Annualized Agricultural Non-Point Source Research Articles

Effectiveness and resilience of BMPs to watershed climate adaptation considering the uncertainty of hydrological model and GCMs

Climate change significantly impacts catchment hydrologic and water quality processes. Best management practices (BMPs) can serve as adaptation strategies to negate the impact of climate change on sediment and total nitrogen (TN) loads. One major controversial issue in climate change adaptation studies is the highly uncertain nature of such changes. Previous studies have rarely focused on the combined impact of the uncertainty in climate change and watershed model parameters, which could be the main sources of uncertainty in climate change adaptation research. In this study, the effectiveness and resilience (defined as continued effectiveness under a changing climate) of BMPs in reducing sediment and TN loads were explored under future climate change in the Shanmei Reservoir watershed (SMW) of Southeast China. Climate change projections provided by 10 general circulation models (GCMs) under the SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5 scenarios from 2031 to 2060 were applied in the Annualized Agricultural Nonpoint Source (AnnAGNPS) model to evaluate the effectiveness and resilience of 4 BMPs (riparian buffers (RB), no-tillage (NT), fertilization reduction (FR), and parallel terraces (PT)). The ensemble average of 10 GCMs and 10 behavior parameter groups were adopted to reduce the uncertainty resulting from the hydrological model parameters and GCMs. The results indicated that the average annual temperature and precipitation in the SMW will increase in the future. On a seasonal scale, the average temperature during all seasons will increase, and precipitation will decrease in summer and autumn but increase in spring and winter. The annual sediment and TN loads will decrease, but the loads in spring and winter will increase. BMPs could be effective as climate adaptation strategies for reducing sediment and TN loads under future climate conditions, with PT as the most effective option. Structural BMPs were more effective in reducing sediment and TN loads in spring and winter, whereas nonstructural BMPs were more effective in reducing loads in summer and autumn. BMPs were more resilient when future watershed runoff changes were slight or climate sensitivity was reduced, with higher BMP resilience in spring and winter than in summer and autumn. This study aimed to provide systematic references for watersheds through the implementation of BMPs for mitigating the effects of climate change and extending the boundaries of the AnnAGNPS model application.

Empirical tool development for prairie pothole management using AnnAGNPS and random forest

Watershed models are robust tools that inform management and policy in a variety of sectors, but these models are often neglected through time due to economic or technical constraints. Additionally, they are not readily accessible tools for key decision makers. Conversely, machine learning models are robust alternatives to common hydrologic modeling frameworks. The random forest algorithm specifically is an interpretable predictive tool. We couple Annualized Agricultural Non-Point Source (AnnAGNPS) model output, an abstract, anthropogenic flood risk metric, and develop a random forest model to provide an empirical tool that benefits decision makers in the Des Moines Lobe of the Prairie Pothole Region in north-central Iowa. The developed model has the capacity to predict our flood risk metric (calibration: R2 > 0.9, validation: R2 > 0.7) for individual farmed prairie potholes across a variety of morphologic and management conditions and can be used iteratively to assess alternative actions.

Characterization of prairie pothole inundation using AnnAGNPS under varying management and drainage scenarios

Farmed prairie potholes are small, isolated depressions frequently classified as semi-permanent wetlands that make up a significant portion of land area in the Des Moines Lobe (DML) of the larger Prairie Pothole Region (PPR). Historically, these depressions have been subjected to significant drainage to improve their agricultural capacity. However, many assessments of the economic return of continuing to farm these depressions suggest that continued attempts to produce conventional row crops is not profitable and has other ecological consequences beyond crop drownout. This study expands the existing discussion of land use and drainage alternatives in a watershed modeling context. This study utilized the Annualized Agricultural Non-Point Source (AnnAGNPS) model to individually simulate the long-term hydrology of 6 prairie potholes using a matrix of land use and drainage modifications. Results suggest the presence of artificial drainage is the dominant factor in prairie pothole hydrology, while retirement and no-till practices can provide moderate reductions in flood inundation. Conservation tillage induces minimal change on flood metrics. Results show that average annual maximum inundated surface area is reduced by at most 50% across all simulations and the median annual days flooded could be reduced by 25 days, though this is less consistent when isolating high-precipitation years. Regardless of drainage status, in all scenarios there are, on average, more than two inundations events per year lasting 2–4 days. Longer events occur approximately once per year on average. Area inundation frequency curves suggest up to a 20% reduction in maximum pothole area inundated annually can be achieved at the 2-year return frequency. The availability of this data helps characterize the hydrology of farmed potholes more generally over a wide range of conditions, providing a reference for the prioritization of potholes for conservation or alternative management.

Assessment of Input Parameters and Calibration Methods for Simulating Prairie Pothole Hydrology using AnnAGNPS

HighlightsModel equifinality incurs user-driven decisions when modeling with AnnAGNPS.Prairie pothole models should be calibrated using a limited parameter search space.LiDAR-derived surface areas best represent prairie pothole hydrology in AnnAGNPS.Modeling field-scale prairie pothole spill-and-fill dynamics is currently limited.Abstract. The Annualized Agricultural Non-Point Source (AnnAGNPS) model was used to predict water volume in individual prairie potholes within the Des Moines Lobe of Iowa. AnnAGNPS can represent prairie potholes as static wetland areas located on preferential flow paths at field scale, whereas standard practice of watershed modeling is to fill depressions while preparing input data, and whereas models have lumped parameters at scales too large for individual pothole representation, or are considered too complex for widespread implementation. Here, we assess how key user inputs affect hydrologic calibration of 6 AnnAGNPS prairie pothole models, suggest methods for calibrating and modeling the impact of some potholes at field scale, and explain challenges in modeling prairie potholes with AnnAGNPS. Results indicate that user-defined pothole surface area does not significantly affect statistical calibration of the model. Nash-Sutcliffe Efficiency 0.26 to 0.79, R2 ranged from 0.37 to 0.80 and RSR ranged from 0.46 to 0.86. The area that best simulates pothole hydrology and expected physical parameters is derived from LiDAR and is considered the maximum extent of the pothole. Potholes were calibrated using a stepwise, iterative, limited search space method using multiple representations of their inundation extent. We graphically assessed statistical parameters to identify calibrated models and chose input parameters within reasonable ranges of published literature. This helped formulate our recommendation for the modeled pothole area. Finally, AnnAGNPS is limited in its ability to simulate field-scale pothole networks due to the placement of potholes within the model’s subbasin network. Investigating high-resolution subbasin delineation or more specific wetland placement may alleviate this issue. Keywords: AnnAGNPS, Farmed wetlands, Hydrology, Prairie potholes, Prairie pothole region.

Simulation of Runoff and Sediment Yield Using AnnAGNPS Model at Dawe Watershed, Eastern Hararghe, Ethiopia

The high erosion rates are mainly affecting the developing countries due to intensive cultivation, deforestation; extreme climate hazard, sediment transport and loss of agricultural nutrients were caused by unwise land use practices, intensive cultivation and improper management. Soil conservation is the only known way to protect the productive land. In this study a physically based watershed model, Annualized Agricultural Non-Point Source (AnnAGNPS) pollution model was applied to the Dawe River watershed for simulation of the runoff and sediment yield. The objectives were to estimate potential runoff and sediment yield and to recommend and design appropriate soil and water conservation measures on a sub watershed basis in Dawe watershed of east hararghe zone. Sensitivity analysis, model calibration and validation were also performed. Four highly sensitive parameters were identified and of which CN was the most sensitive one. For model calibration, model efficiencies of 0.742, -231.081 and 0.828 were observed for surface runoff, peak runoff rate, and sediment yield, respectively. The corresponding determination of coefficients was 0.825, 0.1669 and 0.848, respectively. Runoff and sediment yield were well predicted but, peak runoff rate was over predicted. Validation results produced model efficiencies NSE of 0.769, -73.801 and 0.718 for surface runoff, peak runoff rate and sediment yield, respectively. With coefficient of determination (R²) of 0.9215, 0.235 and 0.764 for runoff, peak runoff rate and sediment yield, respectively. Surface runoff and sediment yield simulation were well in the validation stage and peak runoff rate shows the same trend as calibration. Dawe watershed was divided in thirteen sub-watersheds. Runoff and sediment yield for each sub-watershed were quantified. Average annual watershed runoff, average annual soil loss and total annual sediment outflow from Dawe watershed was 194.48mm, 22.467 tons/ha/yr and 354215 tons/yr, respectively. In Dawe watershed, gully, rill and inter-rill erosions were identified as major problems. Thus, check dam and bench terrace designed and vegetative waterway are recommended for intensively cultivated crop land of Dawe watershed.

Using AnnAGNPS to Simulate Runoff, Nutrient, and Sediment Loads in an Agricultural Catchment with an On-Farm Water Storage System

On-farm water storage (OFWS) systems are best management practices that consist of a tailwater recovery (TWR) ditch used with a storage pond to provide irrigation water and improve downstream water quality. These systems have been increasingly implemented in the southeastern US, but the individual and cumulative effects of these systems on a watershed scale are unknown. In this study, the runoff, nutrient, and sediment loads entering a TWR ditch in an agricultural catchment were quantified, and contributing sources were identified using the annualized agricultural non-point source (AnnAGNPS) model. Fields with larger areas and soils with a high runoff potential produced more runoff. The volume of runoff exceeded the TWR ditch storage volume approximately 110 times, mostly during the winter and spring seasons. During years when corn and winter wheat were planted, NO3–N loads increased because these crops need nitrogen fertilization to grow. Planting winter wheat in priority subwatersheds reduced the total phosphorous (TP) and sediment loads by about 19% and 13%, respectively, at the TWR ditch inlet. Planting winter wheat can reduce runoff, TP, and sediment loads but also result in higher NO3–N loads. AnnAGNPS simulations quantified the benefits of an OFWS system to advance the understanding of their impact on water availability and quality at a watershed scale.

Crop conversion impacts on runoff and sediment loads in the Upper Sunflower River watershed

The Mississippi River alluvial floodplain is one of the most productive agricultural regions in the United States. In recent decades, factors of economic forces and government policies have driven crop selection and respective farming management changes in this region. This study quantified the effects of changes in crop conversion and farming management practices to discharge and sediment loads in the Upper Sunflower River watershed. Farming and climate conditions were dynamically characterized in space and time by integrating annual crop yield at the county scale, annual crop irrigation in the field scale, and an enhanced description of precipitation. This information was used as input into the Annualized Agricultural Non-Point Source (AnnAGNPS) watershed pollution model to describe existing conditions and simulate alternative scenarios. Simulations considering high irrigation adoption indicated 15% average annual percent change increase in short-term flow during the months of June, July, and August at the outlet. Conversely, simulations considering future trends in crop conversion to corn/soybean production indicates a potential reduction in average annual sediment loads for clay and silt, respectively, to 4.6 Mg/ha and 0.41 Mg/ha from 5.5 Mg/ha and 0.44 Mg/ha for existing conditions. As irrigation increases and crop production continues to shift towards corn/soybeans, these findings support the development of responsible irrigation management strategies designed for efficient water usage coupled with implementation of in field conservation practices for reduced sediment loads.

The reduction effects of riparian reforestation on runoff and nutrient export based on AnnAGNPS model in a small typical watershed, China.

The continuous deterioration of the aquatic environment in rivers and streams is increasingly causing social and political tensions. To alleviate aquatic environmental problems, especially for the nonpoint source pollution, establishment of riparian forest buffers has been demonstrated as an effective control measure. However, few comprehensive studies of the reduction effects of riparian reforestation on the aquatic environment have been performed, particularly in identifying the suitable widths of reforestation projects. In this paper, the Annualized Agricultural Non-Point Source (AnnAGNPS) model was used to simulate the reduction effects of riparian reforestation on runoff and nutrient loads in Wucun watershed, China. The results showed that 20-m, 40-m, and 60-m widths of riparian buffer reforestation had significant effects on the yearly loads of total nitrogen (TN) and total phosphorus (TP), with reduced rates of 23.21 to 56.2% and 18.16 to 52.14%, respectively. The reduction effect on annual runoff varied from 2.8 to 5.4%. Furthermore, the reduction effect of nutrients performed best during the transition period, while the best runoff reduction was found during the dry period. These distinct reductions indicated that the implementation of riparian forest buffers was capable of reducing the risk and frequency of flooding and eutrophication, especially during the wet and transition periods. Additionally, the 20-m width of riparian buffer reforestation achieved the highest reduction efficiency for runoff, and the 40-m width was the most suitable reforested riparian buffer width for TN and TP. Therefore, 40m may be the optimum buffer width for the implementation of riparian reforestation in the Wucun watershed. These research results provided scientific information on selecting the optimum buffer width for aquatic environmental regulators and managers as the reduction effects of different widths of riparian buffers on runoff and nutrients were different when considering buffer reforestation.

Modelling soil erosion in a Mediterranean watershed: Comparison between SWAT and AnnAGNPS models

In this study, the simulations generated by two of the most widely used hydrological basin-scale models, the Annualized Agricultural Non-Point Source (AnnAGNPS) and the Soil and Water Assessment Tool (SWAT), were compared in a Mediterranean watershed, the Carapelle (Apulia, Southern Italy). Input data requirements, time and efforts needed for input preparation, strength and weakness points of each model, ease of use and limitations were evaluated in order to give information to users. Models were calibrated and validated at monthly time scale for hydrology and sediment load using a four year period of observations (streamflow and suspended sediment concentrations). In the driest year, the specific sediment load measured at the outlet was 0.89 t ha−1 yr−1, while the simulated values were 0.83 t ha−1 yr−1 and 1.99 t ha−1 yr−1 for SWAT and AnnAGNPS, respectively. In the wettest year, the specific measured sediment load was 7.45 t ha−1 yr−1, and the simulated values were 8.27 t ha−1 yr−1 and 6.23 t ha−1 yr−1 for SWAT and AnnAGNPS, respectively. Both models showed from fair to a very good correlation between observed and simulated streamflow and satisfactory for sediment load. Results showed that most of the basin is under moderate (1.4–10 t ha−1 yr−1) and high-risk erosion (> 10 t ha−1 yr−1). The sediment yield predicted by the SWAT and AnnAGNPS models were compared with estimates of soil erosion simulated by models for Europe (PESERA and RUSLE2015). The average gross erosion estimated by the RUSLE2015 model (12.5 t ha−1 yr−1) resulted comparable with the average specific sediment yield estimated by SWAT (8.8 t ha−1 yr−1) and AnnAGNPS (5.6 t ha−1 yr−1), while it was found that the average soil erosion estimated by PESERA is lower than the other estimates (1.2 t ha−1 yr−1).

Characterizing ponds in a watershed simulation and evaluating their influence on streamflow in a Mississippi watershed

ABSTRACTSmall water bodies, such as ponds and wetlands, are common landscape features, but often are not simulated within a watershed modeling framework. The wetland modeling tool, AgWET, uses a GIS framework to characterize the features of ponds and wetlands so that they can be incorporated into watershed simulations using the Annualized Agricultural Non-Point Source (AnnAGNPS) pollution model. AgWET was used to characterize farm ponds on the Goodwin Creek Experimental Watershed in northwest Mississippi and AnnAGNPS simulated watershed hydrology. Monthly streamflow was validated at four watershed gauges with Nash-Sutcliffe efficiency values between 0.91 and 0.94. Ponds influenced watershed hydrology at various scales, with a decrease in average streamflow by 4% at the watershed outlet, 8% at the sub-watershed scale, and an average of 56% immediately downstream of the ponds. AgWET can be used to simulate ponds in watershed assessments for improved results and evaluation of future scenarios.

Lake Nutrient Responses to Integrated Conservation Practices in an Agricultural Watershed.

Watershed-scale management efforts to reduce nutrient loads and improve the conservation of lakes in agricultural watersheds require effective integration of a variety of agricultural conservation best management practices (BMPs). This paper documents watershed-scale assessments of the influence of multiple integrated BMPs on oxbow lake nutrient concentrations in a 625-ha watershed of intensive row-crop agricultural activity during a 14-yr monitoring period (1996-2009). A suite of BMPs within fields and at field edges throughout the watershed and enrollment of 87 ha into the Conservation Reserve Program (CRP) were implemented from 1995 to 2006. Total phosphorus (TP), soluble reactive phosphorus (SRP), ammonium, and nitrate were measured approximately biweekly from 1996 to 2009, and total nitrogen (TN) was measured from 2001 to 2009. Decreases in several lake nutrient concentrations occurred after BMP implementation. Reductions in TP lake concentrations were associated with vegetative buffers and rainfall. No consistent patterns of changes in TN or SRP lake concentrations were observed. Reductions in ammonium lake concentrations were associated with conservation tillage and CRP. Reductions in nitrate lake concentrations were associated with vegetative buffers. Watershed simulations conducted with the AnnAGNPS (Annualized Agricultural Non-Point Source) model with and without BMPs also show a clear reduction in TN and TP loads to the lake after the implementation of BMPs. These results provide direct evidence of how watershed-wide BMPs assist in reducing nutrient loading in aquatic ecosystems and promote a more viable and sustainable lake ecosystem.

Climate Change Impacts on Runoff, Sediment, and Nutrient Loads in an Agricultural Watershedin the Lower Mississippi River Basin

Abstract. Projected climate change can impact various aspects of agricultural systems, including the nutrient and sediment loads exported from agricultural fields. This study evaluated the potential changes in runoff, sediment, nitrogen, and phosphorus loads using projected climate estimates from 2041-2070 in the Beasley Lake watershed in Mississippi, USA, using the Annualized Agricultural Non-Point Source (AnnAGNPS) pollution watershed model. For baseline conditions and model inputs an earlier validated simulation of the watershed was used with an event-based NSE of 0.81 for runoff and 0.54 for sediment without calibration. Fifteen global climate models (GCMs) for the climate change scenario RCP8.5 in Western Mississippi were used. Daily precipitation and air temperature were generated with the weather generator SYNTOR. Daily climate data derived from all 15 GCMS were used in AnnAGNPS simulations to generate ensemble projected loads, and climate data from four GCMs were used in simulations to assess the effectiveness of five different conservation practices for reducing projected loads. Predicted median annual-average pollutant loads increased by 9% to 12% with ensemble projected climate change. However, no-tillage and cover crop conservation practices were predicted to reduce pollutant loads from 20% to 75% below historical levels despite the impacts of climate change. This study suggests that greater implementation of conservation practices can be effective at mitigating water quality degradation associated with projected climate change. Keywords: AnnAGNPS, CMIP5, Soybean, SYNTOR, USDA-CEAP, Water quality.

Evaluation of Alternative Management Practices With the AnnAGNPS Model in the Carapelle Watershed

AbstractThe Annualized Agricultural Non-point Source (AnnAGNPS) model can be used to analyze the effects of management practices on sediment loads in agricultural watersheds. The study was performed in a 506 km2 Mediterranean watershed located in Apulia, Southern Italy, planted with mostly winter wh

Quantifying Spatiotemporal Change in Landuse and Land Cover and Accessing Water Quality: A Case Study of Missouri Watershed James Sub-Region, North Dakota

Spatial causal effects on water quality are essential in identification of vulnerable watersheds. Modelling landuse variables is an effective method of projecting localized impairment. This study presents an integrated index, designed to gauge the ability of an 8-digit Hydrologic Unit Code watershed in its ability to produce clean water. This index, IAPCW, can be successfully applied on a geospatial platform. In this study we utilized IAPCW to address forest cover dynamics of an impaired watershed, that is, Missouri Watershed James Sub-region in North Dakota. Specific parametric functions were analysed and combined within a customized GIS interface to provide a multi-faceted structured technique to derive IAPCW. These included ambient forest cover, housing density, agricultural land, soil erodibility and road density; it can be lucidly ascertained that where a prevailing forest cover undergoes conversion processes, the secondary effect may spur an exponential increase in water treatment costs. These parameters when projected statistically validated temporal and spatial relations of landuse/land cover dynamics to nutrient concentrations especially those that would be noted at the mouth of the watershed. In this study, we found that the levels of Total Dissolved Solids (TDS) were much higher for the years 2014 to 2016 with a discernible increased alkalizing effect within the watershed. When IAPCW was compared to Annualized Agricultural Nonpoint Source (AnnAGNPS), the spatial distribution generated by the AnnAGNPS study showed that fallow areas produced significant amounts of sediment loads from the sub-watershed. These same locations in this study generated a low IAPCW.

Evaluation of the AnnAGNPS Model for Predicting Runoff and Nutrient Export in a Typical Small Watershed in the Hilly Region of Taihu Lake.

The application of hydrological and water quality models is an efficient approach to better understand the processes of environmental deterioration. This study evaluated the ability of the Annualized Agricultural Non-Point Source (AnnAGNPS) model to predict runoff, total nitrogen (TN) and total phosphorus (TP) loading in a typical small watershed of a hilly region near Taihu Lake, China. Runoff was calibrated and validated at both an annual and monthly scale, and parameter sensitivity analysis was performed for TN and TP before the two water quality components were calibrated. The results showed that the model satisfactorily simulated runoff at annual and monthly scales, both during calibration and validation processes. Additionally, results of parameter sensitivity analysis showed that the parameters Fertilizer rate, Fertilizer organic, Canopy cover and Fertilizer inorganic were more sensitive to TN output. In terms of TP, the parameters Residue mass ratio, Fertilizer rate, Fertilizer inorganic and Canopy cover were the most sensitive. Based on these sensitive parameters, calibration was performed. TN loading produced satisfactory results for both the calibration and validation processes, whereas the performance of TP loading was slightly poor. The simulation results showed that AnnAGNPS has the potential to be used as a valuable tool for the planning and management of watersheds.

Evaluation of Surface Runoff Prediction by AnnAGNPS Model in a Large Mediterranean Watershed Covered by Olive Groves

AbstractThe distributed parameter and continuous simulation Annualized Agricultural Non‐Point Source (AnnAGNPS) model was implemented in the watershed Anzur (Spain) covered by olive groves, to assess its prediction capability of surface runoff at the large watershed scale in semi‐arid conditions. A 5‐year database reporting hydrological, geomorphological and land use characteristics of the watershed allowed model implementation. Almost 180 surface runoff events were modelled by AnnAGNPS and compared with the corresponding observations through statistical indexes and grouping the runoff events in order to evaluate the model at different temporal scales (event, monthly and seasonal). AnnAGNPS evaluation showed that, in general, runoff was estimated by the default model with low accuracy at all the investigated time scales, likely, as a result of a simple representation of spatial variability. Calibration (by reducing initial curve numbers (CN) of the olive groves) provided more accurate and satisfactory predictions of event, monthly and seasonal runoff volumes with a low effort in the parameterisation approach. The best model performance was achieved at the event scale. The runoff prediction reliability may be attributable to the AnnAGNPS inaccuracy in adjusting CN values during the continuous simulation of the soil moisture conditions, because estimations of daily evapotranspiration values are quite realistic. Copyright © 2015 John Wiley & Sons, Ltd.

Herramientas informáticas de gestión de terrenos agrarios : potencialidades del modelo AnnAGNPS en Navarra

The Agricultural activity is largely affected by the fluctuating socioeconomic conditions of the region. Thus, an appropriate management of any agricultural land should anticipate potential changes in the land use and their consequences. For this, hydrological models are valuable aids. In Navarre, AnnAGNPS (Annualized Agricultural Non-Point Source Pollution Model) has been extensively evaluated, specifically in dry lands devoted to cereals. In sums, satisfactory results were obtained. In this paper the rudiment of the possible application of this model in the management of the agricultural activity in Navarre is given. In a small grain-sown watershed, different scenarios with alternative potential crops are recreated by the model. In each case, the effect of these changes on the runoff and sediment generation -average annual values- at the watershed outlet are estimated. Overall, AnnAGNPS appears as a promising tool for assessing the effect of the agricultural activity on the environment in the long run; and then for helping implementing adequate land management practices.

Estimation of Runoff, Peak Discharge and Sediment Load at the Event Scale in a Medium‐Size Mediterranean Watershed Using the Annagnps Model

AbstractIn this paper, the Annualized Agricultural Non‐Point Source (AnnAGNPS) model has been used to estimate runoff, peak discharge and sediment load at the event scale in a Mediterranean watershed. The study area is the Carapelle torrent, Southern Italy (area = 506 km2), where continuous rainfall, streamflow and sediment load data are available. Nineteen flood events have been registered in the period 2007–2009 and were used for the application of the model.The aim of the paper is to evaluate the predictive accuracy of the model at the event scale, in a medium‐size watershed, given the specific conditions of the semi‐arid environments. A sensitivity analysis has been carried out to assign the correct parameterization: the mean normalized output variation of the most meaningful input parameters pointed out the influence of the curve number on runoff, peak discharge and sediment load predictions (values greater than 1); the MN Manning's roughness coefficient and K, C and P factors of the universal soil loss equation showed a moderate influence on sediment load simulations (values between 0·5 and 1).The selection of the Soil Conservation Service synthetic storm types has been based on the observed storm events analysis to improve the peak discharge simulations. The model prediction has proved to be good for runoff (R2 = 0·74, NSE = 0·75, W = 0·92) and peak discharge (R2 = 0·85, NSE = 0·70, W = 0·94), and satisfactory for sediment yield (R2 = 0·70, NSE = 0·63, W = 0·91). The relative error is lower for high events; this result is quite interesting in semi‐arid environments, where most of the annual sediment yield is concentrated in a few, severe events. Copyright © 2013 John Wiley & Sons, Ltd.

Effects of critical source area on sediment yield and streamflow

AbstractThe spatial discretization scale determines the degree to which the spatial heterogeneity of the surface condition and the stream network can be represented. Discretization scale can have a significant impact on watershed model results, with respect to both hydrology and water quality. The impact of critical source area (CSA) on simulated water yield for each cell can be assessed for streamflow and run‐off using the Annualized Agricultural Non‐Point Source (AnnAGNPS) model. The total number of cells simulated for the six watersheds ranged from 8 to 352 for various CSA combinations. Increasing the number of subwatersheds increased water and sediment yield in many watersheds. Increasing the number of cells influences the depiction of land use and soil type accordingly and therefore influences run‐off. The applied CSAs were also compared with topographic parameters such as average cell slope, average channel slope and length to width ratio of each watershed.

三峡库区典型农林流域景观格局对径流和泥沙输出的影响

The Three Gorges Reservoir area is one of China's the most ecologically sensitive areas. Transport and deposition of eroded material as well as substances dissolved in runoff and attached to soil particles lead to declines in water quality, eutrophication and negative impacts to agricultural lands and to waters of the Three Gorges Reservoir. Generally,land use changes,especially the conversion of forests to agriculture in watersheds,are thought to cause these problems. The conversion of cropland to forest as part of an extensive ecological afforestation engineering scheme has played a very important role in reducing runoff and sediment export in this area.Landscape pattern analysis plays an important role in landscape ecology,whose ultimate goal is to link spatial patterns of landscapes with ecological processes and detect status of processes using landscape pattern information. By analyzing the spatial arrangement of relevant factors related to landscapes,landscape pattern analysis can obtain information to predict dynamic patterns of ecological processes. A close relationship exists between land use structure and runoff and sediment export. Landscape patterns have a significant and complex effect on soil erosion and sediment delivery. In this study,a total of 25 typical agroforestry watersheds in Zigui County,Hubei,China,in the Three Gorges Reservoir area were selected to investigate the effects of watershed landscape patterns on runoff and sediment by using RS and GIS techniques. Watershed landscape pattern indices selected were percentage of landscape patch area( PLAND) at the patch-class level,patch richness density,double log fractal dimension( DLFD),Shannon's diversity index( SHDI) and contagion( CONT) at the landscape level. Watershed landscape pattern indices were calculated by FRAGSTATS 4.1. Runoff and sediment exported from 25 watersheds in study area were simulated by the AnnAGNPS( Annualized AGricultural NonPoint Source) model,which is a continuous watershed-scale model that can simulate runoff and sediment in watersheds covering up to 300000 hm2. Correlation between pattern indices and runoff sediment was analyzed using SPSS 19.5 software. The results show a significant positive relationship exists between runoff and CONT and PLANDs of sloping farmlands, terraces,agroforestry terraces and residential land. A significant negative correlation was found between runoff and PLANDs of forestland and shrubland. Soil erosion correlation analysis also showed a significant positive relationship between sediment and DLFD,CONT and PLANDs of sloping farmlands,agroforestry terraces and residential land. A significant negative correlation was found between sediment and SHDI and PLANDs of forestland and shrubland. Multiple correlation coefficients between landscape indices and runoff and sediment were 0.856 and 0.962,respectively,both of which were higher than those of single-factor coefficients. There were linear regression relationships between the landscape indices and runoff and sediment. Landscape pattern significantly influences runoff and sediment delivery in watersheds at the landscape-level. Our results also present empirical evidence for the existence of direct relationship between the landscape patterns in study area and runoff and sediment processes as well as suggest landscape spatial patterns and functional diversity may be linked through feedback mechanisms.