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IMPACT OF MICROWAVE DERIVED SOIL MOISTURE ON HYDROLOGIC 
SIMULATIONS USING A SPATIALLY DISTRIBUTED WATER BALANCE MODEL 
D.-S. LIN, E. F. WOOD, J. S. FAMIGLIETTI 
Department of Civil Engineering and Operations Research 
Princeton University, Princeton, NJ 08544, U.S.A. 
M. MANCINI 
DILAR, Politecnico di Milano, Milano, Italy 
ABSTRACT: 
Spatial distributions of soil moisture over an agricultural watershed with a drainage area of 60 ha 
were derived from two NASA microwave remote sensors, and then used as a feedback to determine 
the initial condition for a distributed water balance model. Simulated hydrologic fluxes over a period 
of 12 days were compared with field observations and with model predictions based on a streamflow- 
derived initial condition. The results indicated that even the low-resolution remotely sensed data 
can improve the hydrologic model’s performance in simulating the dynamics of unsaturated zone soil 
moisture. For the particular watershed under study, the simulated water budget was not sensitive to 
the resolutions of the microwave sensors. 
KEY WORDS: Remote sensing, soil moisture, distributed water balance model. 
1 INTRODUCTION 
The spatial distribution of soil moisture is a critical hydrologic variable which exerts a major 
control on the land surface water and energy balance. Despite its importance, direct usage of this 
information in climatic, hydrologic, or agricultural simulations has not gained widespread applica 
tions. This is in part due to the fact that most models do not consider soil moisture a measurable 
state variable. Also, it is difficult to acquire this information on a consistent and spatially compre 
hensive basis using the conventional techniques. Recent advances in remote sensing technology have 
shown that surface soil moisture distribution can be measured at various resolutions by microwave 
sensors (see Engman, 1990). Since microwave remote sensing techniques can be implemented on high 
altitude platforms, it is likely that such measurements could be obtained routinely over large regions 
in the near future. However, how to incorporate the remotely sensed soil moisture data into the 
modeling framework remains to be a big challenge. On what level of temporal and spatial resolutions 
that will be required by different applications also needs to be addressed. This paper presents a 
spatially distributed water balance model that is capable of using the microwave derived soil mois 
tures to facilitate simulations. Using the model as a tool and data collected during a multi-sensor 
aircraft campaign over an agricultural catchment, we examine the impact of remotely sensed data 
on hydrologic simulations, and the sensitivity of simulated water budget to the resolutions at which 
microwave remote sensing methods can currently provide spatial patterns of soil moisture. 
2 MODEL FORMULATION 
The building block of the distributed hydrologic model is a process-based one-dimensional wa 
ter balance model that accounts for the soil moisture movements in the unsaturated zone of a soil 
column. This one-dimensional model is based on the formulation described in Famiglietti (1992), 
but improves upon the treatment of storage by using a steady-state approximation to represent the 
moisture profile. The lower boundary of this model is at the water table where the steady percolation 
or exfiltration is added to, or extracted from, the regional groundwater system. Lateral subsurface