The stable isotopes of oxygen and hydrogen in the water cycle have become a significant tool to study run-off formation, hydrograph separation, and the origin of precipitation. Precipitation assessment based on isotopic data has a potential implication for moisture sources. In the study, δD and δ18O of precipitation samples collected from six rainfall events were analyzed for stable isotope composition to provide implication of isotopic characteristics as well as moisture sources in Hemuqiao basin within Lake Tai drainage basin, eastern China. In these events, stable oxygen and hydrogen isotopic composition of precipitation had strong variations. Models of the meteoric water line and deuterium excess for different rainfall types (typhoon and plum rain, which is caused by precipitation along a persistent stationary front known as the Meiyu front for nearly two months during the late spring and early summer between eastern Russia, China, Taiwan, Korea and Japan) were established. Compared with plum rain, the moisture source of typhoon events had higher relative humidity and temperature. Moisture transport pathways were traced using the Hybrid Single-Particle Lagrangian Integrated Trajectory Model (HYSPLIT Model, developed by NOAA, Washington DC, U.S.) to verify the linkage with isotopic composition and moisture source. The moisture sources of typhoon events mostly derived from tropical ocean air with higher isotopic value, while that of plum rain events came from near-source local air with lower isotopic value.

To date, floods have become one of the most severe natural disasters on Earth. Flood forecasting with hydrological models is an important non-engineering measure for flood control and disaster reduction. The Xin’anjiang (XAJ) model is the most widely used hydrological model in China for flood forecasting, while the Soil and Water Assessment Tool (SWAT) model is widely applied for daily and monthly simulation and has shown its potential for flood simulation. The objective of this paper is to evaluate the performance of the SWAT model in simulating floods at a sub-daily time-scale in a slightly larger basin and compare that with the XAJ model. Taking Qilijie Basin (southeast of China) as a study area, this paper developed the XAJ model and SWAT model at a sub-daily time-scale. The results showed that the XAJ model had a better performance than the sub-daily SWAT model regarding relative runoff error (RRE) but the SWAT model performed well according to relative peak discharge error (RPE) and error of occurrence time of peak flow (PTE). The SWAT model performed unsatisfactorily in simulating low flows due to the daily calculation of base flow but behaved quite well in simulating high flows. We also evaluated the effect of spatial scale on the SWAT model. The results showed that the SWAT model had a good applicability at different spatial scales. In conclusion, the sub-daily SWAT model is a promising tool for flood simulation though more improvements remain to be studied further.

The geomorphologic instantaneous unit hydrograph (GIUH) is an applicable approach that simulates the runoff for the ungauged basins. The nash model is an efficient tool to derive the unit hydrograph (UH), which only requires two items, including the indices n and k. Theoretically, the GIUH method describes the process of a droplet flowing from which it falls on to the basin outlet, only covering the flow concentration process. The traditional technique for flood estimation using GIUH method always uses the effective rainfall, which is empirically obtained and scant of accuracy, and then calculates the convolution of the effective rainfall and GIUH. To improve the predictive capability of the GIUH model, the Xin’anjiang (XAJ) model, which is a conceptual model with clear physical meaning, is applied to simulate the runoff yielding and the slope flow concentration, integrating with the GIUH derived based on Nash model to compute the river network flow convergence, forming a modified GIUH model for flood simulation. The average flow velocity is the key to obtain the indices k, and two methods to calculate the flow velocity were compared in this study. 10 flood events in three catchments in Fujian, China are selected to calibrate the model, and six for validation. Four criteria, including the time-to-peak error, the relative peak flow error, the relative runoff depth error, and the Nash–Sutcliff efficiency coefficient are computed for the model performance evaluation. The observed runoff value and simulated series in validation stage is also presented in the scatter plots to analyze the fitting degree. The analysis results show the modified model with a convenient calculation and a high fitting and illustrates that the model is reliable for the flood estimation and has potential for practical flood forecasting.

The hydrology response was studied considering the established fact of land use change in Dapoling basin. The whole period was divided into two (1965–1985 and 1986–2012) according to the major land use and land cover change in this region. Xinanjiang model was used to simulate discharge data in the two periods. The hydrologic response to the change could be evaluated by inspecting the response of model parameters and flood elements. The results show that the lag time varied, and the hydrologic elements including the mean runoff depth, flood peak and kurtosis coefficient varied with the rainfall depth. This result is significant for studying the response of runoff characteristic from land use and land cover change.