Common use of Sampling Frequency Clause in Contracts

Sampling Frequency. EO data on optical properties of inland waters are typically available every few days (e.g. Sentinel 2 and 3). For algorithm calibration, the temporal match with the satellite overpass should be short, ranging from 1 hour to ~3 days. The ideal time difference depends on the temporal variability of the water quality parameter of interest, so defining a specific frequency is context dependent. A recommendation is to derive match-ups within 3 hours. Measurements taken during late morning (local time) are closest to the overpasses of optical sensors across the globe. For providing match-up with EO data, sampling frequency of in-situ data should by high, ideally, every day or every few days. Permanent measurement installations help to provide a large number of match-ups, though not all parameters can be measured with automated systems. Sampling frequencies of monthly or less are likely to have fewer close match-ups with satellite data, unless monthly in-situ sampling programmes are explicitly designed to ensure match-up with satellite overpasses. Sampling frequency varies a lot across the different variables and depends on the measurement technique. The frequency ranges from every few minutes for parameters that can be measured automatically using sensors (e. g. temperature, dissolved oxygen, chlorophyll-a fluorescence, turbidity) to parameters that need to be processed in a laboratory, which are more typically sampled at weekly to monthly frequency (e.g. microbiology, cyanobacteria, chlorophyll-a concentration, suspended matter concentration). The possibility of making campaigns though the year or years, also determines the sampling frequency, from months to years. Monthly sampling is most common and corresponds to the sampling frequency typically required for monitoring inland water quality by regulations (e.g. the European Water Framework Directive). However, hourly to weekly sampling can be required for some water quality parameters for research or investigative monitoring purposes to adequately capture natural variability (e.g. diurnal changes in dissolved oxygen or turbidity changes associated with storm events). Permanent measurement stations such as AERONET-OC stations and the GLEON network provide very valuable data sets as they enable match-ups with each overflight and provide key data on reflectance and/or water quality in very many different conditions (light, sun/viewing angles, atmospheric conditions).