Surface hydrology

1
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.

2

• 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.

3
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.

4
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.

5
Green and Ampt model

• f fKn where f is infiltration rate cm/hr
• Knhydraulic conductivity
• f(F1)/(Fz) dimensionless infiltration
• F- ½t-2z ((t-2z)28t)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

6

• t Kn t/ ( delt O ( Hn ?zi)
• 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
• Zi is the thickness and should be summed from i-1
  to n-1
• Zi the thickness of the layer .

7

• Z Kn/ (Hn ?zi) ?zi/Ki
• Where ki is the hydraulic conductivity of layer
  i.
• Kn hydraulic conductivity of layer n containing
  the wetting front.
• Hn is the potential head
• http//weather.nmsu.edu/teaching_Material/soil470/
  green-ampp-inf.pdf

8
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.

9
Mannings equation is combined with the kinematic
wave equations to compute velocity

• v 1/n x R2/3 x S1/2
• 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)

10
kinematic wave equations is the continuity
equation

• http//doctorflood.rice.edu/ceve101/Handouts/Ch04a
  .ppt
• Simple continuity equation is I-0del S/t
• Where iinflow
• O outflow
• 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

11
Differential equation for kinematic-flood-routing

• http//www.alanasmith.com/theory-Kinematic-Flood-R
  outing.htm
• http//www.webs1.uidaho.edu/ch/presentations/kw_ap
  plications.ppt259,4,Kinematic Waves
• http//www.webs1.uidaho.edu/ch/presentations/kw_ap
  plications.ppt262,7,Kinematic Waves

12
Curve Number runoff

• http//www.ecn.purdue.edu/runoff/documentation/scs
  .htm