Influence Of Cascade Reservoirs On Carbon Dioxide Emission From Surface Water In The Yellow River Source Region

Rivers are an important link between the elements of land and sea,and reservoirs would alter the sediment and runoff processes in natural rivers,which in turn will affect the amount of carbon released from natural rivers.Six reservoirs of the cascade reservoirs represented by Longyangxia–Liujiaxia section in the Yellow River source region were selected.The partial pressure of carbon dioxide?p CO₂?in the water of the cascade reservoirs were quantitatively estimated using shake flask method and the water-air interface CO₂ emission flux?FCO₂?were measured by Li-7000 static chamber method in April 2016,August2016 and May 2017.The temporal and spatial characteristics of pCO₂ and FCO₂ were also analyzed.Based on the field measurement results of hydrochemical indicators,including pH,DO,T,Alk and DOC,together with reservoir operating characteristics,the impact of cascade reservoir construction on river FCO₂ was analyzed.The main conclusions are included as follows:1.The longitudinal water chemistry parameters of cascade reservoirs during the sampling period showed significant temporal variability.pH in April was higher than August,with an average of 8.6 and 7.4.The DO concentration was basically the same in August and April,with an average of about 100%.In August,the DO had a maximum value of 138%in the Liujiaxia reservoir.DOC was slightly higher in August than April,with an average of 7.78 mg/L and 4.78 mg/L.The overall variation trend of Alk was the same in April and August,and the average in April of215 mg/L was significantly higher than 150 mg/L in August.Monitoring of water temperature along vertical direction showed that there was a clear stratification of T in Longyangxia and Liujiaxia reservoirs in August 2016,which will affect photosynthesis,respiration and water environment parameters in water body.2.The construction of cascade reservoirs has significantly increased the FCO₂ of rivers in the upper reaches of the Yellow River.The vertical p CO₂ rises first and then decreases.The p CO₂ reaches the maximum value of 820?atm in the Nina reservoir.The averaged pCO₂ was 630?atm in April and August.In Longyangxia reservoir pCO₂ of 740?atm in April was significantly higher than p CO₂ of 525?atm in August and.The pCO₂ in Nina reservoir was significantly higher in August?950?atm?than in April?650?atm?.The pCO₂ gradually increased with the increase of water depth in August,and reached the maximum of 1270?atm and 950?atm in the Longyangxia and Liujiaxia reservoirs.The vertical distribution of pCO₂ was even in April.3.The construction of cascade reservoirs has significantly reduced the FCO₂ of rivers in the upper reaches of the Yellow River.After entering the first-level reservoir-Longyangxia reservoir,the FCO₂ decreased from 1.5-1.8 g C/m²/d to about 0.4gC/m²/d.The reduction effect was significant.The longitudinal change of FCO₂ in the cascade reservoir was not obvious.The FCO₂ gradually decreased before the dam,but the FCO₂ after the dam rapidly increased.The FCO₂ of the cascade reservoir was slightly higher in April with an average of 240 gC/m²/yr than that in August with an average of 270 gC/m²/yr.4.The CO₂ content in Longyangxia and Liujiaxia reservoirs is similar.Inorganic carbon contributes more CO₂ emission in Longyangxia reservoir than that in Liujiaxia reservoir.Most of the CO₂ in the Liujiaxia reservoir comes from organic carbon.This is mainly because Longyangxia reservoir located in high-altitude area with a low water temperature which may limit photosynthesis in water body.The impact of the construction of Longyangxia-Liujiaxia cascade reservoirs on the FCO₂production in the source area of the Yellow River is evaluated by field experiment and principle analysis,we concluded that the Longyangxia-Liujiaxia cascade reservoirs are the source of the CO₂ emission to air.This is of great significance for accurately assessing the FCO₂ in the Yellow River Basin and evaluating the impact of cascade reservoir operation on river CO₂ emissions.