diff --git a/docs/examples/calibration/calibration.ipynb b/docs/examples/calibration/calibration.ipynb
index a97dedf..9513800 100644
--- a/docs/examples/calibration/calibration.ipynb
+++ b/docs/examples/calibration/calibration.ipynb
@@ -895,7 +895,9 @@
     "\n",
     "Dong, J., & Ochsner, T. E. (2018). Soil texture often exerts a stronger influence than precipitation on mesoscale soil moisture patterns. Water Resources Research, 54(3), 2199-2211.\n",
     "\n",
-    "Patrignani, A., Ochsner, T. E., Montag, B., & Bellinger, S. (2021). A novel lithium foil cosmic-ray neutron detector for measuring field-scale soil moisture. Frontiers in Water, 3, 673185."
+    "Patrignani, A., Ochsner, T. E., Montag, B., & Bellinger, S. (2021). A novel lithium foil cosmic-ray neutron detector for measuring field-scale soil moisture. Frontiers in Water, 3, 673185.\n",
+    "\n",
+    "Schrön, M., Köhli, M., Scheiffele, L., Iwema, J., Bogena, H. R., Lv, L., Martini, E., Baroni, G., Rosolem, R., Weimar, J., Mai, J., Cuntz, M., Rebmann, C., Oswald, S. E., Dietrich, P., Schmidt, U., and Zacharias, S.: Improving calibration and validation of cosmic-ray neutron sensors in the light of spatial sensitivity, Hydrol. Earth Syst. Sci., 21, 5009–5030, https://doi.org/10.5194/hess-21-5009-2017, 2017."
    ]
   },
   {