1：Experimental Design and Method Selection:

The study selected the Yufuhe River Basin, a typical karst watershed in northern China, to study the composition and source of DOM using fluorescence spectroscopy and PARAFAC. The properties and behavior of FDOM in the recharge, runoff, and discharge systems in this karst watershed were conducted to explore the FDOM transformation.

2：Sample Selection and Data Sources:

22 surface water and ground water samples were collected in May 2016 according to different land use and lithology. They were packed into 500-mL plastic bottles for chemical and fluorescence analyses without bubbles.

3：List of Experimental Equipment and Materials:

A WTW instrument (WTW muti-3420) for pH, conductivity, temperature, and dissolved oxygen (DO) tests; a portable turbidimeter (HACH 2100Q) for turbidity measurement; a Shimadzu UV-2450PC UV-Vis spectrometer for absorbance spectra; a Hitachi F-7000 fluorescence spectrometer for 3D excitation-emission matrix spectra of DOM.

4：Experimental Procedures and Operational Workflow:

The collected water samples were filtered into glass bottles pre-combusted at 550 °C for 6 h through glass fiber filters (0.22 μm). Absorbance spectra were obtained between 200 and 800 nm at 1-nm intervals. The three-dimensional excitation-emission matrix spectra of DOM were measured using a Hitachi F-7000 fluorescence spectrometer.

5：22 μm). Absorbance spectra were obtained between 200 and 800 nm at 1-nm intervals. The three-dimensional excitation-emission matrix spectra of DOM were measured using a Hitachi F-7000 fluorescence spectrometer. Data Analysis Methods:

5. Data Analysis Methods: Fluorescence EEMs were modeled by PARAFAC, which uses an alternating least-squares algorithm to minimize the sum of squared residuals across the dataset and to estimate the underlying structure of the EEM. The DOMfluor toolbox of the Matlab software package was used for the PARAFAC model’s implementation and validation.