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In dryland ecosystems, runoff is mainly generated in bare areas, which are also more susceptible to water erosion than vegetated areas. These bare areas are often covered and protected by biological soil crusts (BSCs), which modify numerous physicochemical surface properties involved in runoff and erosion processes. BSCs are considered as one of the most important stabilizing factors in the soil surface, but most previous research has concentrated only on patch or hillslope-scale effects of BSCs, and their effect at coarser scales has rarely been studied. In this article, we present a new approach based on previous surface cover quantification for including the effects of BSCs in physically-based runoff and erosion modeling. The Limburg Soil Erosion Model (LISEM) was used to parameterize and simulate the effects of BSCs on runoff and erosion in a small semiarid catchment characterized by fine-textured soils and predominantly covered by BSCs. Paired model simulations under two scenarios, with and without including the effects of BSCs, were run under different rainfall intensities to evaluate the effect of BSCs on runoff and erosion under different rainfall conditions. Runoff and erosion rates recorded in the field at the catchment outlet were predicted much more accurately when BSCs were included because there was less overestimation of runoff rate, maximum runoff peaks and erosion rates in the areas dominated by BSCs. The proposed approach enables BSCs to be included in spatially distributed runoff and erosion models, improving their predictions, and may be used for evaluating how the effect of human activity on BSCs affects catchment-scale water erosion. [ABSTRACT FROM AUTHOR]

Soil sealing and crusting are common, widespread processes in arid and semiarid soils that strongly affect runoff and erosion. In these soils, biological soil crusts (BSCs) are an essential surface component known to have an important role in water processes, and are also strong erosion protection agents. However, these crusts are also highly vulnerable to disturbances, which seriously affect their pedological and ecological roles. The aims of this study were to examine whether penetration resistance (PR) depends on BSC developmental stage and how removal of BSCs affects PR, under dry and wet soil conditions, and to analyse the relationship between PR and runoff and soil detachment in these soils. This research was conducted in two semiarid ecosystems representative of crusting processes in SE Spain (El Cautivo and Las Amoladeras, Almeria province). Four pairs of plots containing the main BSC types identified at both sites were selected and two treatments were considered, one in each plot in the pair: i) unaltered BSC and ii) BSC removal by scraping. Two intense rainfall simulation experiments were performed one after another in these plots. Runoff was measured during both, and water samples were collected for further determination of sediment yield. PR was measured in undisturbed BSC and scalped soil on dry soil prior to rainfall, on wet soil after the first and second rainfalls, and on dry soil seven months after BSC removal. Crust PR was higher at Las Amoladeras (3.13 ± 0.54 kg cm − 2 ) than at El Cautivo (1.52 ± 0.74 kg cm − 2 ) due to soil compaction from trampling by livestock in the former. PR varied with BSC development, with less developed cyanobacteria BSCs showing higher resistance (under dry and wet conditions) than more developed lichen BSCs. BSC removal was followed by the formation of a new seal (physical crust) upon raining, which however, did not result in increased PR. No significant relationship was found between PR and runoff, but there was a weak negative relationship between PR and soil detachment, because in BSCs, which had higher PRs, soil detachment rates were much lower than in recently scalped soils. Seven months after BSC removal, PR was significantly higher in scalped soils than for undisturbed BSCs. Our results show that PR was not a good indicator of hydrological and soil detachment response of BSCs or the soil sealing during rain immediately after BSC removal, but it was a good indicator of physical soil crust formation in scalped soils over time. [ABSTRACT FROM AUTHOR]

Abstract: The natural landscapes of semiarid areas worldwide comprise a series of scattered patches of shrubs within a matrix of biologically crusted soils (BSC). As BSCs are considered ecosystem engineers the relationships between the BSC and the shrub patches determine system functioning. The objective of our study was to investigate long-term effects of agricultural practices on biological soil crusts and their influence on hydrological aspects of a semiarid ecosystem. During 1991, we experimentally simulated five of the area''s agricultural practices; 1) Scraping – the topsoil was removed to a depth of 2 cm, 2) Spraying – phototrophic organisms were chemically killed with herbicide, 3) Mowing – perennial vegetation was cut and spread to simulate grazing practices, 4) Car track – a heavy roller was used to simulate car-tracks, 5) Control – undisturbed natural plots. Sixteen years later, in 2007, these agricultural practices were found to have a long-term effect on the crusted soil surface and the related soil-surface properties. Mowing and car-track treatments led to decreased overland runoff and increased hydraulic conductivity, whereas scraping and spraying treatments led to increased overland runoff production and decreased hydraulic conductivity. We conclude that the practices had a long-term residual impact on BSC succession and related soil surface properties, which affected the hydrological processes and system functioning. [Copyright &y& Elsevier]

Summary: Runoff in arid and semiarid areas is characterized by high spatial and temporal variability. The spatial component is largely associated with the high spatial heterogeneity of soil surface attributes, such as vegetation and rock fragment covers, topography, and soil crust typology. Biological soil crusts (BSCs) are a common soil cover in arid and semiarid areas, and they play an essential role in local hydrological processes, since they affect many soil surface attributes associated with hydrologic properties. Although several publications have reported on the influence of BSCs on runoff at microplot spatial scales, only a few have examined their influence on larger spatial scales. Moreover, very few studies have analyzed the effect of BSCs on runoff under natural rain conditions. This is difficult, since a complex pattern of interactions among rainfall properties, BSC characteristics and some local characteristics, such as topography or type of soil is expected. In addition, in order to achieve a realistic model of how BSCs and rainfall affect runoff, it would be necessary to consider the level of human-driven degradation of BSCs. In this study, runoff was analyzed in plots with varying cover of cyanobacterial and lichen BSCs at microplot and small hillslope scales for two hydrological years in badlands in SE Spain. Structural equation modelling (SEM) was applied to test both direct and indirect relationships of BSC cover, slope gradient, rainfall characteristics and runoff. Our model showed that rainfall characteristics were the main factors controlling runoff yield. The slope positively affected runoff at small hillslope scales, but did not influence runoff at microplot scales. Runoff decreased at both scales with increased lichen-BSC cover. However, this effect was only significant during low-intensity events. Under high rainfall intensities, neither the BSC cover nor the slope had a causal effect on runoff. Our results suggest that incorporation of BSC-crusted surfaces in models should improve their capabilities for low-intensity rainfall events in semiarid areas similar to ours, but would be less important for high-intensity events. In addition, our approach, which takes direct and indirect relationships of factors affecting runoff into consideration, provides a very accurate picture of the process. [Copyright &y& Elsevier]

Abstract: Soil surface roughness has a strong influence on runoff and erosion, affecting surface storage capacity, water flow routing and velocity, and modifying runoff rates. It also reduces soil detachment by raindrops and the shear effect of water flow on the soil surface. In arid and semiarid ecosystems, biological soil crusts (BSCs) commonly appear in clearings between plants. Depending on the dominant component in the BSC community, the roughness of the soil surface may vary considerably, changing the hydrologic and erosive response of the soil. The aim of this study is to determine the effect of physical and biological crusts on soil surface roughness and their influence on runoff and erosion. For this purpose, we set up open plots containing different BSC types and treatments and recorded runoff and total erosion from all the events for 18months. Micro-topographic indexes were calculated from high-resolution digital surface models of the plots built from terrestrial laser scanner height data. After comparing different spatial resolutions and indexes, we concluded that only the local Random Roughness index in a 40-mm moving window provided a precise estimation of the roughness induced by BSCs, though it did not have a direct effect on runoff response. The best relationship between microtopography and runoff on biologically crusted soils was found for surface storage capacity, which appears as a powerful predictor of the runoff coefficient on long temporal scales. Sediment yield was not well predicted by any of the micro-topographic indexes studied. The only index that was significantly related to sediment yield was the local Random Roughness in a 40mm moving window, but even this explained only a third of the erosion variance. [Copyright &y& Elsevier]

To better understand the key role of biological soil crusts (BSCs) in redistribution processes of limited water resources in semi-arid sandy ecosystems, surface runoff over BSCs in a sandy area in the southern Mu Us sandy area was observed in this study. The results indicated that runoff occurred twice among six rainfall events during the experimental period. Runoff yields varied among types of BSCs, and that they increased in the following order: light algae crusts, dark algae crusts and moss crusts. These findings showed that runoff yields were increasing with the development of BSCs. The percentage of runoff in individual rainfall events increased from light algae crusts to moss crusts, which indicated that the redistribution ratios of different types of BSCs differed. Surface runoff of BSCs may be influenced by rainfall, rain intensity, degree of water saturation of the BSCs before the rain and degree of development of BSCs. Formation of BSCs on the surface of sand dunes changed the spatial distribution pattern of water in this semi-arid sandy ecosystem, which increased the heterogeneity of resources such as water and nutrients. Therefore, making appropriate disturbances of BSCs during storm season is beneficial to maintaining the balance of natural resources in the semi-arid sandy ecosystem. [ABSTRACT FROM AUTHOR]

Abstract: In arid zones, runoff is frequently generated as a result of the crust development on the soil surface. This crust is a thin layer of greater density, high shear strength, finer pores and has a lower saturated hydraulic conductivity than the underlying soil. The objective of the research reported herein was to study the factors that influence the generation of runoff in small plots under natural rainfall conditions. Factors studied were crust permeability, roughness, soil salt content and time gaps between the rain showers. The field trial was carried out in the Mashash experimental runoff farm in Israel''s Negev desert. Runoff was measured on eight plots using a tipping-bucket system (resolution 0.01 mm s−1). Rainfall intensity was recorded on-site with a rainfall gauge (resolution 0.25 mm s−1). Two treatments were studied: long-term rainfall-induced crusts (LTC) that had developed over a period of years (three plots), and complete destruction of the crust (ICU) by cultivation to a depth of 0.2 m with a rotary tiller before the beginning of the trial (five plots). Surface roughness was characterized by the surface RMS height obtained from laser micro-relief measurements before and during the season. Prior to the onset of rain, roughness was similar for all the ICU plots. One month thereafter, roughness had decreased sharply, but exhibited no further change until the end of the season. Roughness of LTC plots did not change during the season and was lower than that of the ICU plots. After ~21 mm of cumulative rain, the average runoff yield was similar for both ICU and LTC plots, even though roughness in the former did not reach the low values of the LTC plots. Although the variability in roughness among individual LTC plots was very small, large differences were observed in the collected runoff. The same phenomenon was observed for the ICU plots. Moreover, the runoff yields in two ICU plots were consistently higher than those in two LTC plots while three other ICU plots produced much less runoff. Apparent saturated hydraulic conductivity (AHC) was measured on mounds and depressions. In the ICU plots the average AHC of the mounds was markedly higher than that of the depressions while the AHC values of mounds were very similar. No mounds or depressions were observed in the LTC plots, and their AHC was similar to that of depressions in ICU plots. The results indicate that the apparent saturated hydraulic conductivity of the upper soil layer was not directly linked to the runoff generation. Treatment effect was significant only for the first two rainfall events, but the presence of salts in the upper soil layer significantly affected runoff generation during the last four rain events of the season. Analysis of runoff and time gaps between the runoff-producing rain showers showed a clear relationship between runoff yields and average rainfall intensity, the degree of correlation between them improving with a decrease in the length of the gap. [Copyright &y& Elsevier]