1. Introduction
Coastal estuarine areas generally have a large river flow and high amounts of suspended matter, nutrients, and other substances, which results in plumes that affect ocean water quality [
1,
2,
3,
4,
5]. Similarly, precipitation events bring large amounts of dissolved and particulate substances into lakes [
6,
7,
8]. During a precipitation event, the local water quantity increases, which increases the water flow into the lake and forms a plume in the estuary area of the lake similar to the formation of plumes in coastal estuarine areas. Therefore, the large amounts of substances that flow into lakes as a result of precipitation events are bound to have an impact on the water quality. At present, there are numerous studies on plumes in the estuaries of major rivers that flow into the ocean [
9,
10,
11]. However, relevant research on the impacts of plumes in inland lakes is still rare.
Traditionally, the water quality constituents for inland water have been obtained from in situ-measured samples. The use of a limited number of samples to represent the overall temporal and spatial distributions of water quality constituents has proven to be problematic in lake waters with high spatial and temporal variations, especially for lakes with large spatial scales [
12,
13]. Satellite remote sensing technology, as a large-scale observation method, can acquire an entire spectrum of lake water characteristics. Many scholars have made remarkable achievements in this field in recent decades through continuous advances in remote sensing technology [
14,
15].
MODIS (Moderate Resolution Imaging Spectroradiometer) data are high temporal resolution monitoring data that are widely used for inland water monitoring, and these data are acquired by Terra MODIS and Aqua MODIS with a temporal resolution of one day. Many studies have used MODIS data to research the optical characteristics and water quality parameters of inland water [
16,
17,
18]. With advances in satellite technology, COMS (Communication, Ocean and Meteorological Satellite), which is the world’s first geosynchronous orbit satellite, was equipped with a Geostationary Ocean Color Imager (GOCI) sensor and launched on 27 June, 2010 [
19]. The main purpose of this satellite is to continuously observe the ocean color around the Korean Peninsula and study any rapid changes in the ocean, land, and atmosphere from a stationary platform at a high temporal resolution. One of the most important features of GOCI data is the high temporal resolution, as it improved the temporal resolution from days to hours. GOCI can obtain eight images of the same area from 8:00 am to 15:00 pm each day. The high temporal resolution of GOCI data can be used to study the movement trends of optically active constituents over a long period of time from a macroscopic view.
Although GOCI was designed for ocean waters [
20,
21], many studies have evidenced that it also can be applied to inland waters. For example, some scholars developed a new method for estimating the total suspended solid concentration in Chinese turbid inland lakes using GOCI data [
22]. Others used GOCI data to estimate the chlorophyll concentrations in turbid inland lakes [
23,
24].
Lake Taihu is a typical shallow water body, and the area is 2338 km
2 [
25]. Because the water quality parameters are moving along with the lake current rapidly [
26,
27,
28], GOCI data with extremely high time resolution is suitable for the work to monitor the lake. For instance, Huang used GOCI to monitor the distribution of suspended matter and algae in Lake Taihu [
29,
30], and Bao monitored chlorophyll concentration in Lake Taihu by GOCI [
31]. The plumes can affect Lake Taihu water quality in a short period of time, therefore MODIS data used in the existing study is obviously unable to accurately obtain the variation tendency of the plume [
8]. GOCI data, therefore, have great potential and application value.
Lake Taihu and the Tiaoxi River Basin were used as the study areas in this research. GOCI data and in situ observation data were used to evaluate the total suspended material (TSM) concentration following heavy precipitation events in the Tiaoxi River Basin. Throughout this research, we study the relationship between lake water quality parameter and river plumes and provide a basis for further research on the changes in lake water quality caused by external inputs.
4. Discussion
4.1. The Use of High Temporal Resolution Satellites to Monitor the Diffusion of Matter in Inland Waters
Inland water bodies, such as lakes, are constantly in motion and through these movements, various dissolved solids and materials also migrate and spread. Prior to the launch of COMS, MODIS data, with a temporal resolution of 1 day, were used to monitor large inland waters [
47]. Although the temporal resolution of MODIS data was higher than that of Landsat 8 (16-day revisit time), MERIS (3-day revisit time), and GF1 (2-day revisit time) data, the monitoring of water quality parameters was still lacking [
48,
49,
50]. GOCI data now make such monitoring possible.
The 2013 monitoring data (including two consecutive observations from 10:15 am to 12:15 pm on 12 October and from 10:15 am to 12:15 pm on 14 October (
Figure 6) show that heavy precipitation in the Tiaoxi River Basin transported large amounts of suspended matter into Lake Taihu that diffused significantly with the lake flow. As seen from the October 5 image (
Figure 6a), the suspended matter concentration in the waters of Lake Taihu was still relatively low before the occurrence of precipitation. After the heavy precipitation event in the Tiaoxi River Basin on 12 October, there was a clear plume that carried high concentrations of suspended matter at the mouth of the river. Over time, the plume spread and migrated further. As a result, the concentration of suspended matter in Lake Taihu also changed; the suspended matter concentration in the water body decreased from the original high concentration as suspended plumes were transported to the northeast and the area of water affected by heavy precipitation continued to increase. From 12–13 October 2013, the distributions of suspended matter in Lake Taihu with concentrations above 100 mg/L are shown in
Figure 8. The areas where these concentrations were found encompassed 218 km
2, 221 km
2, 234 km
2, and 678.5 km
2. By 12:15 pm on 14 October, the entire area of Lake Taihu was affected by this heavy precipitation event (
Figure 6h).
4.2. Analysis of the Factors Influencing Plume Diffusion
According to previous research [
51,
52,
53], under normal circumstances, the suspended matter concentration in Lake Taihu is mainly affected by the resuspension of sediments. However, 60% of the flow into Lake Taihu comes from the Tiaoxi River [
54], so when heavy rainfall occurs in the basin, a large amount of suspended matter that is carried by the river will be transported into the lake in a short period of time and thus increase the TSM concentration. The scope of influence by the river flow pattern is not limited to the southern part of Lake Taihu at the estuary of the Tiaoxi River; it will also follow the flow to the northwest region, and then affect most areas of Lake Taihu, with the exception of only the southern part of the lake.
We use heavy precipitation in early October 2013 as an example. From 6–7 October, due to the peripheral cloud systems of Typhoon No. 23 (
Figure 9), the maximum daily precipitation in the Tiaoxi River Basin was 319.0 mm (Bingkeng Station on 7 October). According to the annual hydrological report, the rainfall stations within the basin were monitored, and the mean daily precipitation values were calculated by the Tyson polygon method. The values from 5–8 October were 4.5 mm, 116.2 mm, 164 mm, and 9.6 mm. According to national grade of precipitation, this precipitation level was heavy rainstorm.
As the only station to monitor sediment discharge in the Tiaoxi River Basin in the annual hydrological report, we used the measured values provided by the Gangkou Station to evaluate the amount of suspended sediments carried by the river. According to the analysis of data from the annual hydrological report, the annual maximum daily mean sediment concentration (0.789 kg/m
3) and the maximum daily mean sediment transport rate (649 kg/s) at the Gangkou Station occurred on 8 October, which was one day after the occurrence of heavy precipitation. On 7 and 9 October, the mean daily sediment contents were also much higher than the mean value measured at the monitoring station, which were observed at 0.429 kg/m
3 and 0.348 kg/m
3, respectively (
Figure 10). According to the river flow and the suspended sediment content monitored by the Gangkou Station (
Table 5), in events of heavy precipitation in the Tiaoxi River Basin, the amounts of water and suspended sediments in the river drastically increased. The correlation coefficient between daily mean flows and daily mean sediment transport rate and daily mean sediment concentration were 0.781 (
p < 0.05) and 0.715 (
p < 0.05), respectively. As a result, the amounts of water and suspended matter that were transported into Lake Taihu also greatly increased, which thus affected the water quality of the lake.
According to the wind speed and direction data from 12–14 October 2013, recorded by the Taihu Laboratory for Lake Ecosystem Research Taihu Station of the Chinese Academy of Sciences, the wind direction in Lake Taihu was mainly southeasterly during this period (
Figure 11). The reason for this result was likely the continuation of the impacts caused by Typhoon No. 23 in October 2013. Due to the influence of the wind direction, the plume with high suspended matter concentrations on the surface of Lake Taihu migrated to the north and further moved to the eastern region of the lake.
These results show that the plume in the estuary of the Tiaoxi River is affected by the intensity of precipitation in the basin. When heavy precipitation occurs, the flow of the Tiaoxi River will increase significantly, and a large amount of sediment will flow into the channel along with the rain, causing the amount of water and suspended matter flowing into Lake Taihu to sharply increase. When an algal bloom occurs in the southern part of Lake Taihu at the estuary of the Tiaoxi River, the algal bloom moves and spreads to the northwest part of the lake, along with the plumes from the Tiaoxi River, and may further affect the distribution of alga throughout the lake.
4.3. Analysis of the Relationship Between Precipitation and River Sediment Discharge
Furthermore, we used the sediment discharge data monitored by the Gangkou Station to calculate the monthly mean sediment concentration from 2006 to 2013. The annual total precipitation data were also calculated by the Tyson polygon method in the upper reaches of the Gangkou Station in the West Tiaoxi River Basin (
Table 6).
The results indicated that the sediment discharge from the West Tiaoxi River significantly increased from 2006 to 2011. The annual mean sediment concentration increased from 0.17 kg/m3 in 2006 to 0.39 kg/m3 in 2011, and the annual sediment discharge also increased from 2.00 × 105 t in 2006 to 5.09 × 105 t in 2011. Since 2012, there has been a drastic decline in sediment discharge from the West Tiaoxi River. The annual mean sediment discharge decreased to 0.11 kg/m3 in 2012, and in 2013 it further decreased to 0.08 kg/m3. However, there were no obvious changes in the annual total precipitation in the West Tiaoxi River Basin.
According to the information collected on the official website of the Anji County Government [
55], from 1999 to 2009, more than 30 million tons of sand was mined from the West Tiaoxi River, and after comparing the aerial photographs of 1999 and 2009, it was found that the river channel of the West Tiaoxi River widened nearly three times. Further studies found that a large number of farmland and beach land on both sides of the West Tiaoxi River became water bodies, and more importantly, the average height of the river bed reached –5m, and some areas even reached –10m, greatly exceeding the control elevation of –3m. Visual observation shows that the West Tiaoxi River changes from clear to turbid after the commercial sand excavation activities. These direct and indirect evidences indicate that the suspended matter content in the water has increased significantly. These sand excavation activities have a great influence on the suspended sediment content in the river channel. Sand excavation machines causes high water turbidity during the sand mining operation, which significantly increases the suspended matter content in the water body [
56,
57,
58,
59,
60]. In response to this environmental problem, the government of Anji County has banned commercial sand excavation activities in the West Tiaoxi River since 2012, which led to an obvious decline in the suspended sediment content in West Tiaoxi River since then.
After eliminating the influence of commercial sand excavation activities, we analyzed the correlation between the monthly mean sediment concentration and the monthly mean precipitation and the correlation between the monthly mean sediment concentration and monthly mean flows at the Gangkou Station from 2012 to 2013 in the West Tiaoxi River Basin. The result indicated that the parameters exhibited a positive correlation of 0.5937 and 0.5173 respectively, with a coefficient of determination of 0.3457 and 0.2676, respectively (
Figure 12a,b). The lower-level correlation data of these two groups indicate that sediment concentration is correlated with precipitation and flow, but the relationship was not significant. Combined with the results analyzed in
Section 4.2, we considered that only when extreme weather conditions such as heavy precipitation occurs in the basin, sediment concentration may have a significant correlation with precipitation and flow. This is because when the precipitation is small, water mainly infiltrates into the soil, only when the precipitation reaches a certain grade, surface runoff will occur, and large amounts of sediment and nutrients will enter the river, which will affect the water environment.
Figure 13 is a histogram of the mean monthly precipitation and mean monthly sediment discharge in the West Tiaoxi River Basin from 2012 to 2013. This figure shows that from 2012 to 2013, three months exhibited abnormal increases in sediment concentration in the river at the Gangkou Station, which occurred in August and November 2012 and October 2013. The quantities were 0.249 kg/m
3, 0.226 kg/m
3, and 0.265 kg/m
3. Except for November 2012, precipitation reached its maximum in the months when the maximum sediment concentration appeared. The precipitation was 462.275 mm in August 2012 (failure to obtain GOCI data for similar periods due to weather conditions), and 322.525 mm in October 2013 (corresponding to the results of GOCI data,
Figure 6). This result indicates that when heavy precipitation occurs in the Tiaoxi River Basin, the sediment concentration in the river will increase significantly, and a plume will form after the water flows into Lake Taihu, which will affect the suspended matter concentration in the lake and cause disturbances to Lake Taihu’s water environment. The abnormal increase in sediments in November 2012 may be due to the small number of illegal sand excavation activities in the river.
4.4. Implication for Water Resources Management
The GOCI-derived TSM products in 2011 and 2013 show that the plume of the Tiaoxi River had a great influence on the spatial distribution of TSM in Lake Taihu after heavy precipitation events, and it would make the TSM concentration sharply rise in a short period. These results clearly suggest a significant influence to Lake Taihu water resources from heavy precipitation events in the basin. The issue of water resources security in Lake Taihu has received much attention in recent years. Therefore, the urgent need for high temporal resolution monitoring of the water environment in Lake Taihu has been realized. Water environment monitoring is somewhat limited by using traditional methods and requires a large quantity of time and labors. Thus, the approach developed in this study, while relatively simple, importantly demonstrates the ability to rapidly acquire information about the concentration of TSM from GOCI data. Validated TSM products from GOCI data measurements can provide supporting information derived from existing field sampling activities and can therefore help management agencies make prudent decisions when the water resources are affected. Therefore, we recommend that future lake monitoring plans include satellite water color remote sensing to aid in the interpretation of spatial and temporal patterns of important water quality parameters.
5. Conclusions
In this study, GOCI data, with a high temporal resolution, were used to study the spatial distribution of the TSM concentration in Lake Taihu after heavy precipitation events in the Tiaoxi River Basin. In addition, the influence of the plume on the water quality of Lake Taihu and the hydrological factors affecting the plume were analyzed. Through the analysis of 16 GOCI images after heavy precipitation events in September 2011 and October 2013, we found that the post-precipitation plume had a great influence on the spatial distribution of the TSM concentration in the lake which, in turn, changed the water quality of the entire lake. Moreover, when algal blooms occur in the estuary, a large amount of inflow from the basin will disperse the algal blooms, which may increase the algae concentration in other areas of the lake. Combined with the analysis of data from the hydrological stations within the basin, the plumes entering Lake Taihu from the Tiaoxi River were mainly affected by the precipitation intensity in the basin, as well as the water volume and suspended sediment content in the river channel. In addition, the sand excavation activity in the West Tiaoxi River Basin had a great impact on the sediment discharge from the river at that time.
These results show that high temporal resolution satellite data can be used to continuously observe plumes in inland lakes. To better understand the effects of the plumes on nutrients in lakes, it is necessary to further integrate the basin runoff yield and runoff models to calculate the non-point pollution in the basin. Meanwhile, in situ observations can be used to monitor the rivers and the plumes impacts of heavy precipitation on the transport of suspended matter and nutrients. These in situ observed data can be used to further improve the simulation accuracy of remote sensing models and to characterize this hydrological process in a more specific way. In addition, because the GOCI images have a spatial resolution of 500 m, we could not determine important details for the spatial distribution water parameters in the inland lake. In future studies, satellite data with higher spatial resolution, such as GF4 satellite data, should be used for additional research.