20
crops are produced, the range of values for each of the components becomes con-
siderably greater and more variable in dryland farming regions. Average annual
precipitation in dryland farming regions is generally less than 500 mm, so ET
amounts are commonly below 250 mm even when annual cropping is practiced, and
perhaps about 350 mm when a fallow period is used prior to seeding a crop. The T/
ET values usually range between 0.50 and 0.75 with the lower values associated
with limited water. Sinclair and Weiss ( 2010 ) stated that the TR for crops like maize
and sorghum is about 220 when grown in a somewhat average transpiration envi-
ronment, but could be about 280 for an arid environment and as low as 160 for a
humid transpiration environment. HI values can also vary considerably. Prihar and
Stewart ( 1990 ) reported upper values of HI for maize, grain sorghum, and wheat of
about 0.60, 0.56, and 0.47, respectively. However, the lower values depended
mainly on the amount of stress. Under dryland farming conditions, HI values are
often below 0.30 and under extreme water stress, values can equal or approach zero.
Fig. 7 A graphical model of the water balance for a wide range of climatic conditions (Source:
Ponce 1995 )
B.A. Stewart and S. Thapa