Document Type

Thesis - Open Access

Award Date


Degree Name

Master of Science (MS)

Department / School

Agricultural and Biosystems Engineering

First Advisor

Erin Cortus

Second Advisor

Todd P. Trooien


CN Model, runoff, runoff modeling, SWAT Model


The capability to be able to consistently and accurately model any problem has potential time and money savings. The present study aimed to determine if the Natural Resources Conservation Service’s Curve Number (CN) model or the more detailed Soil and Water Assessment Tool (SWAT) Model can consistently and accurately model runoff events from small agricultural fields in Eastern South Dakota. The overall goal was to better understand models used to predict runoff and determine if they can produce accurate estimates of runoff from the watersheds being studied. Runoff measurements were collected from an agricultural field located south of Coleman, South Dakota. The field of study was under a conventional tillage, two-year corn-soybean rotation. Three distinct watersheds make up the farm field with one flume at the lowest point for each watershed. Four years of collected rain data at the site (2013- 2016) along with two years of runoff data (2015-2016) were used for modeling and subsequent statistical comparison. This study examined the collected runoff totals for rain events and evaluated the accuracy and precision of model predictions to the observed runoff totals. For the two years studied, almost all predicted runoff events using the CN method were higher than the measured runoff totals. On average, the CN model over-predicted runoff totals by 300%. Adjustments to three areas of the CN model (initial abstraction ratio, curve number, and watershed size) improved comparison statistics. However, none of the changes made to the CN model produced satisfactory statistical results. The 2015 and 2016 runoff events were then compared to the SWAT model predictions. The first model run was with SWAT’s recommended settings and produced higher runoff than was observed. Adjustments to three areas of the SWAT model (curve number (CN2), potential evapotranspiration method (IPET), and daily curve number calculation method (ICN)) were combined to improve comparison statistics. The SWAT model using Penman-Monteith method + Potential Evapotranspiration method + 10% decrease in CN2 produced the closest approximation to measured values relative to all models, including the CN model. This variation of the SWAT model did not produce a consistent and accurate model for all three watersheds, but it did produce partially acceptable results for the largest watershed. Neither the CN nor SWAT produced a model that was statistically acceptable at predicting runoff for the studied agricultural watersheds. For further research it was decided to focus on changes to the SWAT model. Given this, the next steps would be: produce fewer outlets during the startup of the SWAT model, test the three watersheds as one large watershed in SWAT, and/or take soil samples at the testing site to determine if the soil is more hydrophilic than modeled.

Library of Congress Subject Headings

Runoff -- South Dakota -- Mathematical models.
Agricultural pollution -- South Dakota -- Mathematical models.
Watersheds -- South Dakota.
Hydrologic models -- Evaluation.



Number of Pages



South Dakota State University



Rights Statement

In Copyright