Surface hydrology The primary purpose of the WEPP surface hydrology component is to provide the erosion component with the duration of rainfall excess, the rainfall intensity during the period of rainfall excess, the runoff volume, and the peak discharge rate. A secondary purpose is to provide the amount of water which infiltrates into the soil for the water balance and crop growth/residue decomposition calculations which are in turn used to update the infiltration, runoff routing, and erosion parameters.

Spatially, the program predicts detachment and deposition at each of a minimum of 100 points on a hillslope, and the sum totals of these values are divided by the number of years of simulation in order to have average data at each point. Spatially, the program predicts detachment and deposition at each of a minimum of 100 points on a hillslope, and the sum totals of these values are divided by the number of years of simulation in order to have average data at each point.

Processes The sequence of calculations relevant to surface hydrology are infiltration, rainfall excess, depression storage, and peak discharge Infiltration is computed using an implementation of the Green- Ampt Mein- Larson model for unsteady intermittent rainfall.

Green and ampt infiltration The Green-Ampt model is the first physically- based equation describing the infiltration of water into a soil. This model yields cumulative infiltration and infiltration rates as implicit functions of time The model is a lot more complex than the simple infiltration function but is more of a water balance infiltration function instead of a simple time power function. The model is a lot more complex than the simple infiltration function but is more of a water balance infiltration function instead of a simple time power function.

Green and Ampt model f= f*Kn where f is infiltration rate cm/hr Kn=hydraulic conductivity f*=(F+1)/(F*+z*) dimensionless infiltration F*- ½{t*-2z*+ ((t*-2z*)^2+8t*)^0.5} Where F* is the dimensionless accumulated infiltration in layer n where the wetting front is located. z* is the dimensionless depth accounting for thickness and conductivity of layers behind the wetting front. Layers i to n-1

t*= Kn t/ ( delt O ( Hn+ ∑zi) t*= Kn t/ ( delt O ( Hn+ ∑zi) Where delt O is the change in volumetric water content as the wetting front passes layer n. Where delt O is the change in volumetric water content as the wetting front passes layer n. Hn is the potential head while the wetting front passes through layer n Hn is the potential head while the wetting front passes through layer n Zi is the thickness and should be summed from i-1 to n-1 Zi is the thickness and should be summed from i-1 to n-1 Zi= the thickness of the layer. Zi= the thickness of the layer.

Z*= Kn/ (Hn+ ∑zi) * ∑zi/Ki Z*= Kn/ (Hn+ ∑zi) * ∑zi/Ki Where ki is the hydraulic conductivity of layer i. Where ki is the hydraulic conductivity of layer i. Kn= hydraulic conductivity of layer n containing the wetting front. Kn= hydraulic conductivity of layer n containing the wetting front. Hn is the potential head Hn is the potential head al/soil470/green-ampp-inf.pdf al/soil470/green-ampp-inf.pdf al/soil470/green-ampp-inf.pdf al/soil470/green-ampp-inf.pdf

Surface hydrology Overland flow processes are conceptualized as a mixture of broad sheet flow occurring in interrill areas and concentrated flow in rill areas. Broad sheet flow on an idealized surface is assumed for overland flow routing and hydrograph development. Overland flow routing procedures include both an analytical solution to the kinematic wave equations and regression equations derived from the kinematicapproximation for a range of slope steepness and lengths, friction factors (surface roughness coefficients),soil textural classes, and rainfall distributions.

Mannings equation is combined with the Mannings equation is combined with the kinematic wave equations to compute velocity v = 1/n x R^2/3 x S^1/2 Where : n = coefficient of roughness (typically 0.3)coefficient of roughness v = Water velocity down the channel (m / sec) R = Hydraulic radius (m) = cross sectional area (m2) / wetted perimeter (m)wetted perimeter S = Gradient of channel (m / 100m)

kinematic wave equations is the continuity equation h04a.ppt h04a.ppt h04a.ppt h04a.ppt Simple continuity equation is I-0=del S/t Simple continuity equation is I-0=del S/t Where i=inflow Where i=inflow O= outflow O= outflow Del S is the change in storage and t is time. Del S is the change in storage and t is time. Simple solutions to the deferential equation involve the muskingum method that makes simplifying assumptions Simple solutions to the deferential equation involve the muskingum method that makes simplifying assumptions

Differential equation for kinematic- flood-routing Kinematic-Flood-Routing.htm Kinematic-Flood-Routing.htm Kinematic-Flood-Routing.htm Kinematic-Flood-Routing.htm ations/kw_applications.ppt#259,4,Kinemat ic Waves ations/kw_applications.ppt#259,4,Kinemat ic Waves ations/kw_applications.ppt#259,4,Kinemat ic Waves ations/kw_applications.ppt#259,4,Kinemat ic Waves ations/kw_applications.ppt#262,7,Kinemat ic Waves ations/kw_applications.ppt#262,7,Kinemat ic Waves ations/kw_applications.ppt#262,7,Kinemat ic Waves ations/kw_applications.ppt#262,7,Kinemat ic Waves

Curve Number runoff entation/scs.htm entation/scs.htm entation/scs.htm entation/scs.htm