Effects of Ranchland Water Retention on Water and Nutrient Discharges in the Lake Okeechobee basin

Abstract:

A long-term study was conducted for runoff and water quality evaluation of the water retention (WR) best management practice (BMP) implemented at two pastures (sites1 and 2) in a ranch in the Lake Okeechobee (LO) watershed, Florida. The BMP was implemented by raising the spillage levels using drainage structures in the ditch that drained the wetland and upland areas. Four-year pre-BMP and 5-year post-BMP data from Site 1 and 3-year pre-BMP and 6-year post-BMP data from Site 2 were used to evaluate the BMP. We compared runoff, Total phosphorus (TP) and Total nitrogen (TN) loads and concentrations between pre- and post-BMP periods. Compared to pre-BMP, annual post-BMP runoff reduced for Site 1 (60%) and Site 2 (54%). These reductions were only statistically significant for Site 2 (p = 0.096). For the wetter part of the rainy season (July-October), when damaging excessive flows to the LO occur, the post-BMP runoff reduced significantly (p= 0.049) for Site 1 but not for Site 2. Reductions were mainly due to increased water storage while the reductions in nutrient loads and average concentrations were due to reduced runoff volume as well as P retention by soil and plants. Despite reductions in average concentrations, the wet-season post-BMP TP flow weighted concentration (FWC) increased significantly (p= 0.05) at Site 1. Increased TP FWC was likely due to increased inundation in wetland and its connectivity with upland pasture during the post-BMP period. This connectivity increased the surface transport of P, the preferred mode for particulate and dissolved P transport. Unlike Site 1, the TP FWC at Site 2 decreased significantly (p = 0.002) during the wet season due to absence of P hotspots and their connectivity to the wetland. Results indicate that while pasture WR can reduce runoff from the ranchlands, it may increase the TP loads depending on the topographic and drainage characteristics and presence of P hotspots within a ranch.