为研究已建大型引调水工程运行期输水损失变化,以南水北调中线一期工程为对象，利用水量平衡原理计算得到6个典型渠段2018—2021年不同时间尺度下的输水损失变化过程，并与输水损失经验公式计算结果进行对比。结果表明：不同计算时空尺度下的渠段输水漏损流量结果差异较大，但沿程输水损失率差异较小；入渠流量规模是影响渠段输水漏损流量的主要因素，蒸发量、降雨和冰期冻融等也是造成中线总干渠输水损失时空变化的重要因素，因此仅考虑渠底渗漏损失不能有效反映中线总干渠输水损失变化过程。

The main task of water dispatching of long-distance water transfer projects is to properly determine the amount,process,and scope of water transfer based on overall considering the water input and storage situation of headwater area and the water demand in the water receiving area,of which the water loss is the key parameter in organizing the water dispatching plan,and directly affects the dispatching accuracy.Therefore,there is a great significance to achieve accurate dispatching,ensure water supply safety,and improve dispatching management level.However,previous studies mainly focus on the calculation methods of canal seepage,little was known about the variation of water loss during the operation period of long-distance water transfer projects. By integrating the synchronized flow and water level measurements data,the variation process of water loss in the six typical canal sections of the Middle Route of South-to-North Water Transfer First Stage Project over the annually-averaged,monthly-averaged,and daily-averaged timescales from 2018 to 2021 was calculated based on the principle of water balance,and the calculated water loss was also compared to the results of four empirical canal water loss methods including Davison-Wilson,Mauriz,Molesworth,and Kostiakov methods to evaluate the performance and applicability of empirical methods in estimating water loss in long-distance water transfer projects. The results showed that the water loss of the Middle Route of South-to-North Water Transfer First Stage Project exhibited pronounced spatiotemporal variations.The magnitude and change amplitude of loss discharge in the southern canals of the Yellow River were larger than that of the northern canals,but there was a minor difference in loss rate between them,indicating that the amount of inflow canal discharge is the main factor affecting the canal water loss discharge.The calculated water loss by empirical methods varied greatly.Specifically,the averaged water loss calculated by the methods of DavisonWilson and Kostiakov was in good agreement with the average results by the principle of water balance in part of typical canals,but the change magnitude of their results within canal was much smaller.The calculated canal water loss by the method of Molesworth is significantly greater than the results of other methods as this method underestimated the effect of anti-seepage measures on canal water loss.Overall,the calculated water loss by the empirical method deviated from calculated results by the principle of water balance,suggesting that the change process of water loss of the Middle Route of South-to-North Water Transfer First Stage Project can not be effectively reflected by only considering canal seepage.The impacts of external factors including natural condition changes,operating condition changes,and measurement error on canal water loss were analyzed preliminarily,and the results indicated that evaporation variation,precipitation,and freezing and thawing during the ice period were crucial factors triggering spatial and temporal variations of water loss of the Middle Route of South-to-North Water Transfer First Stage Project.The adjustment of the water dispatching plan affects canal water loss by changing the water contact area in the canal section.Besides,the measurement errors may greatly affect the calculated results of canal water loss when applying the principle of water balance. The calculated water loss of the current study illustrated that it is necessary to consider the dynamic process of water loss,especially in the long-distance water transfer projects where meteorological and engineering operating conditions exhibited significant variation along the canal.