The FAO-56 two-step approach (K_c-ET_o) is commonly used to estimate crop evapotranspiration (ET_c) for various crops and climate conditions. This approach consists of the product of a specific crop coefficient (K_c) and the grass-reference evapotranspiration (ET_o). Crop coefficients were adjusted to regional climate conditions aiming at incorporating crop-specific impacts of relative humidity and wind speed on K_c in addition to climate impacts already represented in ET_o. In the current study, crop water use and actual crop evapotranspiration and transpiration (ET_{c act} and T_{c act}) were studied for four soybean maturity groups under irrigated and rainfed conditions. Field experiments were conducted in Southern Brazil along two seasons (2018/19 and 2019/20). Observations included measurements of the fraction of ground cover (f_c), plant height (h), and soil water content (SWC) to calculate the soil water balance and estimate the soybean yields. Using the soil water balance model SIMDualKc with field collected data allowed estimating the actual and standard K_cb. The model adopts the dual K_c approach and was calibrated and validated using the field observations from, respectively, 2018/19 and 2019/20. Thus, the study provided for accurately estimating and partitioning soybean ET_{c act} into soil evaporation and crop transpiration. Results of fitting observed SWC by model simulated data provided for a set of excellent goodness-of-fit indicators. Therefore, the actual K_cb estimated with SIMDualKc are assumed to have excellent accuracy for computing actual transpiration to be used for supporting soybean irrigation scheduling. This study allowed to evaluate the main water use and yield responses to water stress of diverse soybean maturity groups and to assess the impacts of related water management strategies.

FAO-56 两步法 (K _c -ET _o ) 通常用于估算各种作物和气候条件下的作物蒸散量 (ET _c )。该方法由特定作物系数 (K _c ) 和草参考蒸散量 (ET _o ) 的乘积组成。作物系数根据区域气候条件进行调整，目的是除了 ET _o中已经体现的气候影响之外，还考虑相对湿度和风速对 K _{c 的}作物特定影响。在目前的研究中，作物用水量和作物实际蒸散量和蒸腾量（ET _{c act}和 T _{c act}）对灌溉和雨养条件下的四个大豆成熟组进行了研究。田间实验在巴西南部进行了两个季节（2018/19 和 2019/20）。观测包括测量地被覆盖率 (f _c )、株高 (h) 和土壤含水量 (SWC)，以计算土壤水平衡并估算大豆产量。使用土壤水平衡模型 SIMDualKc 和现场收集的数据可以估计实际和标准 K _cb。该模型采用双 K _c方法，并分别使用 2018/19 和 2019/20 的现场观测进行了校准和验证。因此，该研究为准确估计和划分大豆 ET _{c 行为提供了依据。}分为土壤蒸发和作物蒸腾。通过模型模拟数据拟合观测到的SWC结果，提供了一组优秀的拟合优度指标。因此，假设使用 SIMDualKc 估计的实际 K _cb对于计算用于支持大豆灌溉调度的实际蒸腾量具有极好的准确性。这项研究可以评估不同成熟大豆组的主要用水和产量对水分胁迫的反应，并评估相关水分管理策略的影响。