Radiation Balance over Low-Turbidity Water Artificially Cleaned for Irrigation of Tobacco Grown Under Shading. I. Analysis of the Shortwave and Longwave Components

Abstract

Abstract In eastern sub-humid Bahia, Brazil, surface water stored in lined ponds is used to drip irrigate Sumatra tobacco plants grown under partial shading. This work, as part of a large evaporation study, aimed at monitoring, over two irrigation seasons (2015 and 2016), the water turbidity, the radiation balance (shortwave - SW and longwave - LW components), and the skin water temperature (Tw) with sensors deployed in a raft. The water turbidity was very low (around 2.6 NTU on average) due to filtration and reverse osmosis treatment of water prior to storage. Considering data from both seasons, the net SW radiation Snet was about 96% of incoming SW flux Sg. The daily net LW flux Lnet was consistently negative (-55.1 W/m2 average). The net all-wave radiation Rn corresponded to 77% of Snet. Four days were selected from both seasons to evaluate the effects of cloudiness on the SW and LW components. While Snet decreased with cloudiness due to solar radiation blockage, Lnet also decreased but due to increasing of downward LW radiation Latm with cloudiness. Rn also decreased with cloud cover. Based on the 2015 data, on average, daily Tw (26.1 °C) was higher than air temperature Ta (21.8 °C) due to near-surface absorption of solar radiation with Ta showing a much higher daily amplitude.