Estimating Catchment Runoff Storm Surface depression storage Infiltration Initial abstraction Direct runoff or Effective rainfall Losses

Infiltration methods zSoil Conservation Service (SCS) Curve Number (CN) method zHorton’s equation ‘moving curve’ method zGreen & Ampt model

SCS Curve Number method inches mm P(t) = depth of rainfall Q(t) = depth of runoff Ia = initial abstraction S = potential storage CN = curve number  100 CN depends on soil type and pre-wetting

Horton equation

Green & Ampt model where M= moisture deficit S = suction head K = hydraulic conductivity

Rainfall-Runoff models (1) Losses subtracted from rainfall to get effective rainfall which is then applied to catchment. Rainfall Infiltration Model Losses Catchment Model Runoff Effective rainfall

Rainfall-Runoff models (2) Losses and infiltration calculated along with runoff as part of Runoff Model Rainfall Catchment Model Runoff Losses and infiltration Surface Depression Storage

Calculating the Runoff (1) Runoff from pervious and impervious fractions computed and added together Flow lengths can be:- (a) equal (b) proportional (c) user supplied

Calculating the Runoff (2) Overland flow length can be estimated as area divided by length of stream bank available for inflow. Symmetrical catchment Area = 2.2 ha One-sided catchment Area = 2.4 ha 75m 96m 63m 192m

Calculating the Runoff (3) Overland flow routing choices: zCombine effective rainfall with: ytriangular response function yrectangular response function ysingle linear reservoir response function zCombine infiltration & other losses with outflow from idealized inclined plane. (Similar to SWMM RUNOFF method)