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Topographic models provide a useful tool for understanding gully occurrence in the landscape but require reliable estimates of gully head drainage areas. Modern high‐resolution topography data (collected using structure from motion photogrammetry or light detection and ranging) is increasingly used for topographic studies of gullies, but little work has been done to assess the variability of gully head drainage area estimates using different methods. This study evaluated alternative approaches to using high‐resolution digital elevation models (DEMs) so that gully topographic models can be more readily applied to any area with suitably high‐resolution data. Specifically, we investigated the impact of single‐ or multiple‐direction flow routing algorithms, DEM hydrologic‐enforcement procedures and spatial resolution on gully head drainage area estimation. We tested these methods on a 40 km2 site centred on Weany Creek, a low‐relief semi‐arid landscape draining towards the Great Barrier Reef, Australia. Using a subroutine to separate gully heads into those with divergent or convergent flow patterns upslope, we found that divergent flow conditions occurred at half of 484 studied gullies. Drainage areas estimated by different flow routing algorithms were more variable in these divergent cases than for convergent cases. This variation caused a significant difference between topographic threshold parameters (slope b and intercept k) derived from single‐ or multiple‐direction flow routing algorithms, respectively. Different methods of hydrologic enforcement (filling or breaching) also affected threshold analysis, resulting in estimates of the exponent b being ~188% higher if the DEM was filled than if breached. The testing of the methods to date indicates that a finer resolution (≤2 m) DEM and a multiple‐direction flow routing algorithm achieve the most realistic drainage area estimates in low‐relief landscapes. For Weany Creek we estimated threshold parameters k = 0.033 and b = 0.189, indicating that it is highly susceptible to gully erosion. [ABSTRACT FROM AUTHOR]

Gully rehabilitation can contribute to catchment management by stabilizing erosion and reducing downstream sediment yields, yet the globally observed responses are variable. Developing the technical basis for gully rehabilitation and establishing guidelines for application requires studies that evaluate individual rehabilitation measures in specific environments. An eight‐year field experiment was undertaken to evaluate sediment yield and vegetation responses to several gully rehabilitation measures. The rehabilitation measures aimed to reduce surface runoff into gully head cuts, trap sediment on gully floors and increase vegetation cover on gully walls and floors. The study occurred in a savanna rangeland in northeast Australia. Two gullies were subject to treatments while four gullies were monitored as untreated controls. A runoff diversion structure reduced headcut erosion from 4.3 to 1.2 m2 yr−1. Small porous check dams and cattle exclusion reduced gully total sediment yields by more than 80%, equivalent to a reduction of 0.3 to 2.4 t ha−1 yr−1, but only at catchment areas less than 10 ha. Fine sediment yields (silt and clay) were reduced by 7 and 19% from the two treated gullies, respectively. The porous check dam deposits contained a lower percentage of the fine fraction than the parent soil. Significant regeneration of gully floor vegetation occurred, associated with trapping of organic litter and fine sediment. Increases in vegetation cover and biomass were comprised of native perennial grasses, trees and shrubs. In variable climates, long‐term gully rehabilitation will progress during wetter periods, and regress during droughts. Understanding linkages between rehabilitation measures, their hydrologic, hydraulic and vegetation effects and gully sediment yields is important to defining the conditions for their success. [ABSTRACT FROM AUTHOR]

Abstract: Drainage network extension in semi‐arid rangelands has contributed to a large increase in the amount of fine sediment delivered to the coastal lagoon of the Great Barrier Reef, but gully erosion rates and dynamics are poorly understood. This study monitored annual erosion, deposition and vegetation cover in six gullies for 13 years, in granite‐derived soils of the tropical Burdekin River basin. We also monitored a further 11 gullies in three nearby catchments for 4 years to investigate the effects of grazing intensity. Under livestock grazing, the long‐term fine sediment yield from the planform area of gullies was 6.1 t ha‐1 yr‐1. This was 7.3 times the catchment sediment yield, indicating that gullies were erosion hotspots within the catchment. It was estimated that gully erosion supplied between 29 and 44% of catchment sediment yield from 4.5% of catchment area, of which 85% was derived from gully wall erosion. Under long‐term livestock exclusion gully sediment yields were 77% lower than those of grazed gullies due to smaller gully extent, and lower erosion rates especially on gully walls. Gully wall erosion will continue to be a major landscape sediment source that is sensitive to grazing pressure, long after gully length and depth have stabilised. Wall erosion was generally lower at higher levels of wall vegetation cover, suggesting that yield could be reduced by increasing cover. Annual variations in gully head erosion and net sediment yield were strongly dependent on annual rainfall and runoff, suggesting that sediment yield would also be reduced if surface runoff could be reduced. Deposition occurred in the downstream valley segments of most gullies. This study concludes that reducing livestock grazing pressure within and around gullies in hillslope drainage lines is a primary method of gully erosion control, which could deliver substantial reductions in sediment yield. Copyright © 2018 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: Degradation of coastal ecosystems in the Great Barrier Reef (GBR) lagoon, Australia, has been linked with increased land-based runoff of suspended solids, nutrients and pesticides since European settlement. This study estimated the increase in river loads for all 35 GBR basins, using the best available estimates of pre-European and current loads derived from catchment modelling and monitoring. The mean-annual load to the GBR lagoon for (i) total suspended solids has increased by 5.5 times to 17,000ktonnes/year, (ii) total nitrogen by 5.7 times to 80,000tonnes/year, (iii) total phosphorus by 8.9 times to 16,000tonnes/year, and (iv) PSII herbicides is 30,000kg/year. The increases in river loads differ across the 10 pollutants and 35 basins examined, reflecting differences in surface runoff, urbanisation, deforestation, agricultural practices, mining and retention by reservoirs. These estimates will facilitate target setting for water quality and desired ecosystem states, and enable prioritisation of critical sources for management. [Copyright &y& Elsevier]

Summary: In many agricultural regions investments are made into improved land management and catchment restoration with the aim of reducing sediment and nutrient delivery to downstream water bodies. This paper presents the results of an 8year field study that evaluated the impact of improved grazing land management (GLM) on a 14km2 sub-catchment of the Burdekin River in northern Australia. Land condition recovery and changes to runoff and sediment yield were measured on hillslopes (using three flumes) and at the end of the catchment (using automatic water sampling). Ground cover was monitored at the beginning of the wet season at hillslope and catchment scales using direct measurement and Quickbird satellite imagery. At the catchment scale, improved GLM resulted in an increase in mean ground cover between 2003 and 2007, relative to a control grazing property. However, the total suspended sediment yield from hillslopes did not decline due to the disproportionately high yields from low (<10%) cover sites particularly in high runoff years. In 2007, when there was above-average rainfall, 97% of the hillslope derived fine sediment was coming from less than 3% of the catchment. Catchment sediment yield also increased, associated with higher rainfall and runoff in latter years of the study. In years of above-average rainfall, it is estimated that less than 40% of catchment sediment yield was derived from hillslope erosion. Channel erosion is considered to contribute the remaining sediment. Rehabilitation of hillslope scalds and gullies is likely to be an important companion to improved GLM to reduce catchment fine sediment yields in this landscape. Evaluation of gully and hillslope scald rehabilitation techniques is required, including the economic feasibility of such options. [Copyright &y& Elsevier]

Detailed understanding of gully erosion processes is essential for monitoring gully remediation and requires fine-scale monitoring. Hand-held laser scanning systems (HLS) enable rapid ground-based data acquisition at centimeter precision and ranges of 10–100 m. This study quantified errors in measuring gully morphology and erosion over a four year period using two models of HLS. Reference datasets were provided by Real-Time-Kinematic (RTK) GPS and a RIEGL Terrestrial Laser Scanner (TLS). The study site was representative of linear gullies that occur extensively on hillslopes throughout Great Barrier Reef catchments, where gully erosion is the dominant source of fine sediment. The RMSE error against RTK survey points varied 0.058–0.097 m over five annual scans. HLS was found to measure annual gully headcut extension within 0.035 m of RTK. HLS was, on average, within 6% of TLS for morphological metrics of depth, area and volume. Volumetric change over a 60 m length of the gully and four years was estimated to within 23% of TLS. Errors could potentially be improved by scanning at times of year with lower ground vegetation cover. HLS provided similar levels of error and was relatively more rapid than TLS and RTK for monitoring gully morphology and change. [ABSTRACT FROM AUTHOR]

Land use in the catchments draining to the Great Barrier Reef lagoon has changed considerably since the introduction of livestock grazing, various crops, mining and urban development. Together these changes have resulted in increased pollutant loads and impaired coastal water quality. This study compiled records to produce annual time-series since 1860 of human population, livestock numbers and agricultural areas at the scale of surface drainage river basins, natural resource management regions and the whole Great Barrier Reef catchment area. Cattle and several crops have experienced progressive expansion interspersed by declines associated with droughts and diseases. Land uses which have experienced all time maxima since the year 2000 include cattle numbers and the areas of sugar cane, bananas and cotton. A Burdekin Basin case study shows that sediment loads initially increased with the introduction of livestock and mining, remained elevated with agricultural development, and declined slightly with the Burdekin Falls Dam construction. [ABSTRACT FROM AUTHOR]

Increased erosion related to climate and/or land cover change has adverse impacts on terrestrial and aquatic ecosystems. Mapping of erosion hotspots improves our ability to identify and potentially remediate the most active erosion sources. The topical savannas of the Great Barrier Reef catchments, in northeast Australia, are generating excessive sediment yields, primarily from gully erosion. To reduce the adverse impacts on marine ecosystems, there is an urgent need to identify priority erosion hotspots and implement mitigation measures. While repeat airborne LiDAR surveys allow subtle topographic change detection, they are cost-intensive for catchment-scale applications and their applicability is constrained by data avalability and detection treshold of ground level change. In contrast, satellite-based radar imagery can allow large scale tracking of geomorphic change at high temporal resolution. Here we apply a new method based on Sentinel-1 C-band radar images and Coherence Change Detection (CCD), where large stacks of interferometric coherence images are subdivided with rain gauge time-series for separation of erosion-rich and erosion-free coherence information. After correcting the former with the latter, the resulting corrected coherence maps are compared with differential elevation models derived from multitemporal LiDAR, regional scale gully delineation maps, maps of gullying potential and in-situ field verification. Our results demonstrate the promising potential of this technique in detecting gully erosion hotspots. The coherence loss indicating erosion/deposition is well detected in wide gully morphologies, however, the line-of-sight angle does not allow penetration into narrow linear gullies. Further, CCD detects sheetwash or rill erosion occurring in areas identified as at risk of gully expansion, which is commonly below detection threshold for multitemporal LiDAR datasets. When used with LiDAR-derived geomorphic change mapping and gullying potential maps, CCD allows identification of gully erosion dynamics and forecasting gully evolution and creation, which is critical for supporting mitigation measures. • Sentinel-1 radar coherence stacks map erosion hotspots in northeast Australia. • Erosion-free coherence is used to correct erosion-affected coherence images. • Comparison with DoD, gully maps, gullying potential maps and in situ observations. • Capability is indicated to detect early development and deepening of gullies. • A cost-effective, semi-quantitative mapping of large-scale erosion patterns. [ABSTRACT FROM AUTHOR]

Summary: Poor land condition resulting from unsustainable grazing practices can reduce enterprise profitability and increase water, sediment and associated nutrient runoff from properties and catchments. This paper presents the results of a 6year field study that used a series of hillslope flume experiments to evaluate the impact of improved grazing land management (GLM) on hillslope runoff and sediment yields. The study was carried out on a commercial grazing property in a catchment draining to the Burdekin River in northern Australia. During this study average ground cover on hillslopes increased from ∼35% to ∼75%, although average biomass and litter levels are still relatively low for this landscape type (∼60 increasing to 1100kg of dry matter per hectare). Pasture recovery was greatest on the upper and middle parts of hillslopes. Areas that did not respond to the improved grazing management had <10% cover and were on the lower slopes associated with the location of sodic soil and the initiation of gullies. Comparison of ground cover changes and soil conditions with adjacent properties suggest that grazing management, and not just improved rainfall conditions, were responsible for the improvements in ground cover in this study. The ground cover improvements resulted in progressively lower runoff coefficients for the first event in each wet season, however, runoff coefficients were not reduced at the annual time scale. The hillslope annual sediment yields declined by ∼70% on two out of three hillslopes, although where bare patches (with <10% cover) were connected to gullies and streams, annual sediment yields increased in response to higher rainfall in latter years of the study. It appears that bare patches are the primary source areas for both runoff and erosion on these hillslopes. Achieving further reductions in runoff and erosion in these landscapes may require management practices that improve ground cover and biomass in bare areas, particularly when they are located adjacent to concentrated drainage lines. [Copyright &y& Elsevier]

• Sediment loads are lower in a catchment with cattle excluded for 17 years. • Most of the sediment and nutrient load is derived from a few large runoff events. • Recovery of degraded savanna rangelands is slow and strongly influenced by climate. • Road work can significantly increase sediment and nutrient concentrations and loads. Excess sediment and nutrient yields from degraded rangelands have detrimental off-site ecological and on-site economic impacts, yet the effect of reduced grazing pressure on water quality is not fully understood. This study compares 15-year records of runoff, sediment and nutrient yields amongst three ephemeral headwater catchments (10.5–13.5 km2) with similar wet/dry tropical climate and landform characteristics, but contrasting cattle grazing pressure. The catchments are located within the Burdekin River basin which is the largest source of sediment and particulate nutrients to the Great Barrier Reef, Australia. Estimated mean annual total suspended sediment (TSS), total nitrogen (TN) and total phosphorus (TP) loads are: 0.6 t ha−1, 1.3 and 0.3 kg ha−1, respectively, in a catchment from which cattle were excluded over the study period ('Main Creek'); 1.5 t ha−1, 1.4 and 0.4 kg ha−1, respectively, in a catchment where grazing pressure was reduced from approximately 25 to 13 cattle per km2 at the commencement of monitoring ('Weany Creek'); and 1.4 t ha−1, 3.0 and 0.5 kg ha−1, respectively, in a catchment which maintained higher levels of grazing pressure ('Wheel Creek', approximately 25 head per km2). Higher annual sediment loads in the grazed catchments compared to the ungrazed catchment are likely due to degradation of soil water storage capacity, soil surface protection and gully erosion rates, but could also be influenced by differences in catchment geomorphic attributes (e.g., hydrological and sediment connectivity). Events with runoff >20 mm (16–25% of all events) generated 79–85% of the total runoff volume and 71–78% of the total TSS, TN, and TP loads. At the event timescale, there are not clear differences in runoff, sediment and nutrient loads among the catchments, attributed to variability in catchment conditions (e.g., antecedent soil moisture, rainfall intensity, vegetation) that occur within- and between events. It is concluded that recovery of degraded savanna rangelands following reduction in livestock grazing pressure takes decades and is strongly influenced by climate. Measuring water quality responses to land management change in variable climates requires nested spatial monitoring over long time scales that also includes factors that can influence the response (e.g., climate, soil properties, vegetation and land use). [ABSTRACT FROM AUTHOR]