Lake droughts are the consequence of climatic, hydrologic and anthropogenic influences. Quantification of droughts and estimation of the contributions from the individual factors are essential for understanding drought features and their causation structure. This is also important for policymakers to make effective adaption decisions, especially under changing climate. This study examines Poyang Lake, China's largest freshwater lake, which has been undergoing drastic hydrological alternation in the past decade. Standardized lake stage is used to identify and quantify the lake droughts, and hydroclimatic contributions are determined with a water budget analysis, in which absolute deficiency is defined in reference to normal hydrologic conditions. Our analyses demonstrate that in the past decade the lake droughts worsened in terms of duration, frequency, intensity and severity. Hydroclimatic contributions to each individual drought varied between droughts, and the overall contribution to the lake droughts in the past decade came from decreased inflow, increased outflow, and reduced precipitation and increased evapotranspiration in the lake region. The decreased inflow resulted mainly from reduced precipitation and less from increased evapotranspiration over the Poyang Lake basin. The increased outflow was attributable to the weakened blocking effects of the Yangtze River, which the Three Gorges Dam (TGD) established upstream. The TGD impoundments were not responsible for the increased number of drought events, but they may have intensified the droughts and changed the frequency of classified droughts. However, the TGD contribution is limited in comparison with hydroclimatic influences. Hence, the recently increased droughts were due to hydroclimatic effects, with a less important contribution from anthropogenic influences.
A drought is a temporary lack of water caused by abnormal climatic or environmental influences, among other factors (Kallis, 2008; Mishra and Singh, 2010; and references therein). There are meteorological droughts (abnormal precipitation deficits), hydrological droughts (abnormal streamflow, groundwater, or lake deficits), agricultural droughts (abnormal soil moisture deficits), ecological droughts (abnormal water deficits causing stress on ecosystems) and socio-economic droughts (abnormal failures of water supply to meet economic and social demands) (Tallaksen and van Lanen, 2004; Kallis, 2008; Mishra and Singh, 2010). The drought phenomena may have different temporal features and causation structures (Kallis, 2008; Mishra and Singh, 2010). It is anticipated that droughts would likely increase owing to global climate change (Kallis, 2008; Mishra and Singh, 2010).
Hydrological droughts occur when land-water resources decrease significantly below normal conditions, represented by low water levels in streams, lakes, reservoirs and groundwater as well (Nalbantis and Tsakiris, 2009; Keskin and Sorman, 2010). Streamflow droughts may occur with basin-scale precipitation deficiency and/or excessive evapotranspiration (Zelenhasic and Salvai, 1987; Tallaksen et al., 1997; Kingston et al., 2013). In addition to local precipitation and evaporation, lake droughts involve other hydrological components, including inflows from streams surrounding the lake and outflows out of the lake. Hence, lake droughts can be more complicated than streamflow droughts in causation structure. Furthermore, both inflows and outflows may be affected by human activities, for example, groundwater pumping, reservoir construction or land cover change (Wilcox et al., 2010). Therefore, lake droughts are the consequence of combined climatic, hydrologic and anthropogenic influences. In contrast to floods that have received a great deal of attention in hydrology, droughts are not yet comprehensively understood (Kallis, 2008; Mishra and Singh, 2011). Quantification of lake droughts and clarification of contributions from individual factors are essential for understanding drought features and their causation structure. This is important for policymakers to make effective adaption decisions, especially under changing climate. Site-based drought analysis is a starting point towards integrated theories of drought (Kallis, 2008).
Poyang Lake is China's largest freshwater lake, which has been undergoing hydrological alterations in recent decades (Jiao, 2009; Finlayson et al., 2010; Hervé et al., 2011; Liu et al., 2013; Zhang et al., 2014). The lake is located at the south bank of the Yangtze River, which is a humid monsoon climatic region. Although the region historically experiences significant floods (Shankman and Liang, 2003; Shankman et al., 2006), severe lake droughts have occurred frequently in the past decade, resulting in tremendous hydrological, biological, ecological and economic consequences (Feng et al., 2012; Environment News Service, 2012; Wu and Liu, 2014). Because the lake is the primary part of the well-known Poyang Lake wetland and the lake region serves as an important food base for China, the frequently occurring lake droughts have also received increasing international attention (Jiao, 2009; Finlayson et al., 2010; Liu et al., 2011; Environment News Service, 2012; The Ramsar Convention, 2012; Zhang et al., 2012, 2014).
Lake droughts are usually defined as an abnormal decline in lake stage or lake size. A number of studies have documented this decline in Poyang Lake and its influencing factors (Guo et al., 2012; Zhang et al., 2012; Liu et al., 2013; Lai et al., 2014a; Zhang et al., 2014). Feng et al. (2012) used satellite images with a 250 m spatial resolution and reported that the lake size had a decreasing trend between 2000 and 2010. Liu et al. (2013) revealed an abrupt decrease in the lake size in 2006, mainly in October and November. Zhang et al. (2014) demonstrated that the lake stage fell to its lowest level during the 2000s compared to previous decades, in particular in the autumn recession periods. Since Poyang Lake receives inflows from its surrounding basin and discharges into the Yangtze River via a narrow outlet at the Hukou (Fig. 1), the strong lake–river interaction makes it complex to separate relative impacts of the inflow and outflow on the lake stage (Hu et al., 2007; Lai et al., 2014a). Zhang et al. (2014) employed a hydrodynamic model for Poyang Lake for the separation and declared that the lake decline in the 2000s was primarily ascribed to the weakened blocking effect of the Yangtze River. Compared to climate variability on the lake basin, modifications to the Yangtze River flows have had a much greater influence on the seasonal (September–October) dryness of the lake (Zhang et al., 2014). The modification was largely attributable to the operation of the Three Gorges Dam (TGD), established upstream of the Yangtze River in 2003. Water impoundments of the TGD incurred water level drops with an average estimate of 2 m at the outlet of Poyang Lake in mid-September to November for the period 2003–2008 (Guo et al., 2012; Zhang et al., 2012). Alternatively, Lai et al. (2013) developed a hydrodynamic model for the middle Yangtze River region (CHAM-Yangtze), in which they coupled both Poyang Lake and the Yangtze River to account for the lake–river interactions explicitly. They demonstrated that the lake stage was more sensitive to the alternation in lake inflow compared to the same discharge modification in the Yangtze River (Lai et al., 2014a). The recent extremely low water levels in the Yangtze River resulted mainly from remarkable declines in inflows to the river, rather than solely from the TGD impoundments (Lai et al., 2014b). These studies highlighted the complexity of the multiple influences on Poyang Lake's decline in the complex basin–lake–river system.
Drought differs from low water level and persistent dryness. Water level can be low in seasonal dry seasons, but this does not necessarily constitute a drought (Smakhtin, 2001). Persistent dryness refers to water decrease in a long run, which is usually unrecoverable in the short term (Zhang et al., 2012). Droughts are complex events that have a recurrent feature, and may occur in any season and last several months or longer (Todd et al., 2013). Feng et al. (2012) quantified the drought severity of Poyang Lake in 2011 and showed that the drought was primarily due to low basin-scale precipitation, rather than discharge differences between the lake and the Yangtze River with TGD impoundments. Very recently, Wu and Liu (2014) used satellite-delineated inundation area to quantify two lake droughts in 2006 and 2011. The results indicated that the 2006 drought was mainly attributable to abnormal decrease of water flow in the Yangtze River and the 2011 drought was due to the combined influences of the Poyang Lake basin and the Yangtze River. Although these were two extreme drought events, it is not certain if they explain the more frequently occurring droughts in Poyang Lake as well.
In principle, drought identification, quantification or characterization with a consistent standard is a prerequisite for drought analysis. However, few studies have comprehensively quantified and addressed the Poyang Lake droughts in the 2000s. The current understanding of the Lake's decline in autumn cannot provide a complete explanation to the lake droughts spanning non-autumn seasons. It remains unknown to what extent the climatic, hydrologic and anthropogenic influences have contributed to the lake droughts, which is one of the key issues for developing integrated, interdisciplinary theories on droughts (Kallis, 2008). Especially for practice, clarification of the multiple influences on the recently increased droughts is essential for the effective prevention of droughts.
The complicated causality of lake droughts requires a robust approach for determining the contributions from multiple influences. Analogous to standardized precipitation index, this study utilizes standardized lake index to quantify lake droughts. With the principle of lake water balance, it proposes to define an absolute deficiency for each water component and determine their relative contributions to lake droughts. The approach is applicable to basin-scale water balance, quantifying regional hydroclimatic influences on lake inflow, and subsequently on lake droughts (Sect. 2). Poyang Lake droughts are examined with the proposed approach, in combination with 5-decade hydroclimatic data including the latest satellite products (Sect. 3). The drought features in the 2000s and their causes are subsequently addressed (Sect. 4). Our findings should be valuable for improving our understanding of lake droughts under changing climate conditions and be useful for local water resources management and climate change adaptation.
The main properties of a drought are time of initiation and termination, duration, severity, magnitude and intensity, as well as spatial extent in the case of meteorological or agricultural droughts (Yevjevich, 1967; Dracup et al., 1980; Wilhite and Glantz, 1985; McKee et al., 1993; Mishra and Singh, 2010; Spinoni et al., 2014). Drought initiation time is the beginning of the drought. Termination time is the end, i.e., when the drought ceases. Drought duration is the period between the initiation and the termination (Yevjevich, 1967; Mishra and Singh, 2010). Drought severity is the total, cumulative water deficiency for the duration of the drought. Drought magnitude is a derivative of drought severity, defined as the average water deficit in the drought period (Dracup et al., 1980; Wilhite and Glantz, 1985). Drought intensity usually refers to the largest departure from the normal conditions (McKee et al., 1993; Spinoni et al., 2014). For a given historical period, another important drought statistic is drought frequency, which refers to the number of drought events that have occurred (Mishra and Singh, 2010; Spinoni et al., 2014).
Various indices have been proposed to characterize and quantify the complex features of droughts (Dracup et al., 1980; Keyantash and Dracup, 2002; Mishra and Singh, 2010, 2011). Among these, the standardized precipitation index (SPI) is most commonly used (McKee et al., 1993; Mishra and Singh, 2010). It is a normalized dimensionless index, defined as the difference of precipitation from the mean divided by the standard deviation for a given period, in which a gamma distribution is generally fitted to the long-term precipitation records for each calendar month to account for seasonal differences (McKee et al., 1993). The SPI is simple but capable of quantifying drought features, and has been recently recommended by the World Meteorological Organization (WMO) to characterize meteorological droughts (Hayes et al., 2011). Nevertheless, it was proposed to quantify precipitation deficiency, the SPI methodology has been applied in a similar manner to other hydroclimatic variables, for example, streamflow discharge, soil moisture, reservoir storage and groundwater level (McKee et al., 1993; Sheffield et al., 2004; Vicente-Serrano and López-Moreno, 2005; Mendicino et al., 2008; Shukla and Wood, 2008).
In the case of lake drought, it can be described with lake stage, lake area or
water storage. Of these variables, lake stage is usually continuously
measured and is suitable for drought analysis. In comparison to SPI, the
standardized lake index (SLI) is described as follows
A drought event is discernible with SLI. While a negative SLI indicates the
lake stage is lower than the normal, not all the negatives can be classified
into a drought event. Only when SLI deviates away from the normal by more
than 1 standard deviation (SLI <
In accordance with the definition of SPI by McKee et al. (1993), SLI
represents a departure of lake stage from its normal conditions. The
departure corresponds to a probability of drought intensity, useful for
drought risk analysis, namely, SLI
Geographic location of Poyang Lake, China. The lake is principally fed by five river systems of the Poyang Lake basin. Lake water flows into the Yangtze River via a single outlet at the Hukou. Jiujiang is located 25 km upstream of the Hukou on the Yangtze River. The Three Gorges Dam (TGD) is upstream of the river.
In addition, drought severity may be calculated as follows
A lake drought results directly from an abnormal change in the lake water
budget. A general water balance for lake in a period can be described as
follows
Once the water budget appears abnormal, it suffers from the anomalies of
some or all the water components, namely, low precipitation, high
evapotranspiration, low inflow and/or high outflow. At the monthly scale,
for a water component
During a drought event, the anomaly of lake water storage in month
Notably, Eq. (
Equation (
In addition to the quantification of water deficiency in inputs and outputs
to the lake, it is also important to understand the causes of inflow
deficiency for a complete understanding of hydroclimatic influences on lake
droughts. Lake inflow originates from precipitation on its contributing
basin. Given the water balance for the basin in a period, lake inflow is
described as
In practice, there are often areas ungauged downstream from hydrological
stations. In this case, the lake inflow includes two parts, one from gauged
areas and another from ungauged areas (
In combination with Eqs. (
Further incorporated with Eq. (9b), the lake water budget can be expressed
as
Poyang Lake is located at the northern part of the Poyang Lake basin, a
sub-basin of the Yangtze River basin of China (Fig. 1a). The lake has a
maximum area of 3860 km
Daily lake stage data from five hydrological stations and daily discharge
data from seven control stations were obtained from the Hydrological Bureau
of Poyang Lake. Lake stage data from Xingzi and Hukou were available for
the period 1961–2010, but the data from the other three stations were available only
until 2008. Daily discharge data for the Hukou outlet are available from the
Hydrological Bureau of the Yangtze River Water Resources Commission. Daily
precipitation data from 73 national weather stations within the Poyang Lake
basin are available from the National Meteorological Information Center of
China for the period 1961–2010. Regional evapotranspiration estimates were extracted
from the latest satellite products (MOD16) of the Moderate resolution
Imaging Spectroradiometer (MODIS) (
To identify lake droughts, SLI was calculated with Eq. (
Statistical distribution of monthly average lake stages with a fitted gamma distribution for each calendar month at Xingzi of Poyang Lake in 1961–2010.
Prior to drought quantification with SLI, the monthly lake stage was evaluated for its fit with the gamma distribution in each calendar month (McKee et al., 1993). The statistical evaluation demonstrated the goodness-of-fit at a significant level of 1 % with the Kolmogorov–Smirnov test (Lloyd-Hughes and Saunders, 2002) for all 12 months (Fig. 2). Subsequently, drought initialization, termination, duration, intensity, severity and frequency were determined from the SLI values of Xingzi, with the criteria described in Sect. 2.1. Finally, all the lake droughts were identified and classified into extreme, severe or moderate drought (McKee et al., 1993).
In addition, for the case without the TGD impoundments, the lake stage at
Xingzi was estimated from its highly correlated relationship with Hukou,
To quantify hydroclimatic influences on lake droughts, the water budget
analysis was designed at multi-spatial scales: the lake, lake region and
gauged basin (Fig. 1). At the lake scale, water components include
precipitation, evaporation, groundwater net inflow, inflows from gauged and
ungauaged areas, and outflow (Eq.
Specification of normal hydrologic conditions is a prerequisite for determining water deficiency. First, precipitation data were grouped for the Poyang lake region and the gauged basin. Multi-year means of monthly precipitation were obtained from the observation data for the period 1961–2010. Second, multi-year means of monthly discharge were calculated from the data for inflows and outflow in the period. Third, since evapotranspiration data prior to 2000 were unavailable, the annual evapotranspiration was calculated from the difference between annual precipitation and discharge, respectively, for the lake region and the gauged basin in 1961–2010. The multi-year mean of monthly evapotranspiration was then obtained by distributing the annual value with a monthly weighting factor calculated from the MOD16 time series, with an assumption that the seasonal variation is relatively similar for the period 1961–2010.
Once the normal hydrologic condition was established, the water deficiency
of a water component (Eq.
It should be emphasized that, for the sake of addressing water contribution
at the lake region and the gauged basin, the water amounts (unit in m
Poyang Lake droughts in 2001–2010.
Drought events occurring during the 2001–2010 time period.
Figure 3a illustrates the SLI variation for Poyang Lake in the past decade. Negative values prevail over positive values, indicating the dry
phase dominates the lake for the period. Three extreme, two severe and four
moderate droughts, according to the drought classification criteria (McKee et al.,
1993), occurred in this period. Among the nine cases, three droughts started
in spring, two in summer and four in autumn (Table 1). Drought duration
varied from 2 to 13 months with a mean of 7.3 months and a standard
deviation (SD) of 3.8 months, which demonstrated that the lake droughts
could occur in any month. Drought intensity ranged from
In comparison to the years 1961–2000, the lake droughts changed in terms of
duration, frequency, intensity and severity in the past decade
(Fig. 3b). On average, drought duration extended from 5.6 to 6.2 months.
Drought frequency increased from 6.0 to 9.0 events per decade. Drought
intensity intensified from
Normal variation of water components is a baseline for the quantitative
analysis of drought occurrence as an abnormal change. Figure 4a shows the
multi-year mean of monthly precipitation (
Water components (unit in mm) of the Poyang Lake region and of the gauged basin for the drought periods. The values in parenthesis are the anomaly of a component against the multi-year mean in 1961–2010. All the water amounts are normalized to equivalent water height of the whole Poyang Lake basin.
Lake droughts occur when abnormal change appears in the water budget. Table 2 lists the water components for the lake region during the periods of lake
droughts. The water budgets (
Contribution of each water component (unit in 100 %) to the total water anomaly up the time of peak drought (highest drought intensity) for the lake region and gauged basin.
Drought causes can be traced from the relative contribution of individual
water components. Table 3 shows the ratios of the total water anomaly of a
component to that of the water budget up the time of peak drought (maximum
intensity) for each event (Eq.
Likewise, prior to performing a water budget analysis, it is necessary to
clarify the normal hydrologic condition. Generally, precipitation (
The water budget was
While the inflow reduction resulted from combined hydroclimatic change,
precipitation and evapotranspiration may have made different contributions
to the drought formation. Table 3 shows that the relative contribution for
The above sections detail the lake droughts as abnormal phenomena and the hydroclimatic contribution to individual drought events. Yet, it remains unclear why the droughts strengthened in the past decade, and whether the droughts resulted from a long-term change of hydroclimatic influences or a seasonal combination of these influences.
Figure 6a shows the accumulated anomalies of the water budget from 2001 to 2010.
At the lake region, the water budget (
Figure 6b displays seasonal variation of the water budget during
2001–2010. In comparison to 1961–2000, the water surplus reduced for the
first half of the year, and the water deficit enhanced for the second half
of the year except for August and November. The large reduced surplus
includes March and June, and the enhanced deficit includes July and
September. The reduced surplus and the enhanced deficit would increase the
possibility of drought occurrence and intensify the drought intensity. In
the enlarged water deficit, the
The weakened effects involve climate change in the upper reaches of the
Yangtze River, and water impoundments of the TGD (Guo et al. 2012). Routinely, the
TGD impoundment begins in mid-September and spans 1 to 2 months. Among
all the drought events, none occurred during exactly the same time span.
Accordingly, the TGD impoundments should not be responsible for the
increased drought events. However, the impoundments lowered the lake stage
at the Hukou outlet by 1–2 m for September–October (Guo et al.
2012; Zhang et al., 2012, 2014). Our analysis indicated that the impoundments led to a
change in SLI from
In general, the recently increased droughts were principally attributed to decreased inflow, increased outflow, and reduced local precipitation and increased evapotranspiration at the lake region. First, use of satellite-retrieved evapotranspiration data makes it possible to analyze drought causes from a perspective of water budget with independent measures of major water components. Given that measurement errors are quality controlled, the independent observations are more faithful than model simulations that are susceptible to model uncertainty and empirical parameterization. Indeed, the MOD16 products have been used effectively in water balance studies in the past. Second, in addition to drought quantification, absolute deficiency was defined for water components and water budget in reference to the normal hydrologic state. Individual hydroclimatic contributions were isolated from the total anomalies of the water budget, and the drought causation structure was subsequently distinguished. The quantification approach is straightforward and applicable to separate hydroclimatic influences on droughts, a key issue identified for developing integrated theories of droughts (Kallis, 2008). Third, it is the first time that all the drought events and their causation structure in Poyang Lake were quantified. Most existing studies did not explicitly quantify the droughts but focused on low water levels mainly in autumn seasons. A few studies addressed one-to-two extreme droughts with statistical regression analysis (Feng et al., 2012; Wu and Liu, 2014), without a systematic water balance analysis of the droughts and their statistics in the past decade. The present study completed drought quantification, water budget analysis, isolation of hydroclimatic contributions and clarification of causation structure for the recently increased droughts in Poyang Lake. The results demonstrated that the droughts were due to hydroclimatic factors, with a less important contribution from the TGD influences. Yet, it should be noted that the present study did not address some potential influences, for example, land cover/use change, agricultural water use, soil moisture variation and vegetation dynamics. These factors may affect the hydrological processes at seasonal and annual scales, and subsequently affect lake stage and droughts, which should be taken into consideration in future studies.
This paper used standardized lake stage to identify and quantify droughts on the case of Poyang Lake in China. From a perspective of water budget, it defined an absolute deficiency of the water components and water budget in reference to normal hydrologic conditions to determine hydroclimatic contributions to drought formation. Given 5 decades of hydroclimatic observations and latest satellite products, the water budget analysis was performed in the study area.
Our analyses demonstrated that the lake droughts had strengthened in the past decade, in terms of duration, frequency, intensity and severity. The overall contribution to the lake droughts came from decreased inflow, increased outflow, and reduced precipitation and increased evapotranspiration at the lake region. The decreased inflow resulted mainly from lower basin-scale precipitation and less from increases in evapotranspiration. The TGD impoundments were not responsible for the increased drought events, but they did intensify the droughts and change the frequency of classified droughts. Overall, the TGD contribution was limited, compared with the hydroclimatic influences.
The findings of this study provide an example of intensified lake droughts, and offer an insightful view into the droughts under the hydroclimatic and anthropogenic influences. The methodology proposed for quantification of lake droughts and isolation of hydroclimatic contributions has potential applications to other lakes. Moreover, the results of the study should be useful for local water resources management under climate change.
This work is supported by the 973 Program of National Basic Research Program of China (2012CB417003), the State Key Program of National Natural Science of China (41430855), and the Key Program of Nanjing Institute of Geography and Limnology of the Chinese Academy of Sciences (CAS) (NIGLAS2012135001). We thank David Shankman for his constructive comments on an earlier version of the manuscript, R. Guo for data pre-processing, Y. Chen for providing hydrological data, and X. Lai for sharing simulation data from the CHAM-Yangtze model. We are very grateful to Peter Molnar for his inexhaustible patience, critical but constructive comments, and language editing that have substantially improved the manuscript. The anonymous reviewers are acknowledged for their constructive comments on the early version of the manuscript. Edited by: P. Molnar