Spatial validation of a semi-distributed hydrological nutrient transport model

Poyang Lake, Dongting Lake, and Taihu Lake are the largest freshwater lakes in the middle and lower reaches of the Yangtze River, China. In recent years, the eutrophication level of lakes has increased with the development of the social economy and caused many environmental and social problems. The concentrations of total nitrogen (TN) and total phosphorus (TP) are the key indicators of the degree of eutrophication, but the traditional ground monitoring methods are not capable of capturing such parameters in whole lakes with high spatial-temporal resolution. In this paper, empirical models are established and evaluated between the TN and TP and remote sensing spectral factors in the three lakes using Landsat 8 Operational Land Imager (OLI) satellite data and in-situ data. The results show that the inversion accuracy is higher than 75%. The TN and TP concentrations in the three lakes are inversed based on the Google Earth Engine (GEE) platform from 2014 to 2020 and their spatial-temporal variations are analyzed. The results show that the concentrations of TN and TP in Poyang Lake were decreased by 5.99% and 7.13% over 7 years, respectively, and the TN in Dongting Lake was decreased by 5.25% while the TP remained stable. The temporal changes in TN and TP concentrations displayed seasonal variations. A low concentration was observed in summer and high concentrations were in spring and winter. The average concentrations of TN and TP in Taihu Lake were higher than that of the other two lakes. The TP concentration was increased by 17.3% over 7 years, while the TN concentration remained almost stable. The variation in TN in Taihu Lake was the same as the growth cycle of algae, with higher value in spring and winter and lower value in summer, while the concentration of TP was lower in spring and winter and higher in summer. The spatial distribution of TN and TP concentrations in the three major lakes was significantly affected by human activities, and the concentrations of TN and TP were higher in areas near cities and agricultural activities.

The effects of water inflow–outflow volumes on the in‐lake total phosphorus (TP) concentrations was investigated for the deep, stratified Çubuk II Dam Lake. The fluctuations in the TP concentrations followed a seasonal pattern, with lower values found during the summer, and peak values during wet seasons. Both the seasonal pattern and the horizontal/vertical heterogeneities in the TP concentrations were usually found to be in accordance with those of the water flow parameters. The vertical differences in the TP concentrations provided evidence that an interflow formed at the 5 m and 10 m depths by the water inflow resulted in higher mean TP concentrations at these depths. In contrast, the water outflows had a reducing effect on the TP concentrations, particularly at the 15 m depth, by promoting net TP export. Although the horizontal distribution of the TP concentrations differed slightly between the riverine, transition and lacustrine zones, the differences did not indicate any statistical significance (P > 0.05, multivariate analysis of variance). The path analysis performed on the vertical data proved to be a promising tool in estimating the direct correlations between water flow dynamics and TP concentrations when the decomposition of indirect correlations via dissolved oxygen concentrations and temperature was accounted for in these mutual associations.

Abstract. An examination of total phosphorus (TP) concentrations from 902 lakes in England, Scotland and Northern Ireland (NI), suggest that only Scottish lakes have a high percentage (73%) of oligotrophic waters (TP<10 μg PI ‐1 ). The TP status of upland lakes in NI was greater than Scottish lakes particularly if lake catchments were afforested. Although lowland lakes in NI drain a predominately non‐urbanized landscape, 38% of lakes below 100 m had TP concentrations > 100 μg PI ‐1 and only 29% <35 μg PI ‐1 . English lakes tended to have higher TP concentrations (70% > 101 μg PI ‐1 ) which may reflect P inputs from sewage treatment works (STWs) although lakes draining agricultural catchments frequently produced high TP concentrations. Between 1985 and 1995, annual point source TP inputs to Loch Leven, Scotland, declined by 8 tonnes P or 40% of the 1985 TP loadings to the Loch. As point source inputs were proportionally richer in dissolved molybdate reactive phosphorus (MRP) than diffuse inputs, the MRP loading was reduced by 46%. From 1974 to 1995, TP concentrations in Lough Neagh (NI) increased despite reduced TP inputs from STWs. Partitioning of annual TP loadings from two major inflowing rivers to Lough Neagh, showed river MRP loadings from non‐point sources had been increasing at annual rates of 1.9 and 2.3 kg P km ‐2 . The remaining non‐MRP river loadings had not been influenced by lower TP loads from STWs and showed no tendency to increase with time. Insufficient data is available from other lake systems in the British Isles to judge whether the increase in non‐point source MRP loadings observed in the Lough Neagh catchment has been repeated elsewhere.

In order to find out the distribution of soil organic matter, total nitrogen and total phosphorus contents in gully in loess hilly region and the difference between gully and slope, 29 and 58 samples were taken from gully and slope in Yuanzegou small watershed respectively. The results showed that the contents of total nitrogen, total phosphorus and organic matter in the 0-60cm soil layer ranged 0.091~0.628g/kg, 0.402~1.388g/kg, and 0.826~6.550g/kg respectively. According to the results of the second national soil census, the total nitrogen and organic matter content in the gully are at the lower level of grade 6, the total phosphorus content is at the higher level of grade 2, and the variation coefficient ranges from 21.6% to 33.4%, which is moderate. Pearson correlation analysis shows that soil total nitrogen, total phosphorus and organic matter contents are positively correlated with slope direction and elevation, negatively correlated with slope, and weakly correlated with soil moisture content, clay content and silt content, and negatively correlated with sand content. 0-20cm, the organic matter contents in the gully was higher than that slope 7.19%; 20-40cm and 40-60cm, the organic matter content in the gully was lower than that slope 4.66% and 12.67% respectively, but there was no significant difference. 0-20cm, 20-40cm, 40-60cm, the content of total nitrogen and total phosphorus in gully were higher than that slope, and the total nitrogen content was higher 24.31%, 13.04% and 8.70% respectively, with significant difference only in 0-20cm (p<0.05); The total phosphorus content was higher 15.68%, 13.56% and 15.11% respectively, with significant differences (p<0.05).

Understanding the response of nitrogen fluxes to changes in land use and agricultural practices is crucial for improving the instream water quality prediction. In central Germany, the expansion of bioenergy crops during the last decade led to an increase in fertiliser application rates. The purpose of this study is to investigate the effect of agricultural management changes on the stream nitrogen load of a drinking water reservoir catchment (Weida, 99.5 km2) using a hydrological water quality model. The semi-distributed hydrological water quality model—the HYdrological Predictions for the Environment (HYPE)—was calibrated and validated successfully for discharge and nitrate-N concentrations during the period 1997–2003 (the lowest discharge Nash-Sutcliffe efficiency (NSE) was 0.78). Subsequently, stream nitrogen load of six different land use scenarios and their associated agricultural practice changes were compared to the baseline simulations of the period 2006–2009. Some of these scenarios were designed considering the increased cultivation of bioenergy crops. Results revealed that an increase in mineral fertiliser by 20 % for all crops augmented an increase of monthly stream nitrogen loads in the range of 2–6 % compared to the baseline simulations. Also, it was found that stream nitrogen load increased in scenarios where all or some crop areas were converted to maize and rape, which are the established bioenergy crops in Germany. The increase of nitrogen load resulting from these scenarios differed in terms of magnitude and their temporal patterns, reflecting the importance of timing, the amount of fertiliser applications, and harvesting periods. However, results showed that nitrogen load was reduced in situations when only organic farming or summer barley was used and when rape and maize cropping areas were converted to winter wheat. In this intensively used agricultural catchment, the simulated stream nitrate-N loads quickly responded to fertiliser application changes (increase/decrease). This rapid response could be explained by short residence time of the interflow and baseflow runoff components because of the hardrock geological properties of the catchment.

Soil water-stable aggregates (WSAs) are the basic unit of soil constitution and can contribute to remaining the stable soil constitution. The objective of this study was to clarify the distribution and stability of WSAs and the soil organic carbon (SOC), the total nitrogen (TN), and the total phosphorus (TP) concentrations in 0 - 20 cm and 20 - 40 cm soil layers under the different ages of Robinia pseudoacacia plantations. The 20, 25, 40, and 50 years-old Robinia pseudoacacia plantations were selected. Stepwise regression analysis showed that >5 mm and 1 - 2 mm WSAs, SOC concentration in 2 - 5 mm WSAs, and TN and TP concentrations in < 0.25 mm WSAs were dominant independent variables affecting aggregate stability and that SOC in 0.25 - 0.5 mm WSAs, TN in <0.25 mm and 1 - 2 mm WSAs and TP in 2 - 5 mm WSAs were dominant independent variables affecting SOC, TN, and TP concentrations in bulk soils.

Shifts in precipitation and agricultural intensity increase phosphorus concentrations and loads in an agricultural watershed

The availability of total nitrogen (TN) and total phosphorus (TP), major contributors to eutrophication in lakes, may limit primary productivity, and therefore long-term and large-scale monitoring of TN and TP concentrations in lakes is crucial to improve water quality. This study compared the performance of linear regression (LR), ridge regression (RR), decision tree regression (DTR), random forest regression (RF), and gradient boosting regression tree (GBRT) models in estimating relationships of TN and TP concentrations to remote sensing spectral information in Donghu Lake, Wuhan. Additionally, the spatiotemporal variations and the impact of human activities on TN and TP concentrations in Donghu Lake from 1990 to 2020 were analyzed based on the optimal model. Among various regression models, the machine learning algorithms using GBRT accurately reflected changes in TN and TP (R 2 > 82%) and indicated that from 1990 to 2013, TN and TP concentrations were high, with substantial interannual fluctuations. From 2014 to 2020, the water quality gradually improved, with TN and TP concentrations decreasing by ∼50% and 25%, respectively, compared to the previous period. Spatially, concentrations of TN and TP were notably higher in the bays or along the shorelines compared to the central lake area. The proposed model algorithms could potentially complement in situ measurements with long-term, large-scale predictions of TN and TP concentrations in Donghu Lake, aiding its water quality management.

Causal impact analysis of enhanced phosphorus effluent standard on river water quality

The effect of sampling frequency and wind velocity on estimates of total phosphorus (TP) concentrations were evaluated in Lake Okeechobee, Florida, a shallow, polymictic lake. It was hypothesized that estimates of TP would be affected by sampling frequency because the lake's P-rich bottom sediments are frequently resuspended during wind events. However, differences in estimates of annual mean TP were small (< 5 %) when daily, weekly, bi-monthly, and monthly sampling frequencies were compared. In contrast, estimates of monthly mean TP often varied by 20 to 30 percent among sampling frequencies and differences in monthly maximum and minimum TP values commonly varied by 50 to 250 μg·L−1. Bottom sediments were most likely to be resuspended during the windy season (November-April) when calculated threshold wind velocities were exceeded more than 40 percent of the time. On a monthly basis, wind velocity explained as much as 70 percent of the daily variability in TP during the windy season. In comparison, threshold velocities were exceeded less frequently during the calm season (May – October) and wind velocity only accounted for 5 to 25 percent of the daily variability in TP. The effects of wind-driven mixing events on the chemical and physical attributes of shallow lakes and the influence of sampling frequency on estimates of these attributes should be considered when designing a monitoring program to evaluate short and long-term trends in water quality.

We propose a palaeolimnological method for inferring past total phosphorus (TP) concentrations in lake water from spectrophotometrically-measured sedimentary pigments, particularly total carotenoids (TC). Our approach is based on a highly significant statistical correlation (P < 0.0001) between pigment concentrations (total carotenoids) in the surface sediment of 28 Italian lakes (subalpine, large, deep, shallow, volcanic) and TP concentrations measured in these lakes at overturn when the core was collected. A transfer function was developed from this “training” set, and used to estimate past TP concentrations from pigment concentrations in sediment cores. The results generally agreed with TP values as measured by long-term water quality monitoring programs. Contrasting results were obtained by a comparison with diatom-inferred TP. While the diatom model showed a tendency to overestimate TP values higher than 100 μg l−1, the pigment model correctly estimated TP in lakes when TP was <100 μg l−1, but not when lakes were rich in macrophytes. In fact, lakes with extensive populations of aquatic submersed macrophytes and epiphytes are outliers in terms of the TC versus TP relationship. The root mean square error of prediction of the pigment model is lower than those derived from certain diatom—based inference models. The predicted and residual values are not related to the estimated values and their average is not statistically different from zero. Errors were estimated via a ‘leave-one-out’ re-sampling technique. The proposed method permits rapid and relatively inexpensive determination of reference trophic conditions.

Constructed wetlands (CWs) can remove a high amount of pollutants from wastewater, and therefore play an important role in water purification. In this study, a pilot system to improve the traditional treatment of industrial wastewater from the tannery industry was tested. The main objective of this research was to remove nitrogen, phosphorus, boron, and chromium from a tannery’s industrial wastewater using a horizontal subsurface flow constructed wetland (HSSFCW) formed from three cells, planted with Phragmites australis and operated in batch mode as an ecofriendly system. P. australis was selected due to its ability to adapt to climatic conditions, its wetland and management characteristics, and its high capacity for pollutant absorption. The concentrations of total Kjeldahl nitrogen (TKN), total phosphorus (TP), boron (B), and chromium (Cr) were analyzed in both wastewater and purified water, and the removal efficiencies were calculated. In addition, both the absorption capacity of P. australis in the aerial and root parts and the adsorption capacity of substrates (gravel and washed sand) were analyzed. Results showed that the concentrations of TP and Cr decreased in the wastewater at both hydraulic retention times (HRTs) tested (3 and 7 days), with 3 days being the most effective, showing removal efficiency values of 78% and 48% for TP and Cr, respectively. However, concentrations of TKN and B were not statistically reduced at either HRTs. Regarding the absorption capacity of P. australis, the highest absorption efficiencies for TKN and TP were reported at 7 days in the aerial part of the plants. In contrast, B was retained in roots at HRT of 3 days. Finally, Cr was more significantly absorbed at 3 days by P. australis. Moreover, the substrates also played important roles in the adsorption of nitrogen and boron. Therefore, CWs planted with P. australis could be used as an ecofriendly technique to the reduce pollution load of the wastewater from tannery industry, especially for P and Cr, although in order to increase the removal efficiency of B and N, the combination with other plant species and different retention times should be tested.

Knowledge of tempo-spatial dynamics of water quality and its response to river flow is important for the management of lake water quality because river discharge associated with rainstorms can be an important source of pollutants to the estuary. Total phosphorus (TP), chlorophyll a (Chl-a), and total suspended matter (TSM) are important indexes of water quality and important factors influencing eutrophication and algal blooms. In this study, remote sensing was used to monitor these indexes to investigate the effects of river discharge on the estuary of Taihu Lake by the largest inflow river which is Chendong River using a total of 136 Geostationary Ocean Color Images (GOCI). In situ datasets collected during the four cruise experiments on Taihu Lake between 2011 and 2015 were used to develop the TP, Chl-a, and TSM inversion models based on simple empirical algorithms: 154 points for TP (mg/L), 114 for Chl-a (μg/L), and 181 for TSM (mg/L). The spatial and temporal changes of the concentration of the three parameters in the Chendong River estuary were analyzed by combining the GOCI data, the flow of the Chendong River, and meteorological data throughout the year in 2014. The several key findings are as follows: (1) In summer and autumn, TP, Chl-a, and TSM contents were significantly higher than in winter and spring. TP and Chl-a have a few similar distribution characteristics. And organic suspended matter in summer was the main reason for the increase of the TSM concentration. (2) The severe surface erosion in the rivers cannot be ignored; the high erodibility is an important factor in the increase of TP and TSM concentrations in the estuary. The concentration of the water quality parameter showed exponential decay with distance from the shore. The concentration decreased slowly after 12km and then remained essentially constant. (3) TP content in the Chendong River estuary decreased under steady flow inputs and dramatically increased when the flow became large. The increase in Chl-a content was linked to higher levels of TP and good weather conditions after the rain event. Higher flow rates mainly play a dilution role for the Chl-a concentration. Erosion of the surface soil via rainfall is a major source of TSM to the estuary. This paper firstly analyzes tempo-spatial dynamics of water quality and its response to river flow in estuary of Taihu Lake, helps to further understand the impact of river input on lake water quality, and is important for lake eutrophication.

Effects of Temperature and Other Factors on Summer Phosphorus in the Inner Bay of Quinte, Lake Ontario: Implications for Climate Warming

Nutrients in runoff degrade water quality. The development of schemes to mitigate such degradation requires a characterization of the underlying dynamic processes of nutrient loss. The drinking-water source protection area in the Lake Hongfeng watershed of Guiyang City, the capital of Guizhou Province, China, has been delimited for effective conservation. However, no systematic observations have provided data on nutrient losses from these areas that could support optimal management. We selected one typical watershed in the area. Automatic gauges were installed to record the water levels and calculate runoff rates during 2010 and 2011. A total of 1523 runoff samples were collected at an interval of 3h during a day; total nitrogen (TN), total phosphorus (TP), and chemical oxygen demand (COD) were tested. The results indicated that surface runoff rates were primarily less than 15L/s but rapidly increased 1-30 times 15L/s when it rained. TN, TP, and COD concentrations primarily fluctuated between 0.06 and 18.79mg/L, between 0.01 and 1.57mg/L, and between 0.01 and 160mg/L, respectively. TN and COD concentrations in 98.98% and 52.04% of the runoff samples, respectively, exceeded the upper limit required by the Environmental Quality Standards for Surface Water (EQSSW) in China. Conversely, 94.29% of the runoff samples had lower concentrations than the upper limit of TP concentration. Surface runoff has been seriously polluted by nitrogen and organic pollutants. The occurrence frequency of different runoff rates and TP and COD concentrations showed different distributions, but TN concentrations had a normal distribution. There was a significant relationship between runoff rates and TP concentration and TN, TP, or COD loss. TN, TP, and COD loss primarily occurred on vegetable lands, rice fields, and residential sites. Effectively controlling nitrogen fertilizer that is applied on vegetable lands and paddy fields and managing wastewater and solid waste are urgent. The results reported here will also provide references for many other regions facing similar problems.