CONTENTS

KUMAR et al
For determination of crop ETc the soil moisture was
monitored by delta probe at four locations in various
depths before and after each irrigation and on
intermediate dates in case of incident precipitation.
Effective rainfall was estimated by following standard
procedure (Dastane, 1974) and it amounted to 13.05
mm during crop growing period. The groundwater
contribution (GWc) to ETc was nil. The reference crop
evapotranspiration (ETo) was estimated at specific
crop growth sub-periods based on Penman Monteith
equation (Allen et al., 1998). Thus the data obtained
on ETc of castor and ETo at specific crop growth
sub-periods were used to calculate the crop coefficient
(Kc) as follows
Kc = ETc ÷ ETo
For constructing the crop coefficient curve
(Fig. 1) the crop life of castor was divided into
germination and establishment, vegetative, flowering,
capsule development, seed filling and maturity
periods. To use Kc values for predicting crop ETc
(ETc = Kc.ETo) throughout the crop season, only
ETo estimates on Penman Monteith method from
the new planting site are needed.
As an application of this study in irrigation
water management, the estimates of ETc for castor
crop from the Kc values of highest yielding irrigation
treatment I 6
(drip irrigated at 0.6Epan up to flowering
i.e., 81 DAS and 0.8Epan later on) were used to
determine the actual irrigation requirements for a given
design as follows:
Ir = ETc x Growth period in days
In which, Ir is net irrigation requirement (cm)
for the growth period considered and ETc is crop
evapotranspiration (mm/day).
V at field inlet = (10 ÷ Ea) × (A.Ir ÷1 – LR) m 3
In which, V is gross irrigation requirement
for the period considered (m 3 ); Ea is field application
efficiency (0.9); A is area (1.0 ha); Ir is net irrigation
requirement (cm); LR is leaching requirement (nil);
and Ep is project efficiency under groundwater
irrigation through bore wells (0.9).
Crop Coefficients
Data on crop coefficients (K c
) calculated
based on castor crop ET c
and ET o
derived from
Modified Penman method are presented in Table 1.
The Kc values during the germination and
establishment period were not markedly different from
each other owing to uniform water application, since
the crop was subjected to variable water supply levels
only from 15 DAS. However, at the later growth stages
of vegetative, flowering, capsule development, seed
filling and maturity stages the Kc values were higher
under surface irrigation method as compared to them
under drip irrigated treatments owing to higher
irrigation water depth. The highest value of the Kc
was 1.206 and 1.108 under surface check basin
irrigation (0.8 IW/CPE ratio) and drip irrigation crop
(I 3
), respectively.Likewise the Kc values in I 6
treatment
were comparable to I 8
. Whereas, for I 1
treatment
irrigation at only 0.4Epan at all the crop growth subperiods
(15 to 150 days), the maximum value of the
Kc was 0.566. Thus, the K c
values were primarily a
function of evaporation replenishment factor during
a given crop growth sub-period under drip irrigated
crop and the amount of water applied under surface
check basin irrigated crop. Higher the replenishment
factor i.e., higher the water application level, greater
were the K c
values owing to higher ETc rates since
Kc is a ratio between ETc and ETo.
Crop Coefficient Curve
The crop coefficient curve shown in Fig. 1
was derived from Kc values of I 2
treatment registering
optimal bean yield, maximum net returns with higher
water productivity. For comparision, the Kc curve for
surface check basin irrigated crop (I 8
) raised irrigating
at 0.8 IW/CPE ratio is also depicted in Fig. 1. The
Kc values for I 8
and I 2
treatment varied between 0.436
to 1.206 and 0.411 to 0.804, respectively. The higher
Kc values for surface check basin irrigated castor
crop may be attributed to unavoidable deep
percolation losses and higher soil evaporation owing
to complete wetting of the soil surface (Bucks and
Nakayama, 1982). On the other hand irrigation
scheduling under drip irrigation is evapotranspiration
based with localized wetting pattern (Schwankl et al.,
1996 and Andreu et al., 1997) eliminating deep
percolation losses contributing to lower Kc values.
The Kc value from sowing to establishment was small
in view of very little (incomplete) canopy cover (LAI
= 0.067) and majority of the water loss may be
attributed to evaporation from the soil. Thereafter the
Kc value increased linearly due to increase in crop
ETc as the crop grew rapidly and developed more
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