Use a log(area) ~ log(volume) relationship. From an email thread:
This was the starting point concerning small reservoirs although the log/log curve figure was not in it:
“Estimation of Small Reservoir Storage Capacities in a semi-arid environment. A case study in the Upper East Region of Ghana.” J. Liebe, N. van de Giesen, M. Andreini. Physics and Chemistry of the Earth, 30: 448–454, doi:10.1016/j.pce.2005.06.011, 2005
The method was repeated, updated and varied upon a few times:
“Delineation of small reservoirs using radar imagery in a semi-arid environment: A case study in the Upper East Region of Ghana.” F.O. Annor, N. van de Giesen, J. Liebe, P. van de Zaag, A. Tilmant, S.N.Odai. Physics and Chemistry of the Earth, 34:309-315, doi:10.1016/j.pce.2008.08.005, 2009
“Determining Watershed Response in Data Poor Environments with Remotely Sensed Small Reservoirs as Runoff Gauges.” J. R. Liebe, N. van de Giesen, M. Andreini, M. T. Walter, T. S. Steenhuis. Water Resources Research, 45, W07410, doi:10.1029/2008WR007369, 2009
“Suitability and limitations of ENVISAT ASAR for monitoring small reservoirs in a semi-arid area.” Jens R. Liebe, Nick van de Giesen, Marc S. Andreini, Tammo S. Steenhuis, and M. Todd Walter. IEEE Transactions on Geoscience and Remote Sensing, 47(5):1536-1547, doi:10.1109/TGRS.2008.2004805, 2009
This is the method using "virtual dams"
“Determining water reservoir characteristics with global elevation data.” Van Bemmelen, C. W. T., M. Mann, M. P. de Ridder, M. M. Rutten, and N. C. van de Giesen, Geophysical Research Letters, 43:11,278-11,286, doi:10.1002/2016GL069816, 2016
Use a log(area) ~ log(volume) relationship. From an email thread:
This was the starting point concerning small reservoirs although the log/log curve figure was not in it: “Estimation of Small Reservoir Storage Capacities in a semi-arid environment. A case study in the Upper East Region of Ghana.” J. Liebe, N. van de Giesen, M. Andreini. Physics and Chemistry of the Earth, 30: 448–454, doi:10.1016/j.pce.2005.06.011, 2005
You can find the graph in Fig 30 on page 53 of Jens' MSc thesis: http://www.iahs-icwrs.org/SmallReservoirs/WaterStorageCapacity_ThesisJens.pdf
The method was repeated, updated and varied upon a few times: “Delineation of small reservoirs using radar imagery in a semi-arid environment: A case study in the Upper East Region of Ghana.” F.O. Annor, N. van de Giesen, J. Liebe, P. van de Zaag, A. Tilmant, S.N.Odai. Physics and Chemistry of the Earth, 34:309-315, doi:10.1016/j.pce.2008.08.005, 2009 “Determining Watershed Response in Data Poor Environments with Remotely Sensed Small Reservoirs as Runoff Gauges.” J. R. Liebe, N. van de Giesen, M. Andreini, M. T. Walter, T. S. Steenhuis. Water Resources Research, 45, W07410, doi:10.1029/2008WR007369, 2009 “Suitability and limitations of ENVISAT ASAR for monitoring small reservoirs in a semi-arid area.” Jens R. Liebe, Nick van de Giesen, Marc S. Andreini, Tammo S. Steenhuis, and M. Todd Walter. IEEE Transactions on Geoscience and Remote Sensing, 47(5):1536-1547, doi:10.1109/TGRS.2008.2004805, 2009
This is the method using "virtual dams" “Determining water reservoir characteristics with global elevation data.” Van Bemmelen, C. W. T., M. Mann, M. P. de Ridder, M. M. Rutten, and N. C. van de Giesen, Geophysical Research Letters, 43:11,278-11,286, doi:10.1002/2016GL069816, 2016
And here you can find the accepted but not yet published article on deduction of operating rules: https://www.hydrol-earth-syst-sci-discuss.net/hess-2016-660/