Simulation of Organic Chemical Movement in Hawaii Soils with PRZM: 3. Calibration

his is the third and final part of a multipart paper reporting testing of the EPA's Pesticide Root Zone Model (PRZM) using data from Hawaii. PRZM is a dynamic-conceptual pesticide leaching model. In the first and second parts of the paper results were reported for predicted pesticide movement based upon preliminary PRZM simulations. In this part of the paper a trial-and-error calibration of PRZM is reported for a site in Hawaii. Performance results from the model calibration exercise are quite poor, illustrating the need for multicriteria evaluation procedures

Hyperresolution information and hyperresolution ignorance in modelling the hydrology of the land surface

There is a strong drive towards hyperresolution earth system models in order to resolve finer scales of motion in the atmosphere. The problem of obtaining more realistic representation of terrestrial fluxes of heat and water, however, is not just a problem of moving to hyperresolution grid scales. It is much more a question of a lack of knowledge about the parameterisation of processes at whatever grid scale is being used for a wider modelling problem. Hyperresolution grid scales cannot alone solve the problem of this hyperresolution ignorance. This paper discusses these issues in more detail with specific reference to land surface parameterisations and flood inundation models. The importance of making local hyperresolution model predictions available for evaluation by local stakeholders is stressed. It is expected that this will be a major driving force for improving model performance in the future. Keith BEVEN, Hannah CLOKE, Florian PAPPENBERGER, Rob LAMB, Neil HUNTE

A comparison of techniques used in rainfall-runoff models : model efficiency

A suite of three underlying rainfall-runoff modeling techniques is applied to two data sets and the results used to compare model efficiencies for selected events. Linear regression, unit hydrograph, and quasi-physically based models make up the modeling suite. The two data sets come from a 7.2 KM subwatershed (MCW) near Klingerstown, Pennsylvania and a 0.096 KM2 subwatershed (R-5) near Chickasha, Oklahoma.-Individual model efficiencies are determined on the basis of a sums of squares criterion. These efficiencies are surprisingly poor. Results indicate that the most informative independent linear regression variables for MCW and R-5 are volume of rainfall and average rainfall intensity respectively. There is a general improvement in correlation coefficients and regression model efficiencies for both MCW and R-5 with increases in the number of selected events. The unit hydrograph and quasi-physically based models exhibited predictive prowess only for the R-5 events. The unit hydrograph technique is found to be strongly dependent upon an accurate estimate of spatially-variable excess rainfall. The efficiency of the physically-based, deterministic, distributed model was found to deteriorate drastically with increases in basin size due to the lumping of spatially-variable soil hydraulic properties. Based on this work a definitively superior rainfall-runoff modeling technique is not suggested. Limitations of each of the three models and the efficiency criterion used for their evaluation are discussed. This work provides the foundation for a subsequent investigation to be carried out by the author, to determine if space-time tradeoffs exist across data sets of various rainfall-runoff modeling techniques. Future research will focus on the concept of data-worth and the question of model choice.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat

An assessment of rainfall-runoff modeling methodology

This study reports model performance calculations for three event-based rainfall-runoff models on both real and synthetic data sets. The models include a regression model, a unit hydrograph model and a quasi-physically based model. The real data sets are for small upland catchments from the Washita River Experimental Watershed, Oklahoma; the Mahantango Creek Experimental Watershed, Pennsylvania; and the Hubbard Brook Experimental Forest, New Hampshire. The synthetic data sets are generated with a stochastic-conceptual rainfall-runoff simulator. Model performance is assessed for a verification period that is carefully distinguished from the calibration period. Performance assessment was carried out both in forecasting mode and in prediction mode. The results on the real data sets show surprisingly poor forecasting efficiencies for all models on all data sets. The unit hydrograph model and the quasi-physically based model have little forecasting power; the regression model is marginally better. The performance of the models in prediction mode is better. The regression model and the unit hydrograph model showed acceptable predictive power, but the quasi-physically based model produced acceptable predictions on only one of the three catchments. The performance of the regression and unit hydrograph models, in both forecasting and prediction modes, for synthetic data is much better than for the real catchments. The performance of the quasi-physically based model on a synthetic data set is surprisingly poor. Supplemental data gathered from the Oklahoma catchment was used for a spatial variability analysis of steady-state infiltration rates. These data were then used to re-excite the quasi-physically based model; the new information resulted in improved model performance. The concept of space-time tradeoffs across the hydrologic data sets of competing models is introduced and tested. Results show the existence of space-time tradeoffs within model data sets but not across model data sets. It is the belief of the author that the primary barrier to successful application of physically based models in the field lies in the scale problems that are associated with the unmeasurable spatial variability of rainfall and soil hydraulic properties. The fact that simpler, less data intensive models provided as good or better predictions than a physically based model is food for thought. The model evaluation and space-time tradeoff experiments reported in this study are conceptually linked to data-worth analysis, network-design, and model-choice criteria for future studies.Graduate and Postdoctoral StudiesGraduat

Spatial and temporal variability in the R-5 infiltration data set: Deja vu and rainfall-runoff simulations

This paper is a continuation of the event-based rainfall-runoff model evaluation study reported by Loague and Freeze [1985]. Here we reevaluate the performance of a quasi-physically based rainfall-runoff model for three large events from the well-known R-5 catchment. Five different statistical criteria are used to quantitatively judge model performance. Temporal variability in the large R-5 infiltration data set [Loague and Gander, 1990] is filtered by working in terms of permeability. The transformed data set is reanalyzed via geostatistical methods to model the spatial distribution of permeability across the R-5 catchment. We present new estimates of the spatial distribution of infiltration that are in turn used in our rainfall-runoff simulations with the Horton rainfall-runoff model. The new rainfall-runoff simulations, complicated by reinfiltration impacts at the smaller scales of characterization, indicate that the near-surface hydrologic response of the R-5 catchment is most probably dominated by a combination of the Horton and Dunne overland flow mechanisms

Hydrologic response: Kaho'olawe, Hawaii

The results of a series of physically-based numerical simulations of hydrologic response to large rainfall events on the Kaho'olawe island in Hawaii are presented. An event-based rainfall-runoff and erosion model, called KINEROS, was applied in a geographic information system (GIS) framework, to make quantitative and distributed estimates of infiltration, Horton overland flow generation, and erosion. A digital elevation model was used to delineate individual catchments across the island, and define areas of uniform slope within each catchment. A Landsat MSS scene of the island was also used to classify it into three distinct landcover categories via the normalized difference vegetation index

Land Misuse and Hydrologic Response: Kaho'olawe, Hawai'i

DEDICATION: This paper is dedicated to "Ka'imipono" Rendell D. Tong (13 September 1959-4 January 1995). In his lifetime Rendell supported many environmental efforts in Hawai'i, especially the work reported in this paper, with a passion that was contagious. About Kaho'olawe he once wrote: "I'm looking forward to our continued work to restore Hakioawa ahupua'a [watershed] and to gain a comprehensive scientific observation and understanding of the hydrologic cycle on Kaho'olawe. We are invigorated and proud to be practicing that foundation of Hawaiian cultural values, miilama 'iiina [take care of the land]. So we keep working for the land, physically, spiritually ... for the people of the earth-e kupono e ka po'e honua." The spirit of Ka'imipono lives on in Hawai'i, especially on the island of Kaho'olawe, forever! ABSTRACT: This paper is concerned with the characterization of near-surface hydrologic response for the Hawaiian island of Kaho'olawe, where erosion caused, in part, by surface runoff is the major factor in landscape denudation. New sets of saturated hydraulic conductivity and sorptivity data from 110 sites across Kaho'olawe are presented and analyzed for spatial structure using statistical methods and land cover classification. At a regional scale there was no statistically characterizable spatial structure in either of the new data sets; we characterized the spatial distribution of saturated hydraulic conductivity and sorptivity based upon land cover. Also presented is a suite of runoff simulations for the entire island of Kaho'olawe, based upon the near-surface soil hydraulic property interpretations reported, for 10 separate rainfall events. The hydrologic response simulator used provides a relatively realistic representation of Hortonian overland flow. This study consisted of 700 deterministic-conceptual rainfall-runoff simulations, based upon the 10 rainfall events applied to 70 catchments that were divided into 1529 overland flow planes. Our simulations suggest, for the large events selected for this study, that the maximum island average surface runoff by the Horton mechanism is ca. 20% of rainfall

Simulation of Organic Chemical Movement in Hawaii Soils with PRZM: 1. Preliminary Results for Ethylene Dibromide

Leaching of agricultural chemicals to groundwater is an environmental issue of major concern in Hawaii. Fumigants used by the pineapple industry are a possible source of this contamination. In this paper we report the results of an initial evaluation of the Pesticide Root Zone Model (PRZM) for highly structured Hawaiian soils. We use PRZM to predict the transport of the soil fumigant ethylene dibromide (EDB) for two pineapple fields and compare the simulated concentration profiles with field measurements. Although preliminary, our results suggest that PRZM may be useful in the future for pesticide screening and risk assessment in Hawaii. The work reported here is part of a larger ongoing study concerned with development and application of methodology for assessing potential groundwater contamination by pesticides

Mobilization, Fate, and Transport of Iron in the Anoxic Zone of a Sewage Contaminated Aquifer

The work reported here is the first part of a larger effort focused on efficient numerical simulation of redox zone development in contaminated aquifers. The sequential use of various electron acceptors, which is governed by the energy yield of each reaction, gives rise to redox zones. The large difference in energy yields between the various redox reactions leads to systems of equations that are extremely ill-conditioned. These equations are very difficult to solve, especially in the context of coupled fluid flow, solute transport, and geochemical simulations. We have developed a general, rational method to solve such systems where we focus on the dominant reactions, compartmentalizing them, in a manner that is analogous to the redox zones that are often observed in the field. The compartmentalized approach allows us to easily solve a complex geochemical system as a function of time and energy yield, laying the foundation for our ongoing work in which we couple the reaction network, for the development of redox zones, to a model of subsurface fluid flow and solute transport. Our method (i) solves the numerical system without evoking a redox parameter, (ii) improves the numerical stability of redox systems by choosing which compartment, and thus which reaction network, to use based upon the concentration ratios of key constituents, (iii) simulates the development of redox zones as a function of time without the use of inhibition factors or switching functions, and (iv) can reduce the number of transport equations that need to be solved in space and time. We show, through the use of various model performance evaluation statistics, that the appropriate compartment choice under different geochemical conditions leads to numerical solutions without significant error. The compartmentalized approach described here facilitates the next phase of this effort where we couple the redox zone reaction network to models of fluid flow and solute transport