Agricultural Policy/Environmental eXtender Research Articles

Evaluation of the agricultural policy environmental extender (APEX) for the Chesapeake Bay watershed

The Agricultural Policy/Environmental Extender (APEX) is a complex mechanistic simulation model designed to evaluate agricultural management strategies and their effects on yields, water and soil quality, and resource use. It is one of the main models used in the Conservation Effects Assessment Project (CEAP) assessment program and has been employed to evaluate the effects of conservation practices in the Chesapeake Bay Watershed (CBW). Calibration of model parameters is an important process in order to apply a simulation model to a particular area. The APEX model is normally parameterized for each application using selected datasets from the region. However, the APEX application area is usually very large and all areas may not be well represented by the calibration. Thus, additional tests of APEX without site-specific calibration for crop yield and soil water content can provide information on the stability of the parameters related to these variables in the CBW. The objective of this study is to evaluate the performance of APEX for corn yields and water balance in a new location using parameters previously developed for the CBW. This study used observations from the OPE3 site (Optimizing Production Inputs for Economic and Environmental Enhancement) located at the US Department of Agriculture, Henry A. Wallace Agricultural Research Center, Beltsville, Maryland, USA. Data from the OPE3 site consists of water content at various layers, corn yields, and ET covering the period from 1998 to 2013. The results for total water in the soil profile (up to 100 cm deep) showed that the simulated water amounts were about 4.4 cm higher than the observed values, but the trends were very similar to the observed trends. The statistical error analysis of R-square (RSQ), Nash-Sutcliffe (ENS) efficiency coefficient and Root Mean Square Error (RMSE) are 0.59, 0.51 and 0.21 cm•day−1, respectively. The average simulated yields were 0.3 t•ha−1 (4%) lower than observed yields with an average absolute deviation between APEX and OPE3 of 1.3 t•ha1 where the average observed yields over the period was 5.18 t•ha−1, The year-to-year trends were very similar to the measured trends. The monthly evapotranspiration data also showed reasonable trends between observed data and model results with the model RSQ, ENS and RMSE being 0.56, 0.41 and 4.49 mm•month−1, respectively. These results suggest that APEX parameters for yield and water use simulations calibrated for the CBW can be considered applicable to other areas of the CBW where calibration data are not available.

APEXeditor: A Spreadsheet-Based Tool for Editing APEX Model Input and Output Files

The APEXeditor, an Excel-based tool has been developed using the Visual Basic for Applications (VBA) to provide a graphical user interface (GUI) to the Agricultural Policy Environmental eXtender (APEX) model. APEX, in its native form, requires users to edit text files for modifying input files; therefore a GUI interface can aid users in modification of these files and reduce errors. Microsoft Excel is a popular spreadsheet program that has the largest user base among scientists and researchers, thus providing a relatively common platform in which stages the tool. The APEXeditor requires minimal additional learning to operate the tool for those who already have basic level of knowledge in Excel. The user can load APEX input files into the spreadsheet tool and the GUI offers meta information and provides functions to edit, write, and run the APEX model. Ultimately, the APEXeditor substitutes existing GUI programs such as WinAPEX or ArcAPEX that require installation or additional licensing. A series of scripts were developed as a back-end engine that automates data formatting and editing of linked APEX input ASCII files including database libraries. The simple architecture of the tool helps users maintain the quality of the data and allows error-free editing of APEX model inputs to characterize the system under study. This tool is suitable for all kinds of applications and has been successfully used for the creation of APEX model runs in numerous studies.

Comparing the Effects of Inputs for NTT and ArcAPEX Interfaces on Model Outputs and Simulation Performance

The Agricultural Policy/Environmental eXtender (APEX) model has five different interfaces used to process and build simulation projects. These interfaces utilize different input databases that lead to different model default values. These values can result in different hydrologic, crop growth, and nutrient flow model outputs. This study compared structural and input value differences of the ArcAPEX and Nutrient Tracking Tool (NTT) interfaces. Long-term, water quality data from the Rock Creek watershed, located in Ohio were used to determine the impact of the differences on computation time, parameter sensitivity, and streamflow, total nitrogen (TN), and total phosphorus (TP) simulation performance. The input structures were the same for both interfaces for all files except soils, where NTT assigns three soil files per field, rather than a single one in ArcAPEX. As a result, computation times were three times as long for NTT as for ArcAPEX. There were twelve sensitive parameters in both cases, but the order of sensitivity was different. Both interfaces simulated streamflow well, but ARCAPEX simulated evapotranspiration, TN, and TP better than NTT, while NTT simulated crop yields better than ArcAPEX. However, none of the models met all of the performance criteria for either interface. Therefore, more work is needed to ensure models are properly calibrated before being used for scenario analysis. While it is acceptable for the values to be different from the SSURGO database, there is no documentation explaining the rationale for the modifications from the original source. This is one of the examples that highlights lack of detailed documentation that would be useful to model users. Overall, the results indicate that different interfaces lead to different model simulation results and, therefore, the authors recommend users specify the interface used and any modifications made to the associated databases when reporting model results.

Modeling the Impacts of Conservation Agriculture with a Drip Irrigation System on the Hydrology and Water Management in Sub-Saharan Africa

The agricultural system in Sub-Saharan Africa (SSA) is dominated by traditional farming practices with poor soil and water management, which contributes to soil degradation and low crop productivity. This study integrated field experiments and a field-scale biophysical model (Agricultural Policy Environmental Extender, APEX) to investigate the impacts of conservation agriculture (CA) with a drip irrigation system on the hydrology and water management as compared to the conventional tillage (CT) practice. Field data were collected from four study sites; Dangishita and Robit (Ethiopia), Yemu (Ghana), and Mkindo (Tanzania) to validate APEX for hydrology and crop yield simulation. Each study site consisted of 100 m2 plots divided equally between CA and CT practices and both had a drip irrigation setup. Cropping pattern, management practices, and irrigation scheduling were monitored for each experimental plot. Significant water savings (α = 0.05) were observed under CA practice; evapotranspiration and runoff were reduced by up to 49% and 62%, respectively, whereas percolation increased up to three-fold. Consequently, irrigation water need was reduced in CA plots by about 14–35% for various crops. CA coupled with drip irrigation was found to be an efficient water saving technology and has substantial potential to sustain and intensify crop production in the region.

Long-term simulated runoff and water quality from grain cropping systems on restrictive layer soils

Evaluation of the sustainability of grain production systems requires knowledge of long-term crop yields, and edge-of-field runoff volumes and water quality. When measured data are limited, computer simulation models —calibrated and validated with the available data— are well suited for this type of inquiry. In this study, the Agricultural Policy Environmental Extender (APEX) model was used to investigate how management affects crop yields, runoff volume, and water quality on claypan soils. Specific objectives were to determine the ability of APEX to simulate the variability of corn and soybean yields on claypan soils and the transport of dissolved-phase atrazine and nutrients via surface runoff; and how management affects long-term runoff and associated loss of these constituents. In spite of satisfactory model performance for corn yields and acceptable relative differences for soybean yields, the model was not capable of simulating the effects of management or landscape position on crop yields. Model performance was very good for runoff, and mostly satisfactory for dissolved constituents. Long-term simulation along with uncertainty analysis indicated that a no-till 3-year corn-soybean-wheat rotation with cover crops provides the most benefits to water quality by reducing nutrient transport in runoff. This system appeared to mitigate the negative effects of no-till on the surface transport of non-incorporated dissolved chemicals because of improved fertilizer management in terms of rate, placement, and timing of application. Cover crops appeared to reduce nitrate leaching compared to a 2-year no-till system. This research highlights the need for further research in crop growth modeling as inconsistent crop yield results undermine the usefulness of the model for making recommendations about management. Overall, the long-term simulations support the concept of multiple practices addressing tillage, fertilizer management, longer crop rotations, and use of cover crops to achieve desired environmental and production goals.

APEXSENSUN: An Open‐Source Package in R for Sensitivity Analysis and Model Performance Evaluation of APEX

AbstractThe Agricultural Policy/Environmental eXtender (APEX) model is used to evaluate the impact of different land management strategies associated with water availability, soil and water quality, plant growth, and economics. This article presents APEXSENSUN, an open‐source software package that automates global sensitivity analysis and assists with calibration of the APEX model. APEXSENSUN was developed in R programming language and includes regression‐based, derivative‐based, and variance‐based methods, as well as a regional sensitivity analysis method. Evapotranspiration data measured at a research field located at the United States Department of Agriculture‐Agricultural Research Service Conservation and Production Research Laboratory in Bushland, Texas were utilized to illustrate the main features of APEXSENSUN, which are to identify important parameters and assist with calibration. The results from variance‐based methods were in agreement regarding the ranking of top five sensitive parameters including the soil evaporation‐plant factor, root growth‐soil strength, and the coefficient for adjusting the microbial activity function in the top soil layer. The Fourier amplitude sensitivity testing method required the least number of simulations for calculation of the total‐effect sensitivity indices. The Monte Carlo‐based calibration feature of the package was also tested for calculating the posterior parameter distributions and prediction intervals with a desired level of confidence for output predictions.

Design and development of a web-based interface for the Agricultural Policy Environmental eXtender (APEX) model

The diffuse nature of nonpoint source (NPS) pollution as well as that of the effectiveness of best management practices (BMP) to control NPS pollution necessitates BMP evaluation at the field scale. In this study, a web interface was developed for application of the Agricultural Policy Environmental eXtender (APEX) model at the field scale. The interface contains background databases for field location, soil, agricultural management and climate across the contiguous United States. Users can specify properties to run the model for an individual field and compare the results under various land management and conservation practice choices. A case study was conducted to demonstrate the capability of the web interface for simulating various land management scenarios. This tool can help provide on-site information for NPS pollution management related policies and serve as a communication tool among scientists, engineers, and stakeholders.

Simultaneous calibration of evapotranspiration and crop yield in agronomic system modeling using the APEX model

Reliable estimation of ET is especially important in semi-arid conditions where ET prediction is instrumental in cost-effective management of scarce water resources and crop production. In addition to the role of climate variables and soil characteristics, the actual ET is influenced by dynamic crop characteristics during the growing season. Therefore, accounting for the interaction between crop growth and actual ET can significantly improve the performance of models. In this study, an efficient approach is presented for simultaneous calibration of ET and crop growth parameters for the Agricultural Policy/Environmental eXtender (APEX) model for daily, weekly, and monthly ET. The proposed approach involves the development of an objective function based on a compounded time series comprising of scaled annual ET and crop yield data. An efficient search algorithm based on the DiffeRential Evolution Adaptive Metropolis (DREAM) algorithm was implemented in R language to find the parameter values that minimizes the defined objective function. The simultaneous calibration approach, which utilized annual data, improved ET prediction for annual and finer time scales (25% in RMSE reduction for 3-month, 18% for monthly, 17% for 2-week, 19% for weekly, 17% for 3-day, and 13% for daily time scales). The average of absolute relative error for crop yield predictions was also reduced from 43% to 16% for the calibrated model. The simulated leaf area index (LAI) for the calibrated model (i.e. calibrated using annual ET and crop yield data) was also consistent with the measured LAI values, confirming the validity of the calibrated parameter values.

Grey water footprint reduction in irrigated crop production: effect of nitrogen application rate, nitrogen form, tillage practice and irrigation strategy

Abstract. Grey water footprint (WF) reduction is essential given the increasing water pollution associated with food production and the limited assimilation capacity of fresh water. Fertilizer application can contribute significantly to the grey WF as a result of nutrient leaching to groundwater and runoff to streams. The objective of this study is to explore the effect of the nitrogen application rate (from 25 to 300 kg N ha−1), nitrogen form (inorganic N or manure N), tillage practice (conventional or no-tillage) and irrigation strategy (full or deficit irrigation) on the nitrogen load to groundwater and surface water, crop yield and the N-related grey water footprint of crop production by a systematic model-based assessment. As a case study, we consider irrigated maize grown in Spain on loam soil in a semi-arid environment, whereby we simulate the 20-year period 1993–2012. The water and nitrogen balances of the soil and plant growth at the field scale were simulated with the Agricultural Policy Environmental eXtender (APEX) model. As a reference management package, we assume the use of inorganic N (nitrate), conventional tillage and full irrigation. For this reference, the grey WF at a usual N application rate of 300 kg N ha−1 (with crop yield of 11.1 t ha−1) is 1100 m3 t−1, which can be reduced by 91 % towards 95 m3 t−1 when the N application rate is reduced to 50 kg N ha−1 (with a yield of 3.7 t ha−1). The grey WF can be further reduced to 75 m3 t−1 by shifting the management package to manure N and deficit irrigation (with crop yield of 3.5 t ha−1). Although water pollution can thus be reduced dramatically, this comes together with a great yield reduction, and a much lower water productivity (larger green plus blue WF) as well. The overall (green, blue and grey) WF per tonne is found to be minimal at an N application rate of 150 kg N ha−1, with manure, no-tillage and deficit irrigation (with crop yield of 9.3 t ha−1). The paper shows that there is a trade-off between grey WF and crop yield, as well as a trade-off between reducing water pollution (grey WF) and water consumption (green and blue WF). Applying manure instead of inorganic N and deficit instead of full irrigation are measures that reduce both water pollution and water consumption with a 16 % loss in yield.

Evaluating evapotranspiration estimation methods in APEX model for dryland cropping systems in a semi-arid region

Evapotranspiration (ET), is a major component of the hydrologic budget and therefore it requires accurate estimation. The Agricultural Policy/Environmental eXtender (APEX), a hydrologic and water quality model developed for evaluating the effect of agricultural production management practices on the environment. It has five different methods to simulate ET. The objectives of this study were to: determine the impact of using different ET methods in APEX on sensitive ET parameters in semi-arid environments, determine reasonable range of values for sensitive parameters, and compare performance of these methods using multiple criteria. ET and crop yield data measured in the Lysimeter fields located in the USDA-ARS Conservation and Production Research Laboratory, Bushland, Texas were used to calibrate and validate the model. Results indicated that selection of statistical model performance measure and ET method affects the number of sensitive parameters and parameter rank. The number of sensitive parameters remained relatively stable among ET simulation methods but there was large variability in parameter sensitivity ranks. It is also important that users carefully choose appropriate statistical measure to use depending on the goals of their study. With the exception of maximum rainfall intercept, exponent coefficient rainfall, SCS index coefficient, rain intercept coefficient, and root growth soil strength, all methods had similar ranges of values for the sensitive parameters. In general there were no large differences in the performance of ET methods. However, Penman-Monteith method simulated ET relatively better than the other methods, which may be explained by the fact that it is a physically-based method with many weather variables whose data was available for the study area. However, the study findings indicate that the other ET methods can provide satisfactory results in regions with limited weather data.

Multi-Dimensional Evaluation of Simulated Small-Scale Irrigation Intervention: A Case Study in Dimbasinia Watershed, Ghana

This paper studied the impacts of small-scale irrigation (SSI) interventions on environmental sustainability, agricultural production, and socio-economics using an Integrated Decision Support System (IDSS). The IDSS is comprised of a suite of models, namely the Soil and Water Assessment Tool (SWAT), Agricultural Policy/Environmental eXtender (APEX), and Farm Income and Nutrition Simulator (FARMSIM). The IDSS was applied in Dimbasinia watershed in northern Ghana using irrigation water from shallow groundwater. The watershed has a modest amount of shallow groundwater resources. However, the average annual irrigation water requirement exceeded the average annual shallow groundwater recharge. It was found that the current crop yield in Dimbasinia watershed was only ~40% of the potential crop production. This is mainly related to climate variability, low soil fertility, and land-management practices. For example, application of 50 kg/ha urea and 50 kg/ha DAP doubled maize and sorghum yield from the current farmers’ practices. Better income was obtained when irrigated vegetables/fodder were cultivated in rotation with sorghum as compared to in rotation with maize. Investment in solar pumps paid better dividends and also supplied clean energy. The socio-economic analysis indicated that having irrigated dry season vegetables will improve household nutrition. Since shallow groundwater recharge alone may not provide sufficient water for irrigation in a sustainable manner, surface water may be stored using water-harvesting structures to supplement the groundwater for irrigation. Integrated use of the water resources will also reduce depletion of the shallow groundwater aquifer. We conclude that IDSS is a promising tool to study gaps and constraints as well as upscaling of SSI.

Soybean crop-water production functions in a humid region across years and soils determined with APEX model

Crop production as a function of water use or water applied, called the crop water production function (CWPF), is a useful tool for irrigation planning, design and management. However, these functions are not only crop and variety specific they also vary with soil types and climatic conditions (locations). Derivation of multi-year average CWPFs through field experiments for different locations and soils is time-consuming and expensive, as it requires careful long-term and multi-location field experiments to obtain them. Process based crop system models provide a useful tool for determining CWPFs using short-term field experimental data for calibration and validation. The aim of this study was to determine soybean CWPFs using the Agricultural Policy/Environmental eXtender (APEX) model across three soil types (Vaiden-silty clay, Cahaba-sandy loam, and Demopolis-clay loam) and three weather conditions (14-year average from 2002 to 2015, dry, and wet) of a humid irrigated region in Mississippi, USA. The results showed that the relationship between simulated soybean grain yield (GY) and the seasonal crop evapotranspiration (ET) for each soil under 14-year average weather condition was linear. Compared with the Vaiden soil, the Cahaba and Demopolis soils had slightly higher water use efficiency (WUE) over 14-year average weather conditions. The CWPFs for GY vs supplemental irrigation were cubic polynomials for all soil types and weather conditions, with varying coefficients. The maximum values of irrigation water use efficiency (IWUEmax) derived from these cubic CWPFs varied from 2.58 to 9.89 kg ha−1 mm−1 across soils and weather conditions. The irrigation amount during the growing season required (Imax) to achieve the maximum GY for soybean also had a wide range of values, from 110 to 405 mm. The IWUE and Imax were related to available water holding capacity of soils. The relationship between GY and total plant available water supply (TWS) was also a cubic function, with coefficients varying with soil types and climatic conditions. The yield response factor (Ky) was 1.24 (greater than 1.00) when averaged over 14 years’ weather data, indicating that soybean was very sensitive to water stress even in a humid region like Mississippi. Thus, supplemental irrigation is necessary to increase GY and ensure stability in yields.

Implications of biofuel-induced changes in land use and crop management on sustainability of agriculture in the Texas High Plains

Texas High Plains (THP), which is an important cotton (Gossypium hirsutum L.) growing region in the United States faces challenges from declining/deteriorating groundwater levels/quality, recurring droughts and severe wind erosion. Growing cover crops after harvesting cotton and/or changing land use from cotton to perennial bioenergy crops could not only address above challenges, but also assist in meeting the national biofuel target. The objective of this study is to assess the implications of changes in land use (replacing cotton with Alamo switchgrass (Panicum virgatum L.) and Miscanthus×giganteus) and crop management (growing winter wheat (Triticum aestivum L.) cover crop) on hydrology and wind erosion in the Double Mountain Fork Brazos Watershed using the Agricultural Policy/Environmental eXtender (APEX) model. Simulated average annual wind erosion, total nitrogen (TN) loss to surface water and nitrate-nitrogen (NO3-N) leaching to groundwater reduced by more than 37%, 43% and 73%, respectively, when winter wheat was grown as a cover crop under the 457 mm (18-inch) annual groundwater pumping limit setup by the High Plains Water District. In addition, winter wheat produced about 0.20–0.26 kg m−2 of biomass for biofuel purposes. Land use change from irrigated cotton to switchgrass and rainfed cotton to Miscanthus decreased the TN load, NO3-N leaching and soil loss by wind erosion by > 89% relative to the baseline scenario. Under the groundwater pumping restrictions, multiple harvests of perennial grasses were found to be better in terms of biomass production (>2 kg m−2), and protection of groundwater and soil.

Simulating Runoff from Small Grazed Pasture Watersheds Located at North Appalachian Experimental Watershed in Ohio

Runoff from grazing pasture lands can impact water quality in receiving streams if not well managed. Management consists of conservation practices to reduce runoff and pollutants transport. Simulation models have been effectively used to design and implement these conservation practices. The Agricultural Policy Environmental Extender (APEX), a process-based hydrologic model, was used in this study to simulate the management impacts on surface runoff from three small grazed pasture watersheds located at the North Appalachian Experimental Watersheds near Coshocton, Ohio. Specific objectives of this study were to 1) calibrate the APEX model and test runoff predictions against measured runoff and 2) simulate the long-term impacts of different management scenarios on surface runoff. Results show that the APEX model simulated surface runoff reasonably well with the coefficient of determination (R2) and Nash-Sutcliffe efficiency values varying from 0.49 to 0.72 and from 0.25 to 0.60 for calibration and validation, respectively. After validation, the APEX model was run for 37 yr (1975−2011) for long-term scenarios to analyze the impacts of soil properties and management on surface runoff. Data from this study indicated that keeping the watershed land use as a hay meadow instead of grazing significantly reduced cumulative runoff by 58−67%. Buffer strips of perennial grasses resulted in decreased simulated runoff. To simulate the impacts of soils on runoff, the surface (0−5 cm) soil properties of the toe position were applied to the entire grazed watershed. Subsequently, the increase in soil richness resulted in reduction (≤5%) in surface runoff. The simulation results from the present study demonstrate the benefits of hayed meadow over grazed pasture and further predict the decreased trend of runoff due to soil properties change and buffer strips.

Lysimetric Evaluation of the APEX Model to Simulate Daily ET for Irrigated Crops in the Texas High Plains

Abstract. The NTT (Nutrient Tracking Tool) was designed to provide an opportunity for all users, including producers, to run complex simulation models, such as APEX (Agricultural Policy Environmental eXtender), with the associated required databases. The APEX model currently nested within NTT provides estimates of the changes in nitrogen (N), phosphorus (P), and sediment losses that are associated with management practices specified by the user. Five methods (Penman-Monteith, Penman, Priestley-Taylor, Hargreaves-Samani, and Baier-Robertson) for determining potential evapotranspiration (PET) are available as inputs for estimating actual ET. This study was conducted to evaluate the accuracy of the ET values obtained from the five PET equations currently available in APEX using both onsite measured climate data and data from the NTT standard databases. The mean daily, monthly, and annual ET values predicted by each of the equations in APEX for a lysimeter field at the USDA-ARS Conservation and Production Research Laboratory at Bushland, Texas, was compared to values measured for the 2001-2010 period. APEX generally underestimated ET with all PET methods (mostly during growing seasons) at both the daily and monthly levels but overpredicted for years when cotton was grown as the major cash crop due to overprediction of leaf area index during the senescing stage for cotton. The underprediction of ET in growing seasons was possibly from underprediction of rainfall due to estimation of rainfall for missing data. Overall, APEX was able to adequately (R2 = 0.82 and NSE = 0.80) predict mean monthly ET for major crops grown in the semi-arid Texas High Plains region. These results should reinforce confidence in APEX’s ability to simulate ET accurately for fully irrigated farms. ET predictions with the Hargreaves-Samani and Priestley-Taylor methods, which require limited data compared to the Penman and Penman-Monteith methods, were similar (p > 0.05, one-way ANOVA), with mean errors within 8.7% for measured weather data and 12.6% for NTT-generated weather data for both methods. This is encouraging because of the limited availability of measured climate data for the majority of locations in the world, including the U.S. Keywords: APEX, Evapotranspiration (ET), Irrigation, Lysimeters, NTT, Semiarid regions.

Improved APEX Model Simulation of Buffer Water Quality Benefits at Field Scale

Abstract. Watershed models offer cost-effective means to quantify the environmental benefits of conservation practices. This study evaluated the ability of the Agricultural Policy/Environmental eXtender (APEX) model to simulate the effectiveness of upland buffers for reducing event runoff, sediment, and total phosphorus (TP) loadings. The study used 16 years of monitoring data (1993-2008) from three field-scale row crop watersheds. Two watersheds had buffers installed in the fall of 1997, one with upland agroforestry buffers (grass plus trees) and the second with upland grass buffers; the third watershed was maintained as a control. Objectives were to (1) determine the effect of upland buffers on APEX parameters, (2) evaluate the ability of APEX to estimate upland buffer effectiveness, and (3) evaluate the impact of alternative buffer placement options on water quality outcomes. After modification of the APEX code to improve simulation of infiltration in the buffers, we successfully calibrated APEX for no-buffer and upland buffer conditions for event runoff and TP. However, calibrated parameters from the no-buffer model simulated buffer conditions poorly and resulted in an overestimation of buffer effectiveness. Buffer effectiveness estimated with upland buffer parameters was similar to that obtained from measured data. Scenario analysis indicated that a combination of backslope and footslope agroforestry buffers was more effective than contour placement for reducing average annual edge-of-field runoff and total P losses. These results highlight the complexity of using APEX to assess upland buffers as well as potential problems when using APEX to quantify the benefits of conservation practices not included in the calibration dataset. Keywords: Agroforestry buffers, Buffer placement, Modeling, Phosphorus, Practice effectiveness, Watershed discretization.

Use of soft data for multicriteria calibration and validation of Agricultural Policy Environmental eXtender: Impact on model simulations

Use of soft data and multiple model performance criteria in model calibration and validation are critical to ensure that environmental models capture major hydrologic and water quality processes. The Agricultural Policy/Environmental eXtender (APEX) is a hydrologic and water quality model widely used for evaluating the effect of agricultural production management practices on the environment. However, there are few studies on the impact of soft data on APEX model outputs. This study sought to determine the impact of soft data (i.e., crop yields, evapotranspiration [ET], and drainage) used to constrain calibration on APEX model to simulate outputs (streamflow, total nitrogen [TN], and total phosphorus [TP] losses), as well as the impact of tile drainage on the model outputs to help identify issues with APEX. Long-term measured water quality and quantity data from Rock Creek watershed, northern Ohio, were used for this purpose. Four calibration scenarios were used: (A) streamflow statistics only, (B) streamflow statistics plus soft data, (C) streamflow plus water quality, and (D) streamflow plus water quality plus yields plus soft criteria. The number of parameter sets that meet the criteria decreased from 3,325 to 2, as the number of calibration criteria increased. In general, when soft data were used to constrain the model, the Nash-Sutcliffe efficiency (NSE) values decreased for all components. For example, NSE for streamflow decreased from 0.79 for scenario A to 0.56 for scenario B when the soft data were used, and NSE for TP decreased from 0.34 for scenario C to 0.30 for scenario D. However, the scenarios that used soft data with lower NSE values simulated the biophysical processes more accurately. For all calibration/validation scenarios, streamflow, TN, and TP values were drastically reduced when the model was run without tile drainage. However, there was hardly any tile drainage impact on ET, surface runoff, or crop yields for all scenarios, indicating that improvements in tile drainage, water table, and crop growth routines may be needed. Overall, the results show the importance of constraining the model parameters to produce reasonable values of crop yield and water balance closure to obtain realistic simulations of various management practices.

Simulating the Effects of Agricultural Management on Water Quality Dynamics in Rice Paddies for Sustainable Rice Production—Model Development and Validation

The Agricultural Policy/Environmental eXtender (APEX) model is widely used for evaluating agricultural conservation efforts and their effects on soil and water. A key component of APEX application in Korea is simulating the water quality impacts of rice paddies because rice agriculture claims the largest cropland area in the country. In this study, a computational module called APEX-Paddy (National Academy of Agricultural Sciences, Wanju, Korea) is developed to simulate water quality with considering pertinent paddy management practices, such as puddling and flood irrigation management. Data collected at two experimental paddy sites in Korea were used to calibrate and validate the model. Results indicate that APEX-Paddy performs well in predicting runoff discharge rate and nitrogen yield while the original APEX highly overestimates runoff rates and nitrogen yields on large storm events. With APEX-Paddy, simulated and observed flow and mineral nitrogen yield (QN) are found to be highly correlated after calibration (Nash & Sutcliffe Efficiency (NSE) = 0.87 and Percent Bias (PBIAS) = −14.6% for flow; NSE = 0.68 and PBIAS = 2.1% for QN). Consequently, the APEX-Paddy showed a greater accuracy in flow and QN prediction than the original APEX modeling practice using the SCS-CN (Soil Conservation Service-Curve Number) method.

Evaluation of the APEX Model to Simulate Runoff Quality from Agricultural Fields in the Southern Region of the United States.

The Agricultural Policy Environmental eXtender (APEX) model has been widely applied to assess phosphorus (P) loss in runoff water and has been proposed as a model to support practical decisions regarding agricultural P management, as well as a model to evaluate tools such as the P Index. The aim of this study is to evaluate the performance of APEX to simulate P losses from agricultural systems to determine its potential use for refinement or replacement of the P Index in the southern region of the United States. Uncalibrated and calibrated APEX model predictions were compared against measured water quality data from row crop fields in North Carolina and Mississippi and pasture fields in Arkansas and Georgia. Calibrated models satisfactorily predicted event-based surface runoff volumes at all sites (Nash-Sutcliffe efficiency [NSE] > 0.47, |percent bias [PBIAS]| < 34) except Arkansas (NSE < 0.11, |PBIAS| < 50) but did not satisfactory simulate sediment, dissolved P, or total P losses in runoff water. The APEX model tended to underestimate dissolved and total P losses from fields where manure was surface applied. The model also overestimated sediments and total P loads during irrigation events. We conclude that the capability of APEX to predict sediment and P losses is limited, and consequently so is the potential for using APEX to make P management recommendations to improve P Indices in the southern United States.

Modified APEX model for Simulating Macropore Phosphorus Contributions to Tile Drains.

The contribution of macropore flow to phosphorus (P) loadings in tile-drained agricultural landscapes remains poorly understood at the field scale, despite the recognized deleterious impacts of contaminant transport via macropore pathways. A new subroutine that couples existing matrix-excess and matrix-desiccation macropore flow theory and a modified P routine is implemented in the Agricultural Policy Environmental eXtender (APEX) model. The original and modified formulation were applied and evaluated for a case study in a poorly drained field in Western Ohio with 31 months of surface and subsurface monitoring data. Results highlighted that a macropore subroutine in APEX improved edge-of-field discharge calibration and validation for both tile and total discharge from satisfactory and good, respectively, to very good and improved dissolved reactive P load calibration and validation statistics for tile P loads from unsatisfactory to very good. Output from the calibrated macropore simulations suggested median annual matrix-desiccation macropore flow contributions of 48% and P load contributions of 43%, with the majority of loading occurring in winter and spring. While somewhat counterintuitive, the prominence of matrix-desiccation macropore flow during seasons with less cracking reflects the importance of coupled development of macropore pathways and adequate supply of the macropore flow source. The innovative features of the model allow for assessments of annual macropore P contributions to tile drainage and has the potential to inform P site assessment tools.