Horton’s equation is the most common relationship for describing infiltration capacity in a soil. It describes the decrease in capacity as more water is absorbed by the soil, and has the form:

where f is the infiltration capacity (mm/h) at time t,

f₀ and f_c are the initial and final constant rates of infiltration (mm/h)>>

k is a shape factor (fixed at a value of 2/h in ILSAX)

t is the time from the start of rainfall (h).

The soil type specified in DRAINS determines values for f₀ and f_c.

Horton models are discussed in Chapter 3 of Book 5 of ARR 2019, in Section 3.5.3.4.