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Accurately characterizing rainfall runoff processes in plain farmland, especially at the plot scale with significant micro-topographic features, has presented challenges. Integrated surface–subsurface flow models with high-precision surface flow modules are appropriate tools, yet open-source versions are rare. To address this gap, we proposed an open-source integrated surface–subsurface flow model called the FullSWOF-Plain model, in which the one-dimensional subsurface module Hydrus-1D was integrated with a modified two-dimensional surface water flow module (FullSWOF-2D) using the sequential head method. Various experimental scenarios were simulated to validate the model's performance, including two outflow cases (i.e., 1D and 2D) without infiltration, a classical one-dimensional infiltration case, and two typical rainfall events at the experimental field. The results demonstrate the accuracy of this proposed model, with the Nash–Sutcliffe efficiency (NSE) of the simulated discharge exceeding 0.90 in the experimental field case and the root mean squared error (RMSE) values for soil moisture at five depths consistently below 0.03 cm3/cm3. However, we observed a lag in the simulated response time of soil moisture due to the neglect of preferential flow. The micro-topography significantly influenced ponding time and ponding areas. Lower local terrain normally experienced earlier surface ponding. Scattered surface ponding water first occurred in the ditch and followed in the relatively low areas in the main field. The concentration process of surface runoff exhibited hierarchical characteristics, with the drainage ditch contributing the most discharge initially, followed by the connection of scattered puddles in the main field, draining excess surface water to the ditch through rills. This quantitative study sheds light on the impact of micro-topography on surface runoff in plain farmland areas. [ABSTRACT FROM AUTHOR]

Near-surface vegetation and micro-topography have a great impact on the surface runoff process in the mountainous areas of southwestern China. It is, therefore, necessary to clarify the combined influence on the soil and water loss of steep slopes. In this study, 10 natural runoff plots were set to reveal the effects of cypress spatial pattern and micro-topography on the surface runoff. Pearson correlation coefficient method was applied to determine the correlation between cypress spatial pattern, micro-topography, and characteristic parameters of surface runoff. A response surface method (RSM) was also utilized to analyze the measured cypress spatial pattern and micro-topography in the process of precipitation and surface runoff. The results showed that: 1) The peak flow coefficient in all runoff plots behaved short-duration rainstorm (the rainfall lasted no more than 3 h, and the maximum 1 h rainfall intensity exceeded 30 mm/h) > long-duration rainstorm (the rainfall lasted more than 3 h, and the average rainfall intensity exceeded 2.5 mm/h) > long-duration heavy rainfall (the rainfall lasted more than 3 h, and the average rainfall intensity was between 1.5-2.5 mm/h), indicating that the blocking effect of different underlays on surface runoff decreased, with the increase of rainfall intensity and concentration of precipitation. 2) Five factors were significantly correlated with the peak flow (P<0.05) in the long-duration heavy rainfall or rainstorm, including topographic relief, surface roughness, runoff path density, contagion index of cypress, and stand density of cypress. Nevertheless, there was no significant correlation with the peak flow under the condition of short-duration rainstorm. 3) In long-duration heavy rainfall or rainstorm, the response of peak flow to the composite index were that: a) There was no significant change in the peak flow, when the composite index for cypress spatial pattern (V) was below 20.5, while the composite index of micro-topography (U) was below 10.5 (Low surface roughness, high runoff path density, and low stand density of cypress). b) The peak flow was relatively promoted, as the composite index for cypress spatial pattern increased within a certain range, when U > 9.0 (High surface roughness, low runoff path density, low stand density of cypress, and uniform structure among cypress). c) The peak flow was reduced significantly with the increase of V value, when U < 7.5 and V > 18 (Low runoff path density, while high stand density of cypress). As such, when the V value was adjusted to 41 under the condition of long-duration heavy rainfall (rainstorm), an optimal combination was achieved, where the peak flow was reduced from 39.6 L/10 min (14.2 L/10 min) to 6.3 L/10 min (4.2 L/10 min), while the reduction rate reached 84% (70%). d) Once the V value exceeded the critical one (the specific critical value increased with the increase of U value), the dominant influencing factor of peak flow shifted gradually from cypress spatial pattern to micro-topography. This finding can provide promising theoretical support to accurately adjust the vegetation patterns for the prevention and control of soil erosion in the mountainous areas of southwest China. [ABSTRACT FROM AUTHOR]

The extensive artificially accelerated erosion of spoil heaps on newly engineered landforms is a key ecological management point requiring better understanding. Soil surface roughness is a crucial factor influencing erosion processes; however, study on spoil heap erosion with a view of surface roughness is lacking. This study investigated the erosion processes and the spatiotemporal variation of surface roughness on spoil heaps, and then, analyzed how the roughness affected the hydrological and sediment yield characteristics. Sequences of four artificial rainstorms with constant rainfall intensity (90 mm/h) were applied to cone-shaped spoil heaps (ground radius 3.5 m, height 2.3 m) of a loess soil containing 30 mass percent rock fragments. The surface elevation was sampled by a laser scanner. For the surface roughness indicators, the root mean square height (rmsh) and the correlation length (cl) increased sharply during the first rainfall event, and in the last three rainfall events, rmsh increased slightly and cl showed a relative decrease. The initial rmsh/cl of the whole slope surface ranged from 0.063 to 0.135, and increased with the rainfall sequence, thus, indicating that the spoil heap surface became rougher. Increasing soil roughness in the rainfall sequence delayed the initial runoff time and increased the runoff yield. The average runoff coefficient of the spoil heaps was 0.658. The average erosion rate of each rainfall event can be simulated by a regression equation of the corresponding average runoff rate and median cl (R-square of 0.816). Soil slumping with an average volume of 0.014 m3 occurred in the first two rainfall events, thus, significantly changing the roughness and peak instant erosion rate. Together, the results revealed the effects of surface roughness on the erosion of spoil heaps and would provide a useful reference for soil loss prediction and control.

An adequate representation of micro-topography in spatially explicit, physically based models can be crucial in modeling runoff generation, surface/subsurface flow interactions or subsurface flow patterns in hydrological systems with pronounced micro-topography. However, representation of micro-topography in numerical models usually requires high grid resolutions to capture relevant small scale variations in topography at the range of centimeters to meters. High grid resolutions usually result in longer simulation times, especially if fully integrated model approaches are used where the governing partial differential equations for surface and subsurface flow are solved simultaneously. This often restricts the implementation of micro-topography to plot scale models where the overall model domain is small to minimize computational cost resulting from a high grid resolution. In this study an approach is presented where a highly resolved digital elevation model (DEM) for a hummocky topography in a plot scale wetland model (10 m × 21 m × 2 m), is represented by spatially distributed rill/depression storage zones in a numerical model with a planar surface. By replacing the explicit micro-topography with spatially distributed rill/depression storage zones, important effects of micro-topography on surface flow generation and subsurface transport characteristics (e.g. residence time distributions) are being preserved, while at the same time the number of computational nodes is reduced significantly. We demonstrate that the rill/depression storage concept, which has been used for some time to represent time delays in the generation of surface runoff, can also be used to mimic subsurface flow patterns caused by micro-topography. Results further indicate that the rill/depression storage concept is an efficient tool to represent micro-topography in plot scale models because model computation times drop significantly. As important aspects of surface and subsurface flows induced by micro-topography can be mimicked adequately by applying the rill/depression storage concept on a coarser grid, it may also be a useful tool to represent micro-topography in numerical flow models beyond the plot scale. [ABSTRACT FROM AUTHOR]

Biological soil crusts (BSCs) cover non-vegetated areas in most arid and semiarid ecosystems. BSCs play a crucial role in the redistribution of water and sediments and, ultimately, in the maintenance of ecosystem function. The effects of BSCs on water infiltration are complex. BSCs increase porosity and micro-topography, thus enhancing infiltration, but, at the same time, they can increase runoff by the secretion of hydrophobic compounds and clogging of soil pores upon wetting. BSCs confer stability on soil surfaces, reducing soil detachment locally; however, they can also increase runoff, which may increase sediment yield. Although the key role of BSCs in controlling infiltration-runoff and erosion is commonly accepted, conflicting evidence has been reported concerning the influence of BSCs on runoff generation. Very little is known about the relative importance of different BSC features such as cover, composition, roughness, or water repellency, and the interactions of these attributes in runoff and erosion. Because BSC characteristics can affect water flows and erosion both directly and indirectly, we examined the direct and indirect effects of different BSC features on runoff and erosion in a semiarid ecosystem under conditions of natural rainfall. We built structural equation models to determine the relative importance of BSC cover and type and their derived surface attributes controlling runoff and soil erosion. Our results show that the hydrological response of BSCs varies depending on rainfall properties, which, in turn, determine the process governing overland flow generation. During intense rainfalls, runoff is controlled not only by rainfall intensity but also by BSC cover, which exerts a strong direct and indirect influence on infiltration and surface hydrophobicity. Surface hydrophobicity was especially high for lichen BSCs, thus masking the positive effect of lichen crust on infiltration, and explaining the lower infiltration rates recorded on lichen than on cyanobacterial BSCs. Under low intensity, rainfall volume exerts a stronger effect than rainfall intensity, and BSC features play a secondary role in runoff generation, reducing runoff through their effect on surface micro-topography. Under these conditions, lichen BSCs presented higher infiltration rates than cyanobacterial BSCs. Our results highlight the significant protective effect against erosion exerted by BSCs at the plot scale, enhancing surface stability and reducing sediment yield in both high- and low-magnitude rainfall events. [ABSTRACT FROM AUTHOR]

Abstract: In humid upland catchments wetlands are often a prominent feature in the vicinity of streams and have potential implications for runoff generation and nutrient export. Wetland surfaces are often characterized by distinct micro-topography (hollows and hummocks). The effects of such micro-topography on surface–subsurface exchange and runoff generation for a 10 by 20m synthetic section of a riparian wetland were investigated in a virtual modeling experiment. A reference model with a planar surface was run for comparison. The geostatistically simulated structure of the micro-topography replicates the topography of a peat-forming riparian wetland in a small mountainous catchment in South-East Germany (Lehstenbach). Flow was modeled with the fully-integrated surface–subsurface code HydroGeoSphere. Simulation results showed that the specific structure of the wetland surface resulted in distinct shifts between surface and subsurface flow dominance. Surface depressions filled and started to drain via connected channel networks in a threshold controlled process, when groundwater levels intersected the land surface. These networks expanded and shrunk in a spill and fill mechanism when the shallow water table fluctuated around the mean surface elevation under variable rainfall inputs. The micro-topography efficiently buffered rainfall inputs and produced a hydrograph that was characterized by subsurface flow during most of the year and only temporarily shifted to surface flow dominance (>80% of total discharge) during intense rainstorms. In contrast the hydrograph in the planar reference model was much “flashier” and more controlled by surface runoff. A non-linear, hysteretic relationship between groundwater level and discharge observed at the study site was reproduced with the micro-topography model. Hysteresis was also observed in the relationship between surface water storage and discharge, but over a relatively narrow range of surface water storage values. Therefore it was concluded that surface water storage was a better predictor for the occurrence of surface runoff than groundwater levels. [Copyright &y& Elsevier]

Abstract: Although connectivity is acknowledged as a key factor of a catchment hydrological behaviour, it still misses reference evaluation methodologies. Therefore the objective of the paper is to evaluate different quantitative indicators of hydrological connectivity, using the concepts of structural and functional connectivity commonly used in ecology. The indicators were tested on contrasted numerical fields of micro-topography that present distinct hydrological responses. The investigated structural connectivity indicators, mostly used so far in porous or fractured media, were the semivariogram, the bivariate entropy integral scale, the n-point rectilinear connectivity integral scale, the connectivity function integral scale, the percolation threshold and the Euler number. The first three indicators were not able to discriminate between the three extreme types of micro-topographical fields whereas the last three indicators were successful. However, relating the latter three indicators to the hydrological response of a topographical field appears uncertain. We therefore proposed a functional connectivity indicator by adapting the ‘volume to breakthrough’ concept: the degree of surface connection as a function of the surface storage filling. This indicator was capable of discriminating between the micro-topographical types. It could be used to conceptualize the process of depression filling and therefore may become an effective characteristic of an elementary representative area in large scale hydrologic model. [Copyright &y& Elsevier]

• Overland flow from relatively flat land is largely controlled by hydrological connectivity. • Hydrological connectivity is difficult to monitor / measure in real time. • We propose surface water coverage (A SW) as a proxy for hydrological connectivity. • A clear threshold response of overland flow to A SW was observed. • A SW has substantial potential for the prediction of overland flow initiation. On rough agricultural soils, initiation of overland flow is primarily related to the gradual filling of small depressions. As the volume of water ponding in local depressions increases, the connectivity of those depressions increases, and that connectivity permits flow across the field boundaries. Previous studies have aimed at predicting overland flow connectivity by means of depression storage, but this is exceedingly difficult to quantify during an irrigation or rainfall event. There has been little work linking overland flow connectivity through variables that are more readily-measured than depression storage, such as the proportion of the soil surface covered in water (A SW). We propose using A SW as a proxy for hydrological connectivity, which can be measured using proximal remote sensing. A series of overland flow experiments were conducted in two contrasting plots of ~1.5 m2. Outlet discharge and changes in soil surface covered in water were continuously recorded during the experiments. The experiments demonstrated that the rougher soil surface experienced a delayed initiation of overland flow. The results also showed overland flow initiation was characterised by a distinct connectivity threshold and showed a clear response of overland flow to A SW. We further investigated the hydrological connectivity in twenty-one additional agricultural soils with contrasting micro-topographic conditions. Our results suggest that a predictor based on A SW has substantial potential to predict initiation of overland flow. The prediction is sufficiently early that it could be used in modern variable-rate irrigation systems, in combination with a sensor to measure A SW in real time, to prevent substantial flow from fields during irrigation. [ABSTRACT FROM AUTHOR]

Clarifying the impact of underlay (i.e., the combination of near-surface vegetation and surface micro-topography) on the surface runoff process would provide a significant theoretical basis for the adjustment of vegetation patterns and the control of soil erosion on steep slopes in mountainous areas of southwestern China. In the current study, the runoff process under different rainfall characteristics was observed based on 10 natural runoff plots, and the correlation between the spatial pattern of cypress (Cupressus funebris), micro-topography, and runoff characteristic parameters was tested using the Pearson correlation coefficient method. The coupling effects of the spatial pattern of cypress and micro-topography on surface runoff also were analyzed using the Response Surface Method (RSM). The results showed that (1) under the conditions of long-duration moderate rainfall or long-duration rainstorm, topographic relief, surface roughness, runoff path density, contagion index of cypress, and stand density of cypress were the main reasons for the difference in the peak flow of each runoff plot, while under the condition of the short-duration rainstorm, the factors previously mentioned were no longer the dominant factors; (2) under the conditions of long-duration heavy rainfall or long-duration rainstorm, the common laws reflected by the response of the peak flow to the composite index of the spatial pattern of cypress and micro-topography were that (1) when the composite index of the spatial pattern of cypress (V) was below 21 and the composite index of micro-topography (U) was below 10.5, the peak flow would not be significantly affected; (2) when U > 10.5, increasing the composite index of the spatial pattern of cypress within a certain range would promote peak flow; (3) when U < 7.5 and V > 18, the increase of V value could significantly reduce the peak flow, and on this basis, adjusting the V value to 41, the reduction rate of peak flow could reach 84%. [ABSTRACT FROM AUTHOR]

The extensive artificially accelerated erosion of spoil heaps on newly engineered landforms is a key ecological management point requiring better understanding. Soil surface roughness is a crucial factor influencing erosion processes; however, study on spoil heap erosion with a view of surface roughness is lacking. This study investigated the erosion processes and the spatiotemporal variation of surface roughness on spoil heaps, and then, analyzed how the roughness affected the hydrological and sediment yield characteristics. Sequences of four artificial rainstorms with constant rainfall intensity (90 mm/h) were applied to cone-shaped spoil heaps (ground radius 3.5 m, height 2.3 m) of a loess soil containing 30 mass percent rock fragments. The surface elevation was sampled by a laser scanner. For the surface roughness indicators, the root mean square height (rmsh) and the correlation length (cl) increased sharply during the first rainfall event, and in the last three rainfall events, rmsh increased slightly and cl showed a relative decrease. The initial rmsh/cl of the whole slope surface ranged from 0.063 to 0.135, and increased with the rainfall sequence, thus, indicating that the spoil heap surface became rougher. Increasing soil roughness in the rainfall sequence delayed the initial runoff time and increased the runoff yield. The average runoff coefficient of the spoil heaps was 0.658. The average erosion rate of each rainfall event can be simulated by a regression equation of the corresponding average runoff rate and median cl (R-square of 0.816). Soil slumping with an average volume of 0.014 m3 occurred in the first two rainfall events, thus, significantly changing the roughness and peak instant erosion rate. Together, the results revealed the effects of surface roughness on the erosion of spoil heaps and would provide a useful reference for soil loss prediction and control. [ABSTRACT FROM AUTHOR]