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A nine-year study was conducted to evaluate the effects of on-ranch water retention (WR) on surface flows and nutrient loads at two study sites in the Lake Okeechobee (LO) watershed, Florida. The study sites (Site 1 and Site 2) are comprised of wetland-upland hydrologic units. To implement the WR, a flashboard riser structure was installed in the drainage ditch. Measured surface flow and nutrient concentration data for four pre-WR and five post- WR years for Site 1, and three pre- and six post-WR years for Site 2 were used. Although annual surface flows decreased by 60% and 54% at Site 1 and Site 2, respectively these reductions were not statistically significant due to variability in rainfall. To reduce the effects of variable rainfall, the flows and nutrient data were compared for similar rainfall events (difference <10%). The event-based comparisons showed statistically significant (p <0.05) reductions in surface flow at both sites. Relatively less evapotranspiration and groundwater fluxes during the short span of rainfall events combined with reduced rainfall variability helped unmask the effects of WR with significant flow reductions shown during the post-WR period. Statically significant reduction (p<0.05) in both total phosphorus (TP) and total nitrogen (TN) loads were observed for Site 2, while Site 1 showed moderate evidence (p=0.051) for TP load reduction. Statistically significant reduction in TP load at Site 2 was likely due to its higher elevation and higher P adsorption capacity of the soils than Site 1. Better connectivity of landscape to flow channel combined with the presence of hot-spots (cattle feeding areas) resulted in higher P transport from Site 1, compared to Site 2. Overall, both sites were effective in reducing surface flows with Site 2 performing better than Site 1 in reducing TP load. Selection of sites similar to Site 2 was recommended for Payment for Environmental Services Programs (PES) in Florida. |
