Infiltration Capacity Decay Horton Calculator
Our hydrology & water resources calculator computes infiltration capacity decay horton accurately. Enter your values for instant results.
Formula
f(t) = fc + (f0 - fc) x e^(-kt)
Horton's equation f(t) = fc + (f0 - fc) x e^(-kt) describes how soil infiltration capacity decays exponentially over time. f0 is the initial (maximum) infiltration rate when soil is dry, fc is the final steady-state rate when soil is fully saturated, k is the decay constant (min⁻¹) controlling how quickly capacity drops, and t is elapsed time in minutes. The result f(t) gives instantaneous infiltration capacity in mm/hr, used to determine when rainfall intensity exceeds soil absorption and surface runoff begins.
Frequently Asked Questions
What is Horton\'s infiltration capacity decay model?
Robert Horton (1940) observed that soil infiltration capacity starts high when soil is dry and declines exponentially to a steady final rate as pores saturate: f(t) = fc + (f0 − fc) × e^(−kt). f0 is the initial rate, fc is the final (saturated) rate, k is the decay constant, and t is time. It remains one of the most widely used infiltration models in hydrological practice.
How do I calibrate Horton\'s parameters from field data?
Conduct a double-ring infiltrometer test, recording infiltration rate at regular intervals (e.g., every 5 minutes). Plot rate vs. time on a semi-log scale: the intercept at t = 0 gives f0 and the asymptote gives fc. Fit the exponential curve to estimate k. Alternatively, linearize by computing ln[f(t) − fc] and regressing against time to find k as the negative slope.
What is the difference between Horton and Green-Ampt infiltration models?
Horton\'s model is empirically fitted to observed rate data without explicit physical parameters. Green-Ampt uses a piston-flow assumption with physically measurable inputs: saturated hydraulic conductivity, wetting front suction, and initial water deficit. Green-Ampt is preferred when soil texture data are available; Horton is convenient when time-series rate data are available but soil physical properties are not.
How is Horton\'s infiltration used in stormwater runoff modeling?
HEC-HMS and similar models use Horton infiltration to separate rainfall into runoff-producing excess and infiltration loss. The model integrates f(t) over the storm duration to find total infiltrated depth, then subtracts from rainfall to get direct runoff. Accurate f0, fc, and k values are critical: overestimating fc underestimates peak flood flow in design events.
What equipment is used to measure infiltration rates in the field?
Double-ring infiltrometers (inner ring ~30 cm diameter, outer buffer ring ~60 cm) are the standard ASTM method (ASTM D3385). The inner ring measurement eliminates lateral flow error introduced by the outer buffer. Tension disc permeameters measure infiltration at controlled suctions to characterize unsaturated conductivity. Rainfall simulators measure infiltration under artificial storm conditions.
How does vegetation and land use affect Horton infiltration parameters?
Forests and native grasslands maintain high f0 due to root channels, organic matter, and biological macropores. Urban compaction reduces f0 from 200 mm/hr (native soil) to as low as 5–10 mm/hr, greatly increasing runoff. Agricultural tillage temporarily increases f0 but surface sealing from raindrop impact can reduce it rapidly. Land use change is the primary driver of long-term infiltration shifts.