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

Processes 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 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 The model is a lot more complex than the simple infiltration function but is more of a simple infiltration function but is more of a water balance infiltration function instead water balance infiltration function instead of a simple time power function.of a simple time power function.

Green and Ampt modelGreen 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+ t*= Kn t/ ( delt O ( Hn+ ∑zi)∑zi) Where delt O is the change in Where delt O is the change in

volumetric water content as the volumetric water content as the wetting front passes layer n. wetting front passes layer n.

Hn is the potential head while the Hn is the potential head while the wetting front passes through layer nwetting front passes through layer n

Zi is the thickness and should be Zi is the thickness and should be summed from i-1 to n-1summed from i-1 to n-1

Zi= the thickness of the layer .Zi= the thickness of the layer .

Z*= Kn/ (Hn+ Z*= Kn/ (Hn+ ∑zi) * ∑zi/Ki∑zi) * ∑zi/Ki Where ki is the hydraulic conductivity of Where ki is the hydraulic conductivity of

layer i. layer i. Kn= hydraulic conductivity of layer n Kn= hydraulic conductivity of layer n

containing the wetting front. containing the wetting front. Hn is the potential headHn is the potential head http://weather.nmsu.edu/http://weather.nmsu.edu/

teaching_Material/soil470/green-ampp-teaching_Material/soil470/green-ampp-inf.pdfinf.pdf

Surface hydrologySurface 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 Mannings equation is combined with the 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) v = Water velocity down the channel (m /

sec) R = Hydraulic radius (m) = cross sectional

area (m2) / wetted perimeter (m) S = Gradient of channel (m / 100m)

kinematic wave equations is the continuity equation

http://doctorflood.rice.edu/ceve101/Handohttp://doctorflood.rice.edu/ceve101/Handouts/Ch04a.pptuts/Ch04a.ppt

Simple continuity equation is I-0=del S/tSimple continuity equation is I-0=del S/t Where i=inflowWhere i=inflow O= outflow O= outflow Del S is the change in storage and t is Del S is the change in storage and t is

time. time. Simple solutions to the deferential Simple solutions to the deferential

equation involve the muskingum method equation involve the muskingum method that makes simplifying assumptionsthat makes simplifying assumptions

Differential equation for Differential equation for kinematic-flood-routingkinematic-flood-routing

http://www.alanasmith.com/theory-http://www.alanasmith.com/theory-Kinematic-Flood-Routing.htmKinematic-Flood-Routing.htm

http://www.webs1.uidaho.edu/ch/http://www.webs1.uidaho.edu/ch/presentations/presentations/kw_applications.ppt#259,4,Kinematic kw_applications.ppt#259,4,Kinematic WavesWaves

http://www.webs1.uidaho.edu/ch/http://www.webs1.uidaho.edu/ch/presentations/presentations/kw_applications.ppt#262,7,Kinematic kw_applications.ppt#262,7,Kinematic WavesWaves

Curve Number runoffCurve Number runoff

http://www.ecn.purdue.edu/runoff/http://www.ecn.purdue.edu/runoff/documentation/scs.htmdocumentation/scs.htm