Surface Runoff Generation Research Articles

Combining FDR and ERT for monitoring soil moisture and temperature patterns in undulating terrain in south-eastern Norway

The occurrence of freeze–thaw cycles modifies water infiltration processes and surface runoff generation. Related processes are complex and are not yet fully investigated at field scale. While local weather conditions and soil management practices are the most important factors in both runoff generation and surface erosion processes, local terrain heterogeneities may significantly influence soil erosion processes in catchments with undulating terrain.This paper presents a field-based investigation of spatial and temporal heterogeneities in subsurface soil moisture and soil temperature associated with freezing, thawing, and snowmelt infiltration. The field setup consists of a combination of traditional point measurements performed with frequency domain reflectometry (FDR) and electrical resistivity tomography (ERT). The transect was approximately 70 m long and spanned an entire depression with a north-facing slope (average slope of 11.5%) and a south-facing slope (average slope of 9.7%). The whole depression was entirely covered with stubble.Observed resistivity patterns correspond well to the measured soil moisture patterns. During the observation period, the north facing slope froze earlier and deeper compared with the south facing slope. Freeze–thaw cycles were less pronounced in the north-facing slope than in the south-facing slope. There were also differences in soil temperature and soil moisture patterns between lower and upper parts of the monitored depression. These indicate that initiation and development of runoff related processes, and consequently soil erosion, in regions with freeze–thaw cycles may differ significantly depending on local terrain characteristics. Consequently, it indicates that spatial terrain heterogeneities, especially slope aspects, may be important when studying soil erosion processes, water flow and nutrient leaching in lowlands where patchy snowpacks and dynamic freeze–thaw cycles are predominating.

SMODERP2D—Sheet and Rill Runoff Routine Validation at Three Scale Levels

Water erosion is the main cause of soil degradation in agricultural areas. Rill erosion can contribute vastly to the overall erosion rate. It is therefore crucial to identify areas prone to rill erosion in order to protect soil quality. Research on rainfall-runoff and subsequent sediment transport processes is often based on observing these processes at several scales, followed by a mathematical description of the observations. This paper presents the use of a combination of data obtained by different approaches at multiple scales to validate the SMODERP2D episodic hydrological-erosion model. This model describes infiltration, surface retention, surface runoff, and rill flow processes. In the model, the surface runoff generation is based on a water balance equation and is described by two separate processes: (a) for sheet flow, the model uses the kinematic wave approximation, which has been parameterized for individual soil textural classes using laboratory rainfall simulations, and (b) for rill flow, the Manning formula is used. Rill flow occurs if the critical water level of sheet flow is exceeded. The concept of model validation presented here uses datasets at different scales to study the surface runoff and erosion processes on the Býkovice agricultural catchment. The first dataset consisted of runoff generated by simulated rainfall on plots with dimensions of 2 × 8 m. The second dataset consisted of the runoff response to natural rainfall events obtained from long-term monitoring of 50 m2 plots. These two datasets were used to validate and calibrate the sheet flow and infiltration parameters. The third dataset consisted of occurrence maps of rills formed during heavy rainfalls obtained using remote sensing methods on a field plot with an area of 36.6 ha. This last dataset was used to validate the threshold critical water level that is responsible in the model for rill flow initiation in the SMODERP2D model. The validation and the calibration of the surface runoff are performed well according to the Nash–Sutcliffe efficiency coefficient. The scale effect was evident in the 50 m2 plots where parameters lower than the mean best fit the measured data. At the field plot scale, pixels with measured rills covered 5% of the total area. The best model solution achieved a similar rill cover for a vegetated soil surface. The model tended to overestimate the occurrence of rills in the case of simulations with bare soil. Although rills occurred both in the model and in the monitored data in many model runs, a spatial mismatch was often observed. This mismatch was caused by flow routing algorithm displacement of the runoff path. The suitability of the validation and calibration process at various spatial scales has been demonstrated. In a future study, data will be obtained from various localities with various land uses and meteorological conditions to confirm the transferability of the procedure.

Event-based quickflow simulation with OpenLISEM in a burned Mediterranean forest catchment

Recently burnt areas typically reveal strong to extreme hydrological responses, as a consequence of loss of protective soil cover and heating-induced changes in topsoil properties. Soil water repellency (SWR) has frequently been referred to as one of the explanatory variables for fire-enhanced surface runoff generation but this has been poorly demonstrated, especially at the catchment scale. This study employs a process-based modelling approach to better understand the relevance of SWR in the hydrological response of a small, entirely burnt catchment in central Portugal, in particular by comparing hydrological events under contrasting initial conditions of dry vs wet soils. The OpenLISEM model was applied to a selection of 16 major rainfall runoff events that occurred during the first 2 post-fire years. The automatic calibration procedure resulted in good model performance, but it worsened for validation events. Furthermore, uncertainty analysis revealed an elevated sensitivity of OpenLISEM to event-specific conditions, especially for predicting the events’ total and peak flows. Also, predicted spatial patterns in runoff poorly agreed with the runoff observed in microplots. Model performance improved when events were separated by dry and wet initial moisture conditions, particularly for wet conditions, suggesting the role of variables other than initial soil moisture.

Incorporating Advanced Scatterometer Surface and Root Zone Soil Moisture Products into the Calibration of a Conceptual Semi-Distributed Hydrological Model

The role of soil moisture is widely accepted as a significant factor in the mass and energy balance of catchments as a controller in surface and subsurface runoff generation. The paper examines the potential of a new dataset based on advanced scatterometer satellite remote sensing of soil moisture (ASCAT) for multiple objective calibrations of a dual-layer, conceptual, semi-distributed hydrological model. The surface and root zone soil moisture indexes based on ASCAT data were implemented into calibration of the hydrological model. Improvements not only in the instrument specifications, i.e., better temporal and spatial sampling, but also in the higher radiometric accuracy and retrieval algorithm, were applied. The analysis was performed in 209 catchments situated in different physiographic and climate zones of Austria for the period 2007–2018. We validated the model for two validation periods. The results show that multiple objective calibrations have a substantial positive effect on constraining the model parameters. The combined use of soil moisture and discharges in the calibration improved the soil moisture simulation in more than 73% of the catchments, except for the catchments with higher forest cover percentages. Improvements also occurred in the runoff model efficiency, in more than 27% of the catchments, mostly in the watersheds with a lower mean elevation and a higher proportion of farming land use, as well as in the Alpine catchments where the runoff is not significantly influenced by snowmelt and glacier runoff.

Surface runoff response to long-term land use changes: Spatial rearrangement of runoff-generating areas reveals a shift in flash flood drivers

Nowadays, rapid growths of urban areas and associated land use/land cover (LULC) changes notably affect the surface runoff response to rainfall in urbanised catchments. This effect of urbanisation on surface runoff is frequently addressed since it relates to the increasing incidence of pluvial floods worldwide. Studies on this issue commonly assess runoff changes over several most recent decades. However, human-induced factors likely resulted in increased surface runoff and associated flash floods in rural catchments even in the past centuries. Such long-term changes have been rarely studied and thus are not well documented. This paper aims to fill this gap and assess the surface runoff response to LULC changes in a small catchment in southwestern Slovakia over more than two centuries. To meet this goal, we applied the Soil Conservation Service Curve Number (SCS-CN) method. We mapped past (1784) and present (2018) LULC classes based on historical maps and modern remote sensing data. Then, we estimated surface runoff depths for specific design rainstorms for both periods by executing CN method equations in the ArcGIS environment. Results revealed significant changes in surface runoff conditions between evaluated periods. In total, surface runoff magnitudes increased by ~230 and ~ 130% for 40- and 65-mm design rainstorms, respectively, compared to 1784. Moreover, surface runoff values show different spatial patterns on historical and present-day output raster images revealing spatial redistribution of runoff-enhancing and runoff-preventing areas during the evaluated period. Although both infiltration-enhancing and runoff-enhancing effects of LULC changes were identified, the latter effect associated with urbanised lowland areas was recognised as prevailing at present. In contrast, runoff-promoting conditions were associated with hilly vineyard areas in the past. These findings likely have important implications for understanding the past and present LULC-based controls of surface runoff generation and flash floods initiation.

An iterative runoff propagation approach to identify priority locations for land cover change minimizing downstream river flood hazard

Local infiltration and surface runoff generation depends on local land use, soil type, antecedent soil water content and slope, while runoff accumulation downstream is furthermore determined by re-infiltration along the flow paths. Hence, land use changes can both mitigate and exacerbate runoff accumulation and flood hazard, providing an opportunity to identify optimal locations for land use changes. An optimization method is presented, encompassing the iterative application of a spatially explicit rainfall-runoff model. This method ranks eligible locations (pixels) according to their modelled contribution to accumulated runoff downstream for a given land use change and given rainfall events, thereby guaranteeing maximal or minimal impact. This method was tested for two medium-sized catchments, located in Flanders, Belgium. Three land use changes were considered: afforestation, sealing and practicing winter cover crops. Results show the considerable impact of these land use changes and their locations on runoff accumulation at the outlet: afforestation of all eligible pixels reduces runoff volumes with 67% to 84 %, cover crops reduce runoff volumes in winter with 42% to 37%, while sealing triples runoff. The priority pixels have a larger impact on downstream runoff volume: afforesting or sealing the 20% highest or lowest ranked pixels leads to a reduction of 71% to 54%, resp. an increase of 102% to 115%. These priority pixels are characterized by high flow accumulation, highlighting the importance of enhancing the infiltration capacity in river valleys. The presented procedure allows spatial planners to consider the impact of local land use interventions to flood resistance downstream.

A complex network theory based approach to better understand the infiltration-excess runoff generation thresholds

Understanding hillslope-scale surface runoff generation processes are challenging due to the non-linear interactions among the different hydro-meteorological variables. Often the complex connection between different hydrologic variables is little understood (i.e., the spatial–temporal connection among the same variable and between two different variables). Therefore, in this paper, we present a network theory-based approach to understand the complex connections of hydrological processes. The article mainly focuses on identifying runoff thresholds in grassed (GA) and agro-forested (AgF) hillslopes and on testing of suitability of network theory for explaining the hillslope hydrological processes. After analyzing 22 rainfall events of AgF and 37 events from GA hillslope, our findings highlight that the runoff generation is associated with the peak rainfall intensity, initial soil moisture conditions and rainfall duration. The runoff thresholds of GA hillslope are higher than the AgF hillslope due to its surface resistance and presence of extremely low soil hydraulic conductivity (<6 mm/h) at the upslope location. Further, from network analysis, we found that the network's topology was highly dependent on event characteristics and soil hydraulic conductivity. The strong network connectivity was observed between runoff and soil moisture nodes during high runoff generating events as the runoff and infiltration take place simultaneously. However, during the low runoff generating events, a weak connection was observed between runoff and soil moisture nodes as most rainwater infiltrates over the hillslope. The outcomes of this study indicate the usefulness of network theory in hillslope-scale runoff process understanding to identify the significant connections.

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.

Analysis of surface runoff water and sediment generations using a short-term water budget in the steeply sloping agricultural land of northern Laos

ABSTRACT A field experiment was conducted during the rainy season of 2014, in the steeply sloping agricultural land of northern Laos to determine the factors that cause the generation of surface runoff water and sediment (i.e., soil erosion). Four treatments (upland rice cropping, maize cropping, maize cropping with mulch treatment, and bare land) were installed on a 2 square meters plot sloping 25°, or 45%. The surface runoff water was measured at 10 min intervals, and the total eroded water and sediment were collected after each rainfall event. The total rainfall during the experiment was 1,480 mm, equivalent to more than 80% of the annual rainfall, with the largest rainfall of 413 mm occurring in July. Strong rainfall, exceeding 10 mm in 10 min, occurred frequently (26 times). More than 50% of the surface runoff water and sediment were generated during the mid-rainy season. Short-term water budget analysis revealed that the amount of the surface runoff water and sediment depended on the soil surface conditions and the initial volumetric water content of the soil. Specifically, 1) rapid water infiltration and percolation suppress the surface runoff water and sediment during the early rainy season, 2) water saturation in the surface soil, clogging of soil pores, and sealing induce soil loss during the mid-rainy season, and 3) crust formation and/or clogging of soil pores induces surface runoff during the late rainy season. Mulching suppressed surface runoff generation by facilitating water infiltration into the soil and mitigating the disturbance of the soil surface by raindrops.

Rainfall and water yield in Macizo del Caroig, Eastern Iberian Peninsula. Event runoff at plot scale during a rare flash flood at the Barranco de Benacancil

Floods are a consequence of extreme rainfall events. Although surface runoff generation is the origin of discharge, flood research usually focuses on lowlands where the impact is higher. Runoff and sediment delivery at slope and pedon scale receiving much less attention in the effort to understand flood behaviour in time and space. This is especially relevant in areas where, due to climatic and hydrogeological conditions, streams are ephemeral, so-called dry rivers (“wadis”, "ramblas" or “barrancos”) that are widespread throughout the Mediterranean. This paper researches the relationship between water delivery at pedon and slope scale with dry river floods in Macizo del Caroig, Eastern Iberian Peninsula. Plots of 1x1, 1x2, 1x4, and 2x8 m located in the “El Teularet” Soil Erosion and Degradation Research Station were monitored from 2004 to 2014 to measure soil and water delivery. Rainfall and flow at the dry river Barranco de Benacancil were also monitored. Results show that runoff and sediment discharge were concentrated in few events during the 11 years of research. A single flood event was registered in the channel on September 28, 2009, however, the runoff was registered 160 times at the plots. Runoff discharge was dependent on the size of the plots, with larger plots yielding lower runoff discharge per unit area, suggesting short runoff-travel distance and duration. Three rainfall events contributed with 26% of the whole runoff discharge, and five achieved 56% of the runoff. We conclude that the runoff generated at the plot scale is disconnected from the main channel. From a spatial point of view, there is a decrease in runoff coefficient along the slope. From a temporal point of view, the runoff is concentrated in a few rainfall events. These results show that the runoff generated at plot and slope scale does not contribute to the floods except for rainfall events with more than 100 mm day-1. The disconnection of the runoff and sediment delivery is confirmed by the reduction in the runoff delivery at plot scale due to the control of the length of the plot (slope) on the runoff and sediment delivery.

Effects of urban residential landscape composition on surface runoff generation

Lawns have long been a primary feature of residential landscapes in the United States. However, as population growth in urban areas continues to rise, water conservation is becoming a key priority for many municipalities. In recent years, some municipalities have begun to offer rebate programs which incentivize removal of turfgrass areas and conversion to alternative ‘water-efficient’ landscapes, with the goal of reducing outdoor water use. The environmental impacts and changes to ecosystem services associated with such landscape alterations are not well understood. Therefore, a 2-year continuous research project was conducted at the Urban Landscape Runoff Research Facility at Texas A&M University to evaluate rainfall capture and runoff volumes associated with several commonly used residential landscape types (including, St. Augustine grass Lawn, Xeriscaping, Mulch, Artificial Turf, and Sand-capped Lawn) and to characterize the flow dynamics of surface runoff in relation to rainfall intensity for each landscape. The results demonstrate that runoff dynamics differ between landscapes, but also change over time as the newly converted landscapes become established. Following the initial months of establishment, the effects of landscape type on runoff volumes were significant, with Artificial Turf and Xeriscaping generating greater runoff volumes than Mulch and St. Augustine grass Lawns for most runoff events, which is partially due to the low infiltration rate of such landscapes. Overall, Artificial Turf and Xeriscaping showed the greatest cumulative runoff volumes (>400 L m−2), whereas Water Efficient- Mulch, Sand-capped Lawn and St. Augustine grass Lawn had a significantly lower cumulative runoff volumes, ranging from 180 to 290 L m−2. Information from this research should be useful to municipalities, water purveyors, and homeowner associations as they weigh the long-term hydrological impacts of lawn removal and landscape conversion programs.

Integrating depression storages and their spatial distribution in watershed-scale hydrologic modeling

Surface depressions are important topographic characteristics for surface runoff initiation, and the spatial distribution of depressions may further affect the timing and quantity of surface runoff reaching channels and outlets. However, many hydrologic models simulate the fill-spill processes for depression-dominated areas in a lumped manner and release outflows from depressions to channels or outlets directly. As a result, the progressive formation of contributing area (CA) and the dynamic runoff contribution process are not well characterized. The objective of this study is to improve depression-oriented hydrologic modeling by incorporating the influence of depression storages and their spatial distribution into the simulation of surface runoff generation and flow routing. To achieve this objective, a modified depression-oriented variable contributing area (MD-VCA) model is developed to simulate the depression-dominated catchment response during rainfall events, which employs a new depressional time-area zone scheme to deal with the spatially distributed depression storages, tracks the intrinsic changing patterns of the connected areas and depression storage, simulates the connected area-based surface runoff generation dynamics, implements a new CA-based surface runoff routing technique, and quantifies the likelihood of occurrence of outlet CA and runoff contributions using the joint probability distribution associated with depression storages and their spatial distribution. The performance of the MD-VCA model was evaluated through the application to a depression-dominated watershed in the Prairie Pothole Region of North Dakota. Simulation results demonstrated that the MD-VCA model was able to simulate the threshold-controlled overland flow dynamics under different rainfall conditions, and it effectively revealed the influences of depression storages and their spatial distribution on surface runoff generation and propagation processes.

СТРУКТУРА СКЛОНОВОЙ ЛОЖБИННОЙ СЕТИ КРИОЛИТОЗОНЫ В ОКРЕСТНОСТЯХ г. АНАДЫРЯ

Water runoff on the slopes of the permafrost zone passes through a network of hollows of various structures and patterns in plan. This is clearly distinguishable on aerial and satellite images, but often poorly distinguished on the ground. The study is aimed at recognizing the reasons for the difference in the structure of the drainage network, which is necessary for understanding the mechanisms of surface runoff generation and the modern relief formation in the permafrost areas. In the vicinity of Anadyr (Chukotka, Russia), three morphological types of the hollow network were identified. In the areas of distribution of each type of hollow network field observations were made, and the morphometric characteristics of the slopes to which they are associated were obtained from the digital surface model and satellite images. Each type of the hollow network corresponds to definite range of altitude, the type of relief, the slope gradient and profile, the composition of the deposits, and the depth of seasonal thawing. It has been established that although all types of troughs are used by temporary streams, they have different mechanisms of development. The hollows with a fan structure form the densest parallel network with a distance of 10–40 m apart. These are characteristic of concave slopes of interfluves with close bedding of bedrock and are the result of fluvial modeling of irregularities formed as a result of uneven sliding of the slope material (gelifluction). The intertwining troughs are confined to the slopes of the volcanic hills, the steepest of the studied ones. Fresh silty sediments have been observed at their bottoms and their formation is associated with erosional processes. Single hollows located at a considerable (200–300 m) distance from each other are formed in loose quaternary deposits with a developed network of ice-wedge-polygons. The role of thermokarst is significant in their deepening.

Efficacy of orchard terrace measures to minimize water erosion caused by extreme rainfall in the hilly region of China: Long-term continuous in situ observations

Terracing and rainfall characteristics notably influence the water erosion processes. However, an extensive long term in situ quantitative evaluation of the approaches to control the water erosion in different orchard terraces has not yet been performed, especially considering the increasing frequency and severity of extreme rainfall events due to the global climate change. In this study, six types of orchard terraces, including slope land as the control (SLck), level terrace with bare (LTb) and vegetation taluses (LTv), outward (OTv) and inward terrace (ITv) with vegetation taluses and level terrace having front mounds and back ditches with vegetation taluses (MDLTv), were used to analyze the effects of extreme and ordinary rainfall events on the surface runoff and soil erosion. According to the measured data for twelve consecutive years, 356 natural rainfall events were divided into extreme and ordinary rainfall, based on the World Meteorological Organization standard. The results indicated that more severe surface runoff and sediment loss occurred under extreme rainfall: the runoff coefficient and soil loss under extreme rainfall were 2.6 and 11.5 times those under ordinary rainfall, respectively. The sediment yield (contribution rate, 42.9%) exhibited a higher sensitivity to extreme rainfall events compared to that of the surface runoff generation (contribution rate, 16.4%). Moreover, the reduction in the surface runoff and sediment in the extreme rainfall case differed for different orchard terraces. The average surface runoff coefficient and soil loss amount decreased in the following order: SLck>LTb>OTv>LTv>ITv>MDLTv. Nevertheless, the highest and lowest contributions of the extreme rainfall to the sediment yield occurred in the LTb (64.8%) and MDLTv (21.7%) plots, respectively. Therefore, severe talus erosion caused by extreme rainfall should be monitored, and a combination of vegetation taluses and front mounds and back ditches on the platforms is recommended as a sustainable strategy to prevent extreme water erosion when transforming slope land into orchard terraces.

Modelling impacts of spatially variable erosion drivers on suspended sediment dynamics

Abstract. Suspended sediment load in rivers is highly uncertain because sediment production and transport at catchment scale are strongly variable in space and time, and they are affected by catchment hydrology, topography, and land cover. Among the main sources of this variability are the spatially distributed nature of overland flow as an erosion driver and of surface erodibility given by soil type and vegetation cover distribution. Temporal variability mainly results from the time sequence of rainfall intensity during storms and snowmelt leading to soil saturation and overland flow. We present a new spatially distributed soil erosion and suspended sediment transport module integrated into the computationally efficient physically based hydrological model TOPKAPI-ETH, with which we investigate the effects of the two erosion drivers – precipitation and surface erodibility – on catchment sediment fluxes in a typical pre-Alpine mesoscale catchment. By conducting a series of numerical experiments, we quantify the impact of spatial variability in the two key erosion drivers on erosion–deposition patterns, sediment delivery ratio, and catchment sediment yields. Main findings are that the spatial variability in erosion drivers affects sediment yield by (i) increasing sediment production due to a spatially variable precipitation, while decreasing it due to a spatially variable surface erodibility, (ii) favouring the clustering of sediment source areas in space by surface runoff generation, and (iii) decreasing their connectivity to the river network by magnifying sediment buffers. The results highlight the importance of resolving spatial gradients controlling hydrology and sediment processes when modelling sediment dynamics at the mesoscale, in order to capture the key effects of sediment sources, buffers, and hillslope hydrological pathways in determining the sediment signal.

Impacts of land-use and climate changes on surface runoff in a tropical forest watershed (Brazil)

ABSTRACT Surface runoff generation capacity can be modified by land-use and climate changes. Annual runoff volumes have been evaluated in a small watershed of tropical forest (Brazil), using the Soil and Water Assessment Tool (SWAT) model. Firstly, the accuracy of SWAT in runoff predictions has been assessed by default input parameters and improved by automatic calibration, using 20-year observations. Then, the hydrological response under land uses (cropland, pasture and deforested soil) alternative to tropical forest and climate change scenarios has been simulated. SWAT application has showed that, if forest was replaced by crops or pasture, the watershed’s hydrological response would not significantly be affected. Conversely, a complete deforestation would slightly increase its runoff generation capacity. Under forecasted climate scenarios, the runoff generation capacity of the watershed will tend to decrease and will not be noticeably different among the representative concentration pathways. Pasture and bare soil will give the lowest and highest runoff coefficients, respectively.

Evaluation of surface ponding and runoff generation in a seasonally frozen drained agricultural field

Surface runoff is often poorly quantified in hydrologic studies of subsurface drained fields, as it is a relatively minor component of the water balance and difficult to measure in large fields. However, conservation practices that seek to mitigate pollutant transport through subsurface drainage may increase surface runoff, and therefore it needs to be better understood. The goal of this study was to determine the frequency and extent of occurrence of surface ponding and runoff, and to understand their generation processes in a seasonally frozen, subsurface drained agricultural field in eastern Indiana. Three different methods were used. Surface ponding was monitored with a time-lapse camera at the edge of the field for three years. A water table threshold for surface ponding was determined using photo evidence of ponding together with water table depth measurements and used to estimate ponding. The DRAINMOD hydrologic model was calibrated and validated by comparing model predictions of subsurface drainage and water table depth with 10 years of field observations and used to predict surface ponding and runoff. The simulation results indicated that surface runoff represented 1–10% of annual precipitation, while subsurface drainage represented between 26 and 45%. On average, 45% of simulated ponding occurred during the cold season (December-March) indicating the importance of soil freezing and snow accumulation. However, during parts of the cold season, DRAINMOD simulations of snow accumulation and melt were poor, resulting in drain flow under-prediction and runoff over-prediction during these periods. Water table depth measurements above the defined threshold provided a simple alternate for prediction of saturation excess ponding events in the absence of direct measurements. Results from both simulations and observations indicated that all of the ponding events in this location were generated by saturation excess rather than infiltration excess processes.

Impact of Multiple Vegetation Covers on Surface Runoff and Sediment Yield in the Small Basin of Nverzhai, Hunan Province, China

Vegetation plays a significant role in controlling soil erosion. However, the effects of each vegetation type on soil erosion have not been fully investigated. In order to explore the influence of multiple vegetation covers on soil erosion and surface runoff generation, 10 different vegetation types, typical of the Nverzhai small basin, have been selected for this study. Regional precipitation, surface runoff, and sediment yield were measured from 2007 to 2018. The wettest year recorded was 2012. Recorded data confirmed that July was the wettest month in this region while January and December were the driest months. Furthermore, surface runoff and sediment yield associated with different vegetation types gradually decreased after 2013, which is the quantification of the consequences due to afforestation processes started in this area. Surface runoff and sediment content recorded for the configuration of sloping farmland were the largest between the different investigated vegetation types. The smallest were the broad-leaved mixed forest, the coniferous mixed forest, and shrubs. Finally, a significant linear positive correlation was found between rainfall and surface runoff, as well as sediment yield (R2 = 0.75). This suggests that climate change implications could be limited by using the more efficient vegetation covering. This research indicates that the ground cover is a key element in controlling soil and water loss, as well as vegetation measures, with high ground cover (i.e., broad-leaved trees). These measures should be strongly recommended for soil erosion control and surface runoff reduction. Moreover, these outcomes can be very helpful for vegetation restoration and water conservation strategies if implemented by local authorities.

Temporal evolution of measured and simulated infiltration following wildfire in the Colorado Front Range, USA: Shifting thresholds of runoff generation and hydrologic hazards

Destructive flash floods and debris flows are a common menace following wildfire. The restoration of protection provided by forests from post-fire floods and debris flows depends on the recovery of infiltration and attendant reduction of infiltration-excess surface runoff generation. This work examines seven years of post-fire infiltration measurements and temporal relations fit to soil-hydraulic properties from the Colorado Front Range, USA, to assess infiltration recovery with increasing time since fire. Point-scale Green-Ampt simulations of infiltration across a full spectrum of rainfall events are used to evaluate infiltration changes and shifts in surface runoff generation thresholds with post-fire temporal recovery. Measured and simulated infiltration generally recovered monotonically with increasing time since fire. This indicates a reduced vulnerability to infiltration-excess runoff generation as time elapses, with the greatest risk in the first two years after the fire. The threshold for infiltration-excess runoff advances with increasing time to rainfall events with higher intensity and greater return intervals; by the third year after wildfire, only extreme events (30–100 year recurrence) generate surface runoff and by the fifth and seventh year even extreme rainfall events typically fail to generate surface runoff. Remotely-sensed vegetation indices indicate linked, or at least contemporaneous, recovery of understory vegetation and field-saturated hydraulic conductivity at this field site, suggesting coincident recovery of multiple hillslope properties impacting surface runoff generation. This work indicates the importance of coupled assessments of hillslope property recovery and stochasticity of high-intensity rainfall for flash flood and debris flow hazards arising from infiltration-excess runoff generation; these hazards likely shift to subsurface mechanisms with increasing time since fire. Post-fire hazard assessments using static hillslope properties could fail to predict flash floods and debris flows associated with infrequent extreme rainfall events that strike during the post-fire recovery period when hillslope properties are partially recovered.