Hortonian Overland Flow Research Articles

Revegetation impacts on runoff generation processes in a typical loess catchment

AbstractSince 1999, large‐scale ecosystem restoration has been implemented in the Loess Plateau, effectively increasing regional vegetation coverage. Vegetation restoration has significantly elevated the saturated hydraulic conductivity (Ks) of the near‐surface soil layers and increased the vertical heterogeneity of the Ks profile. Many studies have examined the change of runoff due to revegetation, yet the impacts of Ks profile on the soil moisture distribution and runoff generation processes were less explored. In this study, numerical simulations were conducted to investigate how changes in the Ks profile caused by vegetation restoration influenced the hydrological responses at event scale. The numerical simulation results show that the increase of surface Ks caused by vegetation restoration can effectively reduce runoff at event scale. Moreover, the enhancement of vertical heterogeneity of Ks profiles can significantly change the vertical profile of soil water content, prompting more water to percolate into the deep soil layer. When rainfall exceeds a threshold, the accumulation of soil water above the relatively less permeable layer can cause short‐term saturation in shallow soil layers, resulting in a transient perched water table. As a result, after the vegetation restoration in the Loess Plateau, though Horton overland flow is still the main runoff generation mechanism, there is a possibility of the emergence of Dunne overland flow under the high vegetation coverage (e.g., NDVI larger than 0.5). This emergence of new runoff generation mechanism, saturation excess runoff, in the Loess Plateau due to the vegetation restoration could provide scientific guidance for water and sediment movement, soil and water conservation practices, and desertification control in the Loess Plateau.

Detrimental effects of tourist trails on soil system dynamics in Ubatuba Municipality, São Paulo State, Brazil

People continue to search for idyllic havens of natural beauty, whether to visit for themselves or for others to appreciate and admire. These are usually sites with natural elements that integrate environmental functions. However, increasing tourist activities conducted without respect and awareness for the natural environmental can result in destructive and detrimental issues, particularly when soil physicochemical properties are changed by excessive tourist footfall along abandoned trails. Soil erosion and runoff are two examples of these issues. This study has analysed runoff and soil loss on a trail in the Ubatuba Municipality (São Paulo State, Brazil) to understand how the soil processes have been modified by tourists' actions. This location has many serene and innate attractions, such as mountains, rivers, waterfalls and beaches, which draw the attention of many people and it is the geosystemic relation of Ubatuba that is being transformed. To understand the impacts, a suite of soil characterisation techniques (clay mineralogy, bulk density and physicochemical analysis) were adopted to analyse soils, together with continuous hydrological measuring and monitoring. Three erosion plots were monitored for four years, with runoff and loss soil measured (2013–2017). Clay minerals, indicative of more weathered soils, were identified. Porosity (40.5%) and bulk density (∼1.6 g/cm3) showed that the trail floor is compacted, compared to surrounding soils. Consequently, these properties, together with the occurrence of biocrusts on the soil surface of trail, hamper rain infiltration and increase runoff, give rise to Hortonian overland flow (HOF). A direct relationship was found between soil saturation and runoff: during periods of saturation, runoff was equivalent to ≤50% of rain. The trail soil characteristics changed during the monitoring period. Analysis of sediment texture shows there is 18.2% more silt than the trail soil (31.5%). The trend is similar for clay, with 18.4% more clay in the transported sediment, compared to the original soil. Therefore, problems regarding water infiltration and soil loss may influence the loss of local geodiversity. Enhanced erosion is increasing soil degradation, which points to an imbalanced relation between tourist activities and environmental functions. Consequently, it is proposed that the insights gathered from this study could be beneficial to land managers across the region, and beyond, and also contribute towards wider debates and guidance on sustainable tourism.

The role of biocrust-induced exopolymeric matrix in runoff generation in arid and semiarid zones – a mini review

Abstract Although playing an important role in shaping the environment, the mechanisms responsible for runoff initiation and yield in arid and semiarid regions are not yet fully explored. With infiltration-excess overland flow, known also as Hortonian overland flow (HOF) taking place in these areas, the uppermost surface ‘skin’ plays a cardinal role in runoff initiation and yield. Over large areas, this skin is composed of biocrusts, a variety of autotrophs (principally cyanobacteria, green algae, lichens, mosses) accompanied by heterotrophs (such as fungi, bacteria, archaea), which may largely dictate the infiltration capability of the surface. With most biocrust organisms being capable of excreting extracellular polymeric substances (EPS or exopolymers), and growing evidence pointing to the capability of certain EPS to partially seal the surface, EPS may play a cardinal role in hindering infiltration and triggering HOF. Yet, despite this logic thread, great controversy still exists regarding the main mechanisms responsible for runoff generation (runoff initiation and yield). Elucidation of the possible role played by EPS in runoff generation is the focus of the current review.

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

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

The timing and magnitude of changes to Hortonian overland flow at the watershed scale during the post‐fire recovery process

AbstractExtreme hydrologic responses following wildfires can lead to floods and debris flows with costly economic and societal impacts. Process‐based hydrologic and geomorphic models used to predict the downstream impacts of wildfire must account for temporal changes in hydrologic parameters related to the generation and subsequent routing of infiltration‐excess overland flow across the landscape. However, we lack quantitative relationships showing how parameters change with time‐since‐burning, particularly at the watershed scale. To assess variations in best‐fit hydrologic parameters with time, we used the KINEROS2 hydrological model to explore temporal changes in hillslope saturated hydraulic conductivity (Ksh) and channel hydraulic roughness (nc) following a wildfire in the upper Arroyo Seco watershed (41.5 km2), which burned during the 2009 Station fire in the San Gabriel Mountains, California, USA. This study explored runoff‐producing storms between 2008 and 2014 to infer watershed hydraulic properties by calibrating the model to observations at the watershed outlet. Modelling indicates Ksh is lowest in the first year following the fire and then increases at an average rate of approximately 4.2 mm/h/year during the first 5 years of recovery. The estimated values for Ksh in the first year following the fire are similar to those obtained in previous studies on smaller watersheds (<1.5 km2) following the Station fire, suggesting hydrologic changes detected here can be applied to lower‐order watersheds. Hydraulic roughness, nc, was lowest in the first year following the fire, but increased by a factor of 2 after 1 year of recovery. Post‐fire observations suggest changes in nc are due to changes in grain roughness and vegetation in channels. These results provide quantitative constraints on the magnitude of fire‐induced hydrologic changes following severe wildfires in chaparral‐dominated ecosystems as well as the timing of hydrologic recovery.

How water flow components affect sediment dynamics modeling in a Brazilian catchment

Water flows that move slowly in the landscape are not simulated by spatially and physical distributed erosion models and can contribute significantly during rainfall events in the streamflow, making model calibration complex. The goals of this study were: (i) to assess the relative contribution of subsurface and surface runoff to the streamflow at the outlet and (ii) to investigate to what extent the presence of runoff, other than Hortonian overland flow, affects erosion model performance. The Arvorezinha catchment (1.23 km2 – Southern Brazil) is characterized by the cultivation of tobacco under minimum cultivation system on shallow soils. From October 2011 until October 2013, we set up a complementary monitoring program of water chemistry and samples were collected during rainfall-runoff events to analyze dissolved silicon (DSi). The observed hydrograph was separated into two components using DSi: i) surface flow and ii) groundwater + soil water flow. The OpenLISEM model was used to assess the impact of different runoff sources and understand the hydrological and erosive process dynamics in the catchment. For this, a manual calibration procedure was used and repeated for two approaches: with and without runoff separation. For the six rainfall events evaluated, 62% of the hydrograph is formed by groundwater + soil water, especially in autumn–winter and for low-medium magnitude events (up to Qpeak = 800 L s−1). The dilution effect was observed during rainfall events, when liquid discharge increases, DSi concentration and suspended sediment concentration decreases. The performance of physics-based models in a catchment with very fragile soils and with frequent changes in land use, is impacted by runoff separation.

Convergent development of experimental landforms with differing rainfall and substrate properties

AbstractFour runs of experimental landform development, with the same uplift rate, different rainfall intensity, and the same material of different permeability adjusted by the degree of compaction, showed complicated effects of rainfall and mound‐forming material. In the run with more rainfall on less permeable material, low separated ridges developed in the uplifted area, because abundant overland flow promoted valley erosion and slope processes from early stages. In the run with less rainfall on less permeable material, valley incision proceeded mostly in major valleys where surface water converges. Canyons developed during early stages and later a high massive mountain emerged. The effect of rainfall difference, however, appeared completely opposite on more permeable material accompanied by lower shear strength. In the run with more rainfall on more permeable material, a massive mountain similar to that with less rainfall on less permeable material appeared, and low separated ridges appeared in the run with less rainfall on more permeable material as in the run with more rainfall on less permeable material. In the former case, similar amount of water available for Hortonian overland flow in early stages estimated from rainfall rate and permeability can explain the development of similar landforms. In the latter case, while abundant surface water with more rainfall on less permeable material made fluvial erosion active from early stages, the deficiency in surface water with less rainfall on more permeable material apparently attenuated fluvial erosion but possibly accentuated slope processes and slope failures by seepage water flow through more permeable material of low shear strength. The active erosion from early stages apparently resulted in the development of enduring similar low landforms later in the dynamic equilibrium stage. These experimental results indicate that similar landforms can emerge from different environmental and lithologic controls, and that process does not necessarily follow from form.

Exploring the Dominant Runoff Processes in Two Typical Basins of the Yellow River, China

Storm runoff in basins is comprised of various runoff processes with widely disparate infiltration and storage capacities, such as Hortonian overland flow (HOF), saturated overland flow (SOF), sub-surface flow (SSF), and deep percolation (DP). Areas may be classified according to these various runoff processes based on the soil characteristics, geology, topography, and land-use. This study analyzes changes in runoff components in the Jialu River basin and the Fen River (Jingle sub-basin) during runoff generation from 1980 to 2013 using the runoff segmentation method. Based on the decision scheme, the dominant runoff process (DRP) in the basins was distinguished using geographic information system (GIS) tools. The impact of different runoff process distributions on the changes in the runoff for the basin was determined. The results show that the floods in the Jialu River basin and Jingle sub-basin were dominated by overland flow components. Compared with 1980–1999, the proportion of overland flow components for 2000–2013 in two basins showed a decreasing trend by 8.3% and 7.1%, respectively, while the interflow and underground runoff components increased. In addition, HOF was the DRP in the Jialu River basin and Jingle sub-basin from 2000 to 2013. The area of the rapid runoff processes (HOF, SOF1, and SSF1) in the Jialu River basin and Jingle sub-basin accounted for 89% and 78% of the entire basin, respectively. In contrast, the slow runoff processes (SOF2, SSF2, and DP) accounted for 11% and 22% of the entire basin, respectively. The runoff of the Jingle sub-basin was substantially lower than that of the Jialu River basin under the same rainfall conditions, because of the influence of the distribution of different runoff processes. Compared with the Jialu River Basin, the peak discharge and runoff of Jingle sub-basin were 190.4 m3/s and 2.85 mm lower on average, respectively. The results of this study provide useful information to understand land-use changes and formulate management practices to reduce flooding in the Yellow River.

Assessing plot‐scale impacts of land use on overland flow generation in Central Panama

AbstractLand use in Panama has changed dramatically with ongoing deforestation and conversion to cropland and cattle pastures, potentially altering the soil properties that drive the hydrological processes of infiltration and overland flow. We compared plot‐scale overland flow generation between hillslopes in forested and actively cattle‐grazed watersheds in Central Panama. Soil physical and hydraulic properties, soil moisture and overland flow data were measured along hillslopes of each land‐use type. Soil characteristics and rainfall data were input into a simple, 1‐D representative model, HYDRUS‐1D, to simulate overland flow that we used to make inferences about overland flow response at forest and pasture sites. Runoff ratios (overland flow/rainfall) were generally higher at the pasture site, although no overall trends were observed between rainfall characteristics and runoff ratios across the two land uses at the plot scale. Saturated hydraulic conductivity (Ks) and bulk density were different between the forest and pasture sites (p < 10−4). Simulating overland flow in HYDRUS‐1D produced more outputs similar to the overland flow recorded at the pasture site than the forest site. Results from our study indicate that, at the plot scale, Hortonian overland flow is the main driver for overland flow generation at the pasture site during storms with high‐rainfall totals. We infer that the combination of a leaf litter layer and the activation of shallow preferential flow paths resulting in shallow saturation‐excess overland flow are likely the main drivers for plot scale overland flow generation at the forest site. Results from this study contribute to the broader understanding of the delivery of freshwater to streams, which will become increasingly important in the tropics considering freshwater resource scarcity and changing storm intensities.

Comparing overland flow processes between semiarid and humid regions: Does saturation overland flow take place in semiarid regions?

Two main processes of runoff are commonly thought to take place in a drainage basin: infiltration-excess overland flow, known also as Hortonian overland flow (HOF), and saturation overland flow (SOF). While HOF mainly takes place in arid and semiarid regions once the infiltration capacity of the upper skin of the soil (usually ~1 mm thick) is surpassed, SOF may takes place in wet humid and subhumid climates. SOF takes place once the groundwater table reaches the surface (mainly close to the stream channel), or due to an impermeable subsurface soil layer. Despite the low precipitation in semiarid regions, a number of publications advocated the occurrence of SOF also in semiarid regions, subsequently underplaying the centrality of biocrusts to effectively generate runoff. These publications are mainly based on moisture measurements that were conducted with dielectric-based sensors. In the present work, it will be argued that: (a) the amount and duration of the rain cannot possibly result in SOF, (b) the location of the alleged SOF, as reported by some publications (at the up- or mid-slope), is unlikely to facilitate SOF, (c) the likelihood that the electrical pulses produced by the dielectric-based sensors will distinguish between completely saturated and nearly saturated conditions is low, (d) in certain occasions even when the moisture content, as detected by these sensors, was slightly below saturation, the authors tended to assume that the soil was saturated, (e) while claiming that the subsurface soil layer reached saturation, the upper layer of the soil (where overland flow takes place) was occasionally reported to remain below saturation, (f) no supportive evidence for the occurrence of SOF or even subsurface flow was provided, such as visible wet belts with dense vegetation. It is therefore concluded that the occurrence of SOF in semiarid regions should be cautiously considered and re-examined.

Effects of critical zone structure on patterns of flow connectivity induced by rainstorms in a steep forested catchment

Flow connectivity in the hillslope–riparian–stream (HRS) system describes the hydrological linkage between upland water and the channel network. However, the time and form of the establishment of HRS connectivity are not adequately understood. Herein, we examined how hillslope structure (topography and soil) and rainfall influence HRS connectivity in a steep, forested, zero-order catchment at the Hemuqiao Hydrological Experimental Station in Southeast China, from July 2016 to November 2017. To this end, surface and subsurface flow, soil moisture, and soil hydraulic conductivity (Ks) were observed, and soil dye staining experiments were conducted. Two patterns of HRS connectivity, namely saturation connectivity that initiates at the soil–bedrock interface (SCSB) and saturation connectivity at different soil horizons (SCSH), were identified. The persistence time of SCSH connectivity was short (<1 h), and the contribution of the perched interflow in different soil horizons to the total runoff was relatively small (0.6–3.0%). Instead, we found that the soil–bedrock interface acted as an important impeding layer that established HRS connectivity. That is, among the rainfall events during which HRS connectivity was established, 90% were established through SCSB connectivity and only 10% were established through SCSH connectivity. We further evaluated the time required to established HRS connectivity and found that SCSB connectivity required more time (4.6–67.4 h) than SCSH connectivity (<1 h). We further found that rainfall intensity determined the initiation of connectivity and that the time required for HRS connectivity decreased exponentially with increasing rainfall intensity (R2 = 0.67). Finally, we found that subsurface saturation excess flow, rather than Hortonian overland flow, was the main contributor to the flood peak during large events. In these events, the total volume of the runoff and flood peak were five and eight times higher than that of subsurface outflow, respectively. These results provide a clearer understanding of runoff generation and can narrow the gap between experiments and models for further development of hydrological theories and methods.

The dominant runoff processes on grassland versus bare soil hillslopes in a temperate environment - An experimental study

Abstract This paper aimed to investigate the dominant runoff processes (DRP’s) at plot-scale in the Curvature Subcarpathians under natural rainfall conditions characteristic for Romania’s temperate environment. The study was based on 32 selected rainfall-runoff events produced during the interval April–September (2014–2017). By comparing water balance on the analyzed Luvisol plots for two types of land use (grassland vs. bare soil), we showed that DRP’s are mostly formed by Hortonian Overland Flow (HOF), 47% vs. 59% respectively. On grassland, HOF is followed by Deep Percolation (DP, 31%) and Fast Subsurface Flow (SSF, 22%), whereas, on bare soil, DP shows a higher percentage (38%) and SSF a lower one (3%), which suggests that the soil-root interface controls the runoff generation. Concerning the relationship between antecedent precipitation and runoff, the study indicated the nonlinearity of the two processes, more obvious on grassland and in drought conditions than on bare soil and in wet conditions (as demonstrated by the higher runoff coefficients). Moreover, the HOF appeared to respond differently to rainfall events on the two plots - slightly longer lag-time, lower discharge and lower volume on grassland - which suggests the hydrologic key role of vegetation in runoff generation processes.

Assessing spatially distributed infiltration capacity to evaluate storm runoff in forested catchments: Implications for hydrological connectivity

We assessed spatial distribution of infiltration capacity under forest canopies to simulate runoff processes in a steep small catchment in Japan. High resolution LiDAR data were used to assess light availability to understory vegetation, which in turn was converted into an understory vegetation cover ratio. Spatially distributed infiltration capacities based on data from field rainfall-runoff experiments were incorporated into the TopoTube model using a relationship between infiltration capacity and vegetation cover. Effects of spatial distribution of infiltration capacity and its influence on hydrological connectivity and runoff processes were examined based on numerical simulations in the following three infiltration scenarios for seven natural storms: (1) spatially distributed infiltration capacity; (2) uniform infiltration capacity using the weighted mean of the spatial distribution in the drainage basin; and (3) uniform infiltration capacity using the arithmetic mean of field measurements. Although saturated subsurface flow dominated storm runoff in all simulations, Hortonian and saturated overland flow were significant for storm runoff response in smaller catchments (headwaters and zero-order basins). Simulations using spatially distributed infiltration capacities showed more Hortonian overland flow compared to using uniform infiltration because of areas with low infiltration located along channels. On the other hand, no differences in runoff were apparent between spatially-distributed and uniform infiltration scenarios in larger catchments. Our study also indicates that differences in contributions of subsurface and groundwater flow at the catchment scale need to be examined based on observations and simulations in nested catchments. Our findings suggest that including detailed patterns of vegetation ground cover are important for understanding hydrogeomorphic connectivity within catchments.

Impacts of Antecedent Soil Moisture on the Rainfall-Runoff Transformation Process Based on High-Resolution Observations in Soil Tank Experiments

An experimental soil tank (12 m long × 1.5 m wide × 1.5 m deep) equipped with a spatially distributed instrument network was designed to conduct the artificial rainfall-runoff experiments. Soil moisture (SM), precipitation, surface runoff (SR) and subsurface runoff (SSR) were continuously monitored. A total of 32 rainfall-runoff events were analyzed to investigate the non-linear patterns of rainfall-runoff response and estimate the impact of antecedent soil moisture (ASM) on runoff formation. Results suggested that ASM had a significant impact on runoff at this plot scale, and a moisture threshold-like value which was close to field capacity existed in the relationship between soil water content and event-based runoff coefficient (φe), SSR and SSR/SR. A non-linear relationship between antecedent soil moisture index (ASI) that represented the initial storage capacity of the soil tank and total runoff was also observed. Response times of SR and SM to rainfall showed a marked variability under different conditions. Under wet conditions, SM at 10 cm started to increase prior to SR on average, whereas it responds slower than SR under dry conditions due to the effect of water repellency. The predominant contributor to SR generation for all events is the Hortonian overland flow (HOF). There is a hysteretic behavior between subsurface runoff flow and soil moisture with a switch in the hysteretic loop direction based on the wetness conditions prior to the event.

Geological and geotechnical land zoning for potential Hortonian overland flow in a basin in southern Brazil

This paper presents a new procedure to perform geological and geotechnical land zoning at the 1:50,000 scale for potential Hortonian overland flow (Rpot values). The procedure was applied to the Ribeirão do Feijão Basin, São Paulo State, Brazil. The zoning was conducted using a procedure that combines rainfall data, engineering geological mapping, infiltration modelling, and anthropogenic and geomorphic aspects. The association between the geological-geotechnical units defined by the engineering geological mapping and the land use types generated 29 different infiltration condition categories, which were used to divide the basin into 94 territorial units. The in situ infiltration test and geotechnical data for each infiltration condition category combined with several rainfall event types were used as inputs to the Philip model to obtain the values of the Rpot and infiltration values. The Rpot and infiltration values estimated by the proposed procedure agreed well with the values estimated by the Chu model. The 94 territorial units were grouped into 4 classes based on the variability and magnitude of the potential Hortonian overland flow. The first class represents the areas where infiltration predominates (59 km2) for all selected rainfall events, and the second class represents areas of infiltration and flooding (11 km2). Infiltration is predominant in the third class; however, overland flow can be generated (133 km2) depending on the intensity and duration of the rainfall event. The fourth class represents the main source areas of overland flow generation and runoff (40 km2). The areas considered in the third and fourth classes are affected by erosion processes due to the potential generation of Hortonian overland flow. The results indicated that the proposed procedure is adequate for identifying, characterizing, classifying and dividing a region into units with distinct geological and geotechnical zones that are associated with potential Hortonian overland flow. Therefore, the procedure is an appropriate tool for regional planning and water management at the 1:50,000 scale because the results define land units with specific environmental characteristics. The application of the procedure is not expensive and uses common equipment but requires experienced or trained professionals to perform the field and laboratory work.

How can the grasslands under rainfall events modify water balance in drought conditions

AbstractTaking into account the well-established influence of hillslopes grasslands on runoff processes, the purpose of this study was to investigate how grasslands can affect the water flow pathways on hillslopes, in drought conditions. This study was performed in experimental grassland at plot-scale (e.g., Festuca pratensis), in temperate humid continental climatic conditions of Curvature Subcarpathians, Romania. The rainfall, evapotranspiration, and soil moisture daily data, respectively 208 rainfall and 16 rainfall-runoff events data measured in grassland hills during the growing season (1 April up to 30 September 2015 and 2016) were used. Our results suggest that a runoff event response in extreme drought conditions occurs on grasslands only if precipitation exceeds the threshold of 31 mm Hortonian overland flow (HOF), while this threshold drops to 17 mm during moderate droughts and up to 8 mm for wet conditions. The rainfall events up to 16 mm proved to be insufficient to completely saturate the soil. Therefore, HOF has only a minor contribution in drought conditions, on grassland and light on bare soil. A complementary and negative effect of grasslands in drought conditions is the water resources suppressing on hillslopes.

Soil physical characteristics of a degraded tropical grassland and a ‘reforest’: Implications for runoff generation

Imperata grassland soils are widely perceived as having poor physical and chemical properties that render them unproductive and prone to erosion. They are therefore increasingly targeted for reforestation across the tropics. To better understand how reforestation and forest growth affect soil hydrological processes we compared the soil physical characteristics for an Imperata grassland and a 23-year-old ‘reforest’ on Leyte Island, the Philippines. Saturated hydraulic conductivity (Ksat) was determined in the field (Amoozemeter, 20–90 cm depth) and the laboratory (small-core permeametry). Core-based values of Ksat were (much) lower than field-based values, suggesting macropores were not sampled adequately with the small cores. Ksat decreased exponentially with depth in both land-cover types, with a median field-measured Ksat of the grassland of 2.1 mm h−1 at the surface and 2.9 mm h−1 at 20–40 cm depth, declining to ≤1 mm h−1 below 60 cm. Corresponding values for the reforest were 59 (at 20 cm), 37 (at 40 cm) and 7.3 mm h−1 (at 60–100 cm depth). Reforest Ksat-values down to 60 cm depth were significantly higher than corresponding values in the grassland, but the difference disappeared at 90 cm depth. Organic carbon content in the top 40 cm of soil was slightly higher in the reforest than the grassland. Bulk density was higher and porosity marginally lower in the grassland than the reforest at all depths considered. The median five-min rainfall intensity (June 2013–May 2014) was 3.2 mm h−1, suggesting that >50% of the rainfall might generate Hortonian overland flow in the grassland. Overland flow is unlikely in the reforest where lateral flow is likely to be generated around 60 cm depth for ~30% of rain-time, versus 2–3% between 20 and 60 cm. Within the limitations of the space-for-time substitution approach, these results suggest that 23 years of forest development at Manobo had a positive effect on hillslope hydrological functioning.

The Development of a 1-D Integrated Hydro-Mechanical Model Based on Flume Tests to Unravel Different Hydrological Triggering Processes of Debris Flows

Many studies which try to analyze conditions for debris flow development ignore the type of initiation. Therefore, this paper deals with the following questions: What type of hydro-mechanical triggering mechanisms for debris flows can we distinguish in upstream channels of debris flow prone gullies? Which are the main parameters controlling the type and temporal sequence of these triggering processes, and what is their influence on the meteorological thresholds for debris flow initiation? A series of laboratory experiments were carried out in a flume 8 m long and with a width of 0.3 m to detect the conditions for different types of triggering mechanisms. The flume experiments show a sequence of hydrological processes triggering debris flows, namely erosion and transport by intensive overland flow and by infiltrating water causing failure of channel bed material. On the basis of these experiments, an integrated hydro-mechanical model was developed, which describes Hortonian and saturation overland flow, maximum sediment transport, through flow and failure of bed material. The model was calibrated and validated using process indicator values measured during the experiments in the flume. Virtual model simulations carried out in a schematic hypothetical source area of a catchment show that slope angle and hydraulic conductivity of the bed material determine the type and sequence of these triggering processes. It was also clearly demonstrated that the type of hydrological triggering process and the influencing geometrical and hydro-mechanical parameters may have a great influence on rainfall intensity-duration threshold curves for the start of debris flows.

Hydrological dynamics in a small catchment with silvopastoral land use in SW Spain

Hydrological processes were investigated since the year 2000 in a semiarid rangeland with disperse cover of evergreen holm oaks. In this article results are presented on the spatial variation of rainfall in the catchment using a network of 6 raingauges. Results are also presented on soil water dynamics, which were studied under different vegetation covers and varying soil depths. Furthermore, the temporal variation of runoff generation in the basin outlet is reviewed, taking into account different timescales. The following results can be highlighted: Spatial variation of rainfall in the catchment at annual or monthly scale was low. However, large dispersion exists in the case of individual rainstorms, when the differences between the 6 raingauges varied strongly, both for daily values as well as maximum intensities. Events with high spatial variation are presumably related with small, localized storms, particularly those of high intensity and short duration, and those showing low variation are probably of frontal origin. Soil moisture presented seasonal patterns clearly dependent on factors such as rainfall or evapotranspirative demand. Spatial distribution of soil moisture was mainly affected by physiographic terrain characteristics and vegetation covers. Generally, in dry hydrological years, soil moisture was lower beneath than beyond the tree canopies, while the opposite was observed in wet years. Dry situations increase the intensity and duration of soil water deficits, enhance the evaporative demand and increment water storage capacity by covers. Channel flow in dehesas depends highly on the antecedent moisture conditions and particularly on the water content of the sediment-filled valley bottoms. If these areas become saturated, high amounts of runoff are produced during rainfall events. Under high intensity rainstorms Hortonian overland flow is rapidly generated on hillslopes producing a quick channel response. Interannual runoff variation is very high, the more frequent low rainfall years provoke very low total water flow, contrasting with less frequent years of above average rainfall, which are the ones producing high total runoff. It can be concluded that the hydrological dynamics in Mediterranean rangelands with a disperse tree cover are complex, both regarding soil water and catchment runoff, being highly variable in time. Future changes in land use or climate could affect notably the hydrological dynamics of the catchment, and hence similar upland basins. For example, a notable increase of tree cover would reduce water availability of pasture plants, particularly during dry years. A decrease of annual rainfall amounts and/or increased drought intensities, as a consequence of climate change, would enlarge periods of soil moisture deficit for pasture plants, as well as a reduction of runoff.

Identifying Dominant Runoff Processes at a Regional Scale – A GIS - Based Approach

Abstract Identifying landscapes with similar hydrological characteristics is useful for the determination of dominant runoff process (DRP) and flood prediction. Several approaches used for DRP-mapping differ in respect to time and data requirement. Manual approaches based on field investigation and expert knowledge are time consuming and difficult to implement at regional scale. Automatic GIS-based approach on the other hand require simplification of data but are easier to implement and it is applicable on regional scale. In this study, GIS-based automated approach was used to identify the DRPs in Anambra area. The result showed that Hortonian Overland Flow (HOF) has the highest coverage of 1508.3 Km2 (33.5%) followed by Deep Percolation (DP) with coverage of 1455.3 Km2 (32.3%). Subsurface Flow (SSF) is the third dominant runoff process covering 920.6 Km2 (20.4%) while Saturated Overland Flow (SOF) covers the least area of 618.4 Km2 (13.7%) of the study area. The result reveal that considerable amount of precipitated water would be infiltrated into the subsurface through deep percolation process contributing to groundwater recharge in the study area. However, it is envisaged that HOF and SOF will continue to increase due to the continuous expansion of built-up area. With the expected increase in HOF and SOF and the change in rainfall pattern associated with perpetual problem of climate change, it is paramount that groundwater conservation practices be considered to ensure continued sustainable utilization of groundwater in the study area.