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Background and aims: Agroforestry systems combining trees with crops or pastures have been widely used to reduce water, soil, and nutrient losses and associated water pollution from agricultural lands in both temperate and tropical regions. However, reviews on improvement/efficiency and the scope of such reductions by soil, management, climate, and hydrological processes are limited. Methods: This paper synthesized the available evidence on the reduction in surface runoff, soil erosion, nutrient, and pollutant losses (e.g., herbicides, pesticides, and antibiotics) to quantify the effectiveness of agroforestry systems on water quality improvement based on published studies. Results: On average, agroforestry systems reduced surface runoff, soil, organic carbon, and related nutrient losses by 1–100%, 0–97%, –175–92%, and –265–100%, respectively, with average values of 58%, 65%, 9%, and 50%, respectively. They also lowered herbicide, pesticide, and other pollutant losses by –55–100% (49% on average). Conclusions: Reduction efficiency of agroforestry systems is site-dependent and varies widely depending on different biophysical factors. A comprehensive science-based review is needed to generalize agroforestry design and site adaptability for water and soil conservation where climatic, geographical, ecological, and socio-economic conditions are relatively similar in the world. [ABSTRACT FROM AUTHOR]

Background and Aims Root mechanical traits, including tensile strength (T r), tensile strain (ε r) and modulus of elasticity (E r), are key functional traits that help characterize plant anchorage and the physical contribution of vegetation to landslides and erosion. The variability in these traits is high among tree fine roots and is poorly understood. Here, we explore the variation in root mechanical traits as well as their underlying links with morphological (diameter), architectural (topological order) and anatomical (stele and cortex sizes) traits. Methods We investigated the four tropical tree species Pometia tomentosa, Barringtonia fusicarpa, Baccaurea ramiflora and Pittosporopsis kerrii in Xishuangbanna, Yunnan, China. For each species, we excavated intact, fresh, fine roots and measured mechanical and anatomical traits for each branching order. Key Results Mechanical traits varied enormously among the four species within a narrow range of diameters (<2 mm): <0.1–65 MPa for T r, 4–1135 MPa for E r and 0.4–37 % for ε r. Across species, T r and E r were strongly correlated with stele area ratio, which was also better correlated with topological order than with root diameter, especially at interspecific levels. Conclusions Root topological order plays an important role in explaining variability in fine-root mechanical traits due to its reflection of root tissue development. Accounting for topological order when measuring fine-root traits therefore leads to greater empirical understanding of plant functions (e.g. anchorage) within and across species. [ABSTRACT FROM AUTHOR]

Structure from Motion with Multi-View Stereo photogrammetry (SfM-MVS) is increasingly used in geoscience investigations, but has not been thoroughly tested in gullied savanna systems. The aim of this study was to test the accuracy of topographic models derived from aerial (via Unmanned Aerial Vehicle, 'UAV') and ground-based (via handheld digital camera, 'ground') SfM-MVS in modelling hillslope gully systems in a dry-tropical savanna, and to assess the strengths and limitations of the approach at a hillslope scale and an individual gully scale. UAV surveys covered three separate hillslope gully systems (with areas of 0.412-0.715 km2), while ground surveys assessed individual gullies within the broader systems (with areas of 350-750 m2). SfM-MVS topographic models, including Digital Surface Models (DSM) and dense point clouds, were compared against RTK-GPS point data and a pre-existing airborne LiDAR Digital Elevation Model (DEM). Results indicate that UAV SfM-MVS can deliver topographic models with a resolution and accuracy suitable to define gully systems at a hillslope scale (e.g., approximately 0.1 m resolution with 0.4-1.2 m elevation error), while ground-based SfM-MVS is more capable of quantifying gully morphology (e.g., approximately 0.01 m resolution with 0.04-0.1 m elevation error). Despite difficulties in reconstructing vegetated surfaces, uncertainty as to optimal survey and processing designs, and high computational demands, this study has demonstrated great potential for SfM-MVS to be used as a cost-effective tool to aid in the mapping, modelling and management of hillslope gully systems at different scales, in savanna landscapes and elsewhere. [ABSTRACT FROM AUTHOR]

Bed sediments of streams and rivers may store high concentrations of fecal indicator bacteria (FIB) and pathogens. Due to resuspension events, these contaminants can be mobilized into the water column and affect overall water quality. Other bacterial indicators such as microbial source tracking (MST) markers, developed to determine potential sources of fecal contamination, can also be resuspended from bed sediments. The primary objective of this study was to predict occurrence of waterborne pathogens in water and streambed sediments using a simple statistical model that includes traditionally measured FIB, environmental parameters and source allocation, using MST markers as predictor variables. Synoptic sampling events were conducted during baseflow conditions downstream from agricultural (AG), forested (FORS), and wastewater pollution control plant (WPCP) land uses. Concentrations of FIB and MST markers were measured in water and sediments, along with occurrences of the enteric pathogens Campylobacter , Listeria and Salmonella , and the virulence gene that carries Shiga toxin, stx 2 . Pathogens were detected in water more often than in underlying sediments. Shiga toxin was significantly related to land use, with concentrations of the ruminant marker selected as an independent variable that could correctly classify 76% and 64% of observed Shiga toxin occurrences in water and sediment, respectively. FIB concentrations and water quality parameters were also selected as independent variables that correctly classified Shiga toxin occurrences in water and sediment (54%–87%), and Salmonella occurrences in water (96%). Relationships between pathogens and indicator variables were generally inconsistent and no single indicator adequately described occurrence of all pathogens. Because of inconsistent relationships between individual pathogens and FIB/MST markers, incorporating a combination of FIB, water quality measurements, and MST markers may be the best way to assess microbial water quality in mixed land use systems. [ABSTRACT FROM AUTHOR]

Summary Annual total suspended solid (TSS) loads in the Mae Sa Catchment in northern Thailand, determined with an automated, turbidity-based monitoring approach, were approximately 62,000, 33,000, and 14,000 Mg during the three years of observation. These loads were equivalent to basin yields of 839 (603–1170), 445 (217–462), and 192 (108–222) Mg km −2 for the 74.16-km 2 catchment during 2006, 2007, and 2008, respectively. The yearly uncertainty ranges indicate our loads may be underestimated by 38–43% or overestimated by 28–33%. In determining the annual loads, discharge ( Q ) and turbidity ( T ) values were compared against 333 hand-sampled total suspended solid concentrations (TSS) measured during 18 runoff events and other flow conditions across the three-year period. Annual rainfall varied from 1632 to 1934 mm; and catchment runoff coefficients (annual runoff/annual rainfall) ranged from 0.25 to 0.41. Measured TSS ranged from 8 to 15,900 mg l −1 ; the low value was associated with dry-season base flow; the latter, a wet-season storm. Storm size and location played an important role in producing clockwise, anticlockwise, and complex hysteresis in the Q -TSS relationship. Turbidity alone was a good estimator for turbidity ranges of roughly 10–2800 NTU (or concentrations approximately 25–4000 mg l −1 ). However, owing to hysteresis and high sediment concentrations that surpass the detection limits of the turbidity sensor during many annual storms, TSS was estimated best using a complex multiple regression equation based on high/low ranges of turbidity and Q as independent variables. Turbidity was not a good predictor of TSS fractions >2000 μm. Hysteresis in the monthly Q -TSS relationship was generally clockwise over the course of the monsoon season, but infrequent large dry-season storms disrupted the pattern in some years. The large decrease in annual loads during the study was believed to be related to depletion of fine sediment delivered to the stream by several landslides occurring the year prior to the study. The study indicated the importance of monitoring Q and turbidity at fine resolutions (e.g., sub-hourly) to capture the TSS dynamics and to make accurate load estimations in this flashy headwater stream where hysteresis in the Q -TSS signature varied at several time scales. [ABSTRACT FROM AUTHOR]

Summary: We examined the effect of forest thinning on runoff generation at plot and catchment scales in headwater basins draining a Japanese cypress (Chamaecyparis obtusa) forest. We removed 58.3% of the stems (corresponding to 43.2% of the basal area) in the treated headwater basin (catchment M5), and left the control catchment (M4) untreated. In both catchments, we monitored overland flow from hillslope plots and stream runoff from catchments at basin outlets over a 2-year pre-thinning period and a 2-year post-thinning period. Paired catchment analysis revealed that annual catchment runoff increased 240.7mm after thinning. Delayed runoff increased significantly, while quick runoff followed similar patterns in the pre- and post-thinning periods. Flow duration in the ephemeral channel in catchment M5 increased from 56.9% in the pre-thinning period to 73.3% in the post-thinning period. Despite the changes in hydrological responses at the catchment scale, increases in overland flow were not significant. The increased availability of water in the soil matrix, caused by decreased interception loss and evapotranspiration, increased base flow after thinning. Based on the summarized data of previous studies together with this study, the effects of forest thinning on increases in runoff were less than partial harvesting in which the managed areas were concentrated within a watershed. We demonstrated that the effect of forest thinning was strongly scale dependent, an important finding for optimizing water and forest management in forested watersheds. [Copyright &y& Elsevier]

Numerical models are invaluable for the provision of real-time and forecasting information that can be used to examine estuarine hydrodynamics, particularly during times of flood or contaminant release. However, model outputs are associated with uncertainty; this necessitates the use of data assimilation (DA) techniques to improve model accuracy. We used an open-source DA tool to effectively assimilate Lagrangian drifter data into an estuarine hydrodynamic model using an ensemble Kalman filter (EnKF) algorithm. Our aims were to (i) evaluate the potential of drifter data for improving the accuracy of model estimates, and (ii) reduce the challenge and programming effort required for assimilation of such datasets, to make this technique accessible, for a broader range of users. We showed that assimilation of Lagrangian data obtained from prompt deployment of drifters in estuaries can lead to significant improvement (here, up to 54%) in modelled velocity fields. • Drifter data was assimilated into a hydrodynamic model using open-source software. • Assimilation of drifter data reduced model errors by up to 54%. • Data assimilation performance was strongly dependent on frequency of assimilation. [ABSTRACT FROM AUTHOR]

Data assimilation (DA) is an essential element for the next generation of operational forecast systems for estuaries, to improve estuarine management. With limited resources and prohibitive cost to collect observations for such system, sensor choice and location is of prime importance in improving hydrodynamic model performance. In this study, we examine an optimal ensemble-based DA platform for improving the hydrodynamic modelling of a shallow estuary. Using an ensemble Kalman filter (EnKF), a set of synthetic (twin) experiments was conducted to test different DA scenarios covering observation types (i.e. water level and velocity) and noise modelling. We also evaluated the impact of the observation location on the DA performance by performing an observing system simulation experiment (OSSE). Results revealed that the assimilation of a single variable can significantly enhance the accuracy of the variable being assimilated, while the level of improvement for another variable is smaller. However, the best model estimates were obtained via a multivariate EnKF (i.e. both observations are assimilated). EnKF was robust to under and overestimation of the model errors, although overestimation led to slightly greater improvements. Our analysis showed that model performance is more sensitive to velocity observation location, rather than water level. These findings suggest that locations with strong velocity gradients are the locations where the hydrodynamic model needs to be enhanced, and accordingly, they are the preferable locations to deploy a velocity sensor. • An observing system simulation experiment to assess impact of observation type and location on model estimates in an estuary. • The assimilation of different types of observations resulted in different effects on model estimates. • Results showed that the assimilation efficiency is more sensitive to velocity observation location, rather than water level. [ABSTRACT FROM AUTHOR]

• Vertical unsaturated flow can contribute to rapid storm runoff responses. • Runoff-storage relationships in runoff models are derived from the Richards equation. • Runoff models must aim at specifying each effect of vertical and lateral processes. Numerical experiments on vertical unsaturated flow using the Richards equation were conducted to examine the physical basis of why storm runoff responses from mountainous watersheds in tectonically active regions could be simulated by simple runoff models with a runoff-storage power-law relationship. Pressure head propagation transmitted through vertical unsaturated flow could produce a rapid response of the outflow from bottom of the soil column, regardless of the inherent soil hydraulic properties, similar to storm runoff responses observed on a hillslope in a 'constant allocation period' when a large constant portion of rainfall is allocated to storm runoff. Additional experiments were conducted both for an increasing stage of downward flux in response to increasing rainfall intensity and for a recession stage without rainfall, to investigate the detailed characteristics of vertical unsaturated flow. The creation of a wetting front during the increasing stage delayed the increase of outflow, but this delay was small during a constant allocation period. An interdependent relationship between the outflow rate and the total storage of the soil column throughout the recession stage was approximated by a power-law equation derived from relationships between total storage and constant outflow under steady-state conditions. The exponent of the power-law equation approached a maximum of unity as the column length decreased, and it approached the minimum value obtained from the intrinsic relationship between soil hydraulic conductivity and volumetric water content as the column length increased. The interdependence of the outflow rate with the total storage generally detected only in the unsaturated zone might cause a low sensitivity of outflow responses to the heterogeneities of soil hydraulic properties, justifying the application of simple runoff models in heterogeneous mountain watersheds. [ABSTRACT FROM AUTHOR]

Erosion and mass wasting processes on mountain slopes can benefit from or be adversely affected by the presence of biopores formed by plant root systems or soil fauna. The relationship between biopores and subsurface flow during rainstorms is poorly understood. Here, we examined the link between subsurface flow and biopores formed through different processes, including soil faunal activity and abundance of fine and coarse roots. As the distribution of biopores is influenced by the type of vegetation present, we investigated the effect of plant diversity (forest with or without understorey vegetation) on the pattern of water infiltration throughout the soil. We hypothesized that increased species diversity would enhance the extension of subsurface flow because biopores would be distributed throughout the soil profile and that more coarse roots would create large biopores, increasing subsurface flow. In situ experiments were conducted on hillslopes with plantations of rubber trees (Hevea brasiliensis) growing on terraces, or with secondary mixed forests, in the tropical zone of Yunnan province, China. Three sites with Ferralsol soils and different vegetation types were examined: (1) plantation with no understorey; (2) clear-cut plantation with understorey; and (3) secondary mixed forest with understorey. Irrigation experiments with Brilliant Blue FCF dyed water were performed upslope of trees at each site and staining patterns resulting from infiltrated dyed water were examined at two different scales. After dye irrigation, soil was removed in 1.0 × 0.8 m slices starting 1 m downslope and soil profiles were photographed for subsequent mapping of dyed areas in the profile (macroscale). Each profile was then divided into a 0.1 × 0.1 m grid (microscale) and burrows formed by macrofauna and fine and coarse root densities were measured. At the macroscale, the greatest lateral extension in subsurface flow occurred in the natural forest and the least in the rubber tree plantation with no understorey vegetation. At the microscale, and in all types of vegetation, fine roots significantly increased the incidence of subsurface flow compared to coarse roots and macrofauna activity. We conclude that in tropical Ferralsols, fine roots, and hence understorey vegetation, play a positive role in promoting subsurface flow and therefore reducing water erosion and mass wasting processes. Thus, planting mixtures that include a diversity of species and strata could significantly improve soil conservation. • Hydrological processes in soil are affected by biopores. • Irrigation tests in forests show that fine roots increase preferential flow. • Coarse roots and macrofaunal activity had little effect on preferential flow. • Preferential flow was highest in mixed forest compared to rubber tree plantations. • Diversifying planting mixtures would reduce runoff and improve soil conservation. [ABSTRACT FROM AUTHOR]

Overland transport of fecal bacteria in water and their resuspension from bed sediments are important transport mechanisms that help explain the transport of enteric pathogens in watersheds. In this study, multiyear monitoring along with regression relationships between sediment and fecal indicator bacteria (FIB) were used to investigate annual loading in the South Fork Broad River watershed, located in northeastern Georgia, USA. Suspended transport was the dominant transport mechanism contributing to in-stream total annual loads for sediment (81.4-98.1%) and FIB (>98%). Annual bedload transport of FIB was small and Escherichia coli (up to 1.8%) contributed more to annual bedload FIB than enterococci (≤0.03%). Bedload contributions of FIB increased with the duration of critical discharge exceedance, indicating a prolonged risk of exposure to enteric pathogens during extended periods of high flows, which is important during major storm events. The risk of exposure to enteric pathogens through pathways such as recreational use and drinking water treatment could be much greater because fecal bacteria are released from sediment during higher flows and dominantly transported in suspension when bedload are not actively moving. Therefore, the combined contribution of fecal bacteria from overland and bedload-associated transport should be considered in risk assessments. Discharge, bedload, and FIB data collected over 2 yr in this study can supplement future hydrologic modeling and microbial risk assessment modeling efforts., (© 2020 The Authors. Journal of Environmental Quality © 2020 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America.)

Major earthquakes cause widespread environmental and socioeconomic disruptions that persist for decades. Extensive ground disturbances that occurred during the shallow-focus Kumamoto earthquakes will affect future sustainability of ecosystem services west of Aso volcano. Numbers of earthquake-initiated landslides per unit area were higher in grasslands than forests, likely owing to greater root reinforcement of trees, and mostly initiated on ridgelines and/or convex/planar hillslopes. Most landslides traveled short distances and did not initially evolve into debris flows; resultant sediments and wood accumulating in headwater channels can be mobilized into debris flows during future storms. Fissures along ridgelines may promote water ingress and induce future landslides and debris flows that affect residents downstream. Native grasses are at risk because of habitat fragmentation caused by ground disturbances, extensive damage to rural roads, and abandonment of traditional pasture management practices. Sustainable management of affected areas needs to consider future risk of cascading hazards and long-term socioeconomic impacts.

Although the concept of ecosystem sustainability has a long-term focus, it is often viewed from a static system perspective. Because most ecosystems are dynamic, we explore sustainability assessments from three additional perspectives: resilient systems; systems where tipping points occur; and systems subject to episodic resetting. Whereas foundations of ecosystem resilience originated in ecology, recent discussions have focused on geophysical attributes, and it is recognized that dynamic system components may not return to their former state following perturbations. Tipping points emerge when chronic changes (typically anthropogenic, but sometimes natural) push ecosystems to thresholds that cause collapse of process and function and may become permanent. Ecosystem resetting occurs when episodic natural disasters breach thresholds with little or no warning, resulting in long-term changes to environmental attributes or ecosystem function. An example of sustainability assessment of ecosystem goods and services along the Gulf Coast (USA) demonstrates the need to include both the resilient and dynamic nature of biogeomorphic components. Mountain road development in northwest Yunnan, China, makes rivers and related habitat vulnerable to tipping points. Ecosystems reset by natural disasters are also presented, emphasizing the need to understand the magnitude frequency and interrelationships among major disturbances, as shown by (i) the 2011 Great East Japan Earthquake and resulting tsunami, including how unsustainable urban development exacerbates geodisaster propagation, and (ii) repeated major earthquakes and associated geomorphic and vegetation disturbances in Papua New Guinea. Although all of these ecosystem perturbations and shifts are individually recognized, they are not embraced in contemporary sustainable decision making.