Nocturnal transpiration in field crops: Implications for temporal aggregation and diurnal weighing of vapor pressure deficit

Abstract

Daytime transpiration is driven by evaporative demand (vapor pressure deficit, VPD) and radiative energy, dominantly accounting for daily transpiration (T). Recent modeling approaches recommend weighing VPD for diurnal cycle of T using radiation to predict T, as opposed to using daily mean VPD. This proposition requires field evaluation so that diurnal patterns of water use are represented fairly. In this research, hourly T was measured concurrently for irrigated maize, sorghum, and soybean in a dry sub-humid climate. Various VPD conditioning approaches available to the user that differed in measurement frequency, spectral sampling, and weighing strategies were evaluated to explain variance in T. The locally derived radiation-based weighing coefficient (Frac) showed substantial variability with mean estimates of 0.61 (annual) and 0.58 (growing season), which were lower than the classically recommended Tanner-Sinclair coefficient (0.75). The use of a constant Frac value for weighing VPD was suboptimal to dynamic weighing of VPD for each day, accounting for day-to-day variability. This is primarily because of considerable nighttime water use: Tn demonstrated values that were 6–16% of T across the three crops. Tn was driven by wind speed and VPD during nighttime hours. These field observations of Tn hold implications for (un)suitability of radiation-based weighing procedures, which assume no Tn. Diurnally weighted VPD was suboptimal to using daily mean VPD, as the former does not account for dark period-VPD. Additionally, reference evapotranspiration (ETo), the standardized metric of crop water use was either negative or zero for nocturnal periods when Tn ≠ 0, and thus, failed to appropriately represent Tn. A variable of transpiration, based on daily mean VPD and total daily radiation, was the most effective in explaining T variance in all crops. With increasing asymmetry between daytime and nighttime warming and aridity, it becomes increasingly important to appropriately precondition VPD data to predict T. Caution should be exercised while using radiation-weighted VPD in crops, environments and seasons with non-negligible Tn.