The results of Modified Penman method were larger than Penman-Monteith method in different
growth stage and the whole growth stage, expectly in July and August, the reason is the radiation term
and the aerodynamicist term different towards ET0, the error of the calculating result may cause by the
formula and the parameter. The Relative Error of the evaporation pan method were larger of three forage
crop. The reason is the characteristic of evaporation pan reserving and releasing energy is different from
crop surface, the evaporating capacity was influenced by the wind speed and relative humidity, but
evaporation coefficient was according to average wind speed and average relative humidity. Blaney-
Criddle belong to temperature method, the results were larger than Penman-Monteith method in different
growth stage and the whole growth stage. The Relative Error of two methods is consistent, the reason is
that the difference of temperature is big in the day and night. So the tallest temperature of the day is
mostly factor. Priestley-Taylor and Markkinks method belong to radiation method, the results were
smaller than Penman-Monteith method in different growth stage and the whole growth stage. The reason
is that sunshine is main weather in the area of experiment. So Rn and Ra that are main factors by selected
were smaller than actually.
Table 3 Water requirements and relative error of silage maize by calculating methods
growth stage
PM
EP
MP BC PT MK
ETc
mm
ETc
mm
RE
%
ETc
mm
RE
%
ETc
mm
RE
%
ETc
mm
RE
%
ETc
mm
RE
%
seeding stage 82.32 100.2 21.78 84.29 2.39 90.64 10.11 81.25 -1.3 79.56 -3.35
shooting stage 85.64 106.9 24.93 86.97 1.55 91.35 6.67 80.62 -5.86 81.29 -5.08
heading stage 148.25 160.4 8.22 158.8 7.14 157.3 6.12 137.1 -7.48 140.1 -5.48
anthesis stage 100.92 91.91 -8.93 105.5 4.54 106.3 5.34 91.59 -9.24 87.94 -12.86
whole stage 417.13 459.5 10.18 435.6 4.43 445.6 6.83 390.6 -6.36 388.9 -6.76
Table 4 Water requirements and relative error of elymus dahuricus by calculating methods
growth stage
PM EP MP BC PT MK
ETc
mm
ETc
mm
RE
%
ETc
mm
RE
%
ETc
mm
RE
%
ETc
mm
RE
%
ETc
mm
RE
%
turn green 69.30 79.17 14.24 71.08 2.57 72.58 4.73 66.97 -3.36 64.91 -
6.33
tillering stage 84.68 96.03 13.40 85.19 0.60 88.19 4.15 82.49 -2.59 81.07 -
4.26
shooting stage 89.92 104.83 16.58 91.55 1.81 97.04 7.92 81.07 -9.84 84.29 -
6.26
heading stage 128.25 126.80 -1.13 130.71 1.92 129.71 1.14 127.54 0.55 126.75 1.17
mature stage 106.54 122.81 15.27 109.86 3.12 114.37 7.35 101.81 -4.44 101.78 -
4.47
whole stage 409.39 450.47 10.03 417.31 1.93 429.31 4.87 392.91 -4.03 393.89 -
3.79
3.2. Correlative analysis
In order to analyze the relation of seven methods with PM method and carry out conversion with it.
Author established regression equation that curve pass origin: ETc(Y) = aETc(PM), Y mean seven methods.
According to the experiment and meteorologic data of water-saving irrigation base in XiLinGuole typical