Document Type


Date of Degree

Summer 2015

Degree Name

PhD (Doctor of Philosophy)

Degree In

Civil and Environmental Engineering

First Advisor

Jerald L. Schnoor


A Soil and Water Assessment Tool (SWAT) model was developed for the Iowa-Cedar River Basin (ICRB), a 32,660 km2 watershed dominated by agricultural land cover (∼70%) to simulate hydrology and water quality for the prediction of stream discharge, nitrate loads, and nitrate concentration under climate and land use change scenarios. Iowa exports as much as 20% of the nitrogen entering the Gulf of Mexico at the mouth of the Mississippi, contributing to Gulf hypoxia as well as local threats to water quality in the ICRB. The model utilized a combined autocalibration and sensitivity procedure incorporating Sequential Uncertainty Fitting (SUFI) and generalized additive models. This procedure resulted in Nash-Sutcliffe Efficiency (NSE) goodness-of-fit statistics that met literature guidelines for monthly mean stream discharge (NSE≥0.60) and daily nitrate load (NSE≥0.50). Artificial neural networks coupled with SWAT stream discharges aided in the simulation of daily mean nitrate concentrations that met the literature guideline (NSE≥0.50).

The North American Regional Climate Change Assessment Program (NARCCAP) provided an ensemble of 11 climate change scenarios. NARCCAP is a multi-institutional effort to simulate climate change at the mesoscale by downscaling global circulation models (GCM) with regional climate models (RCM). The resulting GCM-RCM produced synthetic precipitation and temperature time-series that drove the SWAT simulations and scenarios. The land use scenarios were a collaboration with the U.S. Army Corps of Engineers, using a rule-based GIS method to generate scenarios that (1) maximized agricultural productivity, (2) improved water quality and reduced flooding, and (3) enhanced local biodiversity. The SWAT simulations and ensemble climate change scenarios resulted in a warmer and wetter climate with greater and more extreme discharge in all seasons except summer where the models indicate a somewhat higher probability of extreme low flows (p-value<0.05). The land use scenarios for SWAT showed that nitrate load and discharge positively and linearly scale with percent of agricultural land area (p-value<0.05).

Public Abstract

Climate change manifests not merely as global warming; the latter term only starts to describe the impact of human activities on the planet. Extreme floods and droughts are another facet of climate change that are becoming accepted in the scientific discourse as not just independent events, but part of a growing trend. Naturally, different locales will weather different effects under climate change and how those effects emerge depend on how humans shape and use their land. Iowa has a fully developed agrarian economy infamous for releasing large amounts of nutrients into the waters of the United States. Nitrate, a major component and end-product of fertilizer application, is a threat to local water supplies, increasing the cost of drinking water treatment as the source water becomes more contaminated. Nitrate exiting Iowa eventually enters the Gulf of Mexico, accelerating algal blooms that subsequently suffocate the coastal waters deprived of oxygen. This research projects the future by building a computational model capable of simulating water quantity and water quality on the order of decades through the use of the Soil and Water Assessment Tool (SWAT). The model developed succeeded at simulating the past, a test that all models must pass. Applying scenarios created by leading institutions that study climate and land use change, the model foresees the possibility of greater flooding events and nitrate loads in a wetter and warmer future. No prediction is so exact to give the time and place of such events, but what SWAT can give is a sense of the average. An increase in the average implies an increase in the larger values of greater discharge and nitrate runoff.


publicabstract, climate change, iowa, land use change, narccap, nitrate, swat


xiv, 211




Copyright 2015 Lance Olot Le