Agriculture Reference
In-Depth Information
the vegetation. The scheme has seven prognostic variables: three temperature variables
(foliage, soil surface and deep soil), one interception storage variable, and three soil
moisture storage variables. For the upper boundary conditions the following forcing
variables are used: air temperature, water vapor pressure, wind speed, short wave and
long wave radiation and precipitation at a reference level within the atmospheric
boundary layer. The hydrological module is designed as a three-layer model, which is
used to describe the vertical transfer of water in the soil. The LAPS uses the
morphological and physiological characteristics of the plant community for deriving the
coefficients and resistances that govern all the fluxes between the surface and
atmosphere.
In order to simulate partitioning of the soybean canopy water into water balance
components, during short and long period (day, growing season), several simulation were
performed. The corresponding forcing, morphological, physiological and soil data as well
as the observations were inserted from data sets comprising different agroecological
soybean regions: Paragaminas (Brasil), Marchfeld (Austria) and Caumont (France).
1.
I
NTRODUCTION
1.1. Soybean-water Relationship
Irrigation applications for soybean (
Glycine max
), or high amounts of rainfall that occur
during vegetative growth, are normally not beneficial unless soil water levels are extremely
low. In fact, excessive water during the early growth stages may stimulate vegetative growth
without leading to a yield increase. If lodging occurs, vegetative stage irrigation can
substantially depress yields.
The total water used (evapotranspiration usually denoted as ET) by a fully irrigated
soybean crop may vary from 508 mm to 660 mm during the growing season (Figure 1).
About 65 percent of total water use occurs during the Beginning Flower to Beginning Seed
Enlargement reproductive stages. In an average season, the water-use rate will reach a peak of
about 8.1 mm per day during the late flowering and early pod development stages, but during
the mid to late reproductive stages it may average 6.4 mm per day. However, the actual ET
amount in any given year will vary daily. Note that a well-watered soybean crop may
transpire up to 12.8 mm of water on a hot, windy day in late-July or August. The most
important times for soybean plants to have adequate water are during pod development
(Beginning Pod Elongation - End of Pod Elongation) and seed fill (Beginning Seed
Enlargement - End of Seed Enlargement). These are the stages when water stress can lead to
a significant decrease in yield. Irrigation also may be required prior to these stages on sandy
soils (insufficient water-holding capacity) or during very low rainfall years on medium- and
fine-textured soils. However, if water is applied during flowering, it is important to follow
with adequate water during seed fill. Irrigation at flowering typically increases the number of
seeds produced per plant, but any subsequent water stress will reduce the size of those seeds
such that the yield response to an irrigation at flowering may be no more or even less than not
irrigating at flowering. Irrigation at flowering also may increase the potential for a white mold
infection due to the humid in-canopy conditions after the irrigation. Though soybean roots
can reach depths of 1.52 m to 1.82 m, the largest concentration of roots and the majority of
Search WWH ::
Custom Search