In agriculture, soil evaporation is considered a loss of water, and crop transpiration is a crucial factor affecting crop growth and yield. Although there have been numerous studies on evapotranspiration (ET) on farmland, only a few have been conducted on field crop transpiration because of the difficulties in the direct measurement of transpiration and in separate evaluation of evaporation and transpiration under field conditions. In this study, we used the heat-pulse method to measure the sap velocity in sugarcane stems to evaluate field transpiration during summer at the study site on Tanegashima Island (at about 31° N) in Japan. The reference crop ET was determined by the Food and Agriculture Organization (FAO) Penman–Monteith method from on-site meteorological data and was compared with the measured transpiration. The daily transpiration in August ranged from 3.3 mm d⁻¹ to 6.6 mm d⁻¹; the corresponding sap flow varied from 0.27 L d⁻¹ to 0.55 L d⁻¹ per stalk. The daily analysis of ET showed that a standard value of 1.25 for the crop coefficient presented by the FAO is acceptable as the crop coefficient for the mid-season growth stage of sugarcane in the study site. Numerical simulations of Richards equation with the sink term were conducted to examine the response of root water uptake to atmospheric transpiration demand throughout 20 consecutive clear and dry days in August. The decrease in the soil moisture in the root zone depends on the transpiration demand during daytime, and soil moisture is redistributed and recovered during the nighttime. The hourly numerical results in August support a standard value of 1.20 for the basal crop coefficient in the mid-season growth stage, as proposed by FAO. The simulations showed that roots extract water preferentially from the deep and wet soil layers to meet the transpiration demand; transpiration predicted from the sink term agreed well with the actual transpiration measured by the heat-pulse method. Therefore, the numerical analysis of soil moisture dynamics under the atmospheric transpiration demand based on the reference crop ET can be an effective tool to evaluate field sugarcane water consumption.

在农业中，土壤蒸发被认为是水的流失，而作物的蒸腾作用是影响作物生长和产量的关键因素。尽管对农田的蒸发蒸腾量（ET）进行了大量研究，但由于在田间条件下直接测量蒸腾量以及分别评估蒸发量和蒸腾量存在困难，因此仅对田间作物的蒸腾量进行了很少的研究。在这项研究中，我们使用热脉冲法测量了甘蔗茎中的汁液速度，以评估夏季在日本种子岛（约31°N）的研究地点的田间蒸腾作用。参考粮食作物的ET由粮食及农业组织（粮农组织）的Penman-Monteith方法根据现场气象数据确定，并与测得的蒸腾量进行了比较。^-1至6.6 mm d ^-1 ; 相应的树液流量在0.27 L d ^-1至0.55 L d ^{-1之间变化}每根茎。ET的每日分析表明，粮农组织提出的作物系数的标准值为1.25，可以作为研究地点甘蔗中期生长阶段的作物系数。进行了带有下沉项的Richards方程的数值模拟，以研究8月连续20个晴天和干旱天的根系水分吸收对大气蒸腾需求的响应。根部土壤水分的减少取决于白天的蒸腾需求，而土壤水分则在夜间重新分配和恢复。根据粮农组织的建议，8月的每小时数值结果支持在季节中期的基础作物系数的标准值为1.20。模拟结果表明，根系优先从深层和湿润的土壤层中提取水分，以满足蒸腾作用。从下沉期预测的蒸腾量与通过热脉冲法测得的实际蒸腾量非常吻合。因此，基于参考作物ET对大气蒸腾需求下土壤水分动态的数值分析可以作为评价田间甘蔗耗水量的有效工具。