Agriculture Reference
In-Depth Information
tAble 19.2
water Requirement (grams of water Required to Produce grams of
biomass) of Alfalfa at different locations in the great Plains
Pan
Average
a
(mm d
−1
)
water
Requirement/
Pan
water
Requirement
location
growth Period (1912)
Williston, ND
July 29 to September 24
518 ± 12
4.04
128
Newell, SD
August 9 to September 6
630 ± 8
4.75
133
Akron, CO
July 26 to September 6
853 ± 13
5.74
149
Dalhart, TX
July 26 to August 31
1005 ± 8
7.77
129
a
Average daily amount of water evaporated from an open pan of water.
Source:
L.J. Briggs and H.L. Shantz, 1913,
USDA Bureau Plant Industry Bulletin
284.
water becomes increasingly limited, the actual evapotranspiration will also become
increasingly more limited, and crop growth will decrease because of water stress.
Some of the early work that showed the effect of climate on the amount of water
required for plant growth was done by Briggs and Shantz (1917) and is summarized
in Table 19.2. The locations reported in Table 19.2 move from north to south down
the Great Plains, and the climate becomes increasingly hotter, as reflected by the
higher amounts of water evaporated from an open pan. The water requirement for
producing a gram of aboveground biomass of the same crop species was roughly two
times greater at Dalhart, Texas, compared to Williston, North Dakota, and the pan
evaporation amounts were in approximately the same ratios. These were some of the
first data that could be used to show that evaporation and saturation deficit could be
used to normalize the transpiration component.
The generalized relation between transpiration and yield of aboveground biomass
and between evapotranspiration and agronomic yield is shown in Figure 19.1. The
yield of biomass increases as a straight-line function of the amount of transpiration
and the line intersects the origin. The slope of the line will vary with the crop and
with the climate, but the relationship indicates that for every additional amount of
water transpired by the crop, there will be a corresponding amount of biomass pro-
duced. The other line in Figure 19.1 representing the amount of agronomic yield as
a function of evapotranspiration does not pass through the origin, but the relation
is still a straight line after the threshold value of evapotranspiration has been met.
Agronomic yield will depend on the crop and what the harvestable product is for that
crop. For example, for crops like wheat, maize, and sorghum, the agronomic yield
would be the amount of grain produced, while for a crop like alfalfa, it might be the
amount of forage produced. The reason that the line does not pass through the origin
is because there is an evaporation component of evapotranspiration. The relation
shown indicates that for any agronomic crop there is a threshold amount of evapo-
transpiration that must be met before any agronomic yield is produced. However,
once this threshold amount is met, agronomic yield is a straight-line function of
evapotranspiration. Again, the slope of the line will vary depending on the crop and
