Joachim Ingwersen

Multimodel ensembles of wheat growth: many models are better than one.

Crop models of crop growth are increasingly used to quantify the impact of global changes due to climate or crop management. Therefore, accuracy of simulation results is a major concern. Studies with ensembles of crop models can give valuable information about model accuracy and uncertainty, but such studies are difficult to organize and have only recently begun. We report on the largest ensemble study to date, of 27 wheat models tested in four contrasting locations for their accuracy in simulating multiple crop growth and yield variables. The relative error averaged over models was 24-38% for the different end-of-season variables including grain yield (GY) and grain protein concentration (GPC). There was little relation between error of a model for GY or GPC and error for in-season variables. Thus, most models did not arrive at accurate simulations of GY and GPC by accurately simulating preceding growth dynamics. Ensemble simulations, taking either the mean (e-mean) or median (e-median) of simulated values, gave better estimates than any individual model when all variables were considered. Compared to individual models, e-median ranked first in simulating measured GY and third in GPC. The error of e-mean and e-median declined with an increasing number of ensemble members, with little decrease beyond 10 models. We conclude that multimodel ensembles can be used to create new estimators with improved accuracy and consistency in simulating growth dynamics. We argue that these results are applicable to other crop species, and hypothesize that they apply more generally to ecological system models.

Modelling water dynamics with DNDC and DAISY in a soil of the North China Plain: A comparative study

The performance of the DNDC and Daisy model to simulate the water dynamics in a floodplain soil of the North China Plain was tested and compared. While the DNDC model uses a simple cascade approach, the Daisy model applies the physically based Richards equation for simulating water movement in soil. For model testing a three years record of the soil water content from the Dong Bei Wang experimental station near Beijing was used. There, the effect of nitrogen fertilization, irrigation and straw removal on soil water and nitrogen dynamics was investigated in a three factorial field experiment applying a split-split-plot design with 4 replications. The dataset of one treatment was used for model testing and calibration. Two other independent datasets from further treatments were employed for validating the models. For both models, the simulation results were not satisfying using default parameters. After parameter optimisation and the use of site-specific van Genuchten parameters, however, the Daisy model performed well. But, for the DNDC model, parameter optimisation failed to improve the simulation result. Owing to the fact that many biological processes such as plant growth, nitrification or denitrification depend strongly on the soil water content, our findings bring us to the conclusion that the site-specific suitability of the DNDC model for simulating the soil water dynamics should be tested before further simulation of other processes.

Incorporating dynamic root growth enhances the performance of Noah‐MP at two contrasting winter wheat field sites

Interactions between the soil, the vegetation, and the atmospheric boundary layer require close attention when predicting water fluxes in the hydrogeosystem, agricultural systems, weather, and climate. However, land-surface schemes used in large-scale models continue to show deficiencies in consistently simulating fluxes of water and energy from the subsurface through vegetation layers to the atmosphere. In this study, the multiphysics version of the Noah land-surface model (Noah-MP) was used to identify the processes, which are most crucial for a simultaneous simulation of water and heat fluxes between land surface and the lower atmosphere. Comprehensive field data sets of latent and sensible heat fluxes, ground heat flux, soil moisture, and leaf area index from two contrasting field sites in South-West Germany are used to assess the accuracy of simulations. It is shown that an adequate representation of vegetation-related processes is the most important control for a consistent simulation of energy and water fluxes in the soil-plant-atmosphere system. In particular, using a newly implemented submodule to simulate root growth dynamics has enhanced the performance of Noah-MP. We conclude that further advances in the representation of leaf area dynamics and root/soil moisture interactions are the most promising starting points for improving the simulation of feedbacks between the subsoil, land surface and atmosphere in fully coupled hydrological and atmospheric models.

Assessing the relevance of subsurface processes for the simulation of evapotranspiration and soil moisture dynamics with CLM3.5: comparison with field data and crop model simulations

Plant water uptake is a crucial process linking water fluxes in the soil–plant–atmosphere continuum. Soil water extraction by roots affects the dynamics and distribution of soil moisture. Water supply of plants controls transpiration, which makes up for an important fraction of the energy balance at the land surface, and influences soil–vegetation–atmosphere feedback processes. Therefore, efficient algorithms for an accurate estimation of root water uptake are essential in land-surface models that are coupled with climate models, in agricultural crop models that predict water budget and plant growth at the field and plot scale, and in hydrological models. Due to different purposes and demands on computational time, the degree of detail in representing belowground processes varies considerably between these model types. This study investigates the impact of the degree of detail in process descriptions of root growth and water uptake and of information about soil hydraulic properties on simulated seasonal patterns of evapotranspiration and soil moisture in a field study with winter wheat (Triticum aestivum L. cv. Cubus). Evapotranspiration was well simulated by CLM3.5 until the beginning of crop senescence, but it overestimates the water flux through plants in the last three weeks of the vegetation period and showed a lower performance in simulating soil moisture compared to crop models. The best simultaneous fit of soil moisture and latent heat flux was achieved by the crop model XN-SPASS, which consists of the most detailed representation of root growth dynamics. The results indicate the importance of implementing improved belowground process descriptions for advanced simulations with coupled hydrological and atmospheric models.

Investigation of PBL schemes combining the WRF model simulations with scanning water vapor differential absorption lidar measurements

Six simulations with the Weather Research and Forecasting (WRF) model differing in planetary boundary layer (PBL) schemes and land surface models (LSMs) are investigated in a case study in western Germany during clear-sky weather conditions. The simulations were performed at 2 km resolution with two local and two nonlocal PBL schemes, combined with two LSMs (NOAH and NOAH-MP). Resulting convective boundary layer (CBL) features are investigated in combination with high-resolution water vapor differential absorption lidar measurements at an experimental area. Further, the simulated soil-vegetation-atmosphere feedback processes are quantified applying a mixing diagram approach. The investigation shows that the nonlocal PBL schemes simulate a deeper and drier CBL than the local schemes. Furthermore, the application of different LSMs reveals that the entrainment of dry air depends on the energy partitioning at the land surface. The study demonstrates that the impact of processes occurring at the land surface is not constrained to the lower CBL but extends up to the interfacial layer and the lower troposphere. With respect to the choice of the LSM, the discrepancies in simulating a diurnal change of the humidity profiles are even more significant at the interfacial layer than close to the land surface. This indicates that the representation of land surface processes has a significant impact on the simulation of mixing properties within the CBL.

Pesticide transport simulation in a tropical catchment by SWAT

The application of agrochemicals in Southeast Asia is increasing in rate, variety and toxicity with alarming speed. Understanding the behavior of these different contaminants within the environment require comprehensive monitoring programs as well as accurate simulations with hydrological models. We used the SWAT hydrological model to simulate the fate of three different pesticides, one of each usage type (herbicide, fungicide and insecticide) in a mountainous catchment in Northern Thailand. Three key parameters were identified: the sorption coefficient, the decay coefficient and the coefficient controlling pesticide percolation. We yielded satisfactory results simulating pesticide load dynamics during the calibration period (NSE: 0.92-0.67); the results during the validation period were also acceptable (NSE: 0.61-0.28). The results of this study are an important step in understanding the modeling behavior of these pesticides in SWAT and will help to identify thresholds of worst-case scenarios in order to assess the risk for the environment.

Pesticide Transport Pathways from a Sloped Litchi Orchard to an Adjacent Tropical Stream as Identified by Hydrograph Separation

This study was performed to identify the transport pathways of pesticides from a sloped litchi ( Sonn.) orchard to a nearby stream based on a three-component hydrograph separation (baseflow, interflow, surface runoff). Dissolved silica and electrical conductivity were chosen as representative tracers. During the study period (30 d), 0.4 and 0.01% of the applied mass of atrazine and chlorpyrifos, respectively, were detected in the stream after 151 mm of rainfall. Baseflow (80-96%) was the dominant hydrological flow component, followed by interflow (3-18%) and surface runoff (1-7%). Despite its small contribution to total discharge, surface runoff was the dominant atrazine transport pathway during the first days after application because pesticide concentrations in the surface runoff flow component declined quickly within several days. Preferential transport with interflow became the dominant pathway of atrazine. Because chlorpyrifos was detected in the stream water only twice, it was not included in the hydrograph separation. A feature of the surface runoff pathway was the coincidence of pesticide and discharge peaks. In contrast, peak concentrations of pesticides transported by interflow occurred during the hydrograph recession phases. Stormflow generation and pesticide transport depended on antecedent rainfall. The combination of high-resolution pesticide concentration measurements with a three-component hydrograph separation has been shown to be a suitable method to identify pesticide transport pathways under tropical conditions.

Ground water preservation by soil protection : Determination of tolerable total Cd contents and Cd breakthrough times

Taking Cd as an example we introduce a procedure to estimate tolerable total content of heavy metals in soils with regard to a specific ground water quality criterion. Furthermore, we present a piston-flow approach to estimate breakthrough times of a sorptive solute to the ground water. Both procedures are applied to the sandy soils in the 4300 ha wastewater irrigation area Braunschweig, Germany. Applicability of these procedures is tested by numerical simulations. The calculated breakthrough times of Cd for an input concentration of 3 μg L−1 and a mean water flux density of 570 mm yr−1 varies, as a function of depth of water table and sorption characteristics, between 10 and 805 years (mean = 141 years). The deviation between the piston-flow approach and the numerical simulation is on the average 1.6%. We determined a mean tolerable total Cd content of 0.61 mg kg−1 with regard to a ground water quality criterion of 3 μg L−1. The limit of the German sewage sludge regulation (AbfKlarV, 1992) of 1 mg Cd kg−1 exceeds the calculated tolerable total content in 90% of the investigated Ap horizons. Moreover, the results of the numerical simulations show that the limit of 1 mg Cd kg−1 would lead to a concentration in seepage water significantly above 8 μg Cd L−1. We conclude that in the sandy soils of the wastewater irrigation area the current limit of 1 mg Cd kg−1 is not sufficient to keep the Cd concentration in seepage water below 3 μg L−1 and, thus, to ensure ground water protection in the long run. Grundwasserschutz durch Bodenschutz: Ermittlung von tolerierbaren Cd-Gesamtgehalten und Cd-Durchbruchszeiten Am Beispiel des Schwermetalls Cd wird ein Verfahren vorgestellt, um fur ein definiertes Grundwasserqualitatsziel tolerierbare Gesamtgehalte eines Schwermetalls im Boden zu berechnen. Weiterhin wird ein „piston-flow”-Ansatz beschrieben, der die Berechnung der Durchbruchszeit eines sorptiven Stoffes in das Grundwasser erlaubt. Die Verfahren werden exemplarisch auf die Sandboden des 4300 ha grosen Abwasserverregnungsgebietes Braunschweig angewendet und die Qualitat der Aussagen durch numerische Simulationen getestet. Die berechneten Durchbruchszeiten von Cd bei einer Infiltrationskonzentration von 3 μg L−1 und einer Wasserflussdichte von 570 mm a−1 betragen im Untersuchungsgebiet im Mittel 141 Jahre und variieren, in Abhangigkeit vom Grundwasserflurabstand und den Sorptionseigenschaften, zwischen 10 und 805 Jahren. Die Abweichung zwischen dem „piston-flow”-Ansatz und der numeri schen Simulation betragt im Mittel 1.6%. Fur das Grundwasserqualitatsziel 3 μg L−1 wird ein mittlerer tolerierbarer Cd-Gesamtgehalt von 0.61 mg kg−1 bestimmt. Der Grenzwert der Klarschlammverordnung (AbfKlarV, 1992) von 1 mg kg−1 wurde bei 90% der untersuchten Ap-Horizonte den berechneten tole rierbaren Cd-Gesamtgehalt uberschreiten. Nach den Ergebnissen der numerischen Simulation wurden Cd-Gesamtgehalte in Hohe des Grenzwertes der AbfKlarV (1992) im Untersuchungsgebiet zu Sickerwasserkonzentrationen deutlich oberhalb von 8 μg Cd L−1 fuhren. Der Grenzwert der AbfKlarV (1992) ist folglich fur die Sandboden des Abwasserverregnungsgebietes unter der Masgabe eines Qualitatskriteriums von 3 μg Cd L−1 fur einen langfristigen Grundwasserschutz unzureichend.

Simulating pesticide transport from a sloped tropical soil to an adjacent stream.

Preferential flow from stream banks is an important component of pesticide transport in the mountainous areas of northern Thailand. Models can help evaluate and interpret field data and help identify the most important transport processes. We developed a simple model to simulate the loss of pesticides from a sloped litchi (Litchi chinensis Sonn.) orchard to an adjacent stream. The water regime was modeled with a two-domain reservoir model, which accounts for rapid preferential flow simultaneously with slow flow processes in the soil matrix. Preferential flow is triggered when the topsoil matrix is saturated or the infiltration capacity exceeded. In addition, close to matrix saturation, rainfall events induce water release to the fractures and lead to desorption of pesticides from fracture walls and outflow to the stream. Pesticides undergo first order degradation and equilibrium sorption to soil matrix and fracture walls. The model was able to reproduce the dynamics of the discharge reasonably well (model efficiency [EF] = 0.56). The cumulative pesticide mass (EF = 0.91) and the pesticide concentration in the stream were slightly underestimated, but the deviation from measurement data is acceptable. Shape and timing of the simulated concentration peaks occurred in the same pattern as observed data. While the effect of surface runoff and preferential interflow on pesticide mass transport could not be absolutely clarified, according to our simulations, most concentration peaks in the stream are caused by preferential interflow pointing to the important role of this flow path in the hilly areas of northern Thailand.

Micro-scale modeling of pesticide degradation coupled to carbon turnover in the detritusphere: model description and sensitivity analysis

Microbiologically active biogeochemical interfaces are excellent systems to study soil functions such as pesticide degradation at the micro-scale. In particular, in the detritusphere pesticide degradation is accelerated by input of fresh organic carbon from litter into the adjacent soil. This observed priming effect suggests: (i) pesticide degradation is strongly coupled to carbon turnover, (ii) it is controlled by size and activity of the microbial community and (iii) sorption and transport of dissolved carbonaceous compounds and pesticides might regulate substrate availability and in turn decomposition processes. We present a new mechanistic 1D model (PEsticide degradation Coupled to CArbon turnover in the Detritusphere, PECCAD) which implements these hypotheses. The new model explicitly considers growth and activity of bacteria, fungi and specific pesticide degraders in response to substrate availability. Enhanced pesticide degradation due to availability of a second source of carbon (dissolved organic carbon) is implemented in the model structure via two mechanisms. First, additional substrate is utilized simultaneously with the pesticide by bacterial pesticide degraders resulting in an increase in their size and activity. Second, stimulation of fungal growth and activity by additional substrates leads directly to higher pesticide degradation via co-metabolism. Thus, PECCAD implicitly accounts for litter-stimulated production and activity of unspecific fungal enzymes responsible for co-metabolic pesticide degradation. With a global sensitivity analysis we identified high-leverage model parameters and input. In combination with appropriate experimental data, PECCAD can serve as a tool to elucidate regulation mechanisms of accelerated pesticide degradation in the detritusphere.