Water quality related to Conservation Reserve Program (CRP) and cropland areas: Evidence from multi-temporal remote sensing

Water quality is affected by croplands. The Conservation Reserve Program (CRP), where farmers convert croplands to natural land cover (e.g., trees), is expected to improve water quality. However, whether such improvements are achieved alongside cropland area change has not been verified at river-basin scales, due to challenges in large scale observations. Therefore, aiming to quantify the relationship between CRP enrollment, cropland area, and the downstream water quality, we propose an approach that combines archived survey data, water quality monitoring data (total nitrogen content, TN), and remote sensing observations. By constructing the long-term datasets (1999–2014 annually) in Google Earth Engine and conducting multiple linear regression, we explained 79% variation in TN by the area of total CRP enrollment (CRP_all), area of corn and soybeans croplands, and discharge. Moreover, 78% is explained if we consider only water quality targeted conservation practices (CRP_WQ). Our results indicate significant positive correlation between CRP enrollment (both CRP_all and CRP_WQ) and the downstream water quality. Nevertheless, it should be noted that correlation does not necessarily represent causation. While this pioneer effort of quantifying impacts of the CRP on water quality from large scale observations has achieved some success, we call for more research to expand the spatial and/or temporal scales and consider more water quality variables, so as to further enhance our understanding of the coupled natural-and-human system.