Characterization of Nighttime Evapotranspiration and Other Surface Energy Fluxes and Interactions with Microclimatic Variables in Subsurface Drip and Center-Pivot Irrigated Soybean Fields
The lack of knowledge and data on the driving forces of nighttime (nocturnal) evapotranspiration (ET) for various vegetation surfaces under different climatic and management conditions led this study to investigate the magnitude of nighttime ET (ETnight) and its interactions with other nighttime surface energy fluxes, i.e., soil heat flux (Gnight), sensible heat flux (Hnight), and net radiation (Rn _ night), and microclimatic variables, i.e., wind speed at 3 m (u3 _ night), vapor pressure deficit (VPDnight), and air temperature (Tnight). Soybean [(G lycine max (L.) Merr.] canopies under two different irrigation methods in subsurface drip- and center-pivot irrigated (SDI and CP) fields in south central Nebraska were studied. Hourly energy flux and meteorological data from the SDI field for the 2007 and 2008 seasons and from the CP field for 2008 were analyzed. The study period was divided into five sub-periods based on plant and canopy development to evaluate nighttime energy balances and driving forces at various plant growth and development stages. The five sub-periods are: pre-planting (from mid-March to plant emergence, EM), early season (from emergence to full canopy cover, leaf area index, LAI 3, MS), late season (full canopy cover to harvest, LAI < 3, LS), and bare soil (from harvest to mid-November, PH). In 2007, seven nights had greater than 0.50 mm per night of ET, and there were 13 such nights in 2008. Daily ETnight was a maximum of 35% (0.42 mm night-1) and 55% (0.48 mm night-1) of daily total ET (sum of hourly (ETSOH) in 2007 and 2008, respectively. The maximum ETnight usually occurred on windy nights. During LS in 2007, ETnight averaged 5% of ETSOH. In 2008, ETnight was greatest (0.17 mm night-1) during ES; ETnight rates were also high (0.11 mm night-1) during EM. During these two periods, ETnight accounted for 4% and 6% of ETSOH). The variables that had the most impact in driving the ETnight varied with the stage of growth. For example, in 2008, during EM when ETnight averaged over 6% of daily ETSOH at 0.11 mm night-1, ETnight was significantly (p < 0.05) correlated to u3 _ night. ETnight was significantly correlated to Hnight in ES when ETnight was about 4% of ETSOH at 0.17 mm night-1 and during MS when ETnight was at a minimum. However, ETnight was not significantly correlated to any of the microclimate variables during LS when ETnight was -3% of ETSOH at -0.04 mm night-1. ETnight was significantly correlated to u3 _ night and Rn _ night during PH when ETnight was 2% of ETSOH at 0.03 mm night-1. ETnight values at the SDI and CP fields were typically within 0.20 mm night-1 of each other, with ETnight at the CP field averaging 0.03 mm night-1 greater. Maximum ETnight was about the same at the two fields (1.72 and 1.92 mm for SDI and CP) and occurred on the same day. However, negative ET fluxes occurred more frequently and with greater magnitude at the SDI field. The contributions of H and G to ETnight at the CP field were similar to those at the SDI field. While nighttime fluxes away from the canopy were generally greater at CP, the similarity of ETnight at the SDI and CP fields (particularly the maximum ETnight occurring on the same day) suggests that ETnight is highly a function of regional weather conditions, while microclimate conditions contribute to variations in magnitude.