Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Agricultural and Biosystems Engineering

First Advisor

R. S. Kanwar

Second Advisor

T. A. Austin


This research was designed with the goal of simulating tillage effects on quantity and quality of subsurface drain flows. For this purpose, fluctuating water table and subsurface drain water flow components were incorporated into an existing solute transport model (Root Zone Water Quality Model-RZWQM). The modified RZWQM model was further extended to simulate nitrate-nitrogen (NO[subscript]3-N) concentrations in subsurface drainage water. Different tillage systems were characterized in terms of soil physical and hydraulic properties, and this information was used as input to the model. Subsurface drain flows and NO[subscript]3-N concentrations in the drainage water were simulated under different tillage systems for three consecutive years 1990, 1991, and 1992. Simulated results were compared with the observed subsurface drain flows and NO[subscript]3-N concentrations in drainage water to evaluate the performance of the modified RZWQM;Comparison of simulated and field-measured tile flows showed that the model is capable of simulating tillage effects on the subsurface drain flows and NO[subscript]3-N losses with subsurface drain flows. The model usually showed a good response to rainfall and consistent trends between simulated and observed tile flows for all three years. However, the model usually underpredicted peak tile flows. Discrepancies between simulated and observed data were attributed to inaccuracies in the rainfall intensity data, lack of a lateral groundwater flow and/or deep seepage components, and unaccounted for spatial variabilities in the physical and hydraulic properties of the soil;Similarly, simulated and observed NO[subscript]3-N concentrations in the drain water and NO[subscript]3-N losses with the subsurface drain flow usually agreed well. However, a comparison of simulated and observed NO[subscript]3-N concentrations in the soil profile showed a need for improvement in the model. Calibration and validation of NO[subscript]3-N transformation processes under different tillage systems, validated plant growth component, and an improved hydrologic component should improve the predictions of various model outputs.



Digital Repository @ Iowa State University,

Copyright Owner

Piyush Singh



Proquest ID


File Format


File Size

180 pages