Thomas B. Kirchner

Observations and modeling of biomass and soil organic matter dynamics for the grassland biome worldwide

Century is a model of terrestrial biogeochemistry based on relationships between climate, human management (fire, grazing), soil properties, plant productivity, and decomposition. The grassland version of the Century model was tested using observed data from 11 temperate and tropical grasslands around the world. The results show that soil C and N levels can be simulated to within ±25% of the observed values (100 and 75% of the time, respectively) for a diverse set of soils. Peak live biomass and plant production can be simulated within ± 25% of the observed values (57 and 60% of the time, respectively) for burned, fertilized, and irrigated grassland sites where precipitation ranged from 22 to over 150 cm. Live biomass can be generally predicted to within ±50% of the observed values (57% of the time). The model underestimated the live biomass in extremely high plant production years at two of the Russian sites. A comparison of Century model results with statistical models showed that the Century model had slightly higher r2 values than the statistical models. Data and calibrated model results from this study are useful for analysis and description of grassland carbon dynamics, and as a reference point for testing more physiologically based models predictions of net primary production and biomass. Results indicate that prediction of plant and soil organic matter (C and N) dynamics requires knowledge of climate, soil texture, and N inputs.

The Importance of Soil Water in the Recruitment of Bouteloua Gracilis in the Shortgrass Steppe

In the shortgrass steppe region of North America there is a controversy about the ability of the dominant species to recruit from seedlings. The prevailing view is that Bouteloua gracilis is incapable of recruitment from seedlings in areas receiving <380 mm of annual precipitation. A common explanation for this situation is that environmental conditions permitting seedling establishment are infrequent. To assess the frequency of environmental conditions appropriate for the recruitment of B. gracilis we used a soil water simulation model and long—term climatic data in conjunction with detailed information about the ecophysiological requirements for seed germination and growth of seminal and adventitious roots. We found that recruitment events occur as frequently as every 30—50 yr on silty clay, silty clay loam, and silty loam soils, but less than once in 5000 yr on sandy soils. Simulated frequencies of recruitment were sufficient to account for the observed abundance of B. gracilis in 7 of 11 soil textures...

Modeling vegetation structure-ecosystem process interactions across sites and ecosystems

Abstract We describe an approach to investigating and understanding the interactions between vegetation structure and ecosystem processes that uses simulation models as a framework for comparison and synthesis across ecosystems arrayed along environmental gradients. The models are individual-based vegetation simulators and compartment models of nutrient cycling and soil water relations. Applications focus on interactions and feedbacks between vegetation structure (species composition, size structure) and ecosystem processes (water balance, nutrient cycling), and how these relationships vary across environmental gradients. Preliminary results indicate that life-history traits of plants have a profound influence on system-level behaviors, and that differences between grasslands and forests can be attributed largely to contrasting traits of grasses and trees. Experiments with linked vegetation-ecosystem process models diverge from simulations with either model run independently, suggesting the importance of feedbacks between details of vegetation pattern and ecosystem processes. The development of a fully coupled vegetation-ecosystem process model that is sufficiently general to simulate systems dominated by multiple lifeforms presents several conceptual, logistical, and scaling challenges, but also provides for new opportunities in ecosystem theory.

Contaminant Transport through Agroecosystems: Assessing Relative Importance of Environmental, Physiological, and Management Factors

Agroecosystems can become contaminated by atmospherically released radionuclides. The subsequent concentrations of radionuclides in foods are dependent on numerous environmental, physiological, and management factors. We compared four approaches for estimating the relative importance of several of these factors in determining concentrations of 131 I and 137 Cs in milk. A series of sensitivity analyses with Monte Carlo and full-factorial sampling designs was conducted on the PATHWAY model, which simulates radionuclide transport through an agroecosystem. Sensitivity of time-integrated concentrations in milk was estimated as a function of the time of year that fallout was deposited and as a function of time following a spring deposition. The dominant parameters affecting time-integrated concentrations of 131 I in milk were the initial fraction of radionuclides deposited on vegetation, timing and amount of pasture consumption, and the production rate of milk. For time-integrated concentrations of the longer-lived 137 Cs in milk, resuspension was a dominant parameter and pasture use was less important. The sampling designs were compared by ranking the parameters to which the model output is sensitive. The three sampling designs based on parameter variances produced sets of ranks that were similar to each other but differed from the ranking produced by the sampling design based on parameter magnitude. The results indicate which data are most crucial for real-time calculations following an accident and how subsequent dose from ingestion can be most effectively reduced, provide insight into model behavior, and help prioritize future research. This paper demonstrates the importance of variance-based sensitivity analysis.

TIME-ZERO: The integrated modeling environment

Abstract TIME-ZERO is a system of software tools that provides an environment for constructing, running, and analyzing simulation models. Models can include first-order diffrential and difference equations, age-specific population dynamics, and a Markov transition matrix. TIME-ZERO helps one write models in either Basic or Fortran by providing an interactive code generator. The code generator uses menus and explicit prompts to guide one through the process of building a model. TIME-ZERO minimizes the amount of code that one needs to write to create a simulation model, yet allows one to build complex models. The system also has an extensive set of menus to help run models, examine graphical or tabular output from models, perform sensitivity and uncertainty analyses, and select parameters using numerical optimization. This paper describes the basic features of TIME-ZERO.

Validation of pathway, a simulation model of the transport of radionuclides through agroecosystems

ABSTRACT Kirchner, T.B. and Whicker, F.W., 1984. Validation of PATHWAY, a simulation model of the transport of radionuclides through agroecosystems. Ecol. Modelling, 22: 21–44. PATHWAY is a dynamic simulation model of the transport of radionuclides from fallout through the agricultural foodchain. The model was developed to fulfill the need for a realistic, testable, completely time dependent model to estimate radionuclide ingestion by humans exposed to fallout patterns originating at the Nevada Test Site during the 1950s. The model simulates the fate of radionuclides in various management units (pastures, gardens, croplands, etc.) of an agro-ecosystem. The agricultural products currently considered in the model are pasture grass, alfalfa, grains, garden vegetables, milk, eggs, beef, and poultry. Harvested and stored products, such as hay and canned vegetables, are also represented in the model. Animal diets are specified on a daily basis. Thus, the model can be used to simulate the fate of radionuclides from both acute and chronic fallout deposition. The utility and credibility of radionuclide transport simulation models are severely limited without rigorous testing of their predictive capabilities against real observations. This paper reviews exercises testing the overall accuracy and temporal dynamics simulated by the PATHWAY model against 37 sets of observed data. These data include concentrations of 137 Cs, 90 Sr, 140 Ba, and 131 I in milk, beef, alfalfa, and pasture grass from several western states following deposition of the radionuclides in fallout. In addition, the model is tested against observations of concentrations of 131 I in milk following the accident at the Windscale reactor in England. The statistical tests used to compare the predictions of PATHWAY to the observations include a correlation analysis, a paired t-test, and a binomial test. We use the correlation coefficient between observations and predictions through time to compare the dynamics of the simulated and real world system. Plots of the residuals from regression are then examined for bias between the predictions and observations. The significance of any trends in the residuals is evaluated using a runs test. The paired t-test and the binomial test are used to evaluate the accuracy of PATHWAYs predictions. The hypothesis for the paired t-test is that the ratio of predictions to observations is 1. The paired t-test can be used to test hypotheses about ratios because the distributions of observations and predictions appear to be lognormal. However, the paired t-test does not consider uncertainty in the predictions of the model. We use a binomial test to compare the observed data to an interval estimate from PATHWAY. The interval corresponds to a 95% confidence interval on the prediction, and is derived from uncertainty analyses that have been conducted on PATHWAY. PATHWAYs predictions are significantly correlated with observed levels of 137 Cs and 90 Sr in pasture and alfalfa. PATHWAY also simulates the dynamics of 131 I, 140 Ba, and 137 Cs in milk well, but fails to predict what appears to be a long term accumulation of 90 Sr in the agro-ecosystem. PATHWAY predicts the absolute concentrations of 131 I in milk quite well, but tends to predict levels of 140 Ba, 90 Sr, 137 Cs in milk that are different from those observed by factors of 2 to 7. PATHWAY predicts levels of 137 Cs and 90 Sr in pasture and beef within a factor of 2 of those observed.

Distributed processing applied to ecological modeling

Abstract Ecological simulation models are widely used in ecology to help understand and predict the behavior of ecosystems. Many of the questions now being asked in ecology, such as the effects of climate change on ecosystems or the risks to ecosystems from contaminants, require extrapolation in both space and time and often require the collaboration of scientists from several fields of expertise. Models designed to help evaluate such questions are often composites of smaller, discipline-oriented models. Distributed processing techniques can be effectively used to simplify the process of linking two or more models together or of linking models to GIS and data base systems. The linked processes can then be run concurrently on a single computer, or communicate with each other across networks. The most important benefit of applying distributed processing to simulation models is likely to be the facilitation of collaboration among scientists.

ESTIMATION OF DOSE TO HUMAN TISSUES FROM INGESTION OF FOODS EXPOSED TO FALLOUT FROM NUCLEAR WEAPONS TESTS IN NEVADA

This paper provides a brief overview of the methodology developed to estimate the transport of radionuclides through agricultural ecosystems to persons of various ages, lifestyles, and geographic locations. The methodology, embodied in the computer code PATHWAY, was used to convert estimates of fallout deposition to time-dependent concentrations of radionuclides in food products, total intakes by people, and organ-specific doses from 21 radionuclides in fallout from 86 nuclear test events. A summary of model prediction uncertainties, an assessment of predictive accuracy, and a discussion of the relative importance of different exposure pathways is also provided. 14 refs.