Runoff Generation Research Articles

Runoff generation, confluence mechanism, and water balance change of coal mining areas with goaf: Establishment of a runoff prediction model

The formation of goaf in coal mines changes the original hydrological processes. This study investigated the influence mechanism of goaf on surface runoff and constructed a model for predicting hydrological processes in mining areas with goaf. For this purpose, the control basin of Jingle Hydrology Station in Shanxi Province, which has experienced severe coal mining damage, was taken as the research area. First, based on field investigation, indoor simulation experiments of runoff generation and confluence mechanism in goaf were designed. Then, according to the experimental results, the runoff mechanism and water balance change in goaf were analyzed, runoff generation and confluence mechanism in goaf were parameterized and introduced into a hydrological model to simulate floods, and the water balance equation of goaf was introduced into the two-parameter monthly water balance model (MWBM) to simulate annual runoff. Finally, with 1951–2017 as the study period, 1990 was taken as the reference time for the appearance of goaf. By introducing generalized parameters to recalculate 12 flood processes after 1990, the qualification rate of the simulation results increased from 0 to 66.67%. By introducing the goaf water balance equation into MWBM to simulate the runoff process after 1990, the qualification rate of the simulation results of runoff and runoff peak flow increased from 22.22 % to 55.56 %. The introduction of parameters and water balance equation effectively improved the simulation accuracy of the model for predicting floods and annual runoff after large-scale coal mining.

Intense rainfall and debris flows in the Lomond Hills, Fife, 11–12 August 2020

ABSTRACT Over the night of 11–12th August 2020, unusually intense convective rainfall triggered several debris flows along the Lomond Hills escarpment. Rainfall intensities locally exceeded an estimated 0.33% annual exceedance probability. Each debris flow had a different magnitude and physical character depending on the availability of water and sediment and the effectiveness of the vegetation buffer, such that similar-looking micro-catchments responded in different ways. The largest debris flow far exceeded the others in magnitude, extending over 1 km with a descent of 246 m and an estimated volume of c. 1500–3000 m3, causing damage to a forestry road. Debris was entrained from a gullied relict talus, including fallen trees and incision of Lateglacial glaciofluvial sand. Deposit sedimentology and morphology demonstrate an initial debris-flow surge probably happened early in the storm coinciding with the greatest runoff generation, followed by later fluvial incision and sediment reworking. This appears to be the largest such event in the Lomond Hills for more than 90 years and may be characteristic of the landscape response to projected increases in convective rainfall intensities in twenty-first century summers.

Quantifying the impact of landscape changes on hydrological variables in the alpine and cold region using hydrological model and remote sensing data

AbstractQuantifying the impact of landscape on hydrological variables is essential for the sustainable development of water resources. Understanding how landscape changes influence hydrological variables will greatly enhance the understanding of hydrological processes. Important vegetation parameters are considered in this study by using remote sensing data and VIC‐CAS model to analyse the impact of landscape changes on hydrology in upper reaches of the Shule River Basin (URSLB). The results show there are differences in the runoff generation of landscape both in space and time. With increasing altitude, the runoff yields increased, with approximately 79.9% of the total runoff generated in the high mountains (4200–5900 m), and mainly consumed in the mid‐low mountain region. Glacier landscape produced the largest runoff yields (24.9% of the total runoff), followed by low‐coverage grassland (LG; 22.5%), alpine cold desert (AL; 19.6%), mid‐coverage grassland (MG; 15.6%), bare land (12.5%), high‐coverage grassland (HG; 4.5%) and shrubbery (0.4%). The relative capacity of runoff generation by landscapes, from high to low, was the glaciers, AL, LG, HG, MG, shrubbery and bare land. Furthermore, changes in landscapes cause hydrological variables changes, including evapotranspiration, runoff and baseflow. The study revealed that HG, MG, and bare land have a positive impact on evapotranspiration and a negative impact on runoff and baseflow, whereas AL and LG have a positive impact on runoff and baseflow and a negative impact on evapotranspiration. In contrast, glaciers have a positive impact on runoff. After the simulation in four vegetation scenarios, we concluded that the runoff regulation ability of grassland is greater than that of bare land. The grassland landscape is essential since it reduced the flood peak and conserved the soil and water.

Identifying dominant component of runoff yield processes: a case study in a sub-basin of the middle Yellow River

Abstract The effects of long-term natural climate change and human activities on runoff generation mechanism in the middle Yellow River Basin are long-standing concerns. This study analyzed the characteristics of hydro-climatic variables in the meso-scale Tuweihe catchment based on the observed data for the period 1956–2016 and a climate elastic method. The spatial distribution of dominant runoff processes (DRP) following land use changes in case of rainfall was identified. The results show significant decreasing trends in annual runoff, whereas slightly downward trends are identified for annual precipitation and potential evapotranspiration, 1984 is detected as the mutation year of the study period. The average contributions of climate change and human activities to the runoff reduction in the Tuweihe catchment were 33.2% and 66.8%, respectively. In general, the influences of human activities on runoff are applied mostly through the alteration of the catchment characteristics. The dominant runoff processes changes between 1980 and 2015 show significant effects of large-scale soil and water conservation measures in the Tuweihe catchment. We found that Hortonian overland flow (HOF) and fast subsurface flow (SSF1) were the two main processes in 1980 (30.3% and 34.4% respectively), but the proportion of HOF decreased by 9.6% in 2015. The proportions of saturation overland flow (SOF) and SSF have increased to varying degrees, which means that the catchment is more prone to generate subsurface flow processes. Consequently, under similar rainfall conditions, the runoff yield of flood events decreases in the second period.

What affects the hydrological response of rain-on-snow events in low-altitude mountain ranges in Central Europe?

Rain-on-snow (ROS) events influence the hydrological regime of rivers in regions with seasonal snow cover. Although ROS events are often related to floods, they do not always cause severe runoff. During ROS, the snowpack has an ambiguous effect on runoff generation; it can either store a significant portion of rain or amplify runoff by additional snowmelt. There is a need to understand under what circumstances ROS events produce runoff. We analysed eleven years of hourly meteorological, snow water equivalent and streamflow data from 15 catchments located in two mountain ranges in Czechia. We identified 611 ROS events which were further analysed and classified using selected meteorological, snow and runoff indices. The analysis of the runoff response of all ROS events revealed that only 5% of them resulted in high runoff exceeding the 1-year return period, but most of the events (82%) did not cause any significant runoff increase. Employing self-organising maps enabled us to categorise the events and better understand what combination of hydrometeorological characteristics leads to various runoff responses. High volumes of rain together with low snow cover were identified as important factors in the generation of high runoffs. In contrast, a deep and extended snowpack affected by rain under low air temperatures usually caused lower runoffs. The results of this study showed the importance of the snowpack, which can often prevent extreme runoff even when a large amount of rainfall occurs. Understanding the hydrological regime of ROS is becoming even more important with the ongoing decline of the snowfall fraction and subsequent changes in snow storage.

The contributions of the roots, stems, and leaves of three grass species to water erosion reduction on spoil heaps

Soil erosion on spoil heaps is one of the global environmental issues, where grass recovery is often implemented. However, information regarding how the erosion processes and control efficiency on spoil heaps differ as functions of grass types and components is lacking. In this study, a series of artificial rainfalls (48, 72, and 108 mm h−1) were conducted on eight plots (each one was replicated) that were established on steep spoil heaps and treated with three different grass types (Artemisia gmelinii (AG), Medicago sativa (MS), and Cynodon dactylon (CD)), with measurements of the runoff generation, sediment yield, and runoff velocity. Bare plot (BS) was set up as a control. Three scenarios regarding erosion efficiencies that by intact grass (IG), a combination of grass stems and roots (NL), and only grass root (OR), were considered. The results showed: 1) grass recovery can significantly reduce soil loss rate (SLR) on spoil heaps up to 99%, and there was no significant difference in erosion reduction efficiency between grasses with a taproot (AG: 93%) and fibrous root (MS: 92%; CD: 89%) systems under the IG treatments (P > 0.05); 2) The effect of the different grass treatments on SLR reduction depended on the rainfall intensity. The contribution rate of roots to SLR reduction was 68%–90% at a rainfall intensity of 48 mm h−1, while the stem and leaf contribution rates were 39%–156% at intensities of 72 and 108 mm h−1, respectively. While incomplete grass even increased the SLR, especially in the OR treatment, where the contribution rate of root to SLR reduction was −2%– −68%. 3) The IG treatment reduced soil erodibility (Kr) by 81.43%–83.80%, while the Kr values of the OR treatment were 1.51–3.53 times greater, as compared to the BS plots. In most cases, the critical shear stress (τc) on the grass plots was 1.09–2.14 times greater than those in the BS plots. These results would guide the selection of grass species for controlling spoil heaps erosion, also highlight the necessity of maintaining grass to maximize its control efficiency.

Hydraulic and nutrient removal performance of vegetated filter strips with engineered infiltration media for treatment of roadway runoff

As a new strategy for treating excess nutrients in roadway runoff, a self-filtering roadway could be accomplished by including engineered infiltration media within a vegetated filter strip (VFS) located in the roadway shoulder. However, nutrient removal performance will depend on the design to effectively infiltrate roadway runoff and the capacity of subsurface media to sequester or remove nutrients from infiltrated runoff. The objective of this study is to test hydraulic and nutrient removal performance of a roadside VFS over varied rainfall-runoff event sizes and filter widths. Two identical 1:1 scale physical models of roadway shoulders and embankments, one containing engineered media (Treatment model) and the other without (Control model), were tested with simulated rainfall and runoff from 1- and 2-lane roadways. Overall, 32 paired hydraulic experiments and 28 paired nutrient removal experiments were completed to assess performance across frequent and extreme rainfall-runoff events. The results indicate that scalability of performance with filter width varied by parameter. Runoff generation scaled predictably with filter width, as runoff generated close to the pavement and total infiltration increased with filter length. A 6 m-wide VFS containing the engineered media infiltrated all rainfall-runoff except during the most extreme storm events (1-h storms of 76.2 mm and 50.8 mm), where respectively 35% and 22% of rainfall-runoff did not infiltrate and left the system as surface runoff. A majority of phosphorus was retained within a 1.5 m filter while nitrate removal was not observed until 6 m. The Treatment model strongly outperformed the Control model with respect to nitrate (arithmetic mean ± standard deviation of 94 ± 6% reduction vs. 23 ± 64% increase, p < .001) and total nitrogen removal (80 ± 5% vs. 38 ± 23% reduction, p < .001) due to higher rates of microbially-mediated denitrification in the Treatment model. The two models performed comparably with regard to phosphorus reduction (84 ± 9% vs. 82 ± 12% reduction). A minimum 6 m filter width is recommended to ensure sufficient infiltration of runoff and nitrogen removal. Results of this study address uncertainty regarding nutrient removal performance of VFS in urban runoff applications and highlight a potential strategy for standardizing VFS performance across varied soil properties by including engineered media within the filter.

The Waterlogging Process Model in the Paddy Fields of Flat Irrigation Districts

Flat, low-lying agricultural areas such as irrigation districts in southern China have been increasingly vulnerable to flood inundation disasters because of the increased runoff associated with urbanization and climate change. In this study, we developed a waterlogging process simulation model comprising two parts: runoff generation module and runoff confluence module. An improved tank model and hydrodynamic model based on Saint–Venant equations were adopted in the runoff generation and confluence module, respectively. The results show that the model’s relative error and root mean square error are 2.1% and 0.17 mm/h, and the Nash coefficient of the model is 0.91. The relative error of river level simulation was within 5%, and the Nash coefficient was higher than 0.9. The proposed waterlogging simulation model could be a valuable tool for describing the process of waterlogging generation, accumulation, and confluence in the studied irrigation district or other regions with similar climatic conditions.

Contributions of Human Activities and Climatic Variability to Changes in River Rwizi Flows in Uganda, East Africa

This study employed Soil and Water Assessment Tool (SWAT) to analyze the impacts of climate variability and human activities on River Rwizi flows. Changes in land use and land cover (LULC) types from 1997 to 2019 were characterized using remotely sensed images retrieved from Landsat ETM/TM satellites. SWAT was calibrated and validated over the periods 2002–2008 and 2009–2013, respectively. Correlation between rainfall and river flow was analyzed. By keeping the optimal values of model parameters fixed while varying the LULC maps, differences in the modeled flows were taken to reflect the impacts of LULC changes on rainfall–runoff generation. Impacts due to human activities included contributions from changes in LULC types and the rates of water abstracted from the river as a percentage of the observed flow. Climate variability was considered in terms of changes in climatic variables such as rainfall and evapotranspiration, among others. Variability of rainfall was analyzed with respect to changes in large-scale ocean-atmosphere conditions. From 2000 to 2014, the portion of River Rwizi catchment area covered by cropland increased from 23.0% to 51.6%, grassland reduced from 63.3% to 37.8%, and wetland decreased from 8.1% to 4.7%. Nash–Sutcliffe Efficiency values for calibration and validation were 0.60 and 0.71, respectively. Contributions of human activities to monthly river flow changes varied from 2.3% to 23.5%. Impacts of human activities on the river flow were on average found to be larger during the dry (14.7%) than wet (5.8%) season. Using rainfall, 20.9% of the total river flow variance was explained. However, climate variability contributed 73% of the river flow changes. Rainfall was positively and negatively correlated with Indian Ocean Dipole (IOD) and Niño 3, respectively. The largest percentages of the total rainfall variance explained by IOD and Niño 3 were 12.7% and 9.8%, respectively. The magnitude of the correlation between rainfall and IOD decreased with increasing lag in time. These findings are relevant for developing River Rwizi catchment management plans.

Global Isotopic Hydrograph Separation Research History and Trends: A Text Mining and Bibliometric Analysis Study

Scientific research into isotope hydrograph separation (IHS) has rapidly increased in recent years. However, there is a lack of systematic and quantitative research to explore how this field has evolved over time. In this study, the methods of text mining and bibliometric analysis were combined to address this shortcoming. The results showed that there were clear periodical characteristics in IHS studies between 1986 and 2019. High-frequency words, e.g., catchment, stable isotope, runoff, groundwater, precipitation, runoff generation, and soil, were the basic topics in IHS studies. Forest and glacier/snow were the main landscapes in this research field. ‘Variation’, ‘spatial’, and ‘uncertainty’ are hot issues for future research. Today, studies involving the geographical source, flow path, and transit/residence time of streamflow components have enhanced our understanding of the hydrological processes by using hydrometeorological measurements, water chemistry, and stable isotope approaches. In the future, new methods, such as path analysis and ensemble hydrograph separation, should be verified and used in more regions, especially in remote and mountainous areas. Additionally, the understanding of the role of surface water in streamflow components remains limited and should be deeply studied in the future.

Modeling hydrologic responses using multi-site and single-site rainfall generators in a semi-arid watershed

Hydrologic response in a watershed is driven by precipitation. Multi-site rainfall generators can be used to model watersheds using spatially varied rainfall inputs to better analyze how the rainfall variability affects runoff generation. This study adopted both a single-site rainfall generator (CLIGEN) and a multi-site rainfall generator to generate two rainfall data sequences, which were then used to drive the Soil and Water Assessment Tool (SWAT) for runoff simulation. The 148-km2 Walnut Gulch Experimental Watershed and its two sub-watersheds were selected to evaluate the hydrologic response. Runoff calibration was done against measured runoff in the watershed. Statistics showed that the single-site and multi-site rainfall generators gave similar results regarding annual precipitation. However, the multi-site generator performed much better than the single-site generator in both mean summer flow and for the different return period flows. The runoff derived from the single-site generator was significantly over-estimated in all three watersheds. As for the multi-site generator, the derived runoff was satisfactorily predicted in the smaller watersheds but only overestimated in the largest watershed. This indicated that in small to medium sized watersheds, the spatial variability of rainfall could play an important role for hydrologic response because of the heterogeneity of convective rainfall in this semi-arid region, which makes the application of multi-site rainfall generator a better option than the single-site generator.

Responses of Runoff and Soil Loss on Slopes to Land Use Management and Rainfall Characteristics in Northern China.

Soil conservation measures are widely used to control soil erosion and sediment loss; however, their proper usage relies on a deep understanding of the responses of runoff and sediment loss to land management and rainfall characteristics. In the present study, a long-term (2014–2020) monitored dataset derived from ten runoff plots in the upstream catchment of the Miyun Reservoir in Beijing, China, was used to study runoff and sediment loss responses to land use management and rainfall characteristics. The study results show that plots with no soil conservation measures had the highest runoff depth of 75 mm and suffered the highest sediment loss, at a rate of 3200 t km−2 yr−1. The terraced and vegetated plots generated lower runoff depths, with soil loss rates less than 213.0 t km−2 yr−1. With the exception of the contour tillage plots on steep slopes, the vegetation and engineering measures can efficiently reduce runoff and sediment loss, with both runoff and sediment reduction efficiencies higher than 76%. Statistical analyses indicate that, on the plots of bare soil and cultivation without soil conservation measures, runoff and sediment loss were mainly affected by the maximum 30 min rainfall intensity. However, on the plots with soil conservation measures, they were mainly determined by rainfall amount and duration. The sediment loss rate can be well fitted with the runoff depth using a power function. Based on the analyses, water-saving soil conservation measures are recommended for the study area. In addition, the size of terraces should be reconsidered on gentle slopes, and the coverage of forest, shrubs, and grass on slopes should be reduced, thus allowing for more surface runoff generation to ensure drinking water safety. In general, for the study area, soil conservation measures are required on the bare soil and cultivated slopes.

Efficient Hazard Assessment for Pluvial Floods in Urban Environments: A Benchmarking Case Study for the City of Berlin, Germany

The presence of impermeable surfaces in urban areas hinders natural drainage and directs the surface runoff to storm drainage systems with finite capacity, which makes these areas prone to pluvial flooding. The occurrence of pluvial flooding depends on the existence of minimal areas for surface runoff generation and concentration. Detailed hydrologic and hydrodynamic simulations are computationally expensive and require intensive resources. This study compared and evaluated the performance of two simplified methods to identify urban pluvial flood-prone areas, namely the fill–spill–merge (FSM) method and the topographic wetness index (TWI) method and used the TELEMAC-2D hydrodynamic numerical model for benchmarking and validation. The FSM method uses common GIS operations to identify flood-prone depressions from a high-resolution digital elevation model (DEM). The TWI method employs the maximum likelihood method (MLE) to probabilistically calibrate a TWI threshold (τ) based on the inundation maps from a 2D hydrodynamic model for a given spatial window (W) within the urban area. We found that the FSM method clearly outperforms the TWI method both conceptually and effectively in terms of model performance.

Soil Moisture Responses to Rainfall: Implications for Runoff Generation

AbstractSoil moisture is a key control on runoff generation and biogeochemical processes on hillslopes. Precipitation events can evoke different soil moisture responses with depth through the soil profile, and responses can differ among landscape positions along a hillslope. We sought to elucidate the nature of these responses by estimating changes in water content, response time between peak precipitation and peak soil moisture, and wetting front velocities for 43 storms at 45 locations on three adjacent hillslopes within a headwater catchment of the southern Appalachian Mountains (NC, USA). We used a multivariate modeling approach to quantify the relative influences and the predictability of soil moisture responses by a combination of landscape and storm characteristics. We quantified the lag correlations between hillslope mean soil moisture and catchment runoff to demonstrate how storm properties and hillslope‐scale characteristics may influence runoff at the catchment outlet. Soil moisture responses varied widely, and no consistent patterns were observed among response metrics laterally or vertically along hillslopes. In contrast to other studies, we found that the relative influence of hillslope properties and storm characteristics varied with soil moisture responses and during storms. Antecedent conditions and storm depths influenced the strength of lag correlations between soil moisture and runoff, whereas storm mean intensity was correlated with the lag times. These results highlight the utility of intensive observations for characterizing heterogeneity in soil moisture responses, suggesting, among other things, a need for better representation of the subsurface processes in rainfall‐runoff models. Identifying the relative importance of drivers can be beneficial in building parsimonious hydrological models.

The sensitivity of runoff generation to spatial snowpack uniformity in an alpine watershed: Green Lakes Valley, Niwot Ridge Long‐Term Ecological Research station

AbstractSeasonal water storage in high‐elevation alpine catchments are critical sources of water for mountainous regions like the western U.S. The spatial distribution of snow in these topographically complex catchments is primarily governed by orography, solar radiation, and wind redistribution. While the effect of solar shading is relatively consistent from year‐to‐year, the redistribution of snow due to wind is more variable – capable of producing snowpacks that have varying degrees of uniformity across these hydrologically‐important catchments. A reasonable hypothesis is that a warmer climate will cause snowfall to become more dense (i.e. wetter and heavier), possibly leading to less wind redistribution and thus produce a more uniformly distributed snowpack across the landscape. In this study, we investigate the role of increasingly uniform spatial snowpack distributions on streamflow generation in the Green Lakes Valley Niwot Ridge Long Term Ecological Research station, within the headwaters of the Boulder Creek watershed in Colorado. A set of idealized hydrologic simulation experiments driven by reconstructed snowpacks spanning 2001–2014 show that more a more uniform spatial snowpack distribution leads to an earlier melt‐out of 31 days on average and tends to produce less total streamflow, with maximum decreases as large as 7.5%. Isolating the role of snowpack heterogeneity from melt‐season precipitation, we find that snowpack uniformity reduces total streamflow by as much as 13.2%. Reductions in streamflow are largely explained by greater exposure to solar radiation in the uniformly distributed case relative to a more heterogeneous snowpack, with this exposure driving shifts towards earlier snowmelt and changes in soil water storage. Overall, we find that the runoff efficiency from shallower snowpacks is more sensitive to the effects of uniformity than deeper snowpacks, which has potential implications for a warming climate where shallower snowpacks and enhanced sensitivities may be present.

Evaluation of Different Runoff Curve Number (CN) Approaches on Water Regulation Services Assessment in Intermittent Micro Catchment Dominated by Oil Palm Plantation

Surface runoff is a primary driving factor for water regulation services on oil palm plantations as it determines the hydrological components and other biogeochemical process. Therefore, understanding on their interaction and contribution within the watershed system is important to support decision-making system. Here, we applied Soil and Water Assessment Tools (SWAT) model to simulate water regulation services for an intermittent micro-catchment dominated by oil palm plantation in Harapan Landscapes, Batanghari Regency, Jambi Province. In this study, we used two different runoff curve number (CN) approaches in the SWAT model, namely the soil moisture curve number (CN-SM) and the plant evaporation curve number (CN-ET), to evaluate their applicability and uncertainty for assessing water regulation services. SWAT was automatically calibrated and validated against daily observed streamflow data. The results showed that the model performed well as indicated by hydrograph visual interpretation and statistical indicators. The performance was good for calibration and validation for both approaches with high R2 and Nash-Sutcliffe Efficiency (NSE). Also, the uncertainty was acceptable with P-factor &gt;70% and R-factor &lt;1. Differences in CN-SM and CN-ET's conceptual structure have caused variations in the calibrated parameters' best-fit value and their sensitivity to streamflow simulations, which implicated for other components' output water regulation services. However, CN-ET approach was less responsive to area's biophysical conditions for runoff generation than CN-SM one. This implicated that CN-ET generated low soil water storage and an overestimated actual evapotranspiration. This modeling exercise showed selection of a runoff CN approach by considering biophysical characteristics is important for calculating and simulating water balance component in such watershed. The accuracy of the simulation will significantly influence watershed management recommendations to improve water regulation's sustainability.

Characterizing shallow groundwater in hillslope aquifers using isotopic signatures: A case study in the Upper Blue Nile basin, Ethiopia

Study regionRobit-Bata watershed, Upper Blue Nile basin, Ethiopia. Study focusStable isotopes of water (Oxygen-18 and Deuterium) were used as tracers to estimate the contribution of groundwater in shallow hillslope aquifers to streamflow in the Robit-Bata watershed. To assess the spatiotemporal variability of shallow groundwater and develop a hydrograph separation technique, we collected rainfall, shallow groundwater, and streamflow samples and analyzed their δ18O and δ2H isotopic compositions. The local meteoric water line (LMWL) and local evaporative line (LEL) of the study area were determined and compared with the global meteoric water line (GMWL). A standard unweighted two-component isotope-based hydrograph separation model was used to determine the percentage contribution of shallow groundwater to streamflow. New hydrological insights for the regionThe LMWL (δ2H = 8.63·δ18O + 18.2) mostly showed heavy isotopic enrichment relative to GMWL, and the LEL (δ2H = 5.45·δ18O + 6.96) indicated isotopic enrichment compared to Ethiopian lakes. Shallow groundwater responded rapidly to rainfall, with good spatial correlation depending on topographic positions of wells. Pre-event water contributed <50% to peak discharge in July, but >90% when the watershed reached maximum storage. This finding gives insight towards the predominant runoff generation process and has significant implications for sustainable dry season irrigation expansion in the area as the sub-surface flow drains out of the watershed from October onwards reducing water tables in the shallow wells.

Post-fire temporal trends in soil-physical and -hydraulic properties and simulated runoff generation: Insights from different burn severities in the 2013 Black Forest Fire, CO, USA

Burn severity influences on post-fire recovery of soil-hydraulic properties controlling runoff generation are poorly understood despite the importance for parameterizing infiltration models. We measured soil-hydraulic properties of field-saturated hydraulic conductivity (Kfs), sorptivity (S), and wetting front potential (ψf) for four years after the 2013 Black Forest Fire, Colorado, USA, at six sites across a gradient of initial remotely sensed burn severity using the change in the normalized burn ratio (dNBR). These measurements were correlated with soil-physical property measurements of bulk density (ρb), loss on ignition (LOI, a measure of soil organic matter), and ground cover composition to provide insight into causal factors for temporal changes in Kfs, S, and ψf. Modeled infiltration using the Smith-Parlange approach parameterized with measured Kfs, S, and ψf further discerned the role of precipitation intensity on runoff generation.Temporal trends of soil-physical properties and ground cover showed influences from initial burn severity. Trends in soil-hydraulic properties, surprisingly, were not strongly influenced by initial burn severity despite inferred effects of ρb, LOI, and ground cover on trends in Kfs and S. Calculations of dNBR at the time of sampling showed strong correlations with Kfs and S, demonstrating a new approach for estimating long-unburned Kfs and S values, infiltration model parameters after fire, and assessing the time of return to pre-fire values. Simulated infiltration-excess runoff, in contrast, did depend on initial burn severity. Time series of the ratio S2/Kfs ≈ ψf tended to converge between 1 and 10 mm four years after wildfire, potentially (i) defining a long-unburned forest domain of S2/Kfs and ψf from 1 to 10 mm with relatively high Kfs values, and (ii) providing a new post-fire soil-hydraulic property recovery metric (i.e. S2/Kfs ≈ ψf in the range of 1 to 10 mm) for sites in the Rocky Mountains of the USA.

A new urban hydrological model considering various land covers for flood simulation

Rapid urbanization has great potential to adversely impact the local hydrological cycle, the environment, and ecosystems, as well as trigger severe problems, such as urban flooding, waterlogging, and water contamination. Understanding runoff generation mechanisms in urban areas helps identify the impacts of urbanization on runoff response and better simulate urban floods. However, in most urban hydrological models, urban surfaces are coarsely classified into impervious and pervious ones, which likely ignores differential rainfall-runoff responses of different urban surfaces and leads to large discrepancies between simulated and observed runoff. Here we developed the TVGM_Urban model, a new urban hydrological model based on the time variant gain model (TVGM), that represents nonlinear rainfall-runoff relationships for different urban surfaces. We applied the model to the Fenghuangcheng region of Shenzhen City, China and compared the results to those of two commonly used urban hydrological models: the Horton infiltration and Soil Conservation Service Curve Number (SCSCN) model. For each of these models, we conducted model uncertainty analysis using the GLUE method. The results showed that the TVGM_Urban model outperformed the other two models in terms of total runoff and peak flow. This could be due to the consideration of different land covers and both saturation excess and infiltration excess runoff generation in the TVGM_Urban model. In addition, the uncertainty analysis indicated better performance of the TVGM_Urban model in reducing structural uncertainty and prediction uncertainty. This study highlights the need to account for detailed land covers and different runoff generation mechanisms in urban hydrological modeling.

Channel head extraction based on fuzzy unsupervised machine learning method

Channel head extraction is fundamental to understand the catchment hydrological processes, catchment origin, runoff generation and landscape evolution. A spatially constant threshold value such as upslope area, slope-area, and curvature have been widely used for channel head extraction because of their simplicity and efficiency. However, it is very difficult to determine the threshold values within and between catchments with different topography, soil types, and land cover. In this study, a fuzzy unsupervised machine learning method (e.g., the fuzzy c-means clustering) is introduced to determine the channel head locations to avoid the need for a threshold parameter. The topographic attributes (e.g., upslope area, slope, curvature, and elevation) and slope-area combined attributes (e.g., AS2 and S/ln(A)) are used as input variables. The sensitivity of surface terrain resolution is also analyzed in terms of the proposed method's ability to predict channel heads. Two catchments with field mapped channel heads, namely Indian Creek and Mid Bailey Run in Ohio, are selected for investigation and comparison. The proposed method performs well in terms of locating the channel head. The accuracy of identified channel heads from the proposed method is comparable to published state-of-the-art channel extraction methods. Meanwhile, the proposed method has a low computational burden. Our findings also reveal the comprehensive impact of topographic attributes in locating channel heads.