Sediment Modelling of a Catchment to Determine Medium-Term Erosional Trends

: This study was part of a project designed to simulate the long-term landform equilibrium of a rehabilitated mine site. The project utilized event Fine Suspended Sediment (FSS) ﬂuxes in a receiving stream following a rainfall event as an indicator of landform stability. The aim of this study was to use HEC-HMS to determine sediment and discharge quantity upstream to determine how it affects the downstream development of the catchment landform, in terms of sediment changes and geomorphology. Thus, the study focused on hydrology and sediment modelling of the upper catchment with HEC-HMS (Hydrologic Engineering Centre-Hydrologic Modelling System) to determine the daily discharge and sediment output at the catchment outlet. HEC-HMS was used to calibrate the stream discharge and FSS quantities at the catchment outlet to observed continuous discharge and FSS values. The calibration of the HEC-HMS model was carried out for two water years and then the same model parameters were used to validate the model for a third water year. The catchment discharge and FSS were calibrated and validated for continuous rainfall events against observed discharge and FSS data at the catchment outlet. The model was then run for a projected rainfall of 50 years. The denudation rate predicted by the model was 0.0245 mm per year, which falls in the range previously determined for the region. The simulated sediment output was compared to the rainfall trends over the years. As a result, the sediment spikes following a rainfall-runoff event gradually decreased over time. Reducing FSS spikes indicates that the landform gradually attains stability. This modelling study can be used for long-term simulations to determine erosion equilibrium over the years and to quantify sediment yield in catchments for projected time periods.


Introduction
Soil erosion in tropical countries is triggered due to geologically weathered soils, intensive rainfall, inappropriate soil and land management practices, and mining activities.Where mining activities cause disturbance in catchment stream ecosystems, they are impacted by heavy loads of eroded material entering the streams [1], if not managed correctly.The transport of hydrophobic pollutants and heavy metals is often facilitated by suspended sediment particles eroded and taken downstream during heavy discharge events [2].Thus, suspended sediments correspond to particle-related pollutant transport from the catchment across the river reaches.
Stream-suspended mud also acts as an indicator of post-mining landform stability.During landform disturbance such as mining, fine suspended sediment (the silt and clay components of suspended sediment-FSS) spikes have been observed in the receiving waters following the disturbance.As the landform is rehabilitated and reaches equilibrium with the surrounding catchment, FSS spikes following rainfall events return to pre-mining levels [3].The magnitude of FSS spikes following a known rainfall event can be a tool to Land 2023, 12, 1785 2 of 21 measure landform stability [3].Since FSS is highly mobile, many contaminants released by the catchment disturbance attach themselves to FSS and are transported downstream [4].Thus, erosional trends of a catchment imply associated sediment and pollutant transfer within the river reaches.
The quantification of stream sediments at the catchment outlet using gauging stations is a time-consuming process.It involves collecting water samples at high-discharge events which are then dried, sieved, and weighed to quantify sediments of different particle sizes.However, there is a strong correlation between turbidity (NTU) and mud concentration (silt and clay component of suspended sediment) [5].Thus, continuous turbidity (NTU) data collected with turbidimeters at the catchment outlet of a stream can be used to measure changes in FSS load exiting the catchment.
Continuous discharge and FSS data measured at the catchment outlet facilitate catchment hydrology and erosion modelling.Modelling also requires knowledge of model input parameters that depict the catchment characteristics, such as transpiration and surface roughness.This provides an avenue to comment on dynamics of landform stability through simulated erosional trends and catchment erosion quantification based on rainfall events.Hydrology and sediment modelling of the catchment enables simulations by the model for projected future rainfall events.This provides medium-term and long-term erosional trends of the catchment under different rainfall scenarios.
There have been recent studies using SWAT (Soil Water Assessment Tool), which is used to estimate soil erosion and sediment transport considering several factors like precipitation rate, topography, soil type, and vegetation cover [6].SWAT was also used to assess the impact of climate change on stream flow and sediment by incorporating climate induced changes in precipitation and temperature [7].Both HEC-HMS (Hydrologic Engineering Centre-Hydrologic Modelling System) and SWAT are efficient models to predict stream flow and sediment erosion simulations [8].
The broader objective of the project is to validate an approach using an event-based stream FSS discharge relationship in combination with a Landform Evolution Model (LEM) to determine when erosion of a rehabilitated landform is at equilibrium with the surrounding catchment [9].A rainfall event-based stream FSS/discharge relationship was developed using continuous receiving stream monitoring data in which a catchment disturbance caused a change in this relationship [3].To evaluate the change in event stream FSS on account of a probable disturbance due to mining in the catchment, CAESAR-Lisflood LEM is calibrated and validated for the mining catchment.It is then run for future rainfall simulations to show changes over time and changes in response to differences in those rainfall scenarios [10].It was found that CAESAR Lisflood accurately predicts event FSS for a specific discharge for early wet season but underpredicts for the later wet season.To run the LEM for a long term and to investigate how the event FSS/discharge relationship changes across the years, upstream continuous stream discharge and sediment input to the catchment is required.Therefore, the upstream catchment needs to be calibrated and modelled with the help of HEC-HMS to simulate future continuous discharge and sediment input to the lower catchment where the mine resides.
The aim of this study was to use HEC-HMS to determine sediment and discharge quantity upstream of a catchment with mining to determine how those upstream inputs affect the downstream development of the catchment landform, in terms of sediment changes and geomorphology.This study focussed on hydrology and sediment modelling of a catchment with HEC-HMS.The Hydrologic Modelling System (HEC-HMS) is designed to simulate the complete hydrologic processes and soil erosion of dendritic watershed systems [11].The application is used to model the catchment by adjusting the parameters of the model such that an input like an observed rainfall event will give discharge and sediment output at the catchment outlet similar to that measured at the site.In this study, HEC-HMS was calibrated for two individual water years and the same model parameters were used to validate the model for a third water year.Observed discharge, fine suspended quantities, and rainfall used for calibration and validation of the model were obtained from gauging stations.Rainfall for future simulations was computed with the help of a weather generator incorporating climate change factors.The model was then run for a time period of 50 years to explore the medium-term erosional trends of the catchment.The application also facilitates the quantification of sediment mobilized and eroded from the catchment for different time periods.HEC-HMS sediment modelling has not been carried out before for the site under consideration in this paper.Previous studies indicate that the parameters that influence the erosion rate likely to occur on the landform will change in time.A study that focused on temporal trends in hydrology and erosion for the post-mining landform at Ranger mine, Alligator River Region, Northern Territory, Australia, indicates that the erosion rate reduces over the long term and the study was able to quantify these changes in terms of SIBERIA parameter value [12].The SIBERIA Landform Evolution Model (LEM) was used to predict medium-term erosional characteristics for another site in the same region, the Scinto 6 site, and the results show that erosion in gullies reduces over time [13].This study investigated the medium-term erosional trends by determining the amount of sediment transported from the catchment over the years.The catchment under consideration in this study for more detailed modelling of discharge and sediment quantification is the upper Gulungul Catchment in the Alligator River Region.

Study Site
Ranger Mine (12 • 41 S, 132 • 55 E) is an open-cut uranium mine operated by Energy Resources of Australia Ltd. (ERA) in Northern Territory, Australia.The mine is located 8 km east of the township of Jabiru in the Alligator River Region and within the 78 km 2  Ranger Project Area which is surrounded by, but separate from, the World Heritage-listed Kakadu National Park.It has been producing uranium oxide (U 3 O 8 ) since 1981 and is presently undergoing rehabilitation and closure.Mining ceased in 2012, milling of the mined ore ceased in early 2021, and work is now focused on rehabilitation [14].The regional geology, climate, and the location significance are detailed in [10].ERA is responsible for the rehabilitation of Ranger Mine based on laid out principles called Environmental Requirements (ERs).The Australian Commonwealth Government Supervising Scientist has a supervisory role.Environmental Requirements pertaining to erosion equilibrium of the landform require erosion characteristics which, as far as can reasonably be achieved, do not vary significantly from those of comparable landforms in surrounding undisturbed areas.It is expected that the erosion rates will initially be high then tend slowly towards the natural rates.These timeframes are expected to be quite long, and so, the outcome is to use the best available modelling to demonstrate that the erosion characteristics of the final landform will eventually be comparable to natural landscapes.
The Ranger Mine is adjacent to Magela Creek, a left-bank tributary of the East Alligator River [14].Gulungul Creek is a small tributary of Magela Creek that is adjacent to the tailings dam.It is one of the tributaries that would be the first to receive sediment generated from the mine site during and after rehabilitation [15].Gulungul and Magela Creeks are ephemeral, braided, sand-bed streams which carry very large sand loads (bed and suspended bed) and small FSS (Fine Suspended Sediment) loads.The Environmental Research Institute of the Supervising Scientist (ERISS) monitored sites in Gulungul Creek DownStream (GCDS), Gulungul Creek UpStream (GCUS), Magela Creek DownStream (MCDS), and Magela Creek UpStream (MCUS) of the mine (Figure 1).The data obtained from ERISS for this study were continuous discharge (m 3 s −1 ), and turbidity (NTU) and FSS (<63 µm fraction of sediment samples collected in the auto-samplers; mg).Data were measured at a frequency of 6 min for Gulungul Creek GCDS and GCUS from August 2004 to August 2015.Rainfall data for GCDS and GCUS at 10 min intervals were also obtained for modelling Gulungul catchment in CAESAR-Lisflood.
to August 2015.Rainfall data for GCDS and GCUS at 10 min intervals were also obtained for modelling Gulungul catchment in CAESAR-Lisflood.

HEC-HMS Model
HEC-HMS is designed to simulate complete hydrological processes of dendritic watershed systems.The software includes many traditional hydrologic analysis procedures such as event infiltration, unit hydrographs, and hydrologic routing.It also includes procedures necessary for continuous simulation including evapotranspiration, snowmelt, and soil moisture accounting.The physical representation of a watershed is accomplished with a basin model.Hydrologic elements are connected in a dendritic network to simulate

HEC-HMS Model
HEC-HMS is designed to simulate complete hydrological processes of dendritic watershed systems.The software includes many traditional hydrologic analysis procedures such as event infiltration, unit hydrographs, and hydrologic routing.It also includes procedures necessary for continuous simulation including evapotranspiration, snowmelt, and soil moisture accounting.The physical representation of a watershed is accomplished with a basin model.Hydrologic elements are connected in a dendritic network to simulate runoff processes.Available elements include sub-basins, reaches, junctions, reservoirs, diversions, sources, and sinks.Computation proceeds from upstream elements in a down-stream direction [11].Modelling of the Upper Gulungul Catchment involved the following components: 1.
Precipitation methods which can describe an observed (historical) precipitation event or a frequency-based hypothetical precipitation event; 2.
Loss methods which can estimate the amount of precipitation that infiltrates from the land surface into the soil.By implication, the precipitation that does not infiltrate becomes surface runoff; 4.
Direct runoff methods that describe overland flow, storage, and energy losses as water runs off a watershed and into the stream channels generally called transform methods; 5.
Baseflow methods that estimate the amount of infiltrated water returning to the channel; 6.
Hydrologic routing methods that account for storage and energy flux as water moves through stream channels; 7.
Flowchart depicting the model parameters is shown in Figure 2.
Land 2023, 12, x FOR PEER REVIEW 5 of 24 runoff processes.Available elements include sub-basins, reaches, junctions, reservoirs, diversions, sources, and sinks.Computation proceeds from upstream elements in a downstream direction [11].Modelling of the Upper Gulungul Catchment involved the following components: 1. Precipitation methods which can describe an observed (historical) precipitation event or a frequency-based hypothetical precipitation event; 2. Evapotranspiration methods; 3. Loss methods which can estimate the amount of precipitation that infiltrates from the land surface into the soil.By implication, the precipitation that does not infiltrate becomes surface runoff; 4. Direct runoff methods that describe overland flow, storage, and energy losses as water runs off a watershed and into the stream channels generally called transform methods; 5. Baseflow methods that estimate the amount of infiltrated water returning to the channel; 6. Hydrologic routing methods that account for storage and energy flux as water moves through stream channels; 7. Models of naturally occurring confluences (junctions) and bifurcations (diversions); 8. Basin and stream erosion methods [17].9. Flowchart depicting the model parameters is shown in Figure 2. The hydrology, soil erosion, and sediment-routing capabilities of HEC-HMS were applied to the Upper Gulungul Catchment (UGC) (Figure 3), an area of 39.1 km 2 which drains to an imaginary outlet at GCUS.Thus, the catchment modelling was carried out such that the continuous discharge and sediment output of UGC was similar to the observed values at gauging station GCUS (Figure 3).For this study, a 10 m resolution DEM of UGC (Figure 4) was used in HEC-HMS and the watershed was divided into 77 subwatersheds/basins and 38 reaches for modelling.The sub-basins and reaches were delineated by HEC-HMS based on DEM data.The hydrology, soil erosion, and sediment-routing capabilities of HEC-HMS were applied to the Upper Gulungul Catchment (UGC) (Figure 3), an area of 39.1 km 2 which drains to an imaginary outlet at GCUS.Thus, the catchment modelling was carried out such that the continuous discharge and sediment output of UGC was similar to the observed values at gauging station GCUS (Figure 3).For this study, a 10 m resolution DEM of UGC (Figure 4) was used in HEC-HMS and the watershed was divided into 77 sub-watersheds/basins and 38 reaches for modelling.The sub-basins and reaches were delineated by HEC-HMS based on DEM data.

Parameters
The parameters used for modelling are tabulated in Table 1 below and a detailed description of the same follows.

Parameters
The parameters used for modelling are tabulated in Table 1 below and a detailed description of the same follows.Rainwater is infiltrated until rainfall exceeds the infiltration capacity of the soil after which excess precipitation formed on the surface that contributes to the runoff.Deficit and constant loss method was used to compute precipitation losses in this study since this allows for continuous simulation when used with a canopy method that will extract water from soil in response to potential evapotranspiration computed in meteorological model [21].Since the watershed in the study had distinct wet and dry seasons and model calibration was carried out for water years from the start of wet season, the initial storage was 0% and initial deficit equals maximum storage in deficit and constant loss method.The start of wet season in 2011 on the Ranger mine site was estimated as 9 October 2011 and end of wet season as 9 April 2012 [10].The initial deficit, constant loss, and imperviousness for basins were attained from ARR (Australian Rainfall and Runoff) data hub and calibrated accordingly [22].ARR data hub is a tool that can be used to attain design inputs required to undertake flood estimation in Australia.The calibrated value of initial deficit was 175 mm and constant loss rate varied from 0.175 to 0.225 mm/h.The evapotranspiration value for the catchment was taken as 5 mm/day [23].The actual surface runoff calculations for a sub-basin element were performed by a transform method.SCS (Soil Conservation Service) unit hydrograph was used as a transform method in this study.The unit hydrograph is a technique for modelling the transformation of excess precipitation to runoff at the watershed scale.The SCS unit hydrograph method defines a curvilinear unit hydrograph by first setting the percentage of the unit runoff that occurs before the peak flow.The percentage of runoff occurring before the peak is reflected in the Peak Rate Factor (PRF).The default unit hydrograph has a PRF of 484, which was used in this study [24].The catchment lag time is calculated from Equation (1) [18].
Lag Time (min) = Time of Concentration/1.67,Time of Concentration (min) = (107nL 0.333 )/S 0.2 (1 where n is the roughness value (n = 0.035 for bushland), L is overland sheet flow path length (m), S is slope of surface (%), [m/km]/10.L and S are attained from sub-basin characteristics in HEC-HMS.
Recession baseflow method is used to compute baseflow contributions to sub-basin outflow.The two constants, namely recession constant and ratio to peak constant used in the method, can vary from 0 to 1.The calibrated constant values for this study were 0.8 and 0.3, respectively.
Surface erosion can be computed at sub-basin elements using the MUSLE approach.Fine Suspended Sediment (FSS) is the so-called wash load in the streams which originates from soil erosion.MUSLE computes sediment yield originating from soil erosion products.It computes the total sediment yield transported out of the sub-basin on account of soil erosion.Sediment yield using MUSLE is calculated using Equation (2) [25].
where Q surf is the surface runoff volume (acre feet), Q peak is the peak runoff rate (cubic feet per second), K is the soil erodibility factor, LS is the topographic factor, C is the cover and management factor, and P is the support practice factor.The erodibility factor K describes the difficulty of eroding the soil.The topographic factor LS describes the surface's susceptibility to erosion due to length and slope.The practice factor P describes the effect of specific soil conservation practices.The cover factor describes the influence of plant cover on surface erosion.The values for K, LS, C, and P for the watershed were determined from CSIRO (Commonwealth Scientific and Industrial Research Organisation) data raster maps [19,26].The threshold factor is the peak flow less than which there will be no erosion or sediment yield.The calibrated value of threshold factor in this study was 1 m 3 /s.The exponent is used to distribute the sediment load into a time-series sedigraph, which was 0.56 in this study.Gradation curve is the sub-basin sediment particle size distribution of the watershed area where the catchment belongs [27].
The transport potential of the watershed specifies which method to use to calculate the stream flow sediment carrying capacity for non-cohesive sediments.The transport potential method used in this study was the Wilcock and Crow method [28] and the fall velocity method which determines the time required for the deposition of sediment from water column to the stream was the van Rijn method [29].
While a reach element in HEC-HMS conceptually represents a segment of stream or river, the actual calculations are performed by a routing method contained within the reach.The routing method used in this study was Muskingum-Cunge.The initial type of method was selected as assuming that the initial inflow is the same as the initial outflow to the reach from upstream elements.The length and slope of the reach were taken from reach characteristics in HEC-HMS.Manning's n roughness coefficient value was a calibrated value of 0.07.For the space-time method, Auto DX Auto DT Method was selected so that the program will automatically select space and time intervals that maintain numeric stability.The index flow was taken as the average value of hydrograph under consideration.
The reach cross section shape was given as trapezoid and the corresponding parameters of width and side slope were attained from Digital Elevation Model (DEM) reach profiles using GIS (Geographic Information System).
Sediment processes within a reach are directly linked to the capacity of the stream flow to carry eroded soil.The volume ratio method links the transport of sediment to the transport of flow in the reach using a conceptual approach.The bed width was the width of the sediment bed which is used in computing the volume of the upper and lower layers of the bed model.Bed depth is the total depth of the upper and lower layers of the bed.The active bed factor is used to calculate the depth of the upper layer of the bed model.The values for bed width, bed depth, and active bed factor for the catchment were taken as 10 m, 0.2 m, and 1, respectively [30].The initial bed curve defines the distribution of the bed sediment by grain size at the beginning of the simulation.The computed particle size distribution of different grain sizes at GCUS was used as input [10].

Rainfall
The rainfall gauging station at GCUS (Figure 1) gives continuous rainfall data at 10 min intervals.Since GCUS lies at the outlet of the Upper Gulungul catchment, using the same data to account for daily rainfall in the whole catchment was not accurate for calibration and validation of the HEC-HMS model.Thus, daily rainfall data were attained for water years 2012-13, 2013-14, and 2014-15 for modelling purposes from SILO, which is a database of Australian climate data from 1889 to the present [31].SILO gives rainfall point data at 5 km intervals.SILO data from points available across and near the catchment were considered and Voronoi polygons were drawn in QGIS [32] to determine the area of the catchment influenced by each rainfall point data [10].Daily rainfall data from six such data points were sorted to determine the rainfall that influences respective sub-basins of the catchment.
Area Reduction Factors (ARF) were then applied to the rainfall data.The point data gives rainfall intensity at a single point.For large catchments, the rainfall intensity is always not constant across the whole of the catchment during a storm.To account for this, an ARF is included to estimate the average rainfall over the whole of a large catchment [33].The equation for ARF is given in Equation (3).Computed ARF value of 0.89 for the catchment was then multiplied with the point data rainfall intensity available at six SILO points to account for the whole catchment.These data were used as rainfall input for respective sub-basins in the model for calibration and validation.where a, b, c, d, e, f, g, h, and I are ARF parameters specific to the region under study [22].Area is the catchment area and duration is the rainfall duration.AEP is the Annual Exceedance Probability, which is the probability of a given rainfall accumulated over a given duration to be exceeded in any one year.For example, a flood with 1% AEP has a 1 in 100 chance of being exceeded in any year for a region.
For future simulations of over 50 years, simulated rainfall data needs to be used as input with adequate climate change factors incorporated.The Intergovernmental Panel on Climate Change (IPCC) is the United Nations body for assessing the science related to climate change [34].CSIRO and the Australian Bureau of Meteorology (BOM) prepares tailored climate change projection reports for Australia, which provides guidance on climate changes that needs to be considered in planning and projections [35].The projections are based on the outputs from the ensemble of model simulations brought together for the Coupled Model Inter-comparison Project phase 5 (CMIP5) [36].Coupled modelling has facilitated using eight different models for climate projections in four greenhouse gas emission criteria [37].It provides future rainfall dataset available for discrete 30-year future periods and not continuous time series data over 100 years.Each future time series datum is regarded as representative of the mean state of future climate for the specified time period.Multiple future time series cannot be joined to make longer continuous time series.
Since the 30-year future time series cannot be joined to make longer continuous time series, rainfall projections for this study were taken from a different data set.Future rainfall simulations for this study shown in Figure 5 were taken from a project on Tom's Gully, which is 100 km away from Ranger mine [38].The study used the WGEN weather simulator [39] to generate a 1000-year continuous daily rainfall series from year 2025.WGEN is a stochastic weather generator which uses monthly and annual statistics to generate daily time series of precipitation, minimum temperature, maximum temperature, and solar radiation.In the absence of on-site observation data, long-term daily climate data from 1957 to 2019 were sourced from the SILO Australian climate online database and used as reference in WGEN.
Land 2023, 12, x FOR PEER REVIEW 11 of 24 time period.Multiple future time series cannot be joined to make longer continuous time series.
Since the 30-year future time series cannot be joined to make longer continuous time series, rainfall projections for this study were taken from a different data set.Future rainfall simulations for this study shown in Figure 5 were taken from a project on Tom's Gully, which is 100 km away from Ranger mine [38].The study used the WGEN weather simulator [39] to generate a 1000-year continuous daily rainfall series from year 2025.WGEN is a stochastic weather generator which uses monthly and annual statistics to generate daily time series of precipitation, minimum temperature, maximum temperature, and solar radiation.In the absence of on-site observation data, long-term daily climate data from 1957 to 2019 were sourced from the SILO Australian climate online database and used as reference in WGEN.

Model Calibration and Validation
The HEC-HMS model was calibrated with parameters given in Table 1 and described in the previous section.Calibration was performed for two water years (31st August to 31st August) of 2011-12 and 2012-13.Daily rainfall point data from SILO improvised with area reduction factor were used as input for respective sub-basins.The model was run for each water year to obtain daily output of discharge (m 3 s −1 ) and sediment (tonnes) at model output which is GCUS (Figure 3).The continuous discharge and turbidity data at 6 min intervals obtained from ERISS from their monitoring station at GCUS were used to compare the simulated output with the observed site data (Figure 6a,b).The observed discharge at 6 min intervals was averaged to hourly discharge, and turbidity data (NTU) at GCUS were converted to continuous FSS hourly data in tonnes (Table A1 in Appendix A) using Equation ( 4), [3].
Figure 6a,b thus compare the calibrated model output to the observed data for discharge and FSS, respectively.The efficiency of the calibrated model to match the observed values is determined by Nash-Sutcliffe Efficiency (NSE) number generated at the end of each simulation.The NSE value is computed as in Equation (5).
where yt is the observed data values for time period t, ft is the simulated data values for the same period, y is the mean observed data values per time period, and T is the number

Model Calibration and Validation
The HEC-HMS model was calibrated with parameters given in Table 1 and described in the previous section.Calibration was performed for two water years (31st August to 31st August) of 2011-12 and 2012-13.Daily rainfall point data from SILO improvised with area reduction factor were used as input for respective sub-basins.The model was run for each water year to obtain daily output of discharge (m 3 s −1 ) and sediment (tonnes) at model output which is GCUS (Figure 3).The continuous discharge and turbidity data at 6 min intervals obtained from ERISS from their monitoring station at GCUS were used to compare the simulated output with the observed site data (Figure 6a,b).The observed discharge at 6 min intervals was averaged to hourly discharge, and turbidity data (NTU) at GCUS were converted to continuous FSS hourly data in tonnes (Table A1 in Appendix A) using Equation ( 4), [3].FSS = 0.52 × NTU (4) Figure 6a,b thus compare the calibrated model output to the observed data for discharge and FSS, respectively.The efficiency of the calibrated model to match the observed values is determined by Nash-Sutcliffe Efficiency (NSE) number generated at the end of each simulation.The NSE value is computed as in Equation (5).
where y t is the observed data values for time period t, f t is the simulated data values for the same period, y is the mean observed data values per time period, and T is the number of time periods [8].The maximum possible NSE value is 1.0 and occurs if simulated values perfectly match observed values.Larger NSE values denote better model performance.The NSE values for 2011-12 and 2012-13 hydrology calibration in this study were 0.5 and 0.6, respectively.Since the turbidity values give the quantity of FSS using Equation ( 4), which is the combined clay and silt component, the clay and silt quantities from the model output were summed up and compared to the observed values in Figure 6b.
of time periods [8].The maximum possible NSE value is 1.0 and occurs if simulated values perfectly match observed values.Larger NSE values denote better model performance.
The NSE values for 2011-12 and 2012-13 hydrology calibration in this study were 0.5 and 0.6, respectively.Since the turbidity values give the quantity of FSS using Equation ( 4), which is the combined clay and silt component, the clay and silt quantities from the model output were summed up and compared to the observed values in Figure 6b.Since the model performed well for each calibration simulation, it was run for another water year of 2013-14 with the same model parameters used for the calibration study, for model validation.A comparison between observed values and the simulated output for hydrology and FSS are shown in Figure 6a and 6b, respectively.The NSE value for hydrology validation simulation was found to be 0.7, which was a good fit.

Medium Term Erosional Trends
Long-term rainfall simulations obtained from WGEN were used to run the calibrated and validated model for 50 years.The rainfall generated had a uniform average value across the years, as shown in Figure 5.The corresponding simulated discharge at GCUS for 50 years is shown in Figure 7a, which also exhibited a uniform trend.Unlike the rainfall and discharge, the simulated FSS quantities across the years decreased gradually (Figure 7b).Thus, the FSS output of the Upper Gulungul Catchment decreased over the years for a corresponding uniform rainfall event in UGC and corresponding discharge at GCUS.Since the model performed well for each calibration simulation, it was run for another water year of 2013-14 with the same model parameters used for the calibration study, for model validation.A comparison between observed values and the simulated output for hydrology and FSS are shown in Figure 6a and 6b, respectively.The NSE value for hydrology validation simulation was found to be 0.7, which was a good fit.

Medium Term Erosional Trends
Long-term rainfall simulations obtained from WGEN were used to run the calibrated and validated model for 50 years.The rainfall generated had a uniform average value across the years, as shown in Figure 5.The corresponding simulated discharge at GCUS for 50 years is shown in Figure 7a, which also exhibited a uniform trend.Unlike the rainfall and discharge, the simulated FSS quantities across the years decreased gradually (Figure 7b).Thus, the FSS output of the Upper Gulungul Catchment decreased over the years for a corresponding uniform rainfall event in UGC and corresponding discharge at GCUS.
One of the limitations of the model was the use of future rainfall simulations from Tom's Gully project, which is 100 km away from Ranger mine.There may be regional variations in rainfall patterns and intensity between the sites and the use of global climate model at a small scale can also be accompanied with certain uncertainties.For example, vegetation cover has a substantial effect on fine suspended sediment [40,41] and may vary according to season and location.

Sediment Quantification and Total Denudation Rate
The FSS sediment output for each water year from HEC-HMS simulations was 296 tonnes, 172 tonnes, and 287 tonnes at GCUS for 2011-12, 2012-13, and 2013-14, respectively.The total denudation rate of the Upper Gulungul catchment after 50 years of simulations was found to total 1.2269 mm.This accounts to 0.0245 mm year −1 of denudation rate in the catchment as predicted by HEC-HMS.The long-term average total denudation rate calculated from the available observed data at lowlands near Ranger mine is 0.075 ± 0.013 mm year −1 [42].As per CSIRO data, the denudation rates in the upper Gulungul catchment varies from 0.576 tonnes ha −1 year −1 to 19.923 tonnes ha −1 year −1 for different basins [19,26].This equates to 0.0217 mm year −1 to 0.727 mm year −1 for the denudation rate of basins across the catchment.The annual denudation rate predicted by HEC-HMS from 50-year simulations was 0.0245 mm, which is, thus, within the bounds of these estimates.One of the limitations of the model was the use of future rainfall simulations from Tom's Gully project, which is 100 km away from Ranger mine.There may be regional variations in rainfall patterns and intensity between the sites and the use of global climate model at a small scale can also be accompanied with certain uncertainties.For example, vegetation cover has a substantial effect on fine suspended sediment [40,41] and may vary according to season and location.

Sediment Quantification and Total Denudation Rate
The FSS sediment output for each water year from HEC-HMS simulations was 296 tonnes, 172 tonnes, and 287 tonnes at GCUS for 2011-12, 2012-13, and 2013-14, respectively.The total denudation rate of the Upper Gulungul catchment after 50 years of simulations was found to total 1.2269 mm.This accounts to 0.0245 mm year −1 of denudation rate in the catchment as predicted by HEC-HMS.The long-term average total denudation rate calculated from the available observed data at lowlands near Ranger mine is 0.075 ± 0.013 mm year −1 [42].As per CSIRO data, the denudation rates in the upper Gulungul catchment varies from 0.576 tonnes ha −1 year −1 to 19.923 tonnes ha −1 year −1 for different basins [19,26].This equates to 0.0217 mm year −1 to 0.727 mm year −1 for the denudation rate of basins across the catchment.The annual denudation rate predicted by HEC-HMS from 50-year simulations was 0.0245 mm, which is, thus, within the bounds of these estimates.

Conclusions
This study showed that HEC-HMS was effectively calibrated and validated as a catchment model for observed rainfall and its quantified discharge and sediment output from the catchment over 50 years.The model output proved to be a good fit to the observed data.The output from the HEC-HMS model showed that the erosional trends tended to decrease over time, which is in concordance with the previous studies which state that parameters that influence the erosion characteristics of a post-mining landform decreases over time, especially in the first 50 years after rehabilitation.The annual denudation rate of the study catchment predicted by the model after the 50-year simulation is in the same order of magnitude of the natural denudation rate of the area calculated in previous studies.It also falls in the range of denudation rates determined by CSIRO for the catchment area.Thus, the methodology and selection of parameters for calibration and validation of the model in this study seem adequate to predict discharge and sediment output from the catchment in concordance with the field observed data, thus enabling it to predict future erosional trends.
Previous studies on the Ranger Mine site have evaluated landform stability and temporal changes in erosional trends based on gully erosion and incision depths.This study enabled the modelling of continuous discharge and FSS output from the catchment and enabled the evaluation of event FSS output for a specific discharge event.Thus, it enabled the use of the output for evaluating FSS following specific rainfall events as an indicator of disturbance and achieving equilibrium.While this was previously carried out within a catchment [3], the use of HEC-HMS enables upstream inputs to be included.HEC-HMS also allows hourly rainfall inputs, and easily enables differing wet and dry season rainfall scenarios to be included.This was important for its use in tropical regions with highly seasonal rainfall conditions.In contrast, the CAESAR-Lisflood LEM was modelled [10], but while the annual simulations gave event FSS output matching the observed site values for the early wet season, they underpredicted the FSS output for the later part of the wet season.HEC-HMS modelling simulations of FSS gave uniform comparison to observed values throughout the wet season.HEC-HMS also runs faster than CAESAR-Lisflood to predict event discharge and FSS quantities.Moreover, HEC-HMSpredicted output can be used to run CAESAR-Lisflood LEM for the mine site to determine how the mine site affects the downstream development of the catchment landform, in terms of sediment changes and geomorphology.

Figure 1 .
Figure 1.Location of gauging stations at Ranger.Adapted from [16]; copyright Commonwealth of Australia.

Figure 1 .
Figure 1.Location of gauging stations at Ranger.Adapted from [16]; copyright Commonwealth of Australia.

Figure 4 .
Figure 4. Basins and reaches identified in HEC-HMS for Upper Gulungul Catchment.

Figure 4 .
Figure 4. Basins and reaches identified in HEC-HMS for Upper Gulungul Catchment.

Figure 6 .
Figure 6.(a) Calibration and validation results comparing the observed output discharge with model outputs.(b) Calibration and validation results comparing the observed FSS quantities with model outputs.

Figure 6 .
Figure 6.(a) Calibration and validation results comparing the observed output discharge with model outputs.(b) Calibration and validation results comparing the observed FSS quantities with model outputs.

Table 1 .
Parameters used in HEC-HMS for calibration.

Table 1 .
Parameters used in HEC-HMS for calibration.
* Field data obtained from ERISS.