Rice (Oryza sativa L.) constitutes the staple food of about half of the world population. About 150 million hectares are grown annually with rice worldwide, with a total production of about 590 million tons. Nearly 75% of the rice production comes from flooded irrigated rice paddies. Brazil is among the top ten rice producers and is the largest one outside Asia, producing about 12 million tons of rice annually in 2.4 million ha (Conab 2013).
In Brazil, 60% of the rice is produced in about one million hectares (ha) of lowlands in the Rio Grande do Sul State. This region has a subtropical climate and is in the southernmost portion of the country (about 30º to 32º S latitude x 51º to 57º W longitude). A typical agronomic timeline consists in preparing the soil conventionally by tilling the dry soil (dry tillage) for good seedbed preparation in spring (October - November); sowing the “dry” (aerobic) soil in November and flood irrigating with a continuous 5 - 10 cm water layer above soil surface, starting at the onset of tillering (about 30 days after sowing), up to about 15 days before harvest in early fall (March - April). Therefore, most of the growing season which represents about 80% of the total time, takes place under anaerobic water saturated soil conditions (Streck et al. 2011). From around April to October, these lowland areas are kept as fallow, the soil is water-saturated (anaerobic) most of the time and crop residue is either soil-incorporated (tillage) or kept as mulch (no tillage). Such rice production timeline is different from those in most of Asia where rice is produced during two growing seasons along one calendar year (Tsai et al. 2007, Alberto et al. 2011).
A lowland flooded irrigated rice ecosystem differs greatly from any other upland non-irrigated or irrigated crop ecosystem because of the continuous water layer above the soil surface, which strongly affects the surface energy balance components (Tsai et al. 2007, Maruyama and Kuwagata 2010, Alberto et al. 2011, Hatala et al. 2012, Hossen et al. 2012). Furthermore, rice paddies are usually part of a larger area of low lands within a river basin, creating a microclimate with large water availability for evapotranspiration (ET), which increases the atmospheric specific humidity near the surface. Therefore, understanding and describing the energy partitioning variability over flooded rice paddies is essential for a good performance of surface-atmosphere interaction models in lowlands, especially considering that agricultural irrigated areas are usually not represented in such models (Noilhan and Planton 1989, Foley et al. 1996, Walko et al. 2000, Ek et al. 2003).
Information on ET is also important for better understanding the hydrological and biogeochemical cycles as well as the surface energy balance in agricultural and other ecosystems. Evapotranspiration is typically the largest component of the water balance in agricultural areas (Suyker and Verma 2008). ...