Time-Area Method (MSMA) Calculator

Design Rainfall (IDF)

State

Station ID / Name

ARI (years)

Duration (minutes/hours)

IDF Coefficients (λ, κ, θ, η)

Design Rainfall Intensity, 𝑖 (mm/hr)

Total Design Rainfall, 𝑃 (mm)

Normalized Design Rainfall Temporal Pattern

Rainfall Temporal Pattern (mm)

Area Distribution (%)

40%

60%

Pervious Area (%)

Impervious Area (%)

Areas must sum to 100%

Initial loss (mm) Fixed 10 mm – Table 2.6

Soil Type

Continuous loss (mm/hr) Table 2.6 range

3 10 6.5

Soil Properties:

Moderate infiltration rate – balanced (3–10 mm/hr)

Surface Type

Initial loss (mm) Fixed 1.5 mm – Table 2.6

Continuous loss (mm/hr) = 0 – Table 2.6

Surface Properties:

No continuous loss – per Chow et al. (1988)

ℹ Impervious surfaces have zero continuous loss regardless of surface type, per Table 2.6.

Losses & Excess Rainfall (mm)

Time (min)	Rainfall (mm)	Pervious Initial	Pervious Continuous	Imperv. Initial	Imperv. Continuous	Total Loss	Excess (mm)
Total	–	–	–	–	0.00	–	–

Total loss = (Pervious% × PervLoss + Impervious% × ImpervLoss) / 100 per time bin. Impervious CL forced to 0 per Table 2.6 (Chow et al., 1988).

Time Bin	Rainfall (mm)	Loss (mm)	Excess (mm)	Excess (mm/s)