Uncertainty reduction maps with varying spatial correlation length scale (l_s). Maps are shown for l_s = 0, 3, 10, and 30 km. Uncertainty reductions are averaged over September 2015. At small length scales (top panels) uncertainty reduction is confined to small areas of high reduction surrounding observation sites, whereas at larger length scales (bottom panels) uncertainty reduction is more widespread across the county domain. DOI: https://doi.org/10.1525/elementa.375.f7