Joseph E. Berlin, AECOM, 7389 Florida Blvd Suite 300, Baton Rouge, LA 70806 Joseph.Berlin@aecom.com
Most large reservoirs in the U.S. are multi-purpose reservoirs, which may be used for providing water supply, flood risk management, hydropower, recreation, and maintenance of minimum river flows downstream for environmental protection. The storage capacity of a reservoir is typically allocated between several of these conflicting purposes.
When a reservoir is used to provide water supply, whether for agricultural or municipal and industrial (M&I) use, that use is usually given priority. Maintaining water supply requires filling capacity to maintain a minimum reservoir level such that daily demands can be met based upon historical weather patterns and river flows. Flood risk management requires maintaining empty reservoir capacity to prevent excess discharge from the reservoir during significant rain events, and as a use of reservoir capacity competes directly with water supply. Hydropower can be generated any time the reservoir pool exceeds the minimum required for other uses, but the highest revenue is generated during periods of peak power demand. Recreational use generally requires that reservoir pools be maintained within a specified range to allow boating access. Environmental use requires that minimum outflows be maintained, especially during drought conditions, and these water needs can be very high if the goal is to prevent saltwater intrusion into wetlands or deterioration of other flora and fauna downstream of the reservoir.
Agricultural water demand is forecasted based upon changes in farming patterns, such as increased irrigation for new crops, or replacement of groundwater sources from aquifer depletion. M&I water demand forecasting is based primarily upon demographic and economic trends and industrial development. Domestic users may switch to public supplies as aquifers are depleted. New industrial development may consume large amounts of water. Hydraulic fracturing for petroleum exploration may also consume large amounts of water, and this water use is particularly difficult to forecast.
When additional water supply from a reservoir is needed, the forecast of needs is important for determining impacts upon other uses. Software such as HEC-RAS is used to determine the impact of additional withdrawals for water supply upon other reservoir uses.
Texas needs increased water supply because of population growth and a long term drought and has expressed an interest in reallocating the storage capacity of the Toledo Bend Reservoir to water supply. The Toledo Bend Reservoir is jointly controlled by Texas and Louisiana and the portion in Louisiana is by far the State’s largest reservoir. The Toledo Bend Reservoir provides recreation and hydropower, in addition to providing some water supply to Louisiana communities. Some coastal restoration plans call for greater environmental use to reduce saltwater intrusion in southwest Louisiana. The impact upon these other uses will need to be evaluated before an increase in diversions to Texas could be implemented. Although the entire re-allocation analysis performed for a federal reservoir reallocation study is not necessary for a non-federal reservoir, some of the evaluation steps would provide valuable insight to decision makers regarding the impact upon other uses.
Key words: reservoir, allocation, demand, forecasting