Soil-water dynamics, evapotranspiration, and crop coefficients of cover-crop mixtures in seed maize cover-crop rotation fields. I: Soil-water dynamics and evapotranspiration

Cover crops are incorporated into row crop production systems as rotational crops because of their potential contributions to soil and water conservation. However, extremely limited data and information exist in the literature in terms of their water use (i.e., evapotranspiration) rates, especially in the midwestern United States. The primary objective of this research study was to investigate the impact of cover-crop mixtures on soil-water dynamics and measure, analyze, and compare the magnitude and trend of weekly, monthly, and seasonal evapotranspiration (ET) rates from different surface covers, including (1) mixture of cover crops (CC) without maize residue, (2) bare soil, (3) cover crops with maize residue (SCCC), and (4) only seed maize residue (SC) without cover crops. Extensive field research was conducted in three cover-crop growing seasons (2012-2013, 2013-2014, and 2014-2015) in three center pivot-irrigated large-scale seed maize cover-crop rotation production fields (F1, F2, and F3) near Beaver Crossing, Nebraska. The ET_a during 2012-2013 (dry year) was lower in the CC treatment than bare soil and was significantly higher (P < 0.05) in the 2013-2014 and 2014-2015 growing seasons (above average years). In 2013-2014 and 2014-2015, SCCC ET_a was significantly lower (P < 0.05) than those in the CC only treatment. Cumulative ET_a for CC only and SCCC treatments for the 2012-2013 cover-crop growing season was 135 and 142 mm, respectively, while it was 267 and 237 mm, respectively, in the 2013-2014 season, and 417 and 381 mm, respectively, in the 2014-2015. The SCCC and SC only treatments were compared to quantify the potential impact of cover crops on water use. While there were cases when SCCC treatment had lower ET_a values than SC only treatment, when the values from all three fields (F1, F2, and F3) are averaged, results indicated that cover crops did not have positive or negative impact on reducing or increasing evaporative losses when incorporated as a rotational crop in the soil, climate, and management conditions presented in this research study. For example, in the 2012-2013 season the three field average ET_a values for the SCCC and SC only were essentially the same (142 and 145 mm, respectively); in the 2013-2014 season they were exactly the same (237 mm); and in the 2014-2015 season they were 381 and 367 mm, respectively [SCCC treatment had significantly greater ET_a (14 mm) than the SC only treatment], indicating that the cover crops did not have impact in reducing evaporative losses. Substantial interseasonal variations were observed in terms of impact of the same surface covers on ET_a in the field. Soil-water storage (SWS) was significantly lower (P < 0.05) in CC treatment than the bare soil during the 2013-2014 and 2014-2015 seasons and significantly higher in 2012-2013. Surface cover's influence on SWS for the next season's crop and ET rates in a maize cover-crop rotation system can vary substantially, depending on the current year's climatic conditions, especially air temperature (influence on evaporative losses), precipitation (influence on soil-water availability), growth, and water use of the cover crops.