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Cropland is an important land cover influencing global carbon and water cycles. Variability of agricultural carbon and water fluxes depends on crop species, management practices, soil characteristics, and climatic conditions. In the context of climate change, it is critical to quantify the long-term effects of these environmental drivers and farming activities on carbon and water dynamics. Twenty site-years of carbon and water fluxes covering a large precipitation gradient and a variety of crop species and management practices were measured in the inland Pacific Northwest using the eddy covariance method. The rain-fed fields were net carbon sinks, while the irrigated site was close to carbon neutral during the winter wheat crop years. Sites growing spring crops were either carbon sinks, sources, or neutral, varying with crops, rainfall zones, and tillage practices. Fluxes were more sensitive to variability in precipitation than temperature: annual carbon and water fluxes increased with the increasing precipitation while only respiration increased with temperature in the high-rainfall area. Compared to a nearby rain-fed site, irrigation improved winter wheat production but resulted in large losses of carbon and water to the atmosphere. Compared to conventional tillage, no-till had significantly lower respiration but resulted in slightly lower yields and water use efficiency over 4 years. Under future climate change, it is expected that more carbon fixation by crops and evapotranspiration would occur in a warmer and wetter environment.