I’m scoping a hydrologic impact assessment for a 180-acre solar array on clay-loam in the San Joaquin, and HEC-HMS indicates our post-build 10-year peak could jump about 18% without infiltration features. What have you seen work to keep pre/post hydrographs within about ±10% — vegetated swales, micro-basins, amended soils, or something else — while staying maintainable over a 20-year O&M horizon?
On a 160-acre clay‑loam array near Los Banos, we held the 10‑year peak to about ±8% by pairing shallow meadowed swales between rows, compost-amending the top 12 inches, and adding a short train of small check‑dammed micro‑basins at the downslope fence, then modeling with HEC‑HMS Green‑Ampt using field Ksat. If you’re grazing, compaction will nudge peaks back up — plan a light rip every couple years and armor the low points; think of the panels as a thousand tiny roofs. For maintenance details, CASQA’s BMP handbook is a solid baseline: https://www.casqa.org/resources/bmp-handbooks.