Variation characteristics and influencing factors of open-water evaporation in Danjiangkou Reservoir
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Abstract:
The Middle Route of the South-to-North Water Transfer Project is the longest cross-basin water transfer project in the world,which undertakes the task of providing water for nearly 80 million people in north China.The Danjiangkou Reservoir,located in the middle reach of the Hanjiang River,is the water source area of the water transfer project.At present,it remains unclear how much water is evaporated from the reservoir each year.To fill this gap,the Penman method and the remote sensing reservoir surface area were used to estimate the evaporation loss of the Danjiangkou Reservoir from 2000 to 2020.An equilibrium temperature method was used to account for the effect of heat storage change on water evaporation estimation.Water evaporation estimates from the Penman model were validated at the site and whole-reservoir scales.At the two scales,pan evaporation observations were used as benchmark data.The contribution of different meteorological variables (temperature,net radiation,relative humidity,and wind speed) to the trend in annual evaporation was quantified based on a detrending experiment.The difference in the annual evaporation trend calculated by the original and detrended specific meteorological variables can be attributed to the effect of the change in this meteorological variable.The results showed that temperature,net radiation,and wind speed in the reservoir showed a significant (p<0.05) upward trend during the study period,while the relative humidity showed an insignificant (p>0.05) downward trend.Changes in these meteorological variables are all conducive to the increase of water evaporation.The remote sensing reservoir area also showed a significant (p<0.05) increasing trend,and the increasing trend was more pronounced after 2015 due to the heightening of the dam.Also,the intra-annual fluctuation of the reservoir surface area during the period 2015-2020 is remarkably smaller than that during the period 2000-2014 because of the implementation of the water transfer project.The evaporation validation results confirmed that the Penman model can be used as a reliable tool to simulate water evaporation loss of the reservoir.It can well reproduce the temporal variability of the reference evaporation at both the site and whole reservoir scales.Annual evaporative loss of the reservoir simulated by the Penman model showed a significant (p<0.05) increasing trend from 2000 to 2020,with a trend value of 3.4×10-3km3/a.The mean annual evaporation loss of the reservoir was 0.26 km3/a,accounting for 2.7% of the planned annual water transfer (9.5 km3).However,actual annual water transfer was far lower than the planned annual water transfer (it was 6.3 km3/a during the first seven years of operation of the water transfer project).Annual evaporation loss during this period accounts for 4.8% of the actual water transfer.In the future,the amount of evaporation loss from the reservoir is likely to further increase with climate warming.Among the four meteorological variables,the net radiation change contributed the most to the trend of the reservoir′s annual evaporation (72.0%),followed by the changes in temperature (23.6%),relative humidity (2.7%),and wind speed (1.7%).The findings of this study can provide references for the water resources management and planning of the middle route of the South-to-North Water Transfer Project.
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