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

Balanço de Radiação sobre água de Baixa Turbidez Purificada Artificialmente para Irrigação de Fumo Cultivado sob Sombreamento. I. Análise dos Components de Onda Curta e Onda Longa

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 (T_w) 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 S_net was about 96% of incoming SW flux S_g. The daily net LW flux L_net was consistently negative (-55.1 W/m² average). The net all-wave radiation R_n corresponded to 77% of S_net. Four days were selected from both seasons to evaluate the effects of cloudiness on the SW and LW components. While S_net decreased with cloudiness due to solar radiation blockage, L_net also decreased but due to increasing of downward LW radiation L_atm with cloudiness. R_n also decreased with cloud cover. Based on the 2015 data, on average, daily T_w (26.1 °C) was higher than air temperature T_a (21.8 °C) due to near-surface absorption of solar radiation with T_a showing a much higher daily amplitude.

Keywords
solar radiation; atmospheric radiation; water turbidity; tobacco