Climate and Land Use/Cover Change Impacts on the Ecologically Relevant Flow Metrics in the Cahaba River Watershed

Abstract

This study explored the impacts of land use/cover (LULC) and climate change on hydrological responses, particularly low-flow regimes, in the rapidly urbanizing upper Cahaba River basin in north-central Alabama. The Cahaba River is identified as the longest free-flowing river in the state of Alabama, and The Nature Conservancy noted it as one of the only eight “Hotspot of Biodiversity” in the contiguous United States. Past, present, and future potential streamflow responses to LULC and climate changes were analyzed based on ecologically relevant flow metrics. We used 38 key flow metrics that capture high, low, and median flow, as well as flashiness, which are known to have significant impacts on flora and fauna. These flow metrics, thus the ecology, will certainly be affected by LULC and climate change. Daily streamflow was produced from 1988 to 2013 using historical climate and LULC data with the Soil and Water Assessment Tool (SWAT). Streamflow data from the periods of 1988-1993 and 2008-2013 were used for model calibration and validation, respectively. The SWAT-CUP calibration and uncertainty program was used for this purpose. For the base periods, the effects of different land cover maps were also analyzed by using “National Land Cover Data (NLCD)” and “Digitized Landsat 5 TM Data”. Future daily streamflows were generated with SWAT using bias corrected and downscaled CMIP5 climate data for the years 2035 to 2060 with eleven climate models under two different representative concentration pathways (RCP 2.6 and RCP 8.5). For the future LULC data, USGS EROS future projected dataset (250-meter resolution) was used. The daily streamflow from each period were fed into the Indicators of Hydrological Alterations (IHA) software to calculate the 38 flow metrics in each period. Differences in the metrics were assessed, which may hint for increase/decrease in native species’ density that may have occurred in the past or might occur in the future.