Graduate Studies


Date of this Version




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

July 2014


Cover crops are incorporated into the cropping system rotation because of their contributions to soil and water conservation. However, extremely limited data and information exist in the literature in terms of their water use rates and crop coefficients. The main objectives of this research were to (a) investigate the impact of cover crop mixtures on soil water dynamics, (b) measure, document and compare the magnitude and trend of weekly, monthly and seasonal evapotranspiration rates from different land covers including mixture of cover crops (CC), bare soil, cover crops in seed maize residue (SCCC) and seed maize residue (SC), and (c) develop single and dual grass-reference (Kco and Kcbo) and alfalfa-reference (Kcr and Kcbr) crop coefficients for cover crop mixes. This is a long-term research which is a part of the Nebraska Water and Energy Flux Measurement, Modeling and Research Network (NEBFLUX; Irmak, 2010). The research is being conducted for two cover crop growing seasons (2012-2013 and 2013-2014) on three center pivot-irrigated seed maize-cover crop rotation fields (F1, F2 and F3) near Beaver Crossing, Nebraska, U.S.A. All associated microclimatic variables, above the canopy were measured using a Bowen ration energy balance systems (BREBS) installed in field F1. Soil water balance approach was used to measure the actual evapotranspiration rates (ETa) from different land cover treatments. Grass and alfalfa reference (potential) evapotranspiration (ETo and ETr) respectively, were calculated using the Penman-Monteith method with a fixed canopy resistance. Results for 2012-2013 and 2013-2014 cover crop growing season, indicate the high values of cover crop ETa at the beginning and end of the season, while the low values were observed during December and January when an ice and/or snow-covered surface was present. The ETa during 2012-2013 was significantly lower in CC treatments than bare soil and was significantly higher in 2013-2014 growing season. The cumulative ETa for CC and SCCC treatments for 2012 – 2013 cover crop growing season was 187 mm and 178 mm, respectively and 304 and 276 mm, respectively in 2013-2014. Soil water storage (SWS) was significantly higher in CC treatment than bare soil during the whole 2012-2013 cover crop growing season however, significantly lower in 2013-2014 growing season. Kco values for CC treatment ranged from 0.01 to 1.8 while Kcr values ranged from 0.01 to 1.4. Kcbo values for CC treatment ranged from 0.03 to 1.7 while Kcbr values ranged from 0.01 to 1.2. The results indicate that soil water availability for future crop season is dependent on cover crop water consumption during the growing season, which is a strong function of climatic conditions.