Cumulative effects of small reservoir construction
Ignatius, Amber Rose
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Despite the critical importance of water management in the Southeastern U.S., policymakers and water professionals have little information regarding the ecological impacts of the region’s several thousand small artificial reservoirs. Often less than one ha in size, small reservoirs disrupt hydrological connectivity, fragment habitat, distort water-level fluctuations, contribute to evaporative losses, impact sediment distribution, alter water temperature, and modify water chemistry. To address the aggregate effects of small reservoir construction in the Georgia Piedmont, this study fulfills the following main objectives: 1) assessment of a geographic database of reservoir construction patterns and distribution over time; 2) estimation of evaporative losses from small reservoirs using the Soil Water Assessment Tool (SWAT) hydrologic model; and 3) evaluation of physicochemical water alteration trends upstream and downstream of reservoirs within different land cover settings along an urban-rural gradient. Within the Chattahoochee River Watershed study area (313 km2), analysis of historic aerial photography from 1950-2010 revealed the number of reservoirs increased seventeen-fold while the area inundated by water increased nearly six-fold (19 reservoirs covering 0.16% of the study area in 1950 to 329 reservoirs covering 0.95% of the study area by 2010). During the sixty-year period, 33-53% of reservoirs were located on-streams, causing between 10-109 stream fragmentations at any given time. In the second study, SWAT was used to model evaporation from small reservoirs in the 1,937 km2 Upper Oconee Watershed. The inclusion of small reservoirs does not increase the predictive ability of the SWAT streamflow simulation. However, additional evaporation caused by artificially created open water was substantial, averaging 0.015-0.020 km3/year. While water bodies covered only 1.14% of the study area, they contributed to between 2.22-2.75% of basin-wide evapotranspiration. Finally, physicochemical water quality parameters were monitored upstream and along the reaches downstream from nine small reservoirs. Agricultural and forested land covers were inversely correlated (positive and negative correlations, respectively) with reservoir alkalinity, total nitrogen, nitrate, and specific conductivity. Small reservoirs decreased downstream nitrate values and top-release dam structures elevated downstream dissolved oxygen, temperature, and pH. Elevated temperatures and decreased dissolved oxygen values were reduced but still observable 250 m downstream.