Impacts of urbanization and climate change on water quantity and quality in the Carp River watershed

Pressure on water resources has reached unprecedented levels during the last decades because of climate change, industrialization, and population growth. As a result, vulnerability to inappropriate water availability and/or quality is increasing worldwide. In this paper, a Soil & Water Assessment Tool (SWAT) model of the Carp river watershed located in the city of Ottawa, Ontario was calibrated and validated. The model was then used to evaluate the individual and coupled impacts of urbanization and climate change on water quantity (discharge) and quality (nitrogen and phosphorus loads). While most of the watershed is currently rural, the headwaters will undergo rapid urbanization in the future, and there are concerns about possible negative impacts on water quantity and quality. Seven scenarios were developed to represent various watershed configurations in terms of land use and climate regime. Future climate time series were obtained by statistically downscaling the outputs of nine regional climate models, ran under representative concentration pathways (RCP)4.5 and RCP8.5. The impacts were evaluated at the main outlet and at the outlet of an upstream sub-watershed that would be most affected by urbanization. Results show that climate change and urbanization's impacts vary greatly depending on the spatial scale and geographic location. Globally, the annual average discharge will increase between 6.75 and 9.34% by 2050, while changes in annual average nitrogen and phosphorus loads will vary between −1.20 and 24.84%, and 19.15 and 23.81%, respectively. Local impacts in sub-watersheds undergoing rapid urbanization would be often much larger than watershed-scale impacts.

Influences of land use changes on the dynamics of water quantity and quality in the German lowland catchment of the Stör

Understanding the impacts of land use changes (LUCC) on the dynamics of water quantity and quality is necessary to identify suitable mitigation measures that are needed for sustainable watershed management. Lowland catchments are characterized by a strong interaction of streamflow and near-surface groundwater that intensifies the risk of nutrient pollution. This study aims to reveal the relationship between long-term land use change and the water and nutrient balance in a typical lowland catchment in northern Germany. A hydrologic model (Soil and Water Assessment Tool, SWAT) and partial least squares regression (PLSR) were used to quantify the impacts of different land use types on the variations in actual evapotranspiration (ET), surface runoff (SQ), base flow (BF), and water yield (WYLD) as well as on sediment yield (SED), total phosphorus (TP) and total nitrogen (TN) loads. To this end, the model was calibrated and validated with daily streamflow data (30 years) as well as sediment and nutrient data from two water quality measurement campaigns (3 years in total). Three model runs over thirty years were performed using land use maps of 1987, 2010, and 2019, respectively. Land use changes between those years were used to explain the modelled changes in water quantity and quality on the subbasin scale applying PLSR. SWAT achieved a very good performance for daily streamflow values (calibration: NSE = 0.79, KGE = 0.88, PBIAS = 0.3 %; validation: NSE = 0.79, KGE = 0.87, PBIAS = 7.2 %), a satisfactory to very good performance for daily TN (calibration: NSE = 0.64, KGE = 0.71, PBIAS = −11.5 %; validation: NSE = 0.86, KGE = 0.91, PBIAS = 5 %), a satisfactory performance for daily sediment load (NSE = 0.54–0.65, KGE = 0.58–0.59, PBIAS = −22.2 %–12 %), and an acceptable performance for daily TP (calibration: NSE = 0.56, KGE = 0.65, PBIAS = −4.7 %; validation: NSE = 0.29, KGE = 0.22, PBIAS = −46.2 %) in the Stör Catchment. The variations in ET, SQ, BF, WYLD, SED, TP, and TN could be explained to an extent of 61 %–88 % by changes in the area, shape, dominance, and aggregation of individual land use types. They were largely correlated with the major LUCC in the study area i.e. a decrease of arable land, and a respective increase of pasture and settlement. The change in the areal percentage of arable land positively affected the dynamics of SED, TP, TN and negatively affected BF, indicated by a Variable Influence on Projection (VIP) > 1.16 and large absolute regression coefficients (RCs: 0.6–0.88 for SED, TP, TN; −1.65 for BF). The change in pasture area was negatively affecting SED, TP, and TN (RCs: −0.69–−0.12, VIPs > 1) while positively affecting ET (RC: 0.09, VIP: 0.92). The change in settlement percentage had a VIP of up to 1.17 for SQ and positively and significantly influenced it (RC: 1.16, p-value < 0.001). PLSR helped to identify the key contributions from individual land use changes on water quantity and quality dynamics. These provide a quantitative basis for targeting most influential land use changes to mitigate impacts on water quality in the future.

Joint optimization of water allocation and water quality based on multi-objective control in Nanning, China

Studying the change mechanism of water quantity and quality is the basis for joint optimization of water resources system, which is a significant means for modern regional water resources management. A water quantity and quality joint optimization model is built based on multiple control objectives, which includes water demand, observed flow rate, and observed pollutant concentration. Coupled water quantity and water quality model was developed for Nanning, China. The natural water cycle and social water cycle in Nanning City and the associated pollutant transport transformation process are simulated. The results indicate that simulation error of water resources allocation is below 5%, the Nash-Sutcliffe efficiency coefficients of the three hydrological stations are 0.85, 0.88, and 0.85 respectively, and the relative errors of the simulated results of three water quality monitoring stations are all within 1.83%, all of which indicates that the model performs well and the simulation results can reproduce the water use process and pollutant transport transformation process of Nanning in time and space. This study can provide effective support for water resources management in Nanning City.