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Document Type

Thesis - University Access Only

Award Date


Degree Name

Master of Science (MS)

Department / School

Agricultural and Biosystems Engineering

First Advisor

Hal D. Werner


Simulations were conducted to determine the effectiveness of a software package that fully automates center pivot irrigation systems on fields planted to com, soybeans and potatoes for years 1986-2005. A total of seven sites from the Central and Northern Plains were chosen for analysis (Akron, CO, Ames, lA, Brookings, SD, Oakes, ND, Ord, NE, Rock Port, MO, and St. John, KS). System pumping capacities of 37.9, 50.5, and 63.1 liters/second were simulated at each site along with the soil available water holding capacities of 83, 125,and 167mm/meter. The different combinations of pumping capacities and soil available water holding capacities resulted in a total of nine treatments at a given site per year. The software was able to schedule irrigations effectively using the ASCE Penman Monteith equation for estimating reference evapotranspiration (ETr) throughout the growing season for all crop types simulated.
For com, the Akron, CO site had the smallest average ratio of rainfall to crop evapotranspiration (ETc) which resulted in the largest average number of days under minimum allowable soil water content per season (44 days. Treatment 7). In contrast, the Ames, Brookings, Oakes, Ord, and Rock Port sites all averaged five days or less below the minimum allowable water content for all treatments with pumping rates of 50.5 or 63.1 L/s.
A yield model was used to determine if the minimum allowable water contents were set to a correct level for com and to determine the overall effectiveness of the irrigation scheduling software. The minimum allowable balances were formed to be set to the correct levels since relative yield decreased as the number of days below the minimum allowable level increased. This is evidenced at the St. John, KS site where Treatment 1 averaged 31 days below the minimum allowable balance with an average relative yield of 85%while Treatment 6 averaged one day below the minimum allowable balance with an average relative yield of 95%.
For the soybean simulations, Treatment 7 at Akron, CO averaged the greatest number of days below the minimum allowable balance of all the treatments at all the sites (37 days). At several of the sites (Ames, Brookings, Oakes, and RockPort), the scheduling software was able to keep the average days below the minimum allowable balance for all treatments to five or less.
Due to the high setting of the minimum allowable water balance (70% of field capacity during the critical stage of development) and the shallow rooting depth, the average days below minimum allowable were greater for potatoes than com and soybeans. The highest average number of days was Treatment 1 at the Akron site (45 days) and the lowest was Treatment 9 at Ames (one day).
Simulations were conducted using the Jensen-Haise equation to estimate corn ETc at the Brookings and St. John sites. The data from these simulations were then compared to the simulations conducted at the same sites using the ASCE Penman Monteith equation. The average relative yield for all treatments was identical regardless of the method used to determine ET at the Brookings site. At St. John, the relative yield increased an average of3% when using the Jensen-Haise equation.

Library of Congress Subject Headings

Irrigation scheduling -- Computer simulation
Irrigation -- Management


Includes bibliographical references (92-97)



Number of Pages



South Dakota State University


Copyright © 1997 Jared Oswald. All rights reserved