ASSESSMENT OF THE MAIN METHODS FOR ESTIMATING POTENTIAL EVAPOTRANSPIRATION AT SAMARU IN NORTHERN GUINEA SAVANNA ZONE OF NIGERIA
ASSESSMENT OF THE MAIN METHODS FOR ESTIMATING POTENTIAL EVAPOTRANSPIRATION AT SAMARU IN NORTHERN GUINEA SAVANNA ZONE OF NIGERIA
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Date
1999-12
Authors
ANOSIKE, RITA NGOZI
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Abstract
Evapotranspiration is a major component of the water balance
in any ecological system. Several methods have been proposed to
estimate potential evapotranspiration in different parts of the
world. However, the success of most of them has been limited
because of uncertainties in the reliability of the methods under
subhumid tropical environment.
A study was therefore undertaken at the Institute for
Agricultural Research Farm, A.B.U., Zaria (11° ll'N, 07° 38'E,
686m above sea level) to determine the most suitable method for
estimating potential evapotranspiration in Samaru within the
Northern Guinea Savanna Zone of Nigeria.
To achieve this objective, predictive efficiency of five PET
models (BMN, Hamon, Jensen Haise, Penman and Makkink) was examined
against measured microlysimetric PET and open pan evaporation.
The estimates of the five models were computed from the 30 years
(1965-1994) climatic data and compared with pan evaporation.
These five equations were also used to compute PET for two years
(1996-1997) and compared with measured microlysimetric PET and pan
evaporation during this period. It was observed that Hamon,
method overestimated PET throughout the entire year. Jensen Haise
and Penman methods over estimated slightly above the pan
evaporation in some months. BMN method estimated closest to pan
evaporation. Makkink method underestimated in some months and
slightly below the pan evaporation. Only BMN method predicted
magnitude of performance for the two measured PET was in the
order: BMN > Penman > Makkink > Jensen Haise > Hamon. The methods
developed by BMN, Penman and Makkink give comparable results which
were good approximation of measured PET. The recmmended methods
( BMN, Penman and Makkink) are inspite of the observation that
they do not exhibit the same degree of oscillation as measured
microlysimetric PET and pan evaporation and thus the differences
between the maxima and minima are relatively smaller than the
other methods.
To further improve on the efficiency of the three equations,
adjustment factors were incorporated into them; so that they can
estimate as precisely and accurately as the measured PET.
Equations relating BMN, Penman and Makkink equations to measured
PET were developed. The modified BMN, Penman and Makkink
equations are given as follow:
Y = 1.38+0.59 BMN R2 = 0.99
Y= 2.58 + 0.17 Penman R2 = 1.00
Y = 0.5 + 0.99 Makkink R2 = 1.00
Y = -0.08 + 0.99Pan
Y = Microlysimetric PET
For pan evaporation (x)
x = 0.033 + 0.99BMN, R2 =1.00
x = -0.051 + 1.007Penman, R2 = 1.00
x = 3.79 + 0.23Makkink, R2 = 0.95
The BMN method is recommended for application as best
satisfying the objective of the study. It is the simplest, most
The BMN method is recommended for application as best
satisfying the objective of the study. It is the simplest, most
accurate and reliable of the three recommended methods.
It was also observed that the PET is highest in March the
warmest month of the year and lowest in August the wettest month
in this zone.
Path correlation analysis on the contribution of each of the
six important climatic factors (relative humidity, air
temperature, wind speed, soil temperature, sunshine hours and
solar radiation) indicated that microlysimetric potential
evapotranspiration was influenced in the following trends:
sunshine hours (18.47%) > soil temperature (13.84%) > relatively
humidity (11.49%) > wind speed (11.22%) > air temperature (8.29%)
> solar radiation (4.41). Whereas pan evaporation was influenced
as follows: soil temperature (39.19%) > sunshine hours (22.28%) >
air temperature (7.56%) > wind speed (4.05%) > relative humidity
and solar radiation (2.56%).
A comparison of Penman and Monteith equations with measured
soil evaporation beneath maize and sorghum crops showed that the
Monteith equation is a better estimator of soil evaporation than
Penman equation. However, adjustment factors were developed for
both equations.
Description
A Dissertation submitted to the Postgraduate School, Ahmadu Bello
University, in partial fulfillment of the requirements for the
degree of DOCTOR OF PHILOSOPHY in Soil Science
DEPARTMENT OF SOIL SCIENCE
Faculty of Agriculture
Ahmadu Bello University
Zaria, Nigeria
DECEMBER, 1999
Keywords
ASSESSMENT,, METHODS,, ESTIMATING,, POTENTIAL,, EVAPOTRANSPIRATION,, SAMARU,, NORTHERN,, GUINEA,, SAVANNA,, ZONE,, NIGERIA.