ABSTRACT: Despite the growth of large-sample hydrology (LSH) datasets, water quality data remain scarce. Here, we introduce CAMELS-CH-Chem, an extension of CAMELS-CH (Catchment Attributes and Meteorology for Large-sample Studies in Switzerland), incorporating up to 40 water quality parameters for 115 Swiss catchments from 1981 to 2020. The dataset includes hourly and daily time series of measurements of water temperature, dissolved oxygen, pH, and electrical conductivity; as well as (bi)monthly measurements of dissolved organic carbon (DOC), total organic carbon (TOC), alkalinity ( HCO4- ), ammonium ( NH4+ ), NO3- , NO2- , total nitrogen, dissolved reactive phosphorus, total filtered phosphorus, total phosphorus, Ca²⁺, Mg²⁺, Na⁺, K⁺, Cl⁻, SO42- , H4SiO4, SO42- , total hardness,
