ENVIRONMENTAL • RIVER WATER QUALITY

DO sag (Streeter–Phelps)

Compute dissolved oxygen (DO) deficit along a river reach using the classical Streeter–Phelps model. This is an educational tool and uses simplifying assumptions (steady, 1D, complete mixing).

Units: DO & BOD in mg/L, distance in km, velocity in m/s, k_d, k_r in 1/day.

Inputs

DO saturation, DO_sat (mg/L)

Initial DO, DO₀ (mg/L)

Ultimate BOD, L₀ (mg/L)

Deoxygenation rate k_d (1/day)

Reaeration rate k_r (1/day)

Mean velocity v (m/s)

Reach length (km)

Number of points

Results (along the reach)

Distance x (km)	Travel time t (days)	Deficit D (mg/L)	DO (mg/L)
No results yet.

Model used: D(t) = (k_dL₀/(k_r-k_d)) (e^-k_dt - e^-k_rt) + D₀ e^-k_rt, where D = DO_sat - DO. DO(t) = DO_sat - D(t). Travel time t is based on distance / velocity.