Reduction In Sediment Concentration Research Articles

Climate change as main driver of centennial decline in river sediment transport across the Mediterranean region

The analysis of suspended sediment transport and of its variations over time is crucial for understanding environmental evolution and it is the key to future challenges caused by current global warming. The Mediterranean area is a hot spot for global changes, and the variation of precipitation amount and intensity will modify the environment of this region.In this work, we analyse the time series of suspended sediment transport of two rivers located in central Italy by using statistical and artificial intelligence techniques. Our study aims to re-analyze time series of suspended sediment transport, in order to demonstrate that climate change is responsible for the substantial decrease in the amount of sediment over the past century, in relation to the atmospheric teleconnections of the North Atlantic. Anthropic pressures like reforestation, land use change, and dam building have influenced the sediment transport capacity of rivers, causing a reduction of sediment concentration in water.These results are key factors to determine the future management of the Mediterranean areas, where the future scenarios predict a greater drop in yearly precipitation, and therefore a possible further decrease in the sediment transport capacity of the rivers, with major consequences for coastal and fluvial environments.

Evolution characteristics of mid‐channel bars with a typical high/low bank structure in the Middle Yangtze River

AbstractAdjustments in mid‐channel bars (MCBs) often influence the stability of channel regime and the safety of river navigation in anabranching reaches. This study aims to investigate the evolution characteristics of the low and high parts of MCBs (L‐MCBs and H‐MCBs) in the Middle Yangtze River (MYR) and reveal the main factors influencing the deformation of these two components. Results show that the total exposed area of L‐MCBs decreased from 305 to 288 km2 in 2003–2019, whereas the H‐MCBs remained relatively stable. Taking the Nanyang bar with a typical high/low bank structure as an example, the deformation of the L‐MCB mainly occurred in the head zone, while the deformation of the H‐MCB was characterised by bank retreat in the tail zone and floodplain deposition on the top of the H‐MCB. In terms of influencing factors, the impacts of the altered flow–sediment conditions and the MCB's properties (soil composition and morphology characteristics) on the Nanyang bar deformation were investigated. On the one hand, the reduction in sediment concentration was the most direct factor causing the shrinkage of L‐MCBs, and the decrease in flood occurrence frequency made the deformation of H‐MCB less significant. On the other hand, the soil composition of the L‐MCB primarily consisted of fine sand with weak erosion resistance, while the large content of cohesive sediment made the H‐MCB difficult to be eroded by overbank flows; moreover, the riverbank of the H‐MCB in the bar‐tail zone was much steeper than that in the bar‐head zone, causing the deformation of the high part to mainly occur in the tail zone. Furthermore, the deformation of the high part was simulated using a bank erosion model at section scale, and the calculated safety factors of riverbank at a typical cross‐section show that the riverbank of the H‐MCB was the most unstable during the receding period.

Stability of Aquatic Nitrogen Cycle Under Dramatic Changes of Water and Sediment Inflows to the Three Gorges Reservoir

Stability of nitrogen cycle is a key indicator to aquatic health. In recent years, water and sediment inflows to the Three Gorges Reservoir (TGR) have changed significantly. To reveal the effects of such dramatic hydrological changes on aquatic nitrogen cycle, this paper at first analyzed the changing trends of water and suspended sediment discharges of TGR based on dynamic harmonic regression, and found that the intra‐year distribution of water flow was significantly homogenized between flood and dry seasons, with the seasonal variations narrowed by 43.5%–69.9% during 2007–2016, while sediment concentration sharply dropped (the non‐periodic term decreased by 1.48%–2.07%/month). Modified with the effects of sediment concentration variations on nitrification/denitrification rates, the proposed numerical model surprisingly showed that ammonia nitrogen and total nitrogen concentrations in TGR were insensitive to either water flow homogenization or sediment reduction, implying relative stability of microbial community related to nitrogen cycle, which is a positive sign for aquatic health. However, N2 emission varied more violently. The variation range of nitrogen gas (N2) emitted from TGR enlarged by 30% with the homogenization of water inflow from 2010 to 2016, while the annual total N2 emission decreased by 7% due to the reduction of sediment concentration, indicating quick response and strong adaption of the microbial N2 producing process to the environmental changes of TGR, which is beneficial for maintaining ecological functions related to nitrogen cycling. This work helps understanding nitrogen cycle of reservoirs experiencing dramatic changes in water and sediment inflows.

Divergent change patterns observed in hydrological fluxes entering China's two largest lakes

Large lakes are key components of hydrological processes and have critical ecological and economic functions. Streamflow and riverine sediment loads exported from tributary rivers to lakes provide inputs of water and nutrients and greatly impact the health of lacustrine ecosystems. Understanding the spatiotemporal patterns of water and sediment transport dynamics within the basins of large lakes is therefore required for informed management of their resources. In particular, the hydrological processes of large lake basins in western China were not fully explored yet. This study investigated changes in runoff and sediment fluxes entering the two largest lakes in China (Qinghai Lake and Poyang Lake) over the past half century. Our results showed that the regional trends of the two lake basins have changed in divergent way. The patterns of change of streamflow and sediment transport—including the trends, periodicity, and transition points as well as the relationship between streamflow and sediment flux—were quite different. The results obtained by a Sediment Identity method revealed that increases of the precipitation and runoff coefficient controlled the observed increase of streamflow and sediment flux within the Qinghai Lake basin, whereas the decrease of the sediment flux entering Poyang Lake was caused mainly by a reduction in sediment concentration. Climate change was the dominant driver influencing the recent increases in surface water resources and sediment load in the Qinghai Lake basin as well as the long-term fluctuations of streamflow within the Poyang Lake basin. In contrast, the observed abrupt reduction in total sediment flux entering Poyang Lake was due mainly to declining sediment discharge from the Gan River; this decrease is attributed primarily to human activities, especially reservoir construction and soil conservation practices. Overall, this study discovers the divergent change patterns when investigating hydrological processes, and highlights the importance of developing policies for managing soil and water resources within large lake basins to the northwest and southeast of the Hu Line. Management policies should consider the tremendous differences in geography, climate, and population across China.

Lab simulation of soil erosion on cultivated soil slopes with wheat straw incorporation

Sloping lands are prone to heavy soil erosion, resulting in farmland degradation in most mountainous regions. Straw incorporation can enhance resistance to erosion by improving the soil structure and reducing runoff. Laboratory-simulated rainfall experiments were conducted on silt loam soil to measure the effect of wheat straw incorporation on soil erosion and sediment delivery processes. Different wheat straw incorporation rates (WSIRs: 0, 2, 4, and 8 t ha−1), slope gradients (SGs: 10°, 15°, and 20°), and rainfall intensities (RIs: 80, 120, and 160 mm h−1) were evaluated. The results showed that the total rainfall amount was set at 100 mm. The sediment delivery processes were measured using oven-dried runoff samples collected during the specific intervals. The experimental data revealed that sediment delivery processes increased with increasing rainfall duration and decreased with increasing straw incorporation rates, respectively. Straw incorporation significantly reduced soil losses under steep slopes (20°) and during high-intensity rainfall (160 mm h−1). Compared with the control plot, the sediment concentrations, sediment delivery rates, and soil losses of slopes with straw incorporation were reduced by 52%, 62%, and 63%, respectively. The reduced sediment delivery rate and soil loss be attributed to increased soil surface roughness (exposed straw) that intercepted runoff, increased soil infiltration, and reduced flow velocity. The incorporated straw indirectly reduced sediment concentration by improving soil structure, which resulted in a smaller reduction in sediment concentration. The sediment reduction was maximized at 80 mm h−1 or 10°, while the benefits of straw incorporation on soil and water conservation were reduced with higher rainfall intensities and slope gradients. This study can serve as a basis for guiding soil and water conservation on sloping farmlands.

Preliminary Experimental Study on Effectiveness of Aquatic Vegetation on Sediment Transport Capacity

Vegetation affects sediment transport by obstructing the flow and changing the turbulence characteristics. Common sediment transport equations are not applicable to situations with submerged vegetation. A laboratory experiment was carried out in which flow, vegetation, and sediment transport were measured in an open channel model with a 240 cm long section of 50 cm high x 50cm width with submerged vege-tation in a sand bed. Measured data from various vegetation density and height were analyzed to obtain estimates of the trapping efficiency. Results show an increment of the trapping efficiency with 86.7% and 94.2% reduction in sediment concentration compared to a case without vegetation.

Impact of BMPs on water quality: a case study in Big Sunflower River watershed, Mississippi

ABSTRACT Impact of Best Management Practices (BMPs) can be evaluated using hydrologic and water quality models. Although numerous models with diverse function, capability and degree of complexity are available, suitable model for each watershed should be determined. The Soil and Water Assessment Tool (SWAT) and Hydrologic Simulation Program-Fortran (HSPF) models were applied to the Big Sunflower River Watershed (BSRW) and their performances in simulating hydrology and water quality were evaluated. Both the models simulated streamflow, sediment, and nutrient concentrations with model efficiency greater than 50%. The SWAT model simulated streamflow and sediment concentration more accurately than HSPF whereas, for total nitrogen (TN) and total phosphorous (TP) concentrations, the HSPF model simulated equally good as SWAT. Models evaluated effect of two BMPs: vegetative filter strip (VFS) and tailwater recovery pond (TRP) in reduction of streamflow, sediment, TN, and TP concentrations. Average reduction of streamflow due to the implementation of both BMPs in SWAT and HSPF were less than 1% but average reduction of sediment concentration by VFS in SWAT was 26% and in HSPF was 38%. Average reduction of sediment concentration by TRP in SWAT and HSPF were 21% and 30% respectively. VFS reduced TN concentration by 51% in SWAT and by 25% in HSPF, while average reduction of TN concentration by TRP in SWAT and HSPF were 7% and 2% respectively. Similarly, average reduction of TP concentration by VFS in SWAT and HSPF were 56% and 31% respectively and that by TRP in SWAT and HSPF were 2% and 1% respectively. Differences in simulation results based on application of two models were mainly attributed by the modelling mechanism and equations used. The results from this study will provide a broader idea to other modellers and end-users in selecting appropriate model according to their need and type of watershed.

The hydrologic behavior of Loess and Marl soils in response to biochar and polyacrylamide mulching under laboratorial rainfall simulation conditions

Due to the high costs associated with the control of soil erosion, the precise selection of practical techniques towards this end is a prerequisite for sustainable land management. For the proper application of anti-erosion measures, a good knowledge of the behavior of soil erosion and hydrological properties is required. Research has indicated that biochar (BC) and polyacrylamide (PAM) are important soil amendments to enhance the physical and hydrological characteristics of soil. However, little is known about the response and mechanisms underlying the wearing away of Marl and Loess erosion-prone soils treated with individually and with a combination of BC and PAM. Therefore, we tested the effectiveness of these anti-erosion techniques on the variability of 1) hydrological components (i.e., time to runoff, the runoff coefficient, and infiltration), 2) runoff quality components (i.e., pH and electrical conductivity (EC)), and 3) erosion components (i.e., upward-splash, downward-splash, net-splash erosion, soil loss, and sediment concentration) at two locations with the Marl and Loess soils under rainfall simulation conditions. The study treatments consisted of control (200 ml water), BC (800 g m−2), PAM (2 g m−2), and BC (800 g m−2) + PAM (2 g m−2). The treatments were sprayed uniformly over the small plots (0.3 × 0.5 × 0.5 m in dimension; 0.25 m2 in area) with a slope of 20% using three replicates. Rainfall with an intensity of 50 mm h−1 for 0.5 h was applied. The results showed that (a) the use of PAM alone delayed the time to runoff with a rate of 41.4% compared to the control treatment in the Marl soil, and all treatments delayed the time to runoff for the Loess soil (BC = 37.1% and PAM = BC + PAM = 12.9%); (b) among the study treatments, PAM had the greatest effect as it decreased the runoff coefficient by 2.47% and 13.67%, and improved infiltration by 0.02% and 0.13% in Marl and Loess soils, respectively; (c) BC and BC + PAM increased the pH values of the runoff whereas the application of PAM partially reduced the pH compared to the control plot in both the soils studied; (d) the application of BC and BC + PAM significantly (p < 0.05) increased the EC of runoff released from both tested soils. In contrast, PAM had the least influence on the EC of the runoff compared to the control condition; (e) the maximum effect on the reduction of soil loss was attributed to BC for Marl and Loess soils with rates of −14.07% and −83.21%, respectively. Additionally, the maximum benefit of the reduction in sediment concentration was attributed to PAM (-14.87%) and BC (−78.35%), respectively, in Marl and Loess soils; (f) all treatments reduced the upward-splash, downward-splash, and the net-splash erosion for both soils studied in the range of −13.69% (BC + PAM in Loess soil) to −73.68% (BC in Marl soil) in comparison with the control treatment.

Controlling runoff generation and soil loss from field experimental plots through inoculating cyanobacteria

Controlling runoff generation and soil loss from hillslopes is a priority for land management. The capability of cyanobacteria as inoculants has recently been utilized in land management, and as a result various lab-scale studies have been conducted. The present study conducted a field-pilot experiment in Miandoab City, south of West Azerbaijan Province, North West Iran, to evaluate the success of cyanobacteria inoculation in runoff and sediment reduction from an abandoned and degraded land at large plot scale under natural rainfall. On 26 December 2017, the cultured native cyanobacteria (Nostoc sp. and Oscillatoria sp.) was inoculated on the 22.1 × 1.83 m2-plot in three replicates along with three control plots treated by pure water. The runoff and soil loss from the treated plots were measured for seven rainfall events that produced runoff during normal rainy season of the study area i.e., December 2017 to May 2018. Results showed that inoculation of cyanobacteria significantly (p < 0.05) decreased runoff by some 35, 25, 31, 33, 46, 47, and 57% for the recorded rainfall events (1 to 7), respectively. The beneficial reduction of sediment concentration due to the inoculation of cyanobacteria was also observed for rainfall events 1, 2, 3, and 5 at the respective tune of 16, 34, 36, and 135% compared to those of control plots. Soil cracks indicators, roughness, and aggregate stability were also improved by 5.1 to 27%, 14 to 20%, and 13 to 22%, respectively, owing to the inoculation of cyanobacteria. Although vegetation cover at the end of experiment was 2.6 times denser than that in the beginning, there was no significant difference between inoculated and control plots. By and large, field inoculation of cyanobacteria was found as an efficient technique for runoff and soil loss inhibition, particularly in degraded and abandoned drylands.

The impacts of porous rock check dams on a semiarid alluvial fan

Low-tech rock check dams are widely used to address land degradation; however, assessments of their impacts on runoff and sediment are lacking and are often limited to the first few years after construction. In 2008, two small (4.0 and 3.1 ha) instrumented watersheds located 300 m apart on an alluvial fan on the Santa Rita Experimental Range in southern Arizona were treated with 37 porous, loose rock check dams. Ten years after construction, the watersheds are experiencing contrasting responses to treatment. The ratio of runoff to precipitation was higher after check dam construction on one watershed and lower on the other, but not significantly in either case. Statistically significant changes in peak runoff rate are not detectable 10 years after construction at either watershed. However, a statistically significant reduction in sediment concentration was found on one watershed and no change was found at the other. The check dams have altered channel grades. However, backfilling of the dams is nearly complete on one watershed and the other has remaining capacity. The alluvial fan setting poses a complex restoration environment due to high sediment loads that deposit in response to vegetated areas of accumulated sediment creating conditions for channel avulsion and new incising concentrated flow paths. Check dams have a lesser impact on watershed outlet runoff and sediment than on internal watershed channel morphology and vegetation establishment. With monitoring and maintenance, check dams can be an effective tool for grade stabilization, but watershed restoration requires additional practices to address degraded interfluve areas.

Effects of restoration practices on controlling soil and water losses in the Wei River Catchment, China: An estimation based on longitudinal field observations

Understanding the soil and water processes and their interactions over time and space has attracted research and policy attention worldwide. But past studies have rarely adopted an interdisciplinary approach to integrating the primary natural and social forces in assessing the causes of soil and water loss or conservation. The purpose of this study is to evaluate the effects of restoring degraded cropland by means of reestablishing forest/vegetation covers and terracing on water and sediment control in the Wei River Catchment, which was notorious for its high erosion rates but has witnessed tremendous improvement since the beginning of the new century. Based on a unique panel dataset covering 44 counties over a period of 16 consecutive years (2000–2015), our simultaneous equations model was able to capture the structural linkages between water runoff, sediment concentration, and sediment transport. To highlight the crucial role of human interventions, a stepwise regression approach was undertaken. Our empirical results show that the reduction of sediment concentration has been driven largely by improved land cover, induced by implementing the Sloping Land Conversion Program, whereas terracing, as part of the Water and Soil Conservation Project, has played a positive role in mitigating runoff and sediment discharge.

Distribution of erosion intensity in the Jingjiang reach influenced by the Three Gorges Dam

AbstractElucidation of the fluvial processes influenced by dams provides better understanding of river protection and basin management. However, less attention has been given to the erosion intensity distribution of riverbeds and its association with channel morphology and hydrological conditions. Based on hydrological and topographic data, the spatial and temporal distributions of erosion intensity (2002–2014) influenced by the Three Gorges Dam (TGD) were analyzed for the Jingjiang reach of the Yangtze River. The mechanisms underlying the distribution of erosion intensity in response to hydrological conditions were investigated. The results are as follows: (1) The erosion intensities of different discharges were not uniform, and moderate flow (10 000–27 000 m3/s) produced the largest erosion magnitude among all flow ranges. Owing to the hydrological changes caused by flood reduction and prolongation of moderate flow duration after the TGD began operating, up to 70% of the erosion amount was caused by moderate flows. (2) The lateral distribution of erosion intensity was extremely uneven, as the proportion of cumulative erosion of the low‐flow channel within the bankfull channel reached 88% in 2013. This caused the channel to become narrower and deeper. (3) The longitudinal distribution of erosion intensity was inhomogeneous. The erosion intensity in the wide reaches was greater than that in the narrow reaches, leading to smaller differences in channel morphology along the river. (4) Changes in hydrological conditions influenced by the TGD, significant reduction of sediment concentration along with flood abatement, and increased duration of moderate flow discharges were the main factors affecting erosion distribution in the post‐dam period. Our conclusions can be applied to the Yangtze River as a basis for riverbed change estimations, and river management strategies. Copyright © 2018 John Wiley &amp; Sons, Ltd.

Driving forces and their contribution to the recent decrease in sediment flux to ocean of major rivers in China

Understanding the mechanisms behind land–ocean sediment transport processes is crucial, due to the resulting impacts on the sustainable management of water and soil resources. This study investigated temporal trends and historical phases of sediment flux delivered to the sea by nine major rivers in China, while also quantifying the contribution of key anthropogenic and natural driving forces. During the past six decades, sediment flux from these nine major rivers exhibited a statistically significant negative trend, decreasing from 1.92Gtyr−1 during 1954–1968 to 1.39Gtyr−1, 0.861Gtyr−1 and 0.335Gtyr−1 during 1969–1985, 1986–1999 and 2000–2016, respectively. We used a recently developed Sediment Identity approach and found that the sharp decrease in sediment load observed across China was mainly (~95%) caused by a reduction in sediment concentration. Reservoir construction exerted the strongest influence on land–ocean sediment fluxes, while soil conservation measures represented a secondary driver. Before 1999, soil erosion was not controlled effectively in China and reservoirs, especially large ones, played a dominant role in reducing riverine sediments. After 1999, soil erosion has gradually been brought under control across China, so that conservation measures directly accounted for ~40% of the observed decrease in riverine sediments. With intensifying human activities, it is predicted that the total sediment flux delivered to the sea by the nine major rivers will continue to decrease in the coming decades, although at a slower rate, resulting in severe challenges for the sustainable management of drainage basins and river deltas.

Total Phosphorus Concentration Trends in 40 Iowa Rivers, 1999 to 2013.

Excessive phosphorus (P) in rivers is prompting states to develop strategies to reduce P concentrations and export. The goal of this study was to assess the current condition by analyzing trends in total P (TP) concentrations at 40 river monitoring sites in Iowa for the period 1998 to 2013. We used monthly river monitoring data collected by the State of Iowa at ambient sites located beyond the influence of point sources or cities. Study objectives were to assess the presence of any linear trends in the TP concentration data using a time-series method that accounted for temporal correlation in the data and discharge and to combine the trend information from individual sites into an assessment of the statewide rate of change in river TP concentrations. Results indicated that annual TP concentrations were significantly decreasing at 12 sites ( < 0.05), with concentrations ranging from 7.5% (Maple River) to 2.6% (Boyer River) and averaging 4.1% for the 12 sites. No statistically significant change was measured at 28 sites ( > 0.05), although all but four of these sites had decreasing concentrations. As a population of sampled rivers across Iowa, TP concentrations were declining at an annual rate of approximately 2.6%. The decreasing trends are consistent with reported reductions in sediment concentrations measured in the Raccoon River and with decreasing trends detected in other midwestern rivers. We attribute the decreasing TP trends primarily to improvements in conservation and land management. Although reducing nonpoint source TP concentrations in Iowa rivers will be a tremendous challenge for the agricultural community, progress is being made toward meeting this goal with existing programs.

Reducing sediment concentration and soil loss using organic and inorganic amendments at plot scale

Abstract. Various organic and inorganic mulches are used for soil conservation purposes, the effectiveness of which on soil characteristics has not been comprehensively considered from different aspects. The present study surveys the efficiency of straw mulch, manure and TA-200 polyacrylamide with respective rates of 500, 300 and 50 g m−2 in changing sediment concentration and soil loss. The experiments were conducted for sandy-loam soil taken from a summer rangeland, the Alborz Mountains, northern Iran. The experiments were performed under laboratory conditions with simulated rainfall intensities of 30, 50, 70 and 90 mm h−1 and a slope of 30%. The results showed that the straw mulch decreased soil erosion at rate of 45.60% compared to the control plots and performed better than manure (8.98% reduction) and PAM (4.74% reduction). The results showed that the maximum reduction in sediment concentration and soil loss for all soil amendments occurred at the rainfall intensity of 90 mm h−1 with the rates of 58.69 and 63.24% for straw mulch, 14.65 and 13.14% for manure and 20.15 and 23.44% for TA-200.

Performance evaluation of two silt fence geotextiles using a tilting test-bed with simulated rainfall

This paper presents the results of the investigation of the performance efficiencies of silt fence fabrics in turbidity and sediment concentration removal, and the determination of flow-through-rate on simulated construction sites in real time. Two silt fence fabrics, (1) a woven type and (2) a nonwoven type, were subjected to material index property tests and a series of field-scale tests. The fabrics were tested for removal efficiency by varying the rainfall intensities and events for different embankment slopes on a tilting test-bed. Collected influent and effluent samples were analyzed for sediment concentration and turbidity, and the flow-through-rate for each fabric was evaluated. Test results revealed that the woven and nonwoven silt fence achieved 14 and 52 percent average turbidity reduction efficiency, and 23 and 56 percent average sediment concentration removal efficiency, respectively. Evaluation of sediment concentration reduction based on percent removal does not correctly account for the sediment concentration transported and deposited downstream. Fabric flowrates were functions of the rainfall intensity, embankment slope and field conditions, and fluctuates with every rainfall event.

Using the Soil and Water Assessment Tool (SWAT) to Assess Material Transfer in the Layawan Watershed, Mindanao, Philippines and Its Implications on Payment for Ecosystem Services

Soil and water are the most important resources in the watershed. The alterations on the quantity and quality of these resources do not only have on-site impacts, but are experienced by off-site communities as well. To assess the material transfer and identify critical sub-basins in the Layawan Watershed, Mindanao, Philippines; the Soil and Water Assessment Tool (SWAT) Model was used. The model was also used to investigate changes in land use. Results show that a 4% reduction in sediment concentration and sediment yield in the critical sub-basins will be achieved if the community-based watershed management plan is implemented. On the other hand, there will be a 106% increase in sediment concentration and sediment yield if forests are cleared for utilization, primarily for agriculture, in the critical sub-basins. Modeling sediment yield and sediment concentration is important to help policy makers, environmental managers, and development agencies predict the impact of activities on soil and water quality, as well as guide them in the implementation of payments for ecosystem services (PES) schemes. The quantification of ecosystem services has been a major challenge surrounding the success of PES. In the Layawan Watershed, it is shown that land change use will not likely affect water quantity, it will, however, heavily impact water quality. Modeling provides an avenue to manage watersheds effectively and efficiently. SWAT running on open source GIS could help budget-constrained government units and development agencies to better predict the impact of programs and projects on watersheds.

Effects of tillage intensity and anionic polyacrylamide on sediment and nitrogen losses in irrigated wheat field

Degradation of agricultural soils and nutrient losses affected by intensive agriculture and tillage is of environmental and agricultural concerns. These concerns lead to emergence and development of conservative technologies such as conservation tillage systems [reduced tillage (RT) and no-till (NT)] and anionic polyacrylamide (PAM). A study was done in order to investigate the interactive effects of three tillage systems and three PAM concentration on sediment loss, runoff nitrate concentration, nitrogen losses from the soil-plant system and nitrogen recovery. The experimental design was a randomized complete block with split-plot arranged in three replications. The anionic polyacrylamide (PAM) were in three levels of zero (P0), 10 (P10) and 20 (P20) mg L-1 as the main plot and different tillage intensities as the subplot including moldboard plowing plus two disk harrow passes (CT1), one stubble cultivator pass (RT) and moldboard plowing plus one power harrow pass (CT2). The RT treatment relative to CT2 led to soil loss reduction by 23.56% during the first irrigation. The P10 and P20 treatments relative to P0, caused sediment concentration reduction by 98.1 and 98.09% and soil loss reduction by 98.7% and 98.8%, respectively. The RT × P20 treatment had a greater impact in reduction runoff nitrate losses than CT1 × P20 and CT2 × P20 treatments. Losses of N in the fertilized plots and recovery of applied N (RAN) was influenced by both tillage system and PAM application. Key words: Tillage, polyacrylamide, polyacrylamide, soil losses, nitrate.

Hillslope runoff and erosion on duplex soils in grazing lands in semi-arid central Queensland. II. Simple models for suspended and bedload sediment

The use of simple models of soil erosion which represent the main effects of management in grazing lands in northern Australia is limited by a lack of measured parameter values. In particular, parameters are needed for erosion models (sediment concentration v. cover equations) used in daily soil-water balance models. For this research, we specifically avoided equations that use rainfall and runoff rates (e.g. peak flow), as current daily models are limited in their ability to estimate these rates. The resulting models will therefore give poor estimates of soil losses for individual events, but should give good estimates of long-term average erosion and management influences. Runoff and erosion data were available for 7 years on 12 hillslope plots with cover of 10–80%, with and without grazing, with and without tree canopy cover, on a variety of soils according to various soil classification systems. Soils were grouped into those derived from sandstone (SS), mudstone (MS), and eroded mudstone (MSe). These data were used to determine two parameters, i.e. (i) efficiency of entrainment for bare soil and (ii) a cover factor, for simple models of bedload and suspended sediment concentrations. Methods used to fit parameters affected the results; optimising to obtain the minimum sum of squares of errors in soil losses gave better results than fitting an exponential equation to sediment concentration–cover data. The use of a linear slope factor in the sediment concentration models was confirmed with data from plots with slopes 4–8%. Parameters for the bedload sediment concentration model were the same for SS, MS, and MSe soils. Parameters for the suspended sediment concentration model were the same for SS and MS soils, but the MSe soil had a greater efficiency of entrainment for bare soil (about double). The sediment concentration–cover relationships and fitted cover factors were different for suspended and bedload sediment. Thus, the resulting modelled proportion of sediment as suspended load changed with cover, from ~0.3 for bare soil to 0.9 at 80% cover, mimicking the measured data. The cover factor was lower than published values for cultivated soils, indicating less reduction in sediment concentration with greater cover. A compilation of parameter values for the sediment concentration model from published and unpublished sources in grazing and cropping lands is provided.

An evaluation of three wood shred blends for post-fire erosion control using indoor simulated rain events on small plots

The assessment teams who make post-fire stabilization and treatment decisions are under pressure to employ more effective and economic post-fire treatments, as wild fire activity and severity has increased in recent years across the western United States. Use of forest-native wood-based materials for hillslope mulching has been on the rise due to potential environmental, erosion control efficacy, and economic incentives. One concern regarding use of woody materials prepared on or near burned sites is the wide range in the size distribution of the shredded materials. We tested three blends of shredded woody materials, each blend containing different amounts of fine (less than 2.5 cm in length) woody particles. The blends (AS IS with 24% fines, MIX with 18% fines, and REDUCED with 2% fines) were applied at 50 and 70% ground cover to 5-m2 plots containing burned soil placed at 40% slope and evaluated through simulated rain events which consisted of a rain only, a rain plus low flow, and a rain plus high flow period. The REDUCED blend was the optimum for both runoff and sediment concentration reduction under conditions of rainfall and rainfall plus concentrated flow. There was no difference between application rates of 50 and 70% for either of the rainfall plus concentrated flows tested. Our recommendation was that 50% ground cover of the REDUCED blend was adequate for both rainfall and sediment reduction compared to a bare soil. The other two blends were effective in reducing runoff but not sediment concentration compared to a bare soil. The wood shred manufacturing and blending process resulted in two statistically different relationships between application rate and ground cover; relationships were controlled by the amount of fines.