## 1 Introduction  
Surface water is an important source of water, providing most of our basic water needs. However, surface water pollution issues have been very serious in recent years. Surface water pollution, to a great extent, damages the ecological environment and directly affects people's health [1]. The chemical oxygen demand (COD) in surface water is an important indicator of the degree of surface water pollution, which can reflect the level of oxygen-consuming organic pollutants in surface water. The determination of COD is particularly important in the analysis of water pollution [2]. Currently, the determination of COD in China mainly adopts the national standard chemical method [3]. Although the measurements of the national standard chemical method are accurate, the process is cumbersome and generally requires heating, reaction, and other steps. Other reactants are also required, which causing secondary pollution. This method also requires a long sample transfer time and reaction time, which is not convenient for field use [4]. Ultraviolet-visible (UV-Vis) absorption spectroscopy is a physical method for detecting COD concentration, whose process is simple without secondary pollution. This method can measure COD in real time [5]. Through online detection of COD concentration, the pollution status of surface water can be known in real time, which is of great significance to protect the surface water environment. When the concentration of COD in water is detected by ultraviolet-visible absorption spectrometry, the absorbance at 254 nm is usually used to obtain the COD measurements, which is called the single-wavelength method [6]. The single-wavelength method is simple, but its stability is poor, as it is easily interfered with, and the measurement range is limited. [PERSON] proposed a new method for selecting different calibration wavelengths based on the COD value to expand the measurement range. However, this method is essentially a single-wavelength method in different measurement ranges, and also needs a turbidity compensation algorithm to remove turbidity interference [7]. The accuracy of the turbidity compensation algorithm inevitably affected the detection accuracy of COD, so high precision was put forward for the turbidity compensation algorithm. Aiming to improve the accuracy of COD detection, the multi-wavelength method was proposed to detect COD. It is important to select appropriate characteristic wavelengths for the multi-wavelength method. Adaptive weighting algorithm (CARS), random frog, and genetic algorithm were used to determine the characteristic wavelengths of COD in aquaculture water by the multi-wavelength method [8]. These methods to extract characteristic wavelengths improve the accuracy of COD measurement in complex water samples. But each time the program runs, the distribution of selected wavelengths may change [9; 10; 11]. Therefore, these methods have problems of reliability and stability. In order to eliminate the influence of turbidity in COD detection, turbidity compensation is necessary. The absorbance at 546 nm in UV-Vis absorption spectrum was used for turbidity compensation [12]. Although the absorbance at 546 nm is simple for turbidity compensation, the accuracy of COD detection with compensation is not satisfactory. [PERSON] et al. used the fourth derivative method to remove turbidity interference [13]. The fourth-order derivative method has the advantage of no baseline and reduces the signal strength of COD, resulting in the reduction of signal-to-noise ratio, finally affecting the accuracy of COD detection. In this paper, a novel hybrid strategy for detecting COD in surface water was proposed. A new characteristic parameter, the extreme value of the absorption spectrum change rate, was used as a characteristic parameter to determine the characteristic wavelengths. A combined measurement method based on the first derivative method, the permutation entropy method, and the partial least squares method (FDPE_PLS) was proposed.