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Title
Mesures physiques et signatures en télédétection

459
( 4 )
T 1/2
where 9 U represents the rewetted moisture content which is equal to the saturation soil moisture
minus entrapped air fraction, K u is the hydraulic conductivity evaluated at 9 U \ and D is the soil
moisture diffusivity.
2.1.2 Percolation and Capillary Rise. Percolation during the storm event and capillary rise dur
ing the interstorm period are assumed to be in a steady state. Employing the Brooks-Corey soil
characteristic relationships (Brook and Corey, 1974), the net vertical flux in the transmission zone is
estimated using the expression developed by Salvucci (1993):
where g is the steady-state flow rate in the transmission zone, positive for capillary rise and negative
for percolation. z x is the depth of the local water table. Tp c is the air-bubbling tension head, ip sz
is the tension head of the surface zone. A = 2 + 3 B, where B is the Brooks-Corey soil pore size
distribution parameter.
2.1.3 Exfiltration. Analogous to the treatment of infiltration, we can estimate the exfiltration rate,
f e , by taking the minimum of exfiltration capacity, /*, and potential évapotranspiration, e p :
Neglect the effect of gravity and again applying the time condensation approximation, the exfil
tration capacity can be expressed as a function of cumulative exfiltration, F e , and the initial condition
at the start of the interstorm event:
where S e is the desorptivity and is given by Milly (1986) as a function of initial soil moisture 9^:
spatially-distributed data in model parameters, inputs and outputs. Catchment topography is rep
resented by digital elevation model (DEM) information. The local model is applied to each grid
fe = min[f*,e p }
( 6 )
2.1.4 Runoff. Both infiltration excess and saturation excess runoffs are accounted for in the model.
Saturation excess runoff occurs when rain falls on the saturated grid elements which are either ad
jacent to the stream, or have a small storage deficit that can be easily satisfied during the storm.
Infiltration excess runoff is generated on those parts of the watershed where p > /*. No flow routing
is carried out in this model. Thus, the total streamflow volume for the watershed is obtained by
summing the contribution to surface runoff from each grid element in the computational domain and
the subsurface base flow Q j, described below.
2.2 Watershed-Scale Water Balance Model
Given the local water balance model presented in the previous section, we employ an aggregation
scheme similar to that used in Famiglietti (1992) to construct a model suitable for use in watershedscale
watershedscale hydrologic simulations. A raster-based geographic information system (GIS) is used to manage