Evapotranspiration mapping of commercial corn fields in Brazil using SAFER algorithm

ABSTRACT SAFER (Simple Algorithm for Evapotranspiration Retrieving) is a relatively new algorithm applied successfully to estimate actual crop evapotranspiration (ET) at different spatial scales of different crops in Brazil. However, its use for monitoring irrigated crops is scarce and needs further investigation. This study assessed the performance of SAFER to estimate ET of irrigated corn in a Brazilian semiarid region. The study was conducted in São Desidério, Bahia State, Brazil, in corn-cropped areas in no-tillage systems and irrigated by central pivots. SAFER algorithm with original regression coefficients (a = 1.8 and b = –0.008) was initially tested during the growing seasons of 2014, 2015, and 2016. SAFER performed very poorly for estimating corn ET, with RMSD values greater than 1.18 mm d –1 for 12 fields analyzed and NSE values < 0 in most fields. To improve estimates, SAFER regression coefficients were calibrated (using 2014 and 2015 data) and validated with 2016 data, with the resulting coefficients a and b equal to 0.32 and –0.0013, respectively. SAFER performed well for ET estimation after calibration, with r 2 and NSE values equal to 0.91 and RMSD = 0.469 mm d –1 . SAFER also showed good performance (r 2 = 0.86) after validation, with the lowest RMSD (0.58 mm d –1 ) values for the set of 14 center pivots in this growing season. The results support the use of calibrated SAFER algorithm as a tool for estimating water consumption in irrigated corn fields in semiarid conditions.

摘要：作物实际蒸散量反演简单算法（Simple Algorithm for Evapotranspiration Retrieving，SAFER）是一种相对新颖的算法，已成功应用于巴西不同作物、不同空间尺度下的实际作物蒸散量（Evapotranspiration，ET）估算。不过，该算法在灌溉作物监测领域的应用尚属少见，有待进一步研究。本研究针对巴西半干旱区域的灌溉玉米蒸散量估算任务，评估了SAFER算法的应用性能。试验于巴西巴伊亚州圣德塞德里奥的免耕灌溉玉米田开展，灌溉方式采用中心支轴式喷灌系统。研究初始采用带有原始回归系数（a=1.8，b=–0.008）的SAFER算法，对2014、2015及2016年的玉米生长季进行测试。结果显示，该算法对玉米蒸散量的估算效果极差：12个分析田块的均方根偏差（Root Mean Square Deviation，RMSD）均大于1.18 mm·d⁻¹，且多数田块的纳什效率系数（Nash-Sutcliffe Efficiency，NSE）小于0。为优化蒸散量估算效果，研究利用2014和2015年的数据对SAFER算法的回归系数进行校准，并以2016年的数据进行验证，最终得到校准后的系数a=0.32、b=–0.0013。校准后的SAFER算法在蒸散量估算中表现优异，决定系数（coefficient of determination，r²）达0.91，均方根偏差为0.469 mm·d⁻¹。经验证后，该算法仍保持良好性能，决定系数为0.86，且在该生长季的14个中心支轴喷灌田块中取得了最低的均方根偏差（0.58 mm·d⁻¹）。本研究结果证实，经校准的SAFER算法可作为半干旱条件下灌溉玉米田水分消耗量估算的有效工具。