Evapotranspiration and crop coefficient for sprinkler-irrigated cotton crop in Apodi Plateau semiarid lands of Brazil

During the twentieth century, the cotton crop was the main agricultural product in the semiarid regions of Brazil, with over 3.2 million hectares planted. However, due to structural problems, this activity became uncompetitive and economically unfeasible, being virtually wiped out in the eighties. The revival of cotton growing in semiarid lands of Brazil is important to the regional economy. However, the adoptions of new technologies mainly related to the water use efficiency are needed. Thus, accurate ETc estimates are required for efficient irrigation management. The Kc method is a practical and reliable technique for estimating ETc, and has been vastly applied by the farmers in the semiarid lands of Brazil. However, the use of Kc values listed in FAO-56 can contribute to ETc estimates that are substantially different from actual ETc. Hence the importance of determining Kc values experimentally. A field study on sprinkler-irrigated cotton was carried out during the dry seasons of 2008 and 2009 years in the Apodi Plateau, Brazilian semiarid lands. This study aims to determine ETc and the Kc curve values using the Bowen Ratio Energy Balance (BREB) technique. The locally developed Kc curves are compared with generalized FAO Kc values adjusted for local climate and management. The ETc values were 716 mm and 754 mm in 2008 and 2009, respectively. These values were higher than those observed in other areas of Brazilian semiarid. These differences are attributed to weather heterogeneity in the region. The average of Kc values were 0.75, 1.09 and 0.80 for initial, middle and end, of growing season, respectively. These values were lower than the Kc-FAO-Adjusted to local conditions. For this reason, ETc values obtained from Kc-FAO-Adjusted were overestimated by 12% in both the years. The irrigation scheduling based on the Kc-FAO-Adjusted increases production cost and yield loss.

► A field study on irrigated cotton was carried out to obtain evapotranspiration and crop coefficient values.
► The accumulated crop evapotranspiration was 716 and 754 mm for 2008 and 2009 years, respectively.
► The crop coefficient values were 0.75, 1.09 and 0.80 for initial, middle and end, of growth season, respectively.
► Crop evapotranspiration values obtained from FAO crop coefficient adjusted to local conditions were overestimated.
► The irrigation scheduling based on the FAO crop coefficient would require about 10 h of additional irrigation.