Agricultural Policy/Environmental eXtender Model Research Articles

Simulating Agricultural Water Recycling Using the APEX Model

Irrigation plays a vital role in many agricultural crop production regions. Drainage water recycling (DWR) is a popular irrigation water management system that collects excess water drained from cropland fields and stores it in on-site reservoirs for reuse. The efficacy of these systems varies by location, climate, irrigation frequency, and crop demands. Simulating this system would be beneficial for assessing the impact of water and land management practices on agriculture and natural resources. This study presents the development of computational algorithms for DWR simulation with the Agricultural Policy Environmental eXtender (APEX) model, along with the results for 39 testing sites where both reservoir and drainage systems are adopted. Simulating a DWR system with the revised reservoir module, the APEX model simulates irrigation water reuse ranging between 29% and 93%; sediment reduction of around 66%; nitrogen loss reduction of 23% and 73% for the mineral and organic forms, respectively; and phosphorus loss reduction of 22% and 79% for the soluble and sediment-transported forms, respectively. In conclusion, the results provided by the APEX model for sediment loss reduction align with field data, but discrepancies for nitrogen and phosphorus losses emerged from this test.

Calibrating APEX model for predicting surface runoff and sediment loss in a watershed - a case study in Shivalik region of India

The process-based model needs to be calibrated before its applications to derive a reliable estimate. A process-based agricultural policy/environmental eXtender (APEX) model was utilised to assess surface runoff and soil loss of a micro-watershed belongs to the Shivalik region of Himachal Pradesh, India. The model was performed exceptionally well for surface runoff (r2 - 0.81) and sediment yield (r2 - 0.81) with the Nash- Sutcliffe Efficiency assessments of 0.71 for surface runoff and 0.70 for sediment yield. The average annual soil loss is 23.42 t ha-1yr-1 from the watershed. The study revealed that the soil erosion in the Shivalik region increases and identified scrubland as the significant source for generating soil erosion in the watershed. Also, the APEX model could predict surface runoff and soil loss in the Shivalik region. The comprehensive information generated from the study will help to generate a sustainable land use plan in the Shivalik region.

Development of Novel QAPEX Analysis System Using Open-Source GIS

The Agricultural Policy/Environmental eXtender (APEX) model has been used for farm/small watershed management, and the ArcAPEX interface was developed using the ArcGIS extension. However, the interface requires a paid license and limits dynamic applications that reflect various agricultural farming practices. In this study, a novel APEX model interface using Quantum GIS, the QAPEX analysis system, was developed by incorporating open-source-based GIS software for the simulation of water quality impacts of various best management practices reflecting local farming activities. The watershed delineation process running on the QAPEX interface is more flexible than that on the ArcAPEX interface, which renders simulations on hydrology and water quality with considerable precision. The newly developed system can be used to visually interpret simulation results (e.g., flow and load duration curve functions). Therefore, the open-source-based model can be used to derive data for sustainable agricultural policies, with a focus on the field-level application of management practices.

Modelling climate change impact on soil loss and erosion vulnerability in a watershed of Shiwalik Himalayas

Understanding the impact of climate change on soil erosion is required to plan effective soil conservation and management practices in hilly and mountainous landscape. Though, few studies have been executed in Indian Himalayas, much less in Shiwalik Himalayas. Thus, keeping this in view, the aim of the study is to estimate the possible impact of projected climate change scenarios on soil loss and erosion vulnerability using Statistical Downscaling Model (SDSM), MarkSim Weather Generator, Agricultural Policy Environmental eXtender (APEX) model and AHP method. The present study downscaled four climate scenarios on the near-future, noted the 2020 s (2011–2040), mid-future, noted 2050 s (2011–2070) and far-future, noted 2080 s (2071–2100/2095) under two Special Report Emission Scenarios (SRES), A2 and B2 with two Representative Concentration Pathway (RCP), 4.5 and 8.5 scenarios. Both SRES and RCP scenarios predicted an increase in temperature and annual rainfall depth during 21st century. Calibrated APEX model was used to simulate future soil loss. The study exposed that the average annual soil loss might increase up to 46.94%, 38.80%, 35.34% and 25.93% in the A2, B2, RCP 4.5 and RCP 8.5 scenarios respectively from the base period (1985–2014). The vulnerability assessment indicates, most of the areas might be under risk from slightly to highly vulnerable and was found that forest land cover could resist this erosion vulnerability to a certain extent. The findings of the study showed a possibility for climate change to increase the rate of soil loss unless conservation strategies or proper land use plans are remunerated. This study offers scientists and policymakers to a detailed understanding of the future impact of rainfall on soil loss and erosion vulnerability in the Shiwalik Himalayas.

Modeling landscape wind erosion processes on rangelands using the APEX model

Rangelands provide vital ecosystem services and comprise 36% of U.S. and 25% of world lands. Approximately 20 to 73% of these rangelands are already degraded. Soil erosion driven by winds is a significant cause of this degradation. Minimal work has been done to evaluate the influence of wind erosion on rangeland dominated landscapes or to assess the effect of landscape-scale soil and water conservation and management practices on reducing wind erosion due to its inherent complexity and nonpoint source nature. Our approach is to use an integrated and holistic eco-hydrologic model. The main objective of this study is to integrate process-based landscape wind erosion modeling schemes into the Agricultural Policy/Environmental eXtender (APEX) model (version 1905; APEX1905) for simulating horizontal aeolian sediment transport and vertical particles transport on rangelands. We demonstrate the performance and capability of the model using field data collected at the USDA ARS Jornada Experimental Range in New Mexico, U.S. A benchmark APEX1905 model that runs on high-resolution sub-daily wind speed data and daily average vegetation gap distribution derived from monthly field measurements. The benchmark APEX1905 model captured the variability in observed aeolian sediment transport during 2015–2017 reasonably well with an R2 = 0.58 and a RMSE = 2.60. When the highly variable sub-daily measured wind speed and vegetation gap distribution data were substituted with the daily average wind speed and APEX1905 estimated daily vegetation gap, the APEX1905 reproduced well the benchmark model estimates with the performance statistic of R2 = 0.77 and RMSE=0.54 for the horizontal aeolian sediment transport; and R2 = 0.82 and RMSE = 0.85 for vertical particle transport. Overall, provided limited aeolian watershed data sets, the APEX1905 is demonstrated to be reliable in estimating wind erosion in arid, desert rangeland landscapes using daily average wind speed and simulated vegetation characteristics. Benefiting from the long-established algorithms of simulating plant growth dynamics and topsoil moisture content in APEX, APEX1905 offers a robust and process-based estimation of wind erosion in areas where wind and vegetation data are scarce.

The impact of rainfall distribution methods on streamflow throughout multiple elevations in the Rocky Mountains using the APEX model-Price River watershed, Utah.

The hydrology of mountainous watersheds in the western United States is significantly influenced by snow year-round. It is widely known that topography affects precipitation; however, the knowledge of how watershed rainfall designation methods affect streamflow is not well understood for high-relief areas. The objectives of this study were to assess the predictive capability of the Agricultural Policy/Environmental eXtender (APEX) model to simulate streamflow in a snowmelt-dominated watershed with high spatial rainfall variability through (a) allocating weather stations to sub-basins based on a conventional Thiessen polygon method (CM) or a rainfall-elevation-based input (RE) and using an areal average Parameter-Elevation Regression on Independent Slopes Model (PRISM) rainfall designation and (b) improving the snowmelt processes in the Price River watershed, Utah. The updated APEX model with snowmelt parameters significantly improved spring flood simulation. The RE was the most robust method in snowmelt and seasonal streamflow simulations compared with the CM and PRISM rainfall designations. Adapting the APEX model to simulate snow-dominant complex terrains will provide crucial water quantity and quality predictions for reliable environmental and watershed management assessment.

Evaluating the APEX model for alternative cow-calf grazing management strategies in Central Texas

CONTEXTSimulation tools are increasingly used to inform grazing management decisions by assessing livestock performance, as well as environmental and economic impacts. Ability to represent the grazing of multiple pastures (i.e., paddocks) that differ in soil, hydrology, vegetation, and management is critical for reliable grazing management decision support. OBJECTIVEThe main objectives of this study were to: 1) modify APEX (Agricultural Policy/Environmental eXtender) for sub-daily grazing, cow-calf weight gain, and supplemental hay, and 2) evaluate the APEX modifications in terms of simulating biomass, calf weight gain, economic impacts of alternative cow-calf grazing management strategies. METHODSAPEX was modified to enhance its ability to simulate alternative grazing management strategies by including sub-daily grazing among multiple paddocks, supplemental hay estimation, and optional simulation of cow-calf weight gain based either on energy or total digestible nutrients (TDN). Simulation results were evaluated against a 5-year experimental data set from Central Texas comparing multi-paddock rotational grazing and conventional continuous grazing. RESULTS AND CONCLUSIONThe modified APEX model adequately simulated the responses of vegetation biomass (coefficient of determination, R2 = 0.60–0.70), hay consumption (R2 = 0.94–0.95), calf weaning weight (R2 = 0.52–0.65), costs (R2 = 0.98), and profits (R2 = 0.89) to the two grazing treatments across years. Simulations with the energy-based weight gain algorithm showed more pronounced effects on above-ground biomass, whereas the TDN-based algorithm had a more pronounced weight gain response to forage intake and hay quality. No significant differences (p > 0.05) in biomass and calf weaning weight were observed between treatments across the years for measured data and for energy-based APEX simulation; however, the TDN-based algorithm simulated lower calf weaning weight in multi-paddock rotational grazing than in continuous grazing. Measured and simulated data also showed similar profits between the two grazing treatments, but total cost and gross return per ha were greater for continuous grazing. SIGNIFICANCEThe model enhancements for sub-daily grazing, cow-calf weight gain, and supplemental hay improved the potential utility of APEX for assessing environmental and economic impacts of alternative grazing strategies at ranch scale.

Modeling daily surface runoff, sediment and nutrient loss at watershed scale employing Arc-APEX model interfaced with GIS: a case study in Lesser Himalayan landscape

The present study was carried out to evaluate performance of Agricultural Policy Environmental eXtender (APEX) integrated with GIS as ArcAPEX model in simulating surface runoff, sediment and nutrients loss from a watershed located in lesser Himalayan region in Uttarakhand state, India. Daily surface runoff, sediment and nutrient loss data measured at watershed level for the year 2010–2011. Measurement at watershed revealed average daily sediment loss in range of 1.0–1.32 t ha−1 was measured with peak of 3.04 t ha−1 on daily time-step. The loss of total carbon (TC) and total nitrogen (TN) were estimated from 0.49 to 0.58 kg ha−1 and 0.16 to 01.7 kg ha−1, respectively from the watershed on daily basis for the rainfall considered for the modelling. A total of 40 rainy days surface runoff and sediment data were collected and of which half of the events data were used for calibration and remaining for validation. The calibration was done by changing the sensitive parameters. Analysis showed that SCS CN number was found most sensitive to runoff, followed by saturated hydraulic conductivity, available water-holding capacity, CN retention parameter and C factor whereas erosion control practice (P) factor was found to be most sensitive, followed by C factor, sediment routing coefficient, average upland slope and soil erodibility (K) factor for the sediment and nutrient loss. APEX model calibrated for the watershed and it predicted quite well for the surface runoff (r = 0.92, NSE = 0.50), sediment loss (r = 0.88, NSE = 0.61 and nutrients of total carbon (r = 0.78, NSE = 0.59) and fairly for total nitrogen (r = 0.77, NSE = 0.19). The calibration and validation results indicate that APEX model can be satisfactorily used for predicting runoff, sediment and nutrient (TC) loss at watershed scale on daily time-step. The landscape is prone landslips/ collapse of field terraces etc. occurring at high rainfall events was not accounted by the model that may be a reason for under prediction of sediment loss by the model. The study suggests to incorporate landslips/collapse of field terraces, etc., processes in future improved versions of APEX for improving sediment yield prediction in the Himalayan landscape.

Constraints of small-scale irrigated fodder production and nutrition assessment for livestock feed, a case study in Ethiopia

Livestock is an integral part of the agricultural system in sub-Saharan Africa, serving as a food source, income, fertilizer, and power for farming and transportation. However, the productivity of the livestock system has been hampered due to a lack of sufficient quantity and quality feed. This study evaluates the gaps and constraints of fodder and nutritional potential for livestock feed using small-scale irrigation (SSI). The study comprised of 30 randomly selected farmers from two different ecological zones in Ethiopia. Half of the farmers cultivated Napier grass ( Pennisetum purpureum ) in the Robit watershed in northern Ethiopia, and the other half cultivated mixed vetch ( Lathyrus cicera) and oats (Avena sativa) in Lemo watershed in southern Ethiopia. The Soil and Water Assessment Tool (SWAT) and Agricultural Policy Environmental eXtender (APEX) were applied in an integrated manner to assess the impacts of SSI at the watershed and field-scale levels, respectively. The watershed-scale analysis showed that there is a substantial amount of surface runoff and shallow groundwater recharge that could be used for dry season fodder production using irrigation. Field data calibrated APEX model indicated that Napier yield could be maximized with 550 mm of water in Robit watershed. While in the Lemo watershed, maximum vetch and oats yield may be achieved with 250 mm of water. The major constraints for Napier and oats production in the study sites were soil fertility, especially nitrogen and phosphorus, and vetch production was limited by high temperature. Fodder samples were collected at the time of harvest to evaluate feed quality. The nutritional analysis indicated that Napier grass has a higher dry matter and ash (mineral) content compared to oats and vetch. However, vetch has higher crude protein content (18%) compared to Napier (10%) and oats (6%). Overall the study indicated that cultivating vetch provided superior performance in terms of providing quality feed and environmental services. • We selected 30 farmers randomly to grow fodder crops in two zones in Ethiopia; • The gaps and constraints of fodder production and nutritional potential were evaluated; • The biophysical models were used to assess potential resources and constraints; • Among the fodder crops, vetch provided superior performance in terms of providing quality feed and environmental services;

Evaluation of APEX modifications to simulate forage production for grazing management decision-support in the Western US Great Plains

ContextUnderstanding how grazing management decisions influence the productivity and composition of rangeland plant communities is essential for the development of effective strategies to sustainably produce multiple ecosystem goods and services. Informed with experimental measurements, simulation models can advance our understanding and stewardship of rangeland ecosystems. ObjectiveOur main objective was to evaluate the APEX (Agricultural Policy/Environmental eXtender) plant growth modules and grazing animal selectivity in simulating forage production using experimental data collected from both traditional season-long grazing and adaptive rotational grazing management on western rangelands. Specifically, we evaluated APEX's capability to simulate forage productivity and its response to soil types and climate conditions under grazing management options. MethodsCapitalizing on a comparative field study with 20 large pastures (> 123 ha each), APEX modifications were evaluated by comparing simulated forage production with experimental data. The field study evaluated traditional grazing (season long grazing on a single pasture) and an alternative grazing system that utilized collaborative adaptive rangeland management with stakeholders engaged in decision making (such as when and where to rotate a single herd). APEX was modified to include rotational grazing based on a user-defined sequence and automatic rotational grazing based on user-defined forage grazing limits and minimum/maximum grazing durations. Results and conclusionsThe APEX model was able to simulate the relative differences in forage production between grazing treatments, across years, and among soil types; however, APEX underestimated forage production in 2015 and 2017 due to overestimating drought stress for the warm season perennial grass functional group. Simulation of grazing management scenarios showed that the collaborative adaptive management decision criteria resulted in grazing durations that produced more forage than consistent 7- or 14-day rotation intervals. SignificanceThese modifications were needed to capture the complexity of semiarid environments and thus enhance APEX to better assess grazing management decisions on forage production in regions such as the Western US Great Plains.

Alternative cropping systems for groundwater irrigation sustainability in the North China Plain

Over-exploitation of groundwater for agricultural irrigation has attracted worldwide attention because of the rapid groundwater depletion and environmental consequences induced by the current intensified cropping system. The combination of cropping system modeling with field experimentation is a powerful tool to evaluate the long-term consequences of changing water management and cropping systems under inter-annual climate variability. Here, we conducted two field experiments for early maize (EM, 2012–2018) single cropping system and winter wheat – summer maize (WWSM, 2015–2018) double cropping system in the North China Plain (NCP). Field observations were carried out for crop growth, yield, and soil volumetric moisture content (VMC) calibration with the Agricultural Policy/Environmental eXtender (APEX) model. The calibrated APEX model was used to assess the long-term (35 years, 1985–2019) impacts of EM and WWSM cropping systems under four irrigation scenarios on sustainable water and crop productivity using historical weather data. Results showed that the APEX replicated growth, yield, and VMC all reasonably well with R2 > 0.5, percent bias < 25%, and index of agreement > 0.8. The model realistically simulated VMC under full irrigation (FI), while model simulations under reduced irrigation were not that accurate. Neglectable yield losses of EM were observed by changing EM-FI to rainfed (EM-RF) (7.0 vs. 6.8 Mg ha−1) and lower inter-annual yield variability. Shifting WWSM-FI to EM-RF saved 47% evapotranspiration (ET) and 115% net water use (NWU) regardless of the precipitation category years, but caused 54% reduction in yield. Critical irrigation (CI) significantly reduced ET (7%) and NWU (28%) of WWSM, without yield loss, compared to FI, indicating great potentials of water-saving through optimized irrigation strategy. Minimum irrigation (MI) saved ET by 17% and NWU of WWSM by 62%, but led to 15% EY reduction compared with FI. In conclusion, the WWSM cropping system under CI has the potential to maintain yield with less water consumption, while rainfed EM is the best alternative option for mitigating groundwater over-exploitation with a certain extent risk in crop yield losses.

Simulating Soybean–Rice Rotation and Irrigation Strategies in Arkansas, USA Using APEX

With population growth and resource depletion, maximizing the efficiency of soybean (Glycine max [L.] Merr.) and rice (Oryza sativa L.) cropping systems is urgently needed. The goal of this study was to shed light on precise irrigation amounts and optimal agronomic practices via simulating rice–rice and soybean–rice crop rotations in the Agricultural Policy/Environmental eXtender (APEX) model. The APEX model was calibrated using observations from five fields under soybean–rice rotation in Arkansas from 2017 to 2019 and remote sensing leaf area index (LAI) values to assess modeled vegetation growth. Different irrigation practices were assessed, including conventional flooding (CVF), known as cascade, multiple inlet rice irrigation with polypipe (MIRI), and furrow irrigation (FIR). The amount of water used differed between fields, following each field’s measured or estimated input. Moreover, fields were managed with either continuous flooding (CF) or alternate wetting and drying (AWD) irrigation. Two 20-year scenarios were simulated to test yield changes: (1) between rice–rice and soybean–rice rotation and (2) under reduced irrigation amounts. After calibration with crop yield and LAI, the modeled LAI correlated to the observations with R2 values greater than 0.66, and the percent bias (PBIAS) values were within 32%. The PBIAS and percent difference for modeled versus observed yield were within 2.5% for rice and 15% for soybean. Contrary to expectation, the rice–rice and soybean–rice rotation yields were not statistically significant. The results of the reduced irrigation scenario differed by field, but reducing irrigation beyond 20% from the original amount input by the farmers significantly reduced yields in all fields, except for one field that was over-irrigated.

Comparison of Evapotranspiration and Biomass Simulation in Winter Wheat under Conventional and Conservation Tillage Systems using APEX Model

The Agricultural Policy/Environmental eXtender (APEX) is one of the hydrologic and water quality models being used in the USDA-ARS Long-Term Agroecosystem Research (LTAR) to evaluate current and develop sustainable agricultural production systems throughout the United States. Tillage practices used affect agricultural production, soil erosion, and evapotranspiration (ET), which is a major component of the hydrologic cycle. However, there is limited literature on the impact of tillage practices on parameterization and performance of APEX to simulate ET. The goals of this study were to 1) determine model parameters that affect ET simulation in APEX model for winter wheat (Triticum aestivum L.) fields managed under conservation and conventional tillage systems, and 2) evaluate the ability of APEX to simulate ET and daily biomass for each of these tillage systems in central Oklahoma. Tillage system had an impact on the number and type of APEX parameters sensitive for simulation of ET. Therefore, it is essential to perform a sensitivity analysis for study areas with different tillage management practices to determine appropriate parameters for inclusion in the calibration process. Overall, APEX simulated daily, biweekly, and monthly ET and daily biomass relatively well for both tillage systems. The Nash-Sutcliffe efficiency (NSE) for the simulated monthly ET of the calibrated model was 0.98 and 0.96 for conservation and conventional tillage fields, respectively. The percent bias (PBIAS) values for the ET simulation were within 3%, while PBIAS values for daily biomass simulation were within 25%, which indicates that the average simulated ET and daily biomass values were within 3% and 25% of the measured ET and daily biomass, respectively, in both tillage fields. This result indicates that a well-calibrated APEX model using appropriate sensitive parameters can simulate ET and biomass satisfactorily in both tillage systems.

Critical standing crop residue amounts for wind erosion control in the inland Pacific Northwest, USA

Crop residue is an important factor influencing wind erosion of cultivated soils. Establishing soil surface protection afforded by standing crop residue is critical for land managers seeking to reduce or prevent soil loss by wind erosion and the impacts of blowing dust from agricultural lands. The objectives of this study were to evaluate the effect of standing residue on soil wind erosion in the inland Pacific Northwest (iPNW), USA, and test the performance of the plant factor algorithm of the Agricultural Policy/Environmental eXtender (APEX) and Revised Wind Erosion Equation (RWEQ) models in influencing soil loss. The effect of standing winter wheat (Triticum aestivum L.), spring canola (Brassica napus L.), and chickpea (Cicer arietinum) residue on wind erosion, remaining from major commodity crops in the region, was tested in a laboratory wind tunnel using four levels of residue density. The impact of standing residue in controlling wind erosion was compared and analyzed in terms of residue density and their respective frontal area index. Our results show that residue at a density characteristic of the production environment (110 standing residue elements m−1 for winter wheat, 20 standing elements m−1 for canola, and 16 standing elements m−1 for chickpea) provided significant protection to the soil surface from wind erosion. Soil loss at this level of residue density was reduced by 73.3, 53.4, and 60.9% for respectively winter wheat, spring canola, and chickpea (frontal area indexes are 0.172, 0.104, and 0.026 respectively) compared with a surface without residue. The soil surface was found to be at significant risk from wind erosion when residue densities of the three crop types were <50% of the typical production amounts. Although not consistently significant, soil loss decreased as wind direction shifted from parallel to perpendicular with the standing residue row. The APEX model adequately simulated winter wheat and spring canola residue protection but had low accuracy in representing chickpea residue effects relative to the wind tunnel experiments. In contrast, the RWEQ model appeared inadequate in simulating soil loss for the winter wheat and canola treatments but adequately represented chickpea residue effects. Differences in model accuracy for different crop types must be considered by producers and managers to determine whether model information used to select practices to control wind erosion are likely to result in under- or over-protection of soil resources.

Evaluating impacts of climate change on hydrology and total nitrogen loads using coupled APEX-paddy and SWAT models

Climate change is currently one of the most critical issues in watershed management, and typical paddy systems should be addressed by watershed modeling approach in paddy-dominant landscapes. This study is designed to evaluate and enhance the watershed modeling approach currently used to characterize the impacts of climate change on hydrologic and water quality responses while considering a paddy environment. APEX-paddy, which is a newly developed and modified APEX (Agricultural Policy/Environmental eXtender) model for paddy ecosystems, was coupled with SWAT (Soil and Water Assessment Tool) model to take advantage of the strengths of the two models. The resulting hybrid model, SWAPX, was calibrated and validated using observed data from 2008 to 2017 for two sites in the study watershed. Compared to SWAT, the accuracy of SWAPX was improved, showing statistically better results in the downstream including more paddy field areas. Ten GCMs were selected, and the characteristics of these GCMs were evaluated to assess the impacts of climate change scenarios. When applying the climate change scenarios to the SWAPX model, the results indicated that the future streamflow would increase due to increased rainfall. The results also showed that total nitrogen (T-N) loads would increase rapidly in the near future, then decrease gradually through the 2090s (2091–2100). T-N load was affected by the characteristics of rainfall patterns (e.g., daily maximum rainfall and rainfall intensity) occurring in various GCMs. This approach will be helpful for decision-makers in adapting to climate change and evaluating Best management practices (BMP) for paddy-dominant watersheds.

Evaluating the Impact of Climate Change on Paddy Water Balance Using APEX-Paddy Model

This research aims to assess the impact of climate change on water balance components in irrigated paddy cultivation. The APEX-Paddy model, which is the modified version of the APEX (Agricultural Policy/Environmental eXtender) model for paddy ecosystems, was used to evaluate the paddy water balance components considering future climate scenarios. The bias-corrected future projections of climate data from 29 GCMs (General Circulation Models) were applied to the APEX-Paddy model simulation. The study area (Jeonju station) forecasts generally show increasing patterns in rainfall, maximum temperature, and minimum temperature with a rate of up to 23%, 27%, and 45%, respectively. The hydrological simulations suggest over-proportional runoff–rainfall and under-proportional percolation and deep-percolation–rainfall relationships for the modeled climate scenarios. Climate change scenarios showed that the evapotranspiration amount was estimated to decrease compared to the baseline period (1976–2005). The evaporation was likely to increase by 0.12%, 2.21%, and 7.81% during the 2010s, 2040s, and 2070s, respectively under Representative Concentration Pathway (RCP)8.5, due to the increase in temperature. The change in evaporation was more pronounced in RCP8.5 than the RCP4.5 scenario. The transpiration is expected to reduce by 2.30% and 12.62% by the end of the century (the 2070s) under RCP4.5 and RCP8.5, respectively, due to increased CO2 concentration. The irrigation water demand is generally expected to increase over time in the future under both climate scenarios. Compared to the baseline, the most significant change is expected to increase in the 2040s by 3.21% under RCP8.5, while the lowest increase was found by 0.36% in 2010s under RCP4.5. The increment of irrigation does not show a significant difference; the rate of increase in the irrigation was found to be greater RCP8.5 than RCP4.5 except in the 2070s. The findings of this study can play a significant role as the basis for evaluating the vulnerability of rice production concerning water management against climate change.

Adapting the APEX Model to Simulate Detasseling in Inbred Corn for Hybrid Seed Production

HighlightsThe APEX model was adapted to simulate detasseling in inbred corn for hybrid seed production.The adapted model satisfactorily predicted detasseling effects on LAI, grain yield, and N content.An inbred corn model could be applied to evaluate best management practices for inbred corns.Abstract. Hybrid seed corn production comprises approximately 10% of the entire corn acreage in the U.S. Because of seed corn’s high economic value, and to maximize yields, seed corn growers often over-irrigate or apply nitrogen (N) fertilizers equal to or in excess of those recommended for commercial hybrid corn. Detasseling female corn inbred lines during hybrid corn seed production is critical to ensure the purity of seeds. In addition to the removal of tassels, detasseling also results in the removal of several leaves, which may lead to reduced seed yields. The objective of this study was to adapt the Agricultural Policy/Environmental eXtender (APEX) model to simulate the detasseling of female inbred corns in hybrid seed production. An APEX inbred corn model was developed to simulate the effects of detasseling and leaf removals on the development of inbred corn, leaf area index (LAI), grain yield, and grain N content. Growth characteristics of inbred corn were parameterized in APEX using data from a field study conducted in Nebraska. Overall, the APEX inbred corn model successfully predicted the effects of detasseling on LAI, grain yield, and grain N content under the conditions of the field experiment. There was a significant correlation between simulated and measured LAI (Pearson r = 0.86 and R2 = 0.74 at p = 0.05). The computed paired t-test and permutation test p-values indicated no significant differences between measured and simulated LAI. The mean simulation percent difference and percent bias (PBIAS) were respectively 4.2% and 4.7%, while measured and simulated LAI values had an average root mean square error (RMSE) of 0.14. The APEX model predicted grain yield with RMSE of 120 kg ha-1, mean simulation percent difference of 0.48%, and PBIAS of 0.26%. Like LAI, predicted grain yields exhibited significant correlation with field data (Pearson r = 0.99 and R2 = 0.97 at p = 0.05). Similarly, computed paired t-test and permutation test p-values indicated no significant differences between measured and simulated grain yields. Grain N content was predicted with RMSE of 6.75 kg N ha-1, mean simulation percent difference of 1.46%, and PBIAS of 2.45%. Predicted and measured grain N content values were correlated (Pearson r = 0.81 and R2 = 0.65 at p = 0.05), while the t-test and permutation test p-values indicated no significant differences between measured and predicted grain N content. Overall, detasseling effects were better predicted for grain yield than for LAI and grain N content as indicated by a Nash-Sutcliffe efficiency (NSE) of 0.92 compared to NSE values of 0.47 for LAI and 0.43 for grain N content. In conclusion, the hybrid seed corn industry could benefit from the application of inbred corn models that could allow growers to evaluate and identify optimal irrigation and N management practices for inbred corn, similar to the benefits that have been obtained with model simulation for commercial hybrid corn grain production systems. Keywords: APEX parameterization, Detasseling, Inbred corn, Leaf area index.

Conservation tillage effects in the Atlantic Coastal Plain: An APEX examination

The Conservation Effects Assessment Project (CEAP) was established to develop a scientific understanding and methodology for estimating environmental benefits and effects of conservation practices on agricultural landscapes at watershed, regional, and national scales. CEAP9s goal is to improve the effectiveness of conservation practices and programs by quantifying conservation effects and providing the science and education needed to enrich conservation planning, implementation, management decisions, and policy. Field observations and computer-based simulation of the effects of agricultural conservation are important components of CEAP. This research supports the CEAP effort by utilizing well-documented field data in conjunction with the Agricultural Policy/Environmental eXtender (APEX) model to quantify long-term conservation effects. In this study, field data collected at a research site located near Tifton, Georgia, were used to evaluate APEX and to quantify long-term benefits of implementing conservation tillage in the Atlantic Coastal Plain region of the United States. The objectives of this research were to (1) quantify differences in crop yield, hydrology, and sediment transport for conventional and conservation tillage systems in the Atlantic Coastal Plain; (2) develop a calibrated APEX simulation for these tillage systems; and (3) quantify the performance of the APEX model with respect to crop yields, hydrology, and sediment yield. Fourteen years of crop yield, surface runoff, subsurface flow, and sediment transport data were quantified comparing conventional tillage (CT) to strip tillage (ST). No treatment differences were found for either cotton (<i>Gossypium hirsutum</i> L.) or peanut (<i>Arachis hypogea</i> L.) yields. Surface runoff from the CT was found to be 1.7 times that of the ST, while subsurface flow from the ST was found to be 1.7 times that of CT. Total water loss (surface and subsurface) was nearly equivalent for the two systems: 30% of annual rainfall. Satisfactory model performance was found for APEX surface runoff simulations. Mixed results were found for model performance of crop yield while model simulations of subsurface flow and sediment yield were less than satisfactory. The APEX modeling framework provides a useful tool for assessing crop yield and hydrologic differences between tillage management systems in the Atlantic Coastal Plain. Additional refinement of modeling approaches may be necessary to adequately represent subsurface flow and sediment transport in these same systems.

Calibrating the APEX Model for Simulations of Environmental and Agronomic Outcomes on Dairy Farms in the Northeast U.S.: A Step-by-Step Example

HighlightsThis article details the method used to calibrate the APEX model for simulations of dairy farm silage corn production.Useful model outputs, plots, and metrics are discussed, as well as decision points and potential improvements.The article is intended as a helpful starting point for those who are new to APEX.Abstract. This article documents the procedure used to calibrate the Agricultural Policy/Environmental eXtender (APEX) model for a project that aimed to simulate crop yields, runoff, erosion, and nutrient losses on dairy farms under climate change. We describe each step in the calibration process, presenting the model outputs, plots, and metrics that were found to be useful, and discussing the decisions that needed to be made along the way. Calibration improved the performance of the model relative to an uncalibrated “baseline” version, and 0.22 &amp;lt; NSE &amp;lt; 0.49 and PBIAS within ±12% were achieved for most of the model outputs. While the mean annual silage yield was correct (PBIAS = 2%), the model did not accurately capture year-to-year yield variations (NSE = -1.6), and results for nitrogen in runoff were poor (NSE = -0.04, PBIAS = -69%).We therefore also outline several ways in which the method could be improved. Other calibration methods exist, and the procedure presented here will not be applicable in all situations. However, fully documented APEX calibrations are rare in the literature, and the number of non-expert model users may be growing. We therefore anticipate that this paper will serve as a useful point of entry for those who are new to APEX. We also hope that this work contributes to the development of transparent and reproducible procedures for modeling studies that have real-world implications. Keywords: APEX model, Hydrologic modeling, Methodology, Model calibration.

Understanding the effects of pasture type and stocking rate on the hydrology of the Southern Great Plains

Grassland is one of the major biomes in the United States (US) and the world. In the US, the majority of grasslands are concentrated in the Great Plains and has undergone through significant interventions or management changes over the last few decades. A key economy-driven intervention in the Southern Great Plains (SGP) include the introduction of new forage species and conversion of native grassland to introduced pasture to increase productivity and its nutritive value for improved cattle production. Since water is one of the fundamental resources needed to sustain grassland productivity, it is important to understand how such pasture conversion and prevailing cattle grazing practices affect water balance and biomass production in a given pasture system. In this study, the Nutrient Tracking Tool (NTT) with its core APEX (Agricultural Policy Environmental eXtender) model was used to assess the hydrological impacts of the pasture introduction, i.e., native pasture (little bluestem, Schizachyrium halapense) vs. introduced pasture (old world bluestem, Bothriochloa caucasica), and the stocking rate in the SGP. Monthly evapotranspiration (ET) and biomass estimates from NTT compared well with observed data at two USDA-ARS experimental pastures (native and introduced) near El Reno, Oklahoma, for the years 2015 and 2016. Simulated long-term average annual hydrologic fluxes (i.e., ET, runoff, and groundwater recharge) from the introduced pasture were slightly lower than the observed data but not significantly different than those from the native pasture under the current management conditions. NTT predicted higher water yield (runoff and recharge) and significantly lower ET for the introduced pasture than the native pasture. Results suggest that grazing has the potential to alter the hydrological balance in the SGP. For example, the increase in stocking rate within the carrying capacity of the farm decreases ET and increases runoff and groundwater recharge for both pastures. Comparison of estimated biomass production between native and introduced pastures indicated that introduced pastures are more efficient in using the available water and thus produce a higher forage biomass per unit of water in the SGP. This study highlighted the potential significance of considering hydrological and other biophysical impacts of new forage introduction and stocking rate changes for the sustainable management of grazing and pasture systems in the SGP.