Graduate Studies


First Advisor

Dr. Suat Irmak

Date of this Version



Hinn, T. 2020, 'Soybean Productivity, Soil-Water, Evapotranspiration and Water Use Efficiency Response to Variable and Fixed (Uniform) Rate Irrigation Management', MS thesis, University of Nebraska, Lincoln.


A THESIS Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Master of Science, Major: Agricultural and Biological Systems Engineering, Under the Supervision of Professor Suat Irmak. Lincoln, Nebraska: May, 2020

Copyright 2020 Trevor Alan Hinn


Irrigation, which is pertinent for current crop production, is the largest single consumer of freshwater. Awareness of freshwater use is becoming more important as allocation needs increase. Fixed rate irrigation (FRI), is the uniform application depth of irrigation water over cropping fields and is susceptible to inefficient water use. Limited irrigation and variable rate irrigation (VRI) have been introduced in efforts to use less water, meeting sufficient returns in yields, ultimately maximizing water use efficiency. VRI research has been nearly focused on management zone delineation as well as temporal and spatial irrigation strategies, however, conclusive evidence for VRI’s water use and return on investment has yet to be illustrated. Furthermore, VRI’s success comparative to deficit and fixed rate irrigation strategies has yet to be determined in variable soil conditions. To determine these effects, this research investigates soybean growth, yield, productivity, evapotranspiration and soil-water dynamics for three irrigation treatments in three different soil types [Crete silt loam (soil 1 or S1), Hastings silty clay loam (soil 2 or S2) and Hastings silt loam (soil 3 or S3] in the Irmak Research Laboratory field research facilities at the South Central Agriculture Lab (SCAL) in Clay County, Nebraska. Three irrigation treatments were imposed: FRI, VRI and limited FRI in each soil type. The 2018 and 2019 growing seasons, in which this research was conducted, had 591.7 mm and 617.8 mm of precipitation. In general, there was not enough evidence to suggest the superiority of VRI management over FRI, especially limited FRI management, in terms of grain yield or water use efficiency. Soil type explained variation in yield with statistical significance and irrigation treatments did not have significant impact on grain yield. Little distinction exists for LAI among irrigation treatments in both growing seasons. However, there were substantial variations in LAI values among the three soil types. S1 had the maximum LAI and the largest mean for both growing seasons. Plant height for the 2018 growing season had little variance among irrigation treatment and soil type. Similar to the LAI data, variation in 2019 plant heights was a stronger function of soil types than irrigation treatments.

Volumetric water content (VWC) varied among soil types for the same irrigation treatments as well as within soil types between the treatments. Soil type had the largest impact on soil moisture. Evidence of irrigation treatment effects and soybean water use were more apparent in 2019 and were more pronounced in the first and second layers. Mean VWC for the two growing seasons both showed more dependence on soil type and groupings of similar mean VWC in soil types are expected to be similar groupings of soil physical properties. The two growing seasons varied in which soil layer exhibited the largest readings. 2018 showed most variability and lowest mean VWC in the first and third layers while the second layer exhibited the lowest mean VWC and most variability for the 2019 growing season.

For the 2018 growing season, yield comparison for soil types resulted with S1 having had the largest average yield at 5.59 tonnes/ha, S3 had the second largest average at 5.23 tonnes/ha, and the lowest average was in S2 at 5.14 tonnes/ha. Yield among irrigation treatments resulted with FRI 1” having the highest average yield of 5.45 tonnes/ha, VRI had the second highest at 5.29 tonnes/ha, and FRI 0.75” was the lowest with 5.23 tonnes/ha for the growing season. There was less uniformity among soil type and treatment types from visual analysis of the 2019 growing season. The largest average yield for the season was again, in S1, with an average of 4.35 tonnes/ha. S2 had the second largest average yield at 4.22 tonnes/ha, and S3 was the lowest at 3.63 tonnes/ha. Irrigation treatments, however, were more uniform with VRI and FRI 0.75” having the same average yield at 4.18 tonnes/ha, as FRI 1” average yield was 4.02 tonnes/ha.

Crop productivity through ETc and crop water use efficiency did not demonstrate any indication of difference among irrigation treatment or soil type. Irrigation depths for VRI and deficit irrigation treatments were less than those for FRI in the 2018 growing season, without statistical differences in yield. As it was possible to demonstrate increased efficiency by the two irrigation strategies, heavy precipitation during the two growing seasons makes it difficult to provide sole evidence for irrigation treatments. The 2019 growing season provides further evidence of this with FRI receiving no irrigation and having non-statistically different yields.

Advisor: Suat Irmak