Low Sediment Concentrations Research Articles

Numerical Response of Migratory Shorebirds to Prey Distribution in a Large Temperate Arid Wetland, China.

Wuliangsuhai Lake provides important breeding and stopover habitats for shorebirds. The health of this wetland ecosystem is rapidly deteriorating due to eutrophication and water pollution and environmental management is urgently needed. To explore the connections among ecosystem health, prey density, and shorebird populations, we conducted surveys of both the benthic macroinvertebrates and shorebirds in the shorebird habitat of the wetland during the 2011 autumn migration season. The abundance of both shorebirds and benthic macroinvertebrates varied significantly in both space and time. Our data showed a clear association between shorebird populations and the density of benthic macroinvertebrates, which explained 53.63% of the variation in shorebird abundance. The prey density was strongly affected by environmental factors, including water and sediment quality. Chironomidae were mainly found at sites with higher total phosphorus, but with lower sediment concentrations of Cu. Lymnaeidae were mainly found at sites with a higher pH, lower salinity, and lower concentrations of total phosphorus and Cu. Habitats with very high concentrations of total phosphorus, heavy metals, or salinity were not suitable for benthic macroinvertebrates. Our findings suggest that the reductions of nutrient and heavy metal loadings are crucial in maintaining the ecological function of Wuliangsuhai as a stopover habitat for migratory shorebirds.

A note on the Krone deposition equation and significance of floc aggregation

For modeling the rate of deposition of cohesive flocs in estuaries the Krone equation is extensively used. It was derived from flume experiments on muddy sediment from the San Francisco Bay, and is applicable to low suspended sediment concentration environments in which shear-induced aggregation – the growth and break up of flocs – has a limited role. It is shown that the use of this equation can lead to erroneous estimates of the mass deposition flux at typically higher estuarine concentrations. Krone's own experimental data permit the development of a more general equation accounting for the effects of concentration and turbulent shear rate on aggregation. These effects are dramatically observed in a deposition test in which a wire mesh was inserted in the flow to change the turbulent shearing rate and increase deposition. Even with the inclusion of aggregation effect in the general equation, field-based observations from San Francisco Bay suggest that typical flumes generally may not meet the space and time scaling requirements for field application of laboratory data. Thus, although the Krone equation should be eschewed in favor of the general equation, interpretations of model-predicted deposition rate must not be accepted without robust field-based verification.

Concentration-Dependent Flow Stratification In Experimental High-Density Turbidity Currents and Their Relevance To Turbidite Facies Models

Basal divisions of turbidite deposits often show characteristics that are interpreted as expression of high-density layers forming at the bases of turbidity currents; however, this link between deposit characteristics and the flow process occurring in these basal high-density layers is still poorly constrained. Here we present the results of a set of experiments were the flow dynamics of high-density layers within turbidity currents were studied, and linked to their depositional behavior. The experiments showed the formation of three types of flow layers depending on the initial sediment concentration and slope. As the sediment concentration increases, the layers show different rheological properties with characteristics expressions in the velocity and turbulence profiles of the flows. Internal flow instabilities, arising from the different density interfaces within the stratified flows, are shown to be increasingly suppressed as sediment concentrations step up over the different flow layers. Suppression of these internal instabilities in the basal layers is shown to have a distinct effect on variations in the aggradation rate, which at lower sediment concentrations can be directly linked to these instabilities. Suppression of these fluctuations in aggradation rate is here used to link the different type of high-density flow stratification to different turbidite facies.

Temporal Variation of Sedimentation from Surface Runoff from Agricultural Land Uses in Sondu-Miriu Basin, Kenya

The study was conducted at two sites in Sondu-Miriu basin between November 2010 and May 2011, using runoff plots to determine the temporal variation of sediments, organic matter, solids and surface runoff from agricultural land uses. At Kabianga, lower sediment and surface runoff values were observed in natural vegetation while runoff was generally higher in April-May 2011 wet season. While there were significantly lower sediment concentrations and higher runoff volumes, organic content was significantly lower in April-May 2011. Principal Components Analysis correlated surface runoff and erosion to seasonal land use activities particularly during wet seasons depicting the influence of hydrology on surface runoff pollution. In Sondu, sediment concentrations were lower in natural vegetation. Surface runoff was higher in January-March 2011 in natural vegetation but lowest in cropped systems during April-May, 2011. Solids and organic matter were significantly higher in November-December 2011 (p<0.05). PCA linked surface runoff quality to changes in hydrological periods which influenced the nature, quality and quantity of material transported via surface runoff. PCA for River Sondu-Miriu at Kabianga suggested ionic pollution by dissolved substances (r = 0.77-0.99, p<0.01) and sediment load introduced via surface runoff during wet seasons. In Sondu, PCA associated ionic pollutants and sediment loads to hydrological changes relating to seasonality (r = -0.75-0.99, p<0.01). Temporal variations in surface runoff volumes, water quality and sediment load correlated significantly to hydroperiods in Sondu-Miriu basin. Hydrological changes impacted highly in agricultural land uses with reduced land cover which increased surface runoff accompanied by higher sediment loads.

Performance evaluation of environmentally compatible stormwater filtration systems

A horizontal flow multimedia stormwater filter was developed and tested for hydraulic efficiency and pollutant removal efficiency. Suitability of different natural fibres such as jute, sisal, hemp, coir and oil palm was studied as filter media. Furthermore, the efficacy of different fabric filter screens made up of both woven and nonwoven textiles was also assessed. A new terminology, Universal Performance Index, was introduced. Analysis of these indices showed that jute medium, nonwoven sisal screen and media proportion 1:1:1 performed better. All of the filter combinations exhibited 100 % sediment removal at lower sediment concentration in the inflow. Percentage of discarded flow versus reference flow for the stormwater filter combinations showed that the device performed better at lower flow rates. It was also observed that the hydraulic efficiency was directly proportional to the slope and inversely proportional to the cross-sectional area of the filter. Hydraulic efficiency showed a diminishing trend as the sediment level in inflow increases. By using matrix ranking method, the gravel–coir fibre–sand filter in 1:1:1 proportion with woven sisal hemp screens was selected as the best filter combination. The field evaluation of the filter showed 97.24 % efficiency in normalizing pH and 13.27 % efficiency in reducing electrical conductivity. The removal percentages of magnesium and sodium were 32 and 34 %, respectively. But higher removal efficiencies above 70 % were recorded for total solids, nitrates and sulphates.

Does centennial morphodynamic evolution lead to higher channel efficiency in San Pablo Bay, California?

Abstract Measured bathymetries on 30 year interval over the past 150 years show that San Pablo Bay experienced periods of considerable deposition followed by periods of net erosion. However, the main channel in San Pablo Bay has continuously narrowed. The underlying mechanisms and consequences of this tidal channel evolution are not well understood. The central question of this study is whether tidal channels evolve towards a geometry that leads to more efficient hydraulic conveyance and sediment throughput. We applied a hydrodynamic process-based, numerical model (Delft3D), which was run on 5 San Pablo Bay bathymetries measured between 1856 and 1983. Model results shows increasing energy dissipation levels for lower water flows leading to an approximately 15% lower efficiency in 1983 compared to 1856. During the same period the relative seaward sediment throughput through the San Pablo Bay main channel increased by 10%. A probable explanation is that San Pablo Bay is still affected by the excessive historic sediment supply. Sea level rise and Delta surface water area variations over 150 years have limited effect on the model results. With expected lower sediment concentrations in the watershed and less impact of wind waves due to erosion of the shallow flats, it is possible that energy dissipations levels will decrease again in future decades. Our study suggests that the morphodynamic adaptation time scale to excessive variations in sediment supply to estuaries may be on the order of centuries.

New perspectives on deep-water sandstones: Implications

Abstract The emerging new concepts of sandy mass-transport deposits (SMTD) and bottom-current reworked sands (BCRS) have made a big impact on conventional turbidite concepts. Sediment failures near the shelf edge are the common cause of gravity-driven downslope processes. Mass-transport processes, which include slide, slump, and debris flow, exhibit elastic and plastic behaviors due to high sediment concentration (25-100% by volume). Turbidity currents which represent viscous fluid behavior with low sediment concentration (1-23% by volume) are not mass-transport processes. Four common bottom currents are thermohaline, wind-driven, deep tidal, and baroclinic types. A distinctive attribute of BCRS is their traction structures. However, depositional aspects of baroclinic currents associated with internal waves and tides are poorly understood. Seismic facies and geometries are unreliable for distinguishing types of SMTDs and BCRS in the ancient record. The only reliable method of distinguishing a specific depositional facies is by detailed bed-by-bed examination of sedimentological features in core and outcrop. Short-term events that represent only a matter of hours or days (e.g., earthquakes, tsunamis, tropical cyclones, etc.) are more important in triggering sediment failures than periods of sea-level lowstands that represent thousands of years.

Sediment Trapping from Hyperconcentrated Flow as Affected by Grass Filter Strips

To evaluate the effect of vegetative filter strips on sediment trapping, the spatial distribution of deposited sediment, and the size distribution of deposited particles from hyperconcentrated flows, a simulated grass filter strip experiment was conducted with plastic grass using an adjustable slope steel flume. The simulated vegetation cover was 36%, and the inflow sediment concentrations applied were 147, 238, 320, and 429 kg m−3. The sediment concentration in the outflow, and the sediment particle size were determined. The results showed that the grass filter strips trapped most of the sediment from inflow at low sediment concentration. The deposition efficiency decreased with increasing sediment concentration, being 55.2% and 15.7% in the 147 and 429 kg m−3 sediment treatments, respectively. Most of the deposited sediments were distributed in the upper flume. In addition, the grass filter strips mainly trapped the coarse sediment (particle size > 10 μm).

Suspended sediment concentration–discharge relationships in the (sub-) humid Ethiopian highlands

Abstract. Loss of top soil and subsequent filling up of reservoirs in much of the lands with variable relief in developing countries degrades environmental resources necessary for subsistence. In the Ethiopia highlands, sediment mobilization from rain-fed agricultural fields is one of the leading factors causing land degradation. Sediment rating curves, produced from long-term sediment concentration and discharge data, attempt to predict suspended sediment concentration variations, which exhibit a distinct shift with the progression of the rainy season. In this paper, we calculate sediment rating curves and examine this shift in concentration for three watersheds in which rain-fed agriculture is practiced to differing extents. High sediment concentrations with low flows are found at the beginning of the rainy season of the semi-monsoonal climate, while high flows and low sediment concentrations occur at the end of the rainy season. Results show that a reasonably unique set of rating curves were obtained by separating biweekly data into early, mid, and late rainfall periods and by making adjustments for the ratio of plowed cropland. The shift from high to low concentrations suggests that diminishing sediment supply and dilution from greater base flow during the end of the rainfall period play important roles in characterizing changing sediment concentrations during the rainy season.

Introduction to the special issue ‘Tracer Applications in Sediment Research’

Introduction to the special issue ‘Tracer Applications in Sediment Research’

Modeling of soil erosion and sediment transport in the East River Basin in southern China

Soil erosion is a major global environmental problem that has caused many issues involving land degradation, sedimentation of waterways, ecological degradation, and nonpoint source pollution. Therefore, it is significant to understand the processes of soil erosion and sediment transport along rivers, and this can help identify the erosion prone areas and find potential measures to alleviate the environmental effects. In this study, we investigated soil erosion and identified the most seriously eroded areas in the East River Basin in southern China using a physically-based model, Soil and Water Assessment Tool (SWAT). We also introduced a classical sediment transport method (Zhang) into SWAT and compared it with the built-in Bagnold method in simulating sediment transport process along the river. The derived spatial soil erosion map and land use based erosion levels can explicitly illustrate the identification and prioritization of the critical soil erosion areas in this basin. Our results also indicate that erosion is quite sensitive to soil properties and slope. Comparison of Bagnold and Zhang methods shows that the latter can give an overall better performance especially in tracking the peak and low sediment concentrations along the river. We also found that the East River is mainly characterized by sediment deposition in most of the segments and at most times of a year. Overall, the results presented in this paper can provide decision support for watershed managers about where the best management practices (conservation measures) can be implemented effectively and at low cost. The methods we used in this study can also be of interest in sediment modeling for other basins worldwide.

Study on the Tidal Current and Suspended Sediment Transport in Taizhou Bay

Taizhou Bay is an estuary with high tidel range, middle tidal current and low sediment concentration. For the sea floor is very dense, it is stable in the usual water regimen. The numerical model is introduced to simulate the suspended sediment transport in Taizhou Bay. And the recent hydrologic data and the seabed change have been validated by the numerical model. The movement of tidal current and sediment in Dagagn Bay are simulated, and the sediment siltation in port designed is calculated by the model. The results show that the sediment source is from the shoal produced by the ebb current, and the sediment silting is decreased two-thirds by the cofferdam back of the bay, because the way of the suspended sediment is stopped by the cofferdam from the shoal to the harbor.

Performance evaluation and modelling studies of gravel–coir fibre–sand multimedia stormwater filter

A horizontal flow multimedia stormwater filter was developed and tested for hydraulic efficiency and pollutant removal efficiency. Gravel, coconut (Cocos nucifera) fibre and sand were selected as the media and filled in 1:1:1 proportion. A fabric screen made up of woven sisal hemp was used to separate the media. The adsorption behaviour of coir fibre was determined in a series of column and batch studies and the corresponding isotherms were developed. The hydraulic efficiency of the filter showed a diminishing trend as the sediment level in inflow increased. The filter exhibited 100% sediment removal at lower sediment concentrations in inflow water (>6 g L−1). The filter could remove NO, SO and total solids (TS) effectively. Removal percentages of Mg2+ and Na+ were also found to be good. Similar results were obtained from a field evaluation study. Studies were also conducted to determine the pattern of silt and sediment deposition inside the filter body. The effects of residence time and rate of flow on removal percentages of NO and TS were also investigated out. In addition, a multiple regression equation that mathematically represents the filtration process was developed. Based on estimated annual costs and returns, all financial viability criteria (internal rate of return, net present value and benefit–cost ratio) were found favourable and affordable to farmers for investment in the developed filtration system. The model MUSIC was calibrated and validated for field conditions with respect to the developed stormwater filter.

Investigating step‐pool sequence stability

Step‐pool units, common features in steep, narrow streams, are highly dynamic systems that adjust during high flows and active sediment transport conditions. Consecutive step pools in a reach form a step‐pool sequence, and though these features are prevalent in nature, quantifying the stability of such systems is challenging. This study focuses on the statistical relationships between 445 stable sequences of three or more steps and nine geometric and resistance‐based parameters. Step sequence stability is parameterized through a sequence stability parameter, a metric of how long a given step‐pool sequence exists relative to the total run time. Despite variability in the strength of the statistical relationships, the results allow identification of dominant trends between sequence stability and both roughness and geometric parameters. The sediment concentration ratio provides a means for comparing the relative strength of these statistical relationships. As sediment concentration ratios increase, the relations between flow resistance parameters and step stability change from dominantly inverse to dominantly direct. Thus, at higher sediment concentration ratios, sequence stability is more likely to increase with flow resistance, but the reverse is more likely at low sediment concentration ratios. A connection exists between bed morphology, flow resistance, and stability. For the majority of stable step sequences, resistance parameters increase over time, indicating some sequences adjust their geometry to increase stability during flood events. The influence of sediment concentration over the stability‐geometric and stability‐resistance parameter relationships may enable the use of the sediment concentration ratio as a predictor of step sequence stability during a flow event.

Respiratory responses of three Antarctic ascidians and a sea pen to increased sediment concentrations

Glacial retreat and subglacial bedrock erosion are consequences of rapid regional warming on the West Antarctic Peninsula. Sedimentation of fine-grained eroded particles can impact the physiology of filter-feeding benthic organisms. We investigated the effect of increasing concentrations of sediment on the oxygen consumption of suspension feeding species, the ascidians Molgula pedunculata, Cnemidocarpa verrucosa, Ascidia challengeri, and the pennatulid Malacobelemnon daytoni in Potter Cove (South Shetland Islands, Antarctica). In A. challengeri and C. verrucosa, oxygen consumption increased gradually up to a critical sediment concentration (Ccrit) where species oxygen consumption was maximal (Omax in mg O2 g−1dm day−1) and further addition of sediments decreased respiration. Ccrit was 200 mg L−1 for A. challengeri (Omax of 0.651 ± 0.238) and between 100 and 200 mg L−1 for C. verrucosa (Omax of 0.898 ± 0.582). Oxygen consumption of M. pedunculata increased significantly even at low sediment concentrations (15–50 mg sediment L−1). Contrary to the ascidians, sediment exposure did not affect oxygen consumption of the sea pen. The tiered response to sedimentation in the four species corroborates recent field observations that detected a reduction in the abundance of the sensitive ascidian M. pedunculata from areas strongly affected by glacial sediment discharge, whereas sea pens are increasing in abundance. Our investigation relates consequences (population shifts in filter-feeder communities) to causes (glacial retreat) and is of importance for modelling of climate change effects in Antarctic shallow coastal areas.

Hydrology, sediment transport dynamics and geomorphology of a variable flow river: The Mfolozi River, South Africa

The co-efficient of variation for inter-annual streamflow of the Mfolozi River is extremely high at 79%. An analysis of flow frequency indicated that streamflow is skewed towards low-flow values, with a number of extremely large flood events occurring as outliers on the histogram. Streamflow variability in the Mfolozi River may be linked to multiple factors including a large catchment size, a seasonal climate of a dry winter and wet summer, evergreen vegetation in the catchment, variable precipitation and the occurrence of regionally pervasive climatic oscillations. This research aimed to address how streamflow variability impacted upon sediment transport and thus, geomorphology. It was found that sediment transport variability occurred at the intra- and inter-annual scale. Analysis of mean monthly sediment concentration and discharge showed a hysteresis effect, such that sediment concentration peaked prior to discharge in the early wet season. During the late wet season, peak discharges often had unexpectedly low sediment concentrations. Furthermore, data suggested the existence of long-term hysteresis that may be related to decadal-scale climatic oscillations that alter sediment availability and stream capacity, resulting in discharge peaking in 2000 and sediment concentration in 2005. However, more data are required to confirm this relationship. Variability in streamflow appears to share a causal relationship with sediment transport variability, as both are linked to variation in precipitation and the resultant impacts on vegetation growth and evapotranspiration rates. The variability of streamflow and sediment transport has implications for stream and floodplain geomorphology, and the hydrology of variable rivers should be considered when interpreting their geomorphology.

Rapid wetland expansion during European settlement and its implication for marsh survival under modern sediment delivery rates

Fluctuations in sea-level rise rates are thought to dominate the formation and evolution of coastal wetlands. Here we demonstrate a contrasting scenario in which land-use–related changes in sediment delivery rates drive the formation of expansive marshland, and vegetation feedbacks maintain their morphology despite recent sediment supply reduction. Stratigraphic analysis and radiocarbon dating in the Plum Island Estuary (Massachusetts, United States) suggest that salt marshes expanded rapidly during the eighteenth and nineteenth centuries due to increased rates of sediment delivery following deforestation associated with European settlement. Numerical modeling coupled with the stratigraphic observations suggests that existing marshland could survive, but not form under the low suspended sediment concentrations observed in the estuary today. These results suggest that many of the expansive marshes that characterize the modern North American coast are metastable relicts of high nineteenth century sediment delivery rates, and that recent observations of degradation may represent a slow return to pre-settlement marsh extent. In contrast to ecosystem management practices in which restoring pre-anthropogenic conditions is seen as a way to increase ecosystem services, our results suggest that widespread efforts to restore valuable coastal wetlands actually prevent some systems from returning to a natural state.

Bloom development and phytoplankton succession in Lake Winnipeg: a comparison of historical records with recent data

Over the past 40 years, hydroelectric, agricultural and urban watershed development and changing hydrology have transformed Lake Winnipeg into a highly eutrophic reservoir with annual outbreaks of widespread surface algal blooms, shoreline and net fouling, and concerns with intermittent cyanotoxin production. To provide a better understanding of the magnitude of these changes and the major causes, we examine long-term increases in phytoplankton biomass and shifts in phytoplankton species dominance in the context of both in-lake and watershed processes. We compare phytoplankton and water quality data from early (1969) and recent (1994–2007) lake-wide surveys, and information from paleolimnological analysis of sediment cores and satellite remote sensing. Our results demonstrate a recent and dramatic rise in severe algal blooms and increased dominance of cyanobacteria beginning in the mid-1990s, coincident with a large increase in phosphorous loading to the lake. Distinct increases in sediment core accumulation of nutrients and chlorophyll, cyanobacteria and diatom microfossils coincided with hydroelectric and agricultural development, increased Red R discharge and shifts in water transparency patterns across the lake. There has been a dramatic increase in phytoplankton biomass, accompanied by marked shifts in seasonal community composition. Spring diatoms blooms are of shorter duration and increasingly dominated by more eutrophic diatom taxa while summer blooms show reduced taxonomic diversity and an increased predominance of nitrogen-fixing cyanobacteria. Satellite images showed annual development of vast summer surface blooms, mainly in the north basin, with chlorophyll highest in regions of relatively low suspended sediment concentration and high transparency. There is an increasing dominance of potentially toxic cyanobacteria taxa and high levels of microcystins in nearshore samples of surface blooms. The combined effects of nutrient increases, algal species shifts and toxin production represent a potential threat to the sustainability of ecosystem function and productivity.

Macroflocs, fine-grained sediment transports, and their longitudinal variations in the Ems Estuary

Field measurements have been carried out to obtain more quantitative knowledge on the longitudinal distribution of the fine-grained sediment dynamics in the Ems Estuary. Both the short-term (time scale < tidal period) and the long-term (tidally averaged) fine-grained sediment transports have been investigated. It is shown that the short-term erosion/sedimentation cycles are most characteristic and are the building blocks for the ultimate long-term transports. Herein, the macroflocs play a key role. The macroflocs had sizes in the range of 200 to 700 μm, sometimes more than 1 mm, and survived high current velocities. The settling velocities were in the range of 0.5 to 8 mm/s. Turbulence intensities and suspended sediment concentration are of utmost importance for the settling properties of the sediment, but the “flocculation ability” of the suspended sediment is shown to be of equal importance. The suspended sediment concentration is only important at low levels of turbulence. At high levels of turbulence, the limiting properties of turbulence dominate. It is demonstrated that the surface properties of the suspended sediment particles are of prime importance for the flocculation ability of the water/sediment mixture, as do the sizes of the resuspended microflocs. Special attention is given to the longitudinal distribution of the sizes and settling properties of the macroflocs as well as of the sediment transports. The flocculation ability was shown to vary almost one order of magnitude along the estuary. These higher flocculation abilities in the direction of higher salinities are responsible for the increase of the sizes of the macroflocs in the seaward direction. Therefore, notwithstanding the low suspended sediment concentrations at the seaward boundary of the Ems Estuary, large macroflocs and corresponding high settling velocities were observed in this area. It explains that, in the Ems Estuary, no distinct effect was determined of the suspended sediment concentration on the settling properties of the suspended macroflocs.

Hillslope runoff and erosion on duplex soils in grazing lands in semi-arid central Queensland. I. Influences of cover, slope, and soil

Many soils in semi-arid grazing lands develop low pasture cover or bare areas (scalds) under heavy grazing and have a low tolerance to soil erosion, due to low total water-holding capacity and concentration of nutrients in the soil surface. Runoff and erosion was measured for 7 years on 12 hillslope plots with cover (pasture plus litter) ranging from 10 to 80%, slopes from 4 to 8%, with and without grazing, with and without tree canopy cover, on a variety of soils. Soils were grouped into those derived from sandstone (SS), mudstone (MS), and eroded mudstone (MSe). One plot with low cover had a grass filter at the outlet. Runoff was strongly influenced by surface cover and was high with low cover (200–300 mm/year or 30–50% of rainfall). Runoff averaged 35 mm/year or 5.9% of rainfall with &gt;50% cover. All soils fitted the same runoff–cover relationship. The grass filter had no effect on runoff and suspended load, but did reduce bedload. Grass pasture cover and tree litter cover were equally effective in controlling runoff and erosion. Total, bedload, and suspended load sediment concentrations increased linearly with slope in the range 4–8% for plots with low cover, and decreased exponentially with greater cover. Total and bedload sediment–cover relationships were similar for SS, MS, and MSe. However, plots on MSe had higher suspended sediment losses and thus slightly higher total soil losses. For all soils, erosion resulted in low sediment concentrations due to the hard-set surface soil, but total soil losses were high due to the large volumes of runoff generated. Concentration–cover relationships were different for bedload and suspended sediment. Consequently, suspended sediment was 20–40% of total soil loss for bare soil, and increased with cover to about 80% with cover &gt;80%. The proportion of suspended sediment for bare soil was similar to the proportion of dispersed silt plus clay in the surface soil. About 90% of suspended sediment was fine-sized (&lt;0.053 mm). Bedload was mainly coarse and fine sands, which were enriched compared with the surface soil. Grazing in semi-arid pastures should be managed to maintain &gt;50% ground cover to avoid excessive runoff and soil erosion, and degradation of soil productivity, and to maintain good off-site water quality.