it a difficult task to accurately describe the key hydrological processes in a catchment. With the aim to better understand the key hydrological processes and runoff generation mechanisms in the semi-arid meso-scale Kaap catchment in South Africa, a hydrological model was developed using the open source STREAM model. Dominant runoff processes were mapped using a simplified Height Above the Nearest Drainage approach combined with geology. The Prediction in Ungauged Basins (PUB) framework of runoff signatures was used to analyse the model results. Results show that in the headwater sub-catchments of Noordkaap and Suidkaap, plateaus dominate, associated with slow flow processes. Therefore, these catchments have high baseflow components and are likely the main recharge zone for regional groundwater in the Kaap. In the Queens sub-catchment, hillslopes associated with intermediate and fast flow processes dominate. However, this catchment still has a strong baseflow component, but it seems to be more impacted by evaporation depletion, due to different soils and geology, especially in drier years. At the Kaap outlet, the model indicates that hillslopes are important, with intermediate and fast flow processes dominating and most runoff being generated through direct runoff and shallow groundwater components, particularly in wetter months and years. There is a high impact of water abstractions and evaporation during the dry season, affecting low flows in the catchment. Results also indicate that the root zone storage and the parameters of effective rainfall separation (between unsaturated and saturated zone), quickflow coefficient and capillary rise, were very sensitive in the model. The inclusion of capillary rise (feedback from the saturated to unsaturated zone) greatly improved the simulation results.

Exploring Controls on Rainfall‐Runoff Events: 1. Time Series‐Based Event Separation and Temporal Dynamics of Event Runoff Response in Germany

AbstractAnalyzing a response of catchments to rainfall inputs allows for deeper insights on the mechanisms of runoff generation at catchment scale. In this study an automated time series‐based event separation procedure consisting of available base flow separation, runoff event identification, and rainfall attribution methods and of a novel iterative procedure for the adjustment of thresholds used to identify single‐peak components of multiple‐peak events is proposed. Event runoff coefficient, time scale, rise time, and peak discharge of more than 220,000 identified rainfall‐runoff events are then used to analyze dynamics of event runoff response in 185 catchments at multiple temporal scales. In mountainous catchments with poor storage event runoff response is strongly controlled by the characteristics of rainfall and is generated by event‐fed saturation or infiltration excess. A distinct switch between saturated and unsaturated states occurs in these catchments. A weak relation between rainfall and runoff event properties is instead observed in lowland and hilly catchments with substantial storage, where a gradual transformation between functioning states occurs and the response is driven by preevent saturation. The seasonality of their event characteristics is governed by the contribution of snowmelt and the seasonality of the aridity index rather than of rainfall properties. Long‐term changes of total precipitation amount alone do not explain season‐specific long‐term changes of event characteristics that are rather consistent with changes of seasonal indicators of the wetness state. The effects of land use changes are detectable only in a few cases and display themselves mostly in the characteristic response time of catchments.

Regression Approaches for Hydrograph Separation: Implications for the Use of Discontinuous Electrical Conductivity Data

Understanding of runoff generation mechanisms affects the ability to manage streamflow quantity and quality issues. Concerning the baseflow in particular, measurements are almost never available and hydrograph separation is generally applied to characterize its relevant patterns. As an alternative to well-known recursive digital filters and mass balance filtering methods, this paper deals with the use of regression approaches, based on electrical conductivity measurements, as a proxy for total dissolved solids, to separate baseflow from total flow. Particular focus is placed on their flexibility and ability to adapt to discontinuous electrical conductivity data measurements. To illustrate this, we analyze a hydrochemical dataset collected from the Ciciriello experimental catchment (Southern Italy). The main findings are as follows: A comparative analysis suggests that the performance of regressive approaches in the case of daily electrical conductivity measurements is better than that of calibrated recursive digital filters. Weekly monitored electrical conductivity data led to performances comparable to the daily scale monitoring, and even monthly observation leads to a nonsignificant reduction in regression hydrograph filter performance; this shows how spot geochemical data monitoring may present valid and operational alternatives for characterization of baseflow in poorly gauged catchments.

Separation of Scales in Transpiration Effects on Low Flows: A Spatial Analysis in the Hydrological Open Air Laboratory.

The objective of this study was to understand whether spatial differences in runoff generation mechanisms affect the magnitudes of diurnal streamflow fluctuations during low flow periods and which part of the catchment induces the diurnal streamflow signal. The spatiotemporal variability of the streamflow fluctuations observed at 12 locations in the 66‐ha Hydrological Open Air Laboratory experimental catchment in Austria was explained by differences in the vegetation cover and runoff generation mechanisms. Almost a quarter of the volume associated with diurnal streamflow fluctuations at the catchment outlet was explained by transpiration from vegetation along the tributaries; more than three quarters was due to transpiration by the riparian forest along the main stream. The lag times between radiative forcing and evapotranspiration estimated by a solar radiation‐driven model increased from 3 to 11 hr from spring to autumn. The recession time scales increased from 21 days in spring to 54 days in autumn. Observations and model simulations suggest that a separation of scales in transpiration effects on low flows exists both in time and space; that is, the diurnal streamflow fluctuations are induced by transpiration from the riparian vegetation, while most of the catchment evapotranspiration, such as evapotranspiration from the crop fields further away from the stream, do not influence the diurnal signal in streamflow.