Agriculture Reference
In-Depth Information
their maximum potential rate with evapotranspiration - and hence photo-
synthesis and growth - unchecked by soil water supply requires more frequent
irrigation than for deeper and more densely rooted crops.
The need for frequent irrigation and a shallow rooting depth means that
nutrients - in particular, nitrate, which exists solely in the soil solution - are
easily leached below the rooting depth if water enters soil already at field
capacity as a result of rainfall or uneven irrigation. Great importance is now
placed on minimizing such losses of nitrate so as to avoid pollution of aquifers
and rivers. For these reasons, edible alliums are demanding crops in terms of
the irrigation and nutrient management needed to achieve maximum yields
with minimum losses of water and nutrients to leaching or run-off. The
following sections will outline the fundamentals underlying optimum manage-
ment and some of the methods available for achieving near-optimal results in
varying circumstances.
FUNDAMENTALS OF IRRIGATION
The principles underlying the irrigation require-
ments of crops, including onions and garlic, are described by Allen
et al.
(1998). Transpiration is essentially the physical process of water evaporation
from moist surfaces within the leaves into the atmosphere via the stomata on
the leaf epidermis. In a field situation, evaporation directly from moist soil to
the atmosphere also occurs and is particularly important when leaves do not
cover much of the ground - for example, when a crop is small soon after
emergence.
Evaporation of water requires large amounts of energy, which is derived
from solar energy influx, either directly as radiant energy or indirectly as
sensible heat. Therefore, evapotranspiration is governed by energy exchange
and is limited by the amount of energy available. From the principle of energy
conservation it is possible to predict evapotranspiration rates, since the energy
arriving at the surface must equal the energy leaving it during the same period.
Another method of estimating evaporation is the mass transfer method that
considers the transport of water vapour and energy (heat and momentum) to
and from evaporating surfaces in small parcels of air, termed eddies.
Making certain assumptions, evapotranspiration can be calculated from
vertical gradients of air temperature and water vapour concentration above
crop surfaces. The energy balance and mass transfer methods have been
combined to derive the Penman-Monteith equation, which can be used to
calculate the evaporation from a uniform expanse of vegetation using para-
meters that can be measured or calculated from weather data. The resistance
to evaporation in this equation depends on stomatal behaviour and the
'architecture' of the crop surface, in terms of its height and surface roughness.
To simplify this situation, the resistances of different crops have been computed
relative to evapotranspiration from a standard large, uniform expanse of grass
with a height of 0.12 m, a fixed surface resistance of 70 s/m and an albedo (see
below) of 0.23.
