17the late 1800s, reported that 233 units of water were required for one unit of corn
dry matter and 338 units for a unit of wheat dry matter. The studies were done in a
humid environment so it can be assumed that most of the water use was for transpi-
ration. Sinclair and Weiss ( 2010 ) reported that a C 4 grass growing in a somewhat
average transpiration environment (examples include maize and sorghum) have a
transpiration rate of approximately 220 units water for each unit of plant growth. A
C 4 species (includes wheat) growing in the same environment will transpire about
330 units of water for each unit of dry matter produced. Stewart and Peterson
( 2015 ), based on field data, recently estimated that the transpiration rates of maize
and grain sorghum are generally between 225 and 275 g of water per g of biomass
production in the U.S. Great Plains region. Therefore, increased yields over time
like those shown in Fig. 5 cannot be attributed to more water efficient crop cultivars
since no significant change in the units of transpiration required to produce a unit of
biomass for a given species appears to have occurred. Accumulation of plant bio-
mass is directly related to water availability (Sinclair 2009 ). Sinclair states that the
difference in vapor pressure inside and outside the leaf (VPD) controls water loss
through the stomata. VPD in arid regions is large because the vapor pressure of the
atmosphere is very low relative to humid areas. For a given environment, VPD
cannot be controlled—it is what it is. Sinclair further stated “despite claims that
crop yields will be substantially increased by the application of biotechnology, the
physical linkage between growth and transpiration imposes a barrier that is not ame-
nable to genetic alteration.” While many plant scientists disagree, there is little evi-
dence to repute it (Gurian-Sherman 2012 ). Therefore, the increased yields shown in
Fig. 5 are most likely due to improved water management that increased ET and to
improved cultivars with higher harvest index (HI) values. HI is the weight of grain
divided by the weight of aboveground biomass which increased significantly in
Fig. 5 Long-term county-wide dryland wheat grain yield in Whitman County, Washington, super-
imposed with crop-year precipitation from Pullman, WA (Source: Schillinger et al. 2010 )
Dryland Farming: Concept, Origin and Brief History