Estuaries and Coasts (2019) 42:629–640 https://doi.org/10.1007/s12237-019-00518-0 Large Addition of Freshwater to the Tidal Reaches of the Yangtze (Changjiang) River Xuefei Mei1 & Min Zhang2 & Zhijun Dai1 & Wen Wei1 & Weihua Li1 Received: 17 April 2018 /Revised: 30 November 2018 /Accepted: 5 January 2019 /Published online: 18 January 2019 # Coastal and Estuarine Research Federation 2019 Abstract Freshwater discharge through the estuarine reach is always affected by tides and often composed of multiple distributary channels. The current estimation of freshwater input to the ocean by large rivers, therefore, is usually based on discharge measured at some distance upstream of the river mouth. Thus, records, however, may not truly reflect the river discharge to the ocean. In this paper, we consider the case of the Yangtze River where the furthest downstream gauging station is some 600 km upstream of the river mouth. Recent gaugings of the flow at Xuliujing, 69 km upstream of the river mouth, are used here to refine the estimate of the Yangtze flow to the sea. Given the errors in flow gauging, we consider only differences between Datong and Xuliujing > 15% as being significant. The results show that flows exceed this threshold on over 120 days with 63% being days when flow is larger at the downstream station. Most of these days with larger flow are in the period July to September, while most of the days when flow is less downstream occur in the period March to June. Annual flow differences between Xuliujing and Datong are only small percentages of the annual flow at Xuliujing for the years for which we have the records (1.78, 2.93, and 4.81% for normal, wet, and dry years, respectively) though these are still significant volumes of water. Local precipitation is the main driver of flow increases between Datong and Xuliujing in the July to September period, while water abstraction for irrigation between March and June appears to be the main cause of water loss in this reach. We recommend that water budgets for the tidal reaches of large rivers around the world are needed to accurately determine freshwater discharges from the rivers to the ocean and their temporal patterns. Keywords Freshwater discharge to the ocean . Tidal reach . Estuary flows . Water consumption . Yangtze (Changjiang) Estuary Introduction budget and estuarine organism populations (Milliman and Farnsworth 2011; Dai et al. 2011b). The river discharge into the oceans plays an important role in In most rivers, the downstream gauging stations are located the global hydrological cycle providing a critical linkage be- upstream of the tidal limit. In large rivers, these stations are tween continental freshwater and oceanic salt water (Largier usually hundreds kilometers upstream of the river’s mouth et al. 1997;Lu2004; Haddeland et al. 2014). An accurate because of the problems involved in gauging where there are estimation of freshwater flowing to the ocean is crucial in tidal flows and delta distributary channels. For example, the estuarine management, as it directly affects the assessment gauging station at Obidos is used to record the flow from the of material flux towards the ocean, the global biogeochemical Amazon River into the central Atlantic Ocean, and it is ~ 900 km upstream of the mouth (Anthony et al. 2010). Xuefei Mei and Min Zhang contributed equally to this work. Similarly, Kratie, the most downstream station on the Mekong River, is over 500 km away from the mouth (Lauri Communicated by Isaac Santos et al. 2012). Dalles, the control station of the Columbia River, * Zhijun Dai is ~ 300 km upstream of the mouth (Nijssen et al. 2001). [email protected] Corrientes, the last station on the Parana River, is 550 km from the mouth (Antico and Kröhling 2011). Regions between these gauging stations and the river mouths are areas of high 1 State Key Laboratory of Estuarine & Coastal Research, East China Normal University, Shanghai 200062, China precipitation with local tributary inflows and are likely to sig- nificantly increase the freshwater flow into the ocean (Chen 2 Department of Geography, Shanghai Normal University, Shanghai 201234, China et al. 2010; Zhang et al. 2012). Often there are large cities 630 Estuaries and Coasts (2019) 42:629–640 located in these tidal areas, such as Macapa on the Amazon control station that records the discharge from the river to the Estuary and Shanghai at the mouth of the Yangtze River. As a sea (Kuang et al. 2017). consequence, intensive human activities, including irrigation, However, Datong station is 600 km from the river mouth industrial and domestic water consumption, and enhanced (Fig. 1b). Along this tidal reach, there are three populous runoff from urban areas below the control station, can further provinces, Anhui, Jiangsu, and Shanghai (Fig. 1b), which sig- impact the discharge entering the ocean (Nguyen et al. 2008; nificantly affect the river flow by the generation of local run- Chen et al. 2013). Thus, it is necessary to evaluate the water off, abstraction of water from the river, and waste water dis- budget for the tidal reach to assess the river discharge to the charge (Zhang et al. 2012). Besides, in this section of the river, ocean more accurately. water is extracted to supply the eastern route of the south to Over the past decade, several attempts have been made to north water transfer project (SNWDP, Fig. 1a). Since the op- obtain accurate discharge estimations for rivers that interact eration of the first phase of the project in 2013, it has delivered with ocean tides. For instance, Hoitink et al. (2009)employed over 2.00-billion m3 water from Jiangsu Province to Horizontal Acoustic Doppler current profilers to estimate river Shandong Province (CSNWDPC 2017), which further affects discharge in the Berau Estuary, Indonesia, by combining the the river’s flow to the sea. The lower Yangtze River down- index velocity method and velocity profile method stream of Datong also receives flow from the 10.00 × 104-km2 approaches. Kawanisi et al. (2010) used a fluvial acoustic catchment area. Thus, Datong station does not accurately re- tomography system to continuously monitor discharge in the cord the terrestrial freshwater discharge into the East China tidal Ota River Estuary, Japan. In view of the difficulties in Sea (Yang 2013). measuring discharge in tidal rivers, Moftakhari et al. (2013) Aflowgaugingstationhasnow been established at estimated monthly average river flows into San Francisco Bay Xuliujing (Fig. 1b) approximately 531 km downstream from using a tidal discharge estimation method based on tidal con- Datong and located immediately upstream of the first estua- stituents, astronomical forcing and the frictional interaction of rine branch channel. Xuliujing records the discharge from the flow and tides. Similarly, Cai et al. (2014) estimated freshwa- Yangtze River to the East China Sea more accurately than ter discharge in the Yangtze Estuary based on tidal water level Datong though it is still located ~ 100 km upstream of the observations by accounting for the effects of residual water mouthoftheYangtzeasdownstreamofthispointthereare level slope and tidal damping in the estuary. However, these channel branches in the estuary that would further complicate efforts mainly focused on the feasibility of the proposed meth- flow gauging. Here, the neighboring areas of Jiangzu and odology but did not produce continuous flow records for the Shanghai have further unmeasured effects on the total fresh- estuaries examined. Thus, they failed to document how the water flow entering the sea. discharge changed downstream of the last gauging station. Along the Datong-Xuliujing Reach (DX Reach, 117° 37′ E– In this study, we estimate, for the first time, the real flow 121° 59′ E and 30° 46′ N–31° 00′ N, Fig. 1b), there are several from the Yangtze River into the East China Sea on the basis of small tributaries that discharge water into the main stream, and continuous daily in-situ observations at Xuliujing where an together, they are equal to 1.20% of Datong’s annual discharge. automatic monitoring system has been established that can The largest tributary downstream of Datong, the Qingyi River cope with tidal flows (Fig. 1b). Based on the measured daily (Fig. 1), has the largest yearly discharge of these tributaries, discharge at Datong and Xuliujing, we explore in detail the 4.93 billion m3 annually on average, which is around 0.55% pattern of flows from the Yangtze River to the East China Sea of the discharge through Datong station. at the daily, monthly, and yearly levels and explore the main The DX Reach experiences a subtropical monsoon climate, causes for the observed flow patterns. characterized by four distinct seasons and abundant rainfall. Average annual rainfall for the DX Reach catchment is 1100 mm, with around 85% of the total occurring between Study Area May and October. Mean annual water surface evaporation over the region ranges from 950 to 1100 mm, ~ 50% occurring The Yangtze River (Changjiang River in Chinese), the longest from May to August. Eurasian river, is 6300-km long with a catchment area of Owning to the effects of both highly varying river dis- 1.8 × 106 km2 (Chen et al. 2001; Mei et al. 2018). It rises on charge and strong tides, a certain number of key interfaces the Qinghai-Tibet Plateau at an elevation of 6600 m above sea exist along the DX Reach. Specifically, Datong and level and flows into the East China Sea (Fig. 1a). The Yangtze Zhenjiang are the upper limit of tidal backwater influence in River is generally divided into three subsections, namely, the the dry season and the flood season, respectively; Nanjing and upstream section from the source to Yichang, the middle sec- Jiangyin are the upper limit of tidal currents in the dry and tion from Yichang to Hankou, and the lower section from flood seasons, respectively; Xuliujing is the upper limit of Hankou to Datong (Xu et al.