Current evapotranspiration of soybean in the Cerrado

With a cultivated area of ​​approximately 37 million hectares, typically under rainfed conditions, and a national average productivity of 3,3 t/ha and total production of 120,9 t/ha in the 2019/2020 harvest, according to data from National Supply Company (Conab)1, Brazil is the largest soybean producer in the world. A study carried out by the company Agrosatélite and the Brazilian Association of Vegetable Oil Industries (Abiove)2 in 2020 showed that more than half of the area cultivated with soy in the country in the 2018/19 harvest was concentrated in the Cerrado, a biome that accounts for approximately 45 % of the national agricultural area, according to the Brazilian Institute of Geography and Statistics (IBGE)3.

Only 11% of Brazilian soybean crops use irrigation4. Due to the high variability of precipitation, which brings uncertainty regarding production, an increase in the area of ​​irrigated soybeans in the Brazilian Cerrado has been observed in recent years. With around 64% of the irrigated area in Brazil5, which concentrates approximately 80% of all central pivots installed in the country6, the region has faced serious problems of water scarcity in some of its main river basins. If not well planned, the growth of irrigation in the Cerrado could lead to an increase in disputes over the use of water in river basins, which already have low water availability.

In this region, which generally lacks soil, climate and water data that can support development strategies, it is important to generate information that contributes to the sustainability of irrigated agriculture. In this sense, it is also important to develop strategies to reduce the amount of water removed from sources for different uses, which can be made possible through integrated river basin planning that establishes effective strategies to increase the efficiency of different uses, especially irrigation. , which is the main user.

Any strategy that seeks to improve irrigation efficiency must prioritize management adjustments. Therefore, it is essential to improve estimates of current crop evapotranspiration (ETa), and for this it is necessary to consider the specificities of crops and regional characteristics for the development or refinement of technical coefficients, such as the average and basal crop coefficient and plant and soil water stress coefficients.

Another way to make management more efficient is by improving mathematical models developed for management. Among the existing models, the one proposed by Doorenbos and Pruitt7 which calculates the crop's potential evapotranspiration (ETc) through the relationship between the evapotranspiration of a reference crop (ETo) and a crop coefficient (Kc), due to its simplicity and ease programming and operationalization, has been the most used. However, this model does not allow the effects of transpiration and direct evaporation of soil water to be individualized.

Other approaches with great potential for application are those based on methodologies that estimate ETa through the individual calculation of transpiration and direct evaporation of soil water (Es). Es is one of the most significant components of the water balance8. However, it is very little studied. In a production system, Es losses are significant. Klocke et al.9 observed that Es represented about 30% of the evapotranspiration of a corn crop.

In irrigation management, several models, with different approaches, make it possible to estimate crop evapotranspiration (ET), separating Es from transpiration (T). Different approaches generally consider, when calculating Es, the influence of soil cover, which can be calculated using the leaf area index (LAI) or the percentage of soil covered (CC). In a study comparing Es in shaded and non-shaded environments, Raz-Yaseef et al.10 observed that Es values ​​were, on average, twice as high in uncovered areas when compared to shaded areas.

Given the significant growth of irrigated agriculture in the Brazilian Cerrado and the increase in disputes over the use of water, there is a need to think about irrigation in a more strategic way. In this context, it is essential to develop technical irrigation coefficients for new crop varieties and improve irrigation management in the Cerrado region, contributing to improving ETa estimates, especially the Es component.

One of the initiatives in this sense was the study developed by Embrapa in partnership with the Federal University of Viçosa (UFV) with the objective of improving the management of irrigation of soybeans cultivated in the Brazilian Cerrado region through the improvement of methods for estimating evaporation and current crop evapotranspiration.

In some conditions, it is possible to reduce irrigation without reducing productivity. Aiming to increase the productivity of water use, it is essential to understand, especially for new soybean varieties, to what extent the water deficit in the soil influences the characteristics of the plant and its productivity. Another point to be considered, when working with irrigated agriculture, whether with total or deficit irrigation, are the factors that can influence its growth and phenology11.

The results obtained indicated that the application of a water deficit in the soil of 80 to 100% of the available water, when compared to the application of a deficit of 0 to 20%, reduced by half, in general, the average height of the plant, the depth of the root system, the leaf area index and the maximum percentage of soil cover for winter. Both in winter and summer, even without water restrictions, it is recommended to apply a soil water deficit of 20 to 40%, without a drop in productivity. In the summer, in situations of water restriction, a soil water deficit of 60 to 80% can be imposed on soybeans, with a reduction in productivity of around 30%.

The evaluation of the performance of different mathematical concepts for estimating the current evapotranspiration of a new soybean cultivar (BRS 7581RR) subjected to different water deficit conditions indicated that, in the winter experiment, the FAO56 Dual12 model presented the best performance in estimate of the current evapotranspiration of the soybean crop for most treatments. Next, the Jensen and Heermann13 model, followed by the AquaCrop14 model, presented the best performances.

One way to estimate Es is using microlysimeters15. However, the area and height of the microlysimeter can impact the estimation of this variable. When estimating Es, depending on dryness and percentage of soil coverage, it was found that for studies that intend to measure accumulated evaporation over periods longer than five days, it is recommended to use microlysimeters 200 or 300 mm high. The models developed to calculate daily Es, during the first five days after irrigation for different percentages of coverage, showed satisfactory performance. The models developed that calculate daily Es during phase 1 of soil water evaporation16, for all treatments separately, showed satisfactory performance.

The results of the study reinforce the importance of improving the mathematical models developed for irrigation management. These models contribute to improving water use productivity, that is, they make it possible to maintain, and in some cases even increase, crop productivity, which is in line with food security policies, and reduce the demand for blue water17, in line with water security policies.

More information about the Embrapa and UFV study can be obtained in the following scientific articles:

• ALVES, ÉLVIS DA S.; Rodrigues, Lineu N.; CUNHA, FERNANDO F.; FARIAS, DIEGO B.S. Evaluation of models to estimate the actual evapotranspiration of soybean crops subjected to different water deficit conditions. ANALS OF THE BRAZILIAN ACADEMY OF SCIENCES, v. 93, p. 2-16, 2021 (DOI 10.1590/0001-3765202120201801). Available in: https://www.scielo.br/j/aabc/a/HhFVSHVgMR6FzmBnVFKjfDM/?lang=en

• ALVES, E. S.; Lineu Neiva Rodrigues; OLIVEIRA, R. A.; LORENA, D. R. Water deficit on the growth and yield of irrigated soybean in the Brazilian Cerrado region. Brazilian Journal of Agricultural and Environmental Engineering, v. 25, p. 750-757, 2021 (DOI: http://dx.doi.org/10.1590/1807-1929/agriambi.v25n11p750-757). Available in: https://www.scielo.br/j/rbeaa/a/Czt6xq9q33yBpyPDs9dpHpG/?lang=en

• ALVES, E. S.; Lineu Neiva Rodrigues. Improving the estimation of soil water evaporation based on days after wetting. Journal of Agronomy and crop science, p. 1-12, 2022 (DOI: 10.1111/jac.12614). Available in: https://onlinelibrary.wiley.com/doi/abs/10.1111/jac.12614

Notes

1 CONAB. Conab - Brazilian Grain Harvest, V. 7 - 2019/20 HARVEST. Available in: https://www.conab.gov.br/info-agro/safras/graos. Accessed on: 5 Aug. 2020.

2 AGROSATELLITE, ABIOVE. Geospatial analysis of soybean dynamics in the Cerrado biome: 2014 to 2017. Florianópolis, SC, 2018.

3 BRAZILIAN INSTITUTE OF GEOGRAPHY AND STATISTICS (IBGE). Agricultural census. Rio de Janeiro: IBGE, 2017. Available at: https://censos.ibge.gov.br/agro/2017/. Accessed on July 02nd. 2020.

4 SILVA, EHFM; GONCALVES, AO; PEREIRA, RA; FATTORI JUNIOR, IM; SOBENKO, LR; MARIN, FR Soybean irrigation requirements and canopy-atmosphere coupling in Southern Brazil. Agricultural Water Management, 218, 1-7, 2019.

5 BRAZIL. Territorial analysis for the development of irrigated agriculture in Brazil. Brasília: MI, 2014.

6 ALTHOFF, D.; RODRIGUES, L. N. The expansion of center-pivot irrigation in the Cerrado Biome. IRRIGA, vol. 1, no. 1, p. 56–61, 2019.

7 DOORENBOS, J.; PRUITT, W. O. Crop Water Requirements. FAO Irrigation and Drainage Paper 24. Rome, Italy: Food and Agriculture Organization of the United Nations, 1977.

8 RAZ-YASEEF, N.; ROTENBERG, E.; YAKIR, D. Effects of spatial variations in soil evaporation caused by tree shading on water flux partitioning in a semi-arid pine forest. Agricultural and Forest Meteorology, vol. 150, n. 3, p. 454–462, 15 Mar. 2010.

9 KLOCKE, N. L.; HEERMANN, D. F.; DUKE, H. R. Measurement of evaporation and transportation with lysimeters. Transactions of the ASAE, vol. 28, no. 1, p. 183– 0189, 1985.

10 RAZ-YASEEF, N.; ROTENBERG, E.; YAKIR, D. Effects of spatial variations in soil evaporation caused by tree shading on water flux partitioning in a semi-arid pine forest. Agricultural and Forest Meteorology, vol. 150, n. 3, p. 454–462, 15 Mar. 2010.

11 Plant development throughout its different stages: germination, emergence, growth and vegetative development, flowering, fruiting, seed formation and maturation.

12 ALLEN, R. G. et al. Crop Evapotranspiration – Guidelines for Computing Crop Water Requirements. FAO Irrigation and drainage paper 56. Rome, Italy: Food and Agriculture Organization of the United Nations, 1998.

13 JENSEN, ME; HEERMANN, DF Meteorological approaches to irrigation scheduling. 1970.

14 HSIAO, T.C.; HENG, L.; STEDUTO, P.; ROJAS‐LARA, B.; RAES, D.; FERERES, E. AquaCrop - The FAO crop model to simulate yield response to water: III. Parameterization and testing for maize. Agronomy Journal, vol. 101, no. 3, p. 448–459, 2009.

15 Structure used to measure the direct evaporation of water from the soil.

16 Phase in which the evaporation rate is governed predominantly by atmospheric demand.

17 Water from rivers and aquifers that can be used for irrigation.

By Lineu Neiva Rodrigues, researcher at Embrapa Cerrados; It is Elvis da Silva Alves, postgraduate scholarship from the Federal University of Viçosa