¹Andrew Keller originally presented the subject of this paper at the 1998 World Bank Water Week Conference, 15 December 1998, Annapolis, Maryland, USA, in a session on dams. The title of that presentation was "Water Scarcity and the Role of Dams in Development." For this paper, we changed the title, substituting the broader term "storage" for "dams," to reflect the importance of increasing storage, regardless of type, to address water scarcity.

2Flow to sinks is an economic as well as physical concept. Sometimes it can be prevented, but at an unacceptably high cost.

³Structures less than 15 m high and embankment volume less than 0.75 mcm.

⁴For aquifers with specific storage capacities of 10%, a typical value, a 2-meter decline in water level represents about 200 mm of actual water. Thus, where groundwater levels are falling 2 m per year, extractions are exceeding recharge by apprfoximately 200 mm per year.

⁵We note that some have serious reservations about the validity of this sedimentation figure. While we were unable to validate the number, we believe that if correct, it is alarming and important to point out.

⁶Many of the "unsafe" dams in the US were rendered so by changes in the applicable design standards—especially the switch to probable maximum flood (PMF)for spillway capacity. In addition. many dams in the US were built privately with less control of standards than is often the case outside the US.

⁷We believe this number is an anomaly of the reporting in the ICOLD dam registry and that the percentage of multipurpose large dams is likely much greater than 30.

⁸Based on an allowable critical flow velocity in earthen channels of around 1m/s. Lined sections may have twice this flow velocity. Where gradients are shallow, such as in Egypt's Nile Delta, flow rates are much slower.

9Most of the recently decommissioned dams in the United States are hydropower dams, which, besides having adverse environmental Impacts, suffer from dam safety and other issues and are not economical to repair or upgrade.

¹⁰HAD statistics are from Gleick 1993 and the minor tank numbers are derived from Sakthivadivel et al. 1997.

¹¹Small tanks (by definition) only submerge a few feet up the sides of a valley—the rest is valley floor. Large dams flood a lot of non-valley floor area that is usually less productive land.

¹²Note that there are several large storage facilities in Sri Lanka.

¹³These extremely low-cost dams are generally concrete arch or gravity dams. An example of such a low-cost (US$1.00/1,000 m³) dam is the Kariba Dam in Zambia and Zimbabwe, which resulted in one of the world's largest reservoirs (by volume, right behind HAD's Lake Nasser). However, we note that Kariba had high environmental and social costs associated with it, which are not reflected in the dollar cost of the dam.(Kariba is a case study of the International Commission on Dams.)

¹⁴The Los Vaqueros Project was winner of the 1999 Outstanding Civil Engineering Achievement award; largely because of the way it addressed environmental concerns (Hunt 1999).

¹⁵Note that the effect of sedimentation on total reservoir yield for reservoirs with capacities two or more times their mean annual inflow is relatively small compared to losses due to sedimentation in reservoirs with capacities smaller than their mean annual inflow.