Hydrological responses to rainfall events including the extratropical cyclone Gloria in two contrasting Mediterranean headwaters in Spain; the perennial font del Regàs and the intermittent Fuirosos

Catchment hydrological responses to precipitation inputs, particularly during exceptionally large storms, are complex and variable, and our understanding of the associated runoff generation processes during those events is limited. Hydrological monitoring of climatically and hydrologically distinct catchments can help to improve this understanding by shedding light on the interplay between antecedent soil moisture conditions, hydrological connectivity, and rainfall event characteristics. This knowledge is urgently needed considering that both the frequency and magnitude of extreme precipitation events are increasing worldwide as a consequence of climate change. In autumn 2018, we installed water level sensors to monitor stream water and near-stream groundwater levels at two Mediterranean forest headwater catchments with contrasting hydrological regimes: Font del Regàs (sub-humid climate, perennial flow regime) and Fuirosos (semi-arid climate, intermittent flow regime). Both catchments are located in northeastern Spain, where the extratropical cyclone Gloria hit in January 2020 and left in ca. 65 h outstanding accumulated rainfalls of 424 mm in Font del Regàs and 230 mm in Fuirosos. During rainfall events of low mean intensity, hydrological responses to precipitation inputs at the semi-arid Fuirosos were more delayed and more variable than at the sub-humid Font del Regàs. We explain these divergences by differences in antecedent soil moisture conditions and associated differences in catchment hydrological connectivity between the two catchments, which in this case are likely driven by differences in local climate rather than by differences in local topography. In contrast, during events of moderate and high mean rainfall intensities, including the storm Gloria, precipitation inputs and hydrological responses correlated similarly in the two catchments. We explain this convergence by rapid development of hydrological connectivity independently of antecedent soil moisture conditions. The data set presented here is unique and contributes to our mechanistic understanding on how streams respond to rainfall events and exceptionally large storms in catchments with contrasting flow regimes.     

Bootstrap based artificial neural network (BANN) analysis for hierarchical prediction of monthly runoff in Upper Damodar Valley Catchment

Estimation of runoff is a prerequisite for many applications involving conservation and management of water resources. This study is undertaken in the Upper Damodar Valley Catchment (UDVC) having a drainage area of 17513.08 km² for prediction of monthly runoff. Thirty one microwatersheds and 15 sub-watersheds were selected from a total of 716 microwatersheds in the catchment area for this study. The feasibility of using different soil attributes (particle size distribution, organic matter content and apparent density), topographic attributes (primary, secondary and compound), geomorphologic attributes (basin, relief and network indices) and vegetation attribute as Normalized Difference Vegetation Index (NDVI), on prediction of monthly runoff were explored in this study. Principal Component Analysis (PCA) was applied to minimize the data redundancy of the input variables. Ten significant input variables namely; watershed length (km), elongation ratio, bifurcation ratio, area ratio, coarse sand (%), fine sand (%), elevation (m), slope (°), profile curvature (rad/m) and NDVI were selected. The selected input variables were added in hierarchy with monthly rainfall (mm) as inputs for prediction of monthly runoff (mm) using Bootstrap based artificial neural networks (BANN). The performance of the models was tested using Spearman’s correlation coefficient (r), coefficient of efficiency (COE), Root Mean Square Error (RMSE) and Mean Absolute Error (MAE). Best performance was observed for model with monthly rainfall, slope, coarse sand, bifurcation ratio and Normalized Difference Vegetation Index (NDVI) as inputs (r = 0.925 and COE = 0.839). Increase in number of input variables did not necessarily yield better performances of the BANN models. Selection of relevant inputs and their combinations were found to be key elements in determining the performance of BANN models. Annual runoff map was generated for all the microwatersheds utilizing the weights of the best performing BANN model. This study reveals that the specific combinations of soil, topography, geomorphology and vegetation inputs can be utilized for better prediction of monthly runoff.     

Linking runoff response to burn severity after a wildfire

Extreme floods often follow wildfire in mountainous watersheds. However, a quantitative relation between the runoff response and burn severity at the watershed scale has not been established. Runoff response was measured as the runoff coefficient C, which is equal to the peak discharge per unit drainage area divided by the average maximum 30 min rainfall intensity during each rain storm. The magnitude of the burn severity was expressed as the change in the normalized burn ratio. A new burn severity variable, hydraulic functional connectivity Φ was developed and incorporates both the magnitude of the burn severity and the spatial sequence of the burn severity along hillslope flow paths. The runoff response and the burn severity were measured in seven subwatersheds (0·24 to 0·85 km²) in the upper part of Rendija Canyon burned by the 2000 Cerro Grande Fire near Los Alamos, New Mexico, USA. A rainfall–discharge relation was determined for four of the subwatersheds with nearly the same burn severity. The peak discharge per unit drainage area was a linear function of the maximum 30 min rainfall intensity I₃₀. This function predicted a rainfall intensity threshold of 8·5 mm h^?1 below which no runoff was generated. The runoff coefficient was a linear function of the mean hydraulic functional connectivity of the subwatersheds. Moreover, the variability of the mean hydraulic functional connectivity was related to the variability of the mean runoff coefficient, and this relation provides physical insight into why the runoff response from the same subwatershed can vary for different rainstorms with the same rainfall intensity. Copyright © 2007 John Wiley & Sons, Ltd.     

Vegetation response to rainfall seasonality and interannual variability in tropical dry forests

Projected changes in rainfall seasonality and interannual variability are expected to have severe impacts on arid and semi‐arid tropical vegetation, which is characterized by a fine‐tuned adaptation to extreme rainfall seasonality. To study the response of these ecosystems and the related changes in hydrological processes to changes in the amount and seasonality of rainfall, we focused on the caatinga biome, the typical seasonally dry forest in semi‐arid Northeast Brazil. We selected four sites across a gradient of rainfall amount and seasonality and analysed daily rainfall and biweekly Normalized Difference Vegetation Index (NDVI) data for hydrological years 2000 to 2014. Rainfall seasonal and interannual statistics were characterized by recently proposed metrics describing duration, timing and intensity of the wet season and compared to similar metrics of NDVI time series. The results show that the caatinga tends to have a more stable response with longer and less variable growing seasons (3.1 ± 0.1 months) compared to the duration wet seasons (2.0 ± 0.5 months). The ecosystem ability to buffer the interannual variability of rainfall is also evidenced by the stability in the timing of the growing season compared to the wet season, which results in variable delays (ranging from 0 to 2 months) between the peak of the rainfall season and the production of leaves by the ecosystem. The analyses show that the shape and size of the related hysteresis loops in the rainfall–NDVI relations are linked to the buffering effects of soil moisture and plant growth dynamics. Finally, model projections of vegetation response to different rainfall scenarios reveal the existence of a maximum in ecosystem productivity at intermediate levels of rainfall seasonality, suggesting a possible trade‐off in the effects of intensity (i.e. amount) and duration of the wet season on vegetation growth and related soil moisture dynamics and transpiration rates. Copyright © 2016 John Wiley & Sons, Ltd.     

Introduction to special issue on connectivity in water and sediment dynamics

Connectivity has emerged in recent years as a significant conceptual framework within which to address the spatial and temporal variability in runoff and sediment transport. This special issue draws together several of the papers that were presented in the session “Connectivity in water and sediment dynamics: how do we move forwards?” at the 2012 General Assembly of the European Geosciences Union in Vienna, Austria. The papers submitted for this special issue fall into three groups: empirical studies, modelling studies, and conceptual development. Although it may be evident that the concept of connectivity helps us to express the complexity (in terms of water and sediment yields) of landscape responses to rainfall inputs, does it improve our ability to understand or predict those responses? There would still seem to be some way to go in connectivity research before this nagging concern can be assuaged. That it can be will undoubtedly be an important task for a number of ongoing research initiatives. Copyright © 2015 John Wiley & Sons, Ltd.     

Investigating hydrologic connectivity and its association with threshold change in runoff response in a temperate forested watershed

Hydrological studies across varied climatic and physiographic regions have observed small changes in the ‘states of wetness’; based on average soil moisture, can lead to dramatic changes in the amount of water delivered to the stream channel. This non-linear behaviour of the storm response has been attributed to a critical switching in spatial organization of shallow soil moisture and hydrologic connectivity. However, much of the analysis of the role of soil moisture organization and connectivity has been performed in small rangeland catchments. Therefore, we examined the relationship between hydrologic connectivity and runoff response within a temperate forested watershed of moderate relief. We have undertaken spatial surveys of shallow soil moisture over a sequence of storms with varying antecedent moisture conditions. We analyse each survey for evidence of hydrologic connectivity and we monitor the storm response from the catchment outlet. Our results show evidence of a non-linear response in runoff generation over small changes in measures of antecedent moisture conditions; yet, unlike the previous studies of rangeland catchments, in this forested landscape we do not observe a significant change in geostatistical hydrologic connectivity with variations in antecedent moisture conditions. These results suggest that a priori spatial patterns in shallow soil moisture in forested terrains may not always be a good predictor of critical hydrologic connectivity that leads to threshold change in runoff generation, as has been the case in rangeland catchments. Copyright © 2007 John Wiley & Sons, Ltd.     

Changes in ecosystem structure,function and hydrological connectivity control water,soil and carbon losses in semi‐arid grass to woody vegetation transitions

Connectivity has recently emerged as a key concept for understanding hydrological response to vegetation change in semi‐arid environments, providing an explanatory link between abiotic and biotic, structure and function. Reduced vegetation cover following woody encroachment, generally promotes longer, more connected overland flow pathways, which has the potential to result in an accentuated rainfall‐runoff response and fluxes of both soil erosion and carbon. This paper investigates changing hydrological connectivity as an emergent property of changing ecosystem structure over two contrasting semi‐arid grass to woody vegetation transitions in New Mexico, USA. Vegetation structure is quantified to evaluate if it can be used to explain observed variations in water, sediment and carbon fluxes. Hydrological connectivity is quantified using a flow length metric, combining topographic and vegetation cover data. Results demonstrate that the two woody‐dominated sites have significantly longer mean flowpath lengths (4 · 3 m), than the grass‐dominated sites (2 · 4 m). Mean flowpath lengths illustrate a significant positive relationship with the functional response. The woody‐dominated sites lost more water, soil and carbon than their grassland counterparts. Woody sites erode more, with mean event‐based sediment yields of 1203 g, compared to 295 g from grasslands. In addition, the woody sites lost more organic carbon, with mean event yields of 39 g compared to 5 g from grassland sites. Finally, hydrological connectivity (expressed as mean flowpath length) is discussed as a meaningful measure of the interaction between structure and function and how this manifests under the extreme rainfall that occurs in semi‐arid deserts. In combination with rainfall characteristics, connectivity emerges as a useful tool to explain the impact of vegetation change on water, soil and carbon losses across semi‐arid environments. Copyright © 2013 John Wiley & Sons, Ltd.     

Connectivity of post-fire runoff and sediment from nested hillslopes and watersheds

Wildfire increases the potential connectivity of runoff and sediment throughout watersheds due to greater bare soil, runoff and erosion as compared to pre-fire conditions. This research examines the connectivity of post-fire runoff and sediment from hillslopes (< 1.5 ha; n = 31) and catchments (< 1000 ha; n = 10) within two watersheds (< 1500 ha) burned by the 2012 High Park Fire in northcentral Colorado, USA. Our objectives were to: (1) identify sources and quantify magnitudes of post-fire runoff and erosion at nested hillslopes and watersheds for two rain storms with varied duration, intensity and antecedent precipitation; and (2) assess the factors affecting the magnitude and connectivity of runoff and sediment across spatial scales for these two rain storms. The two summer storms that are the focus of this research occurred during the third summer after burning. The first storm had low intensity rainfall over 11 hours (return interval <1–2 years), whereas the second event had high intensity rainfall over 1 hour (return interval <1–10 years). The lower intensity storm was preceded by high antecedent rainfall and led to low hillslope sediment yields and channel incision at most locations, whereas the high intensity storm led to infiltration-excess overland flow, high sediment yields, in-stream sediment deposition and channel substrate fining. For both storms, hillslope-to-stream sediment delivery ratios and area-normalised cross-sectional channel change increased with the percent of catchment that burned at high severity. For the high intensity storm, hillslope-to-stream sediment delivery ratios decreased with unconfined channel length (%). The findings quantify post-fire connectivity and sediment delivery from hillslopes and streams, and highlight how different types of storms can cause varying magnitues and spatial patterns of sediment transport and deposition from hillslopes through stream channel networks.     

How does DEM resolution affect microtopographic characteristics,hydrologic connectivity,and modelling of hydrologic processes?

The resolution of a digital elevation model (DEM) is a crucial factor in watershed hydrologic and environmental modelling. DEM resolution can cause significant variability in the representation of surface topography, which further affects quantification of hydrologic connectivity and simulation of hydrologic processes. The objective of this study is to examine the effects of DEM resolution on (1) surface microtopographic characteristics, (2) hydrologic connectivity, and (3) the spatial and temporal variations of hydrologic processes. A puddle‐to‐puddle modelling system was utilized for surface delineation and modelling of the puddle‐to‐puddle overland flow dynamics, surface runoff, infiltration, and unsaturated flow for nine DEM resolution scenarios of a field plot surface. Comparisons of the nine modelling scenarios demonstrated that coarser DEM resolutions tended to eliminate topographic features, reduce surface depression storage, and strengthen hydrologic connectivity and surface runoff. We found that reduction in maximum depression storage and maximum ponding area was as high as 97.56% and 76.36%, respectively, as the DEM grid size increased from 2 to 80 cm. The paired t‐test and fractal analysis demonstrated the existence of a threshold DEM resolution (10 cm for the field plot), within which the DEM‐based hydrologic modelling was effective and acceptable. The effects of DEM resolution were further evaluated for a larger surface in the Prairie Pothole Region subjected to observed rainfall events. It was found that simulations based on coarser resolution DEMs (>10 m) tended to overestimate ponded areas and underestimate runoff discharge peaks. The simulated peak discharge from the Prairie Pothole Region surface reduced by approximately 50% as the DEM resolution changed from 2 to 90 m. Fractal analysis results elucidated scale dependency of hydrologic and topographic processes. In particular, scale analysis highlighted a unique constant–threshold–power relationship between DEM scale and topographic and hydrologic parameters/variables. Not only does this finding allow one to identify threshold DEM but also further develop functional relationships for scaling to achieve valid topographic characterization as well as effective and efficient hydrologic modelling. Copyright © 2016 John Wiley & Sons, Ltd.     

The importance of surface controls on overland flow connectivity in semi‐arid environments: results from a numerical experimental approach

In semi‐arid environments, the characteristics of the land surface determine how rainfall is transformed into surface runoff and influences how this runoff moves from the hillslopes into river channels. Whether or not water reaches the river channel is determined by the hydrological connectivity. This paper uses a numerical experiment‐based approach to systematically assess the effects of slope length, gradient, flow path convergence, infiltration rates and vegetation patterns on the generation and connectivity of runoff. The experiments were performed with the Connectivity of Runoff Model, 2D version distributed, physically based, hydrological model. The experiments presented are set within a semi‐arid environment, characteristic of south‐eastern Spain, which is subject to low frequency high rainfall intensity storm events. As a result, the dominant hydrological processes are infiltration excess runoff generation and surface flow dynamics. The results from the modelling experiments demonstrate that three surface factors are important in determining the form of the discharge hydrograph: the slope length, the slope gradient and the infiltration characteristics at the hillslope‐channel connection. These factors are all related to the time required for generated runoff to reach an efficient flow channel, because once in this channel, the transmission losses significantly decrease. Because these factors are distributed across the landscape, they have a fundamental role in controlling the landscape hydrological response to storm events. Copyright © 2013 John Wiley & Sons, Ltd.     

Spatial–temporal variability and hydrologic connectivity of runoff generation areas in a North Alabama pasture—implications for phosphorus transport

This study delineated spatially and temporally variable runoff generation areas in the Sand Mountain region pasture of North Alabama under natural rainfall conditions, and demonstrated that hydrologic connectivity is important for generating hillslope response when infiltration‐excess (IE) runoff mechanism dominates. Data from six rainfall events (13·7–32·3 mm) on an intensively instrumented pasture hillslope (0·12 ha) were analysed. Analysis of data from surface runoff sensors, tipping bucket rain gauge and HS‐flume demonstrated spatial and temporal variability in runoff generation areas. Results showed that the maximum runoff generation area, which contributed to runoff at the outlet of the hillslope, varied between 67 and 100%. Furthermore, because IE was the main runoff generation mechanism on the hillslope, the data showed that as the rainfall intensity changed during a rainfall event, the runoff generation areas expanded or contracted. During rainfall events with high‐intensity short‐ to medium‐duration, 4–8% of total rainfall was converted to runoff at the outlet. Rainfall events with medium‐ to low‐intensity, medium‐duration were found less likely to generate runoff at the outlet. In situ soil hydraulic conductivity (k) was measured across the hillslope, which confirmed its effect on hydrologic connectivity of runoff generation areas. Combined surface runoff sensor and k‐interpolated data clearly showed that during a rainfall event, lower k areas generate runoff first, and then, depending on rainfall intensity, runoff at the outlet is generated by hydrologically connected areas. It was concluded that in IE‐runoff‐dominated areas, rainfall intensity and k can explain hydrologic response. The study demonstrated that only connected areas of low k values generate surface runoff during high‐intensity rainfall events. Identification of these areas would serve as an important foundation for controlling nonpoint source pollutant transport, especially phosphorus. The best management practices can be developed and implemented to reduce transport of phosphorus from these hydrologically connected areas. Copyright © 2009 John Wiley & Sons, Ltd.     

Hydrological and sediment yield response to summer rainfall in a small high Arctic watershed

In cold regions, the response and related antecedent mechanisms that produce flood flows from rainfall events have received limited study. In 2007, a small watershed at Cape Bounty, Melville Island, Nunavut, was studied in detail during the melt season. Two rainfall events on June 30 and July 22, totalling 9·2 and 10·8 mm, respectively, represented significant contributions to seasonal discharge and sediment transport in a year with a low winter snowpack. The precipitation events elevated discharge and suspended sediment concentrations to twice the magnitude of the nival melt, and generated the only measurable downstream lacustrine turbidity current of the season. In two days, rainfall runoff transported 35% of the seasonal suspended sediment load, in contrast to 29% transported over the nival freshet. The magnitude and intensity of the rain events were not unusual in this setting, but the rainfall response was substantial in comparison with equivalent past events. Exceptional temperatures of July 2007 generated early, deep permafrost thaw, and ground ice melt. The resultant increase in soil moisture amplified the subsequent rainfall runoff and sediment transport response. These results demonstrate the importance of antecedent moisture conditions and the role of permafrost active layer development as an important factor in the rainfall runoff and sediment transport response to precipitation events. Copyright © 2009 John Wiley & Sons, Ltd.     

Impact of road‐generated storm runoff on a small catchment response

The impact of road‐generated runoff on the hydrological response of a zero‐order basin was monitored for a sequence of 24 storm events. The study was conducted in a zero‐order basin (C1; 0·5ha) with an unpaved mountain road; an adjacent unroaded zero‐order basin (C2; 0·2 ha) with similar topography and lithology was used to evaluate the hydrological behaviour of the affected zero‐order basin prior to construction of the road. The impact of the road at the zero‐order basin scale was highly dependent on the antecedent soil‐moisture conditions, total storm precipitation, and to some extent rainfall intensity. At the beginning of the monitoring period, during dry antecedent conditions, road runoff contributed 50% of the total runoff and 70% of the peak flow from the affected catchment (C1). The response from the unroaded catchment was almost insignificant during dry antecedent conditions. As soil moisture increased, the road exerted less influence on the total runoff from the roaded catchment. For very wet conditions, the influence of road‐generated runoff on total outflow from the roaded catchment diminished to only 5·4%. Both catchments, roaded and unroaded, produced equivalent amount of outflow during very wet antecedent conditions on a unit area basis. The lag time between the rainfall and runoff peaks observed in the unroaded catchment during the monitoring period ranged from 0 to 4 h depending on the amount of precipitation and antecedent conditions, owing mainly to much slower subsurface flow pathways in the unroaded zero‐order basin. In contrast, the lag time in the roaded zero‐order basin was virtually nil during all storms. Copyright © 2009 John Wiley & Sons, Ltd.     

Evaluation of infiltration‐based stormwater management to restore hydrological processes in urban headwater streams

Urbanization threatens headwater stream ecosystems globally. Watershed restoration practices, such as infiltration‐based stormwater management, are implemented to mitigate the detrimental effects of urbanization on aquatic ecosystems. However, their effectiveness for restoring hydrologic processes and watershed storage remains poorly understood. Our study used a comparative hydrology approach to quantify the effects of urban watershed restoration on watershed hydrologic function in headwater streams within the Coastal Plain of Maryland, USA. We selected 11 headwater streams that spanned an urbanization–restoration gradient (4 forested, 4 urban‐degraded, and 3 urban‐degraded) to evaluate changes in watershed hydrologic function from both urbanization and watershed restoration. Discrete discharge and continuous, high‐frequency rainfall‐stage monitoring were conducted in each watershed. These datasets were used to develop 6 hydrologic metrics describing changes in watershed storage, flowpath connectivity, or the resultant stream flow regime. The hydrological effects of urbanization were clearly observed in all metrics, but only 1 of the 3 restored watersheds exhibited partially restored hydrologic function. At this site, a larger minimum runoff threshold was observed relative to the urban‐degraded watersheds, suggesting enhanced infiltration of stormwater runoff within the restoration structure. However, baseflow in the stream draining this watershed remained low compared to the forested reference streams, suggesting that enhanced infiltration of stormwater runoff did not recharge subsurface storage zones contributing to stream baseflow. The highly variable responses among the 3 restored watersheds were likely due to the spatial heterogeneity of urban development, including the level of impervious cover and extent of the storm sewer network. This study yielded important knowledge on how restoration strategies, such as infiltration‐based stormwater management, modulated—or failed to modulate—hydrological processes affected by urbanization, which will help improve the design of future urban watershed management strategies. More broadly, we highlighted a multimetric approach that can be used to monitor the restoration of headwater stream ecosystems in disturbed landscapes.     

Assessing dominant factors affecting soil erosion using a portable rainfall simulator

Investigating the causes of soil erosion is difficult in natural conditions owing to the presence of other factors. Without simplifying the experimental conditions, studying soil behavior with its numerous parameters while considering factors such as vegetation cover, topography, and rainfall is difficult and in most conditions impossible. The application of simulation approaches is therefore necessary to simplify the prototype. In this research, the effects of physical soil factors such as texture and antecedent soil moisture, along with land slope and vegetation cover were evaluated in the Taleghan watershed, Iran, using a rainfall simulator and soil erosion plots. For this purpose, a 89 × 120 cm rainfall simulator producing 24.5 and 32 mm/h rainfall intensities of 30 min duration, as a common condition of the study area, was used at 144 locations over soil erosion plots with dimensions of 95 × 125 cm. Plots had slope classes of 12-20 and 20-30 %, different soil textures, different antecedent soil moistures, and medium to poor vegetation cover conditions. It was found that for 24.5 and 32 mm/h rainfall intensities, the sediment yield had high correlations of-0.771 and -0.796 with vegetation cover and slight correlations of 0.045 and 0.029 with land slope respectively. Regression equations for predicting the sediment yield were also developed for different conditions.     

Integrated erosion control measures and environmental effects in rocky mountainous areas in northern China

Based on observations of runoff plots and field investigations of gully cross-sections, impacts of various soil and water conservation measures on runoff and sediment yield are analyzed for different rainfall conditions. The results show that antecedent rainfall and rainfall intensity are the main factors affecting the runoff and soil erosion processes. Rainfall events with antecedent rainfall can produce high runoff and sediment yield. Large differences in the characteristics of two rainfall events will result in greater variations of total runoff and sediment yield from the same runoff plot. Under the same soil control measure and rainfall condition, soil and water conservation measures can reduce the impacts of antecedent rainfall and rainfall intensity on runoff and soil erosion. Among various measures, level terrace seems to be the greatest for soil conservation purposes. Combining with engineering measures,Vegetation measures is also effective in controlling runoff and soil erosion. In the initial stage of vegetation enclosure measures, engineering measure is necessary to improve the environment for ecological recovery. Gully head protection can control gully erosion effectively, but the effectiveness of gully head protection would be reduced when rainfall intensity increases. Therefore, the design of a gully head protection structure must be based on local hydrological conditions.     

Direct and indirect effects of rainfall and vegetation coverage on runoff,soil loss,and nutrient loss in a semi-humid climate

Soil and nutrient loss play a vital role in eutrophication of water bodies. Several simulated rainfall experiments have been conducted to investigate the effects of a single controlling factor on soil and nutrient loss. However, the role of precipitation and vegetation coverage in quantifying soil and nutrient loss is still unclear. We monitored runoff, soil loss, and soil nutrient loss under natural rainfall conditions from 2004 to 2015 for 50–100 m² runoff plots around Beijing. Results showed that soil erosion was significantly reduced when vegetation coverage reached 20% and 60%. At levels below 30%, nutrient loss did not differ among different vegetation cover levels. Minimum soil N and P losses were observed at cover levels above 60%. Irrespective of the management measure, soil nutrient losses were higher at high-intensity rainfall (Imax30>15 mm/h) events compared to low-intensity events (p < 0.05). We applied structural equation modelling (SEM) to systematically analyze the relative effects of rainfall characteristics and environmental factors on runoff, soil loss, and soil nutrient loss. At high-intensity rainfall events, neither vegetation cover nor antecedent soil moisture content (ASMC) affected runoff and soil loss. After log-transformation, soil nutrient loss was significantly linearly correlated with runoff and soil loss (p < 0.01). In addition, we identified the direct and indirect relationships among the influencing factors of soil nutrient loss on runoff plots and constructed a structural diagram of these relationships. The factors positively impacting soil nutrient loss were runoff (44%–48%), maximum rainfall intensity over a 30-min period (18%–29%), rainfall depth (20%–27%), and soil loss (10%–14%). Studying the effects of rainfall and vegetation coverage factors on runoff, soil loss, and nutrient loss can improve our understanding of the underlying mechanism of slope non-point source pollution.     

Hydrograph responses to geomorphic model watershed characteristics and precipitation variables

The primary objective of the Watershed Model Studies Project, reported herein, was to ascertain the effect of selected watershed characteristics on hydrograph parameters under a rainfall simulator. Since most of the runoff contributing to the peak flow was found to emanate from the lower half of the drainage, a measure of watershed eccentricity utilizing easily measured properties in that area is derived and evaluated as a reliable predictor of peak magnitude. In the process of isolating watershed shape, slope, size, drainage pattern, and soil depth were isolated and, along with rainfall intensity, direction of storm movement, and antecedent moisture conditions, evaluated for the models. Studies were made into the similarities between the models and real world watersheds. Three of the several conclusions are 1) the models exhibit hydrologic responses similar to those of a wide range of real watersheds; 2) watershed shape, of itself, does not have a tremendous effect on peak magnitude, and 3) watershed eccentricity is an effective, easily measured, meaningful, and useful expression of watershed shape insofar as that characteristic affects maximum peak flows and certain time parameters of the hydrograph.     

The effects of topographic depressions on multiscale overland flow connectivity: A high‐resolution spatiotemporal pattern analysis approach based on connectivity statistics

In watershed modelling, the traditional practice of arbitrarily filling topographic depressions in digital elevation models has raised concerns. Advanced high‐resolution remote sensing techniques, including airborne scanning laser altimetry, can identify naturally occurring depressions that impact overland flow. In this study, we used an ensemble physical and statistical modelling approach, including a 2D hydraulic model and two‐point connectivity statistics, to quantify the effects of depressions on high‐resolution overland flow patterns across spatial scales and their temporal variations in single storm events. Computations for both models were implemented using graphic processing unit‐accelerated computing. The changes in connectivity statistics for overland flow patterns between airborne scanning laser altimetry‐derived digital elevation models with (original) and without (filled) depressions were used to represent the shifts of overland flow response to depressions. The results show that depressions can either decrease or increase (to a lesser degree and shorter duration) the probability that any two points (grid locations) are hydraulically connected by overland flow pathways. We used macro‐connectivity states (Φ) as a watershed‐specific indicator to describe the spatiotemporal thresholds of connectivity variability caused by depressions. Four states of Φ are identified in a studied watershed, and each state represents different magnitudes of connectivity and connectivity changes (caused by depressions). The magnitude of connectivity variability corresponds to the states of Φ, which depend on the topological relationship between depressions, the rising/recession limb, and the total rainfall amount in a storm event. In addition, spatial distributions of connectivity variability correlate with the density of depression locations and their physical structures, which cause changes in streamflow discharge magnitude. Therefore, this study suggests that depressions are “nontrivial” in watershed modelling, and their impacts on overland flow should not be neglected. Connectivity statistics at different spatial scales and time points within a watershed provide new insights for characterizing the distributed and accumulated effects of depressions on overland flow.