MODELING THE WATER BALANCE COMPONENTS OF THE SOYBEAN CANOPY BY SOIL-VEGETATIONATMOSPHERE TRANSFER MODEL - Soybean and Wheat Crops: Growth, Fertilization, and Yield - page 56

the crops). Analysis of soil moisture also splits soil texture into two broad categories: sand
and loam. The soil type at Caumont is loam. The parameters used for characterizing the land
surface are summarized in Table 1 [52]. Monthly leaf area index, LAI, fraction vegetation
cover, bare soil roughness length, canopy roughness length, zero plane displacement, canopy
height and root distribution used in the one year integration, are presented in Table 2 [52]. In
the same table, the monthly distribution of the soybean root system during the considered
year is listed. Crop height and zero plane displacement have been estimated from the
measurements. Albedo is based on measurements of radiation, which revealed a nearly
constant value of 0.20 for the albedo during the year. Other albedos and emissivities used in
the LAPS scheme are listed in Table 3 [52]. Because of the focus of this chapter, we used the
observations concerning the water balance components which were available from the
HAPEX-MOBILHY programme. We had available weekly volumetric soil moisture
measurements through the year, based on neutron sounding probes for the top 1.6 m soil
layer, with 0.1 m intervals. Another reference data set, that we have used for estimating the
partitioning of the land surface water simulated by the LAPS, was an approximate water
budget for the first four months (days 0-120), which was generated using the observed
weekly root zone (1.6 m), soil moisture content, accumulated precipitation, and evaporation
estimated by the Penman-Monteith formula [50]. Estimated evaporation was 149.6 mm for
the period indicated. The total precipitation was 368.5 mm. The available observations from
the first four months show very few changes in total soil moisture. The total root zone water
content change was estimated at 22.2 mm, while the generated runoff plus drainage was 241.1
mm during the four months.
3.2. Validation of Hydrological Module of the LAPS
In order to test the performance of the hydrological module we run the LAPS for one year
period with a time step of 1800 s. Atmospheric forcing data, validation data and parameters
representing the land surface properties are obtained from the afore mentioned HAPEX-
MOBILHY experiment. The run was initialized by setting the prognostic variables as follows:
all water stores as saturated, canopy water as zero, snow mass as zero and all temperatures at
279.0 K. After initialization the scheme was running to equilibrium by looping through the
one year forcing data. The equilibrium was reached when the conditions:
| λE m (n+1)- λE m (n) |< υ , | λE st (n+1)- λE st (n) |< υ , | H m (n+1)- H m (n) |< υ and | H st (n+1)- H st (n) |< υ
were satisfied; here ν=0.10 W m -2 . The used symbols have the meaning: λ E m (n) , H m (n) ,
λ E m (n+1) , H m (n+1) are the annual means of latent and sensible heat fluxes for year n and
n+1 , while λ E st (n) , H st (n) , λ E st (n+1) , H st (n+1) are the standard deviations for year n and n+1 ,
respectively. The LAPS was converging after the third iteration. Additionally, the LAPS was
tested using ″Milly criteria″ which requires that the condition ⏐ P a -D a -R a -E a ⏐<1 mm has to be
satisfied. Here, P a , D a , R a and E a denote annual cumulated values of the: precipitation,
drainage, runoff and evapotranspiration. The residual, in this criteria, obtained by the LAPS
was 0.6 mm. A comparison of the predicted total soil water during soybean growing season
with the HAPEX measurements is shown in Fig 6. It shows that there is a general agreement