Geology Reference
In-Depth Information
1.2.3 Data Driven Evapotranspiration Modelling
Evapotranspiration, termed ET for short, is a natural phenomenon which is the
combined process of plant transpiration and soil evaporation. Though this study
focusing on data based modelling with soft computing techniques, we have used
some standard reference evapotranspiration equations for comparison. ET is con-
sidered as the most signi
cant component of the hydrologic budget, apart from
precipitation. Two commonly used evapotranspiration (ET) concepts are: potential
evapotranspiration (ET
p
) and reference evapotranspiration (ET
0
). The ET
p
concept
was introduced in the late 1940s by Penman [
75
]. It de
the amount of water
transpired in a given time by a short green crop, completely shading the ground, of
uniform height and with adequate water status in the soil pro
ned as
“
le
”
. In this de
nition
of ET
p
, the evapotranspiration rate is not related to a speci
c crop and therefore
considered to be a shortfall. On the other hand ET
0
is de
the rate of
evapotranspiration from a hypothetical reference crop with an assumed crop height
of 0.12 m (4.72 in), a
ned as
“
fixed surface resistance of 70 s m
−
1
and an albedo of 0.23,
closely resembling the evapotranspiration from an extensive surface of green grass
of uniform height, actively growing, well-watered, and completely shading the
ground
[
9
]. In the late 1970s and early 1980s, the reference evapotranspiration
concept was popularised among irrigation engineers and researchers which helped
them to avoid ambiguities
”
that
existed in the de
nition of potential
evapotranspiration.
The accurate estimation of reference evaporation is very critical in the context of
many scienti
c and management issues; for example, irrigation system design,
irrigation scheduling, hydrologic and drainage studies, crop production, manage-
ment of water resources, evaluation of the effects of changing land use on water
yields, and environmental assessment. The estimation of ET
0
depends on atmo-
spheric variables, such as air temperature, solar radiation, wind speed, number of
daylight hours, saturated vapour pressure and humidity. The Penman
Monteith
approach recommended by FAO (FAO-PM) is considered as the standard to cal-
culate reference evapotranspiration wherever the required input data are available
[
9
]. Many researchers have made strong recommendations to consider FAO-PM as
the standard method for evaluation of evapotranspiration through their comparative
studies [
8
,
9
,
43
,
45
,
84
,
99
]. Some studies also suggest that the ET estimation
techniques are most appropriate for use in climatic regions similar to where they
were developed [
47
,
75
].
Other modi
-
cations of the Penman equation to estimate evapotranspiration from
a hypothetical grass ET
0
, are the CIMIS Penman equation [
36
,
85
] and ASCE
Penman equations. Doorenbos and Pruitt [
28
] added some modi
cations to the
Penman combination equation, with a wind function that was developed at the
University of California, Davis. This modi
cation was adopted by California Irri-
gation Management Information System (CIMIS) for calculating hourly ET
0
and is
