Effect of Amendment Type and Incorporation Depth on Runoff from Compacted Sandy Soils

Increased runoff rates and volumes from urbanizing watersheds are generally attributed to increased imperviousness within the watershed. While pervious surfaces in urban areas are often credited with having little runoff contribution, soil compaction can reduce infiltration capacity, leading to increased runoff. The objective of the research reported in this paper was to evaluate the hydrologic response of potential treatments for mitigating urban soil compaction. In the lysimeter study of the research reported in this paper, two Florida soils [(1) Arredondo fine sand, and (2) Orangeburg fine sandy loam] were compacted and potential mitigating treatments were applied to evaluate runoff reduction. Treatments combined two incorporation depths [(1) 10 cm, and (2) 20 cm] with three amendment cases [(1) no amendment, (2) compost, and (3) fly ash] and were applied to both soils. A set of compacted lysimeters for each soil were designated as controls and remained compacted (no treatment) throughout the research reported in this paper for comparison with amendment type and depth treatments. Runoff volumes were collected from 19 natural and simulated events over a 5-month period. Natural rainfall events ranged in depth from 4–59 mm over 3–23 h (average intensity, $1–7\mathrm{mm}/\mathrm{h}$), while simulated events ranged from 50–114 mm depths over 32–61 min (average intensity, $91–124\mathrm{mm}/\mathrm{h}$). Bulk densities, cone index profiles, and infiltration rates were also measured on each of the 42 lysimeters. Due to greater infiltration rates and lower bulk densities, tillage with or without compost at either incorporation depth produced significantly less runoff than compacted soils (mean runoff coefficients, $p<0.005$ and 0.03–0.14 compared to 0.19 and 0.46, respectively; mean effective curve numbers, 62–71 and 40–49, compared to 87 and 75, respectively). Fly ash treatments did not significantly reduce runoff production compared to controls and increased runoff production on Arredondo soil. Based on a standard method, runoff production from control lysimeters was similar to dirt roads, while tillage treatments with or without compost produced runoff similar to vegetated open space in fair condition. These results suggest that 10-cm tillage alone may be an effective practice to substantially reduce runoff generated from shallow (20–25 cm) compacted urban soils. Future research should focus on investigating shallower tillage depths and long term runoff reductions of tillage on urban soils, as well as potential effect of tillage and amendments, on landscape plant health and performance.