Agriculture Reference
In-Depth Information
Water requirement
: The inverse of the transpiration ratio. A plant with a tran-
spiration ratio of 0.002 requires 500 g of water to be transpired to produce
1 g of aboveground biomass.
Evaporation
: Evaporation accounts for the movement of water to the air from
sources such as the soil or intercepted precipitation or irrigation.
Evapotranspiration
: The sum of evaporation and transpiration from a defined
area for a specified time divided by that area. The specified time is gener-
ally from date of planting a crop until harvesting the crop.
Potential evapotranspiration
: A representation of the climatic demand for
evapotranspiration; it represents the evapotranspiration rate of a short green
crop, completely shading the ground, of uniform height and with adequate
water status in the soil profile. It is a reflection of the energy available to
evaporate water and of the wind available to transport the water vapor from
the ground up into the lower atmosphere. Evapotranspiration is said to
equal potential evapotranspiration when there is ample water available to
the crop.
Field water supply
: The sum of water that will become available to the crop
during the growing season, the gross amount of seasonal irrigation, and pre-
cipitation received during the growing season. The gross amount of seasonal
irrigation is the amount of water conveyed from the source to the field.
Irrigation efficiency:
: The net amount of water added to the root zone divided
by the amount of water applied from the source.
Water use efficiency
: Water use efficiency is the amount of harvestable crop
product produced per volume of water used by evapotranspiration. For
example, grain would be the harvestable product of crops like wheat,
maize, and sorghum, while harvestable aboveground biomass would be the
harvestable product for a forage crop. The water content of the harvestable
product must also be considered.
PlAnt gRowth-wAteR RelAtIonshIPs
It is important to understand some basic relationships between water use and plant
growth before looking at some of the important advances made during the past few
decades in utilizing water more efficiently for crop production. Climate is the most
important factor for the efficient use of water for crop production, and this is true
whether the crop depends entirely on precipitation, entirely on irrigation, or a com-
bination of the two. There are four climatic characteristics that govern plant water
use; simply stated, these are (1) how hot it is; (2) how sunny it is; (3) how windy it
is; and (4) how dry the air is. These characteristics define the demand of the climate
for water, and when water is not limited, the atmosphere will evaporate sufficient
water to meet this demand. When a crop is growing and supplied with ample water,
the amount of water transpired and evaporated from the soil surface is the amount
of evapotranspiration. If the crop does not have an ample supply of water, it cannot
meet the demands of the climate, referred to as the
potential evapotranspiration
, and
the amount of actual evapotranspiration will be less because water is limiting. As
