Relative Evaporative Losses and Water Balance in Subsurface Drip and Center Pivot–Irrigated Soybean Fields

This research evaluated the relative evaporative losses and water balance components in two soybean [Glycine max (L.) Merr.] fields under subsurface drip irrigation (SDI) and center pivot irrigation (CPI) systems in south-central Nebraska. Meteorological and surface energy balance components, including actual evapotranspiration (ET), above the crop canopy was measured using Bowen ratio energy balance systems installed at the center of both fields. Crop transpiration ($T$) was estimated based on the variable stomatal resistances using the Penman-Monteith equation in conjunction with fractional green canopy cover. Evaporation ($E$) losses were estimated as the difference between measured ET and estimated $T$. Average soil water content (ASWC) in the crop root zone and effective rainfall were estimated using the water balance method. The relative evapotranspiration (${\mathrm{ET}}_{\text{rel}}$) was 99.8% for the SDI field in both years (2007 and 2008), and it was 103.4% in 2008 and 93.9% in 2010 for the CPI field. The mean ${\mathrm{ET}}_{\text{rel}}$ between sites and across years were not significantly different ($P>0.05$). There were revealing differences in the relative contribution of $T$ and $E$ to total seasonal ET between the SDI and CPI fields. In the SDI field, $T$ and $E$ were 88 and 12% of ET, respectively, in 2007, and in 2008 they were 84 and 16% of ET, respectively. In the CPI field, $T$ and $E$ were 78 and 22% of ET, respectively, in 2008, and in 2010 they were 75 and 25% of ET, respectively. At full canopy cover, $T$ contributed more than 85% of the total ET. Although $E$ is assumed negligible at the full canopy cover stages, the data indicated that $E$ was 6 and 9% of ET in SDI fields during the midseason growth stage at full canopy cover in 2007 and 2008, respectively, and 14 and 21% of ET in CPI fields during the same growth stage in 2008 and 2010, respectively. On a 2-year average, the CPI field had approximately 10% higher $E$ losses as compared with the SDI field under these experimental conditions, which can have a large impact on crop water productivity when crop production under water-limiting conditions is considered. The reduction in $E$ in the SDI field can be expected to be greater in arid and semiarid regions than in subhumid regions.