## 1 Introduction
In addition to the parameters of chlorophyll a, suspended solids and yellow substances, the water quality parameters also include turbidity, chemical oxygen demand, five-day biochemical oxygen demand (BOD5), transparency, total phosphorus and total nitrogen, etc. [12; 13; 14; 15; 16; 17; 18; 19; 20]. Multispectral imaging uses different objects to acquire different reflection characteristics for each band spectrum, which is very convenient for resource investigation and environmental exploration. Traditionally, the carriers used to collect multispectral images using satellites and aviation aircraft or ground surveys. At present, some research groups still use satellite telemetry technology to obtain spectral image information for analysis. However, the use of satellite remote sensing influencing systems or the regular monitoring of aquatic ecosystems has two main limitations: First, the significant disturbance to the atmosphere, which may prevent observers from immediately obtaining information for a long time. Second, the poorer spatial resolution of the remote sensing on the satellite. In general, multispectral sensors designed for water applications are mainly used for seawater monitoring (e.g., medium-resolution imaging spectrometers, ocean and land color instrument), but their spatial resolution will be coarser and can detect larger areas. Therefore, the sensors currently used to monitor most inland waters are those originally designed for onshore applications, such as the Sentinel 2 Multispectrometer (MSI) and the Landsat 8 Operational Land Imager. These facts will lead to limitations of the regression model and an inversion of the accuracy. In addition, the multispectral image data collected by traditional satellites and aircraft face the problems of a long cycle time, high cost and the meter-level resolution. Fortunately, with the development of modern telemetry technology and improvements in computing power and camera image resolution, the data obtained in recent years are more accurate and realistic. More importantly, as the emerging development of unmanned aerial vehicle (UAV) technology becomes more mature, image resolution has been reduced from meters to centimeters. Drones are being widely employed in the logistic, commercial, precision agriculture, military defense and national security fields. Recently, a vast amount of research literature, particularly in the multispectral imaging as well as remote sensing results (with satellite or UAV), has been published for the application fields of water quality monitoring or prediction [21; 22; 23; 24; 25], bathymetry modeling [26], atmospheric observation [27], vegetation index evaluation [28; 29; 30; 31] and precision agriculture [32; 33; 34], etc. Using a drone equipped with a multispectrometric instrument to collect multispectral image data during low-altitude flight can make the image resolution reach a level higher than the standard, consequently greatly improving the details of the multispectral image and the ability to interpret the characteristics of ground objects or water bodies. Based on these facts and merits, a drone equipped with a multispectrometer was proposed to acquire image data. The optical raw data were then converted into reflectance for numerical regression analysis. On the other hand, the water samples simultaneously collected were sent to Taiwan SGS Co., Ltd. for biochemical analysis and validation of water quality parameters, which are introduced to the reflectance of five multispectral bands as to obtain the optimal regression models in terms of multispectral band combination for water quality parameters. To explore more possibilities of interpretation models, algorithmic research includes linear, band ratio and even logarithmic regression for multi-band expression. With these water parametric models, Carlson trophic state index (CTSI), representing the benchmark of water pollution status, can be acquired and predicted for the first time through optical image data and sampling analysis results, respectively.