Watershed Runoff Calculator
ANA›Life Services Authority›National Calculator Authority›Watershed Runoff Calculator
.calc-container { max-width: 640px; margin: 2rem 0; padding: 1.5rem; background: #fff; border: 1px solid #ddd; border-radius: 8px; box-shadow: 0 1px 3px rgba(0,0,0,0.06); font-family: system-ui, -apple-system, sans-serif; } .calc-container h3 { font-family: Georgia, serif; font-size: 1.15rem; color: #1a1a1a; margin-bottom: 1rem; padding-bottom: 0.5rem; border-bottom: 2px solid var(--ac, #3d5a80); } .calc-row { display: flex; align-items: center; gap: 0.75rem; margin-bottom: 0.75rem; flex-wrap: wrap; } .calc-row label { min-width: 160px; font-size: 0.9rem; color: #333; font-weight: 500; } .calc-row input[type="number"], .calc-row select { flex: 1; min-width: 120px; max-width: 200px; padding: 0.5rem 0.6rem; border: 1px solid #ccc; border-radius: 4px; font-size: 0.9rem; font-family: system-ui, sans-serif; color: #1a1a1a; background: #fafaf8; } .calc-row input:focus, .calc-row select:focus { outline: none; border-color: var(--ac, #3d5a80); box-shadow: 0 0 0 2px rgba(26,74,138,0.12); } .calc-row .unit { font-size: 0.82rem; color: #888; min-width: 30px; } .calc-btn { display: inline-block; margin-top: 0.5rem; padding: 0.55rem 1.5rem; background: var(--ac, #3d5a80); color: #fff; border: none; border-radius: 4px; font-size: 0.9rem; font-weight: 600; cursor: pointer; font-family: system-ui, sans-serif; } .calc-btn:hover { opacity: 0.9; } .calc-result { margin-top: 1.25rem; padding: 1rem 1.25rem; background: #f0f6fc; border-left: 3px solid var(--ac, #3d5a80); border-radius: 0 6px 6px 0; display: none; } .calc-result.visible { display: block; } .calc-result-label { font-size: 0.78rem; text-transform: uppercase; letter-spacing: 0.06em; color: #666; margin-bottom: 0.25rem; } .calc-result-value { font-size: 1.6rem; font-weight: 700; color: var(--ac, #3d5a80); } .calc-result-detail { font-size: 0.85rem; color: #555; margin-top: 0.5rem; line-height: 1.5; } .calc-note { margin-top: 1rem; font-size: 0.8rem; color: #888; font-style: italic; } .calc-grid { display: grid; grid-template-columns: 1fr 1fr; gap: 0.75rem; margin-top: 0.75rem; } .calc-grid-item { padding: 0.6rem 0.8rem; background: #f8f9fa; border-radius: 4px; border: 1px solid #eee; } .calc-grid-item .label { font-size: 0.75rem; color: #888; text-transform: uppercase; letter-spacing: 0.04em; } .calc-grid-item .value { font-size: 1.1rem; font-weight: 600; color: #1a1a1a; } @media (max-width: 720px) { .calc-row { flex-direction: column; align-items: flex-start; gap: 0.3rem; } .calc-row label { min-width: auto; } .calc-row input[type="number"], .calc-row select { max-width: 100%; width: 100%; } .calc-grid { grid-template-columns: 1fr; } } .calc-chart { margin: 1rem 0; text-align: center; } .calc-chart svg { max-width: 100%; height: auto; } .calc-chart-legend { display: flex; flex-wrap: wrap; justify-content: center; gap: 0.6rem 1.2rem; margin-top: 0.6rem; font-size: 0.8rem; color: #555; } .calc-chart-legend span { display: inline-flex; align-items: center; gap: 0.3rem; } .calc-chart-legend i { display: inline-block; width: 10px; height: 10px; border-radius: 2px; font-style: normal; } .calc-related { max-width: 640px; margin: 2rem 0 1rem; padding: 1.25rem 1.5rem; background: #f8f9fa; border: 1px solid #e8e8e8; border-radius: 8px; } .calc-related h3 { font-family: Georgia, serif; font-size: 1rem; color: #1a1a1a; margin: 0 0 0.75rem; padding-bottom: 0.4rem; border-bottom: 2px solid var(--ac, #3d5a80); } .calc-related-list { list-style: none; padding: 0; margin: 0 0 0.75rem; display: grid; grid-template-columns: 1fr 1fr; gap: 0.4rem 1.5rem; } .calc-related-list li a { font-size: 0.88rem; color: var(--ac, #3d5a80); text-decoration: none; } .calc-related-list li a:hover { text-decoration: underline; } .calc-browse-all { margin: 0.5rem 0 0; font-size: 0.9rem; font-weight: 600; } .calc-browse-all a { color: var(--ac, #3d5a80); text-decoration: none; } .calc-browse-all a:hover { text-decoration: underline; } @media (max-width: 720px) { .calc-related-list { grid-template-columns: 1fr; } }
Watershed Runoff Calculator
Estimate surface runoff depth, volume, and peak discharge using the USDA SCS/NRCS Curve Number (CN) method.
Rainfall Depth (inches)
SCS Curve Number (CN)
Watershed Area (acres)
Time of Concentration, Tc (hours)
Calculate
function watCalc() { var resultDiv = document.getElementById('wat-result'); resultDiv.style.display = 'block';
var P = parseFloat(document.getElementById('wat-rainfall').value); var CN = parseFloat(document.getElementById('wat-cn').value); var A = parseFloat(document.getElementById('wat-area').value); var Tc = parseFloat(document.getElementById('wat-tc').value);
// --- Input Validation --- var errors = []; if (isNaN(P) || P 100) errors.push("Curve Number must be between 1 and 100."); if (isNaN(A) || A 0) { resultDiv.innerHTML = 'Please fix the following:' + errors.map(function(e){ return ''; }).join('') + ''; return; }
// --- SCS Curve Number Method --- // Potential maximum retention: S = (1000 / CN) - 10 [inches] var S = (1000.0 / CN) - 10.0;
// Initial abstraction: Ia = 0.2 * S [inches] var Ia = 0.2 * S;
// Runoff depth Q [inches] // If P Ia) { Q = Math.pow(P - Ia, 2) / (P - Ia + S); }
// Runoff volume V [acre-feet] // V = Q (inches) * A (acres) / 12 var V_acft = (Q * A) / 12.0;
// Runoff volume in cubic feet var V_cuft = V_acft * 43560.0;
// Peak discharge using SCS Rational-like approximation (SCS TR-55 Unit Hydrograph): // qp = (484 * A * Q) / Tc [cfs] // where 484 is the SCS peak rate factor (dimensionless, standard for average conditions) // A in square miles for this formula: A_sqmi = A / 640 var A_sqmi = A / 640.0; var qp = (484.0 * A_sqmi * Q) / Tc;
// Runoff coefficient C (ratio of runoff to rainfall) var C = (P > 0) ? (Q / P) : 0.0;
// --- Format Output --- var html = '### Results '; var rows = [ ["Potential Max Retention (S)", S.toFixed(3), "inches"], ["Initial Abstraction (Ia = 0.2S)", Ia.toFixed(3), "inches"], ["Runoff Depth (Q)", Q.toFixed(3), "inches"], ["Runoff Volume", V_acft.toFixed(3), "acre-feet"], ["Runoff Volume", V_cuft.toFixed(0), "cubic feet"], ["Peak Discharge (qp)", qp.toFixed(2), "cfs"], ["Runoff Coefficient (Q/P)", C.toFixed(3), "dimensionless"] ];
rows.forEach(function(r, i) { var bg = i % 2 === 0 ? '#f4f8fb' : '#ffffff'; html += '' + '' + r[0] + '' + '' + r[1] + '' + '' + r[2] + '' + ''; }); html += '';
if (Q === 0) { html += 'Note: Rainfall does not exceed initial abstraction (P ≤ Ia = ' + Ia.toFixed(3) + ' in). No surface runoff is generated.
'; }
resultDiv.innerHTML = html; }
#### Formulas Used
SCS/NRCS Curve Number Method (TR-55):
- Potential Max Retention: S = (1000 / CN) − 10 [inches]
- Initial Abstraction: Ia = 0.2 × S [inches]
- Runoff Depth: Q = (P − Ia)² / (P − Ia + S) when P > Ia; else Q = 0 [inches]
- Runoff Volume: V = Q × A / 12 [acre-feet]
- Peak Discharge (SCS TR-55): qp = 484 × Ami² × Q / Tc [cfs]
#### Assumptions & References
- Initial abstraction is assumed as Ia = 0.2S (standard assumption; some studies use 0.05S for urban areas).
- Time of concentration (Tc) should be estimated using TR-55 sheet flow, shallow concentrated flow, and channel flow methods.
- References: USDA NRCS TR-55 (1986), NRCS National Engineering Handbook Part 630 (2004).
More Calculators
- Emergency Fund Calculator: How Many Months of Expenses to Save
- Hardwood Floor Refinishing Cost Calculator
- Home Charging Cost Calculator
- Pressure Washing Cost Calculator
- Florida Mold & Mildew Risk Calculator
- Hurricane Season Deep Clean Cost Estimator
- Commercial Property Cap Rate Calculator
- Energy Efficiency Savings Calculator
- SEER Savings Calculator
- Air Filter Replacement Interval Calculator
- Heat Pump vs AC Cost Comparison Calculator
- Austin Climate Load Calculator
Read Next
Study Time Planner ANA › Life Services Authority › National Calculator Authority › Study Time Planner .calc-container { max-width: 640px; margin:...