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Abstract
The study
focused on the assessment of impacts of El Niņo and La Niņa events on
the stream discharge and the water budget of Angat, Bayongan, Mananga,
and Manupali watersheds using WatBal and BROOK 5 model. This was done
by comparing the runoff and water budget of the various watersheds during
the El Niņo and La Niņa years with runoff and water budget during normal
years using t-test. The percent change in runoff and other water budget
components during El Niņo and La Niņa with normal year values as bases
were also computed.
The capability
of the two models to predict runoff was also evaluated in the study by
comparing the model estimated runoff with the observed runoff values using
t-test. The correlation coefficient and average errors were also determined
as supplementary tests of model fitness.
The results
of analyses reveal that runoff generally decreased during El Niņo years.
However, during La Niņa years, runoff increased or decreased depending
on rainfall and temperature. There was marked decrease in rainfall in
some watersheds while in others, rainfall remained the same as during
normal years.
Compared
with normal years, the runoff during El Niņo years generally decreased
while the runoff during La Niņa years either increased or decreased. This
erratic runoff behavior could be attributed to the varying intensity and
duration of El Niņo and La Niņa events which for some years were very
pronounced and persistent while in some years were very weak and short-lived.
Consequently, the rainfall during El Niņo was very abundant in some years,
remaining within normal range in some years or was too little in other
years. Likewise, during La Niņa rainfall was less than normal in some
years while in some years rain was normal and in other years rainfall
was excessive depending on its intensity.
The evaluation
of WatBal and BROOK 5 model show that the models have fairly acceptable
ability to predict runoff. WatBal performed better in larger than in smaller
watersheds while BROOK 5 performed better in smaller than in larger watersheds.
The model showed a common weakness in underestimating large runoff events.
This is likely due to the oversimplified representation of the usually
variable watershed attributes like soil, topography, and vegetation.
Adaptation
measures to adverse hydrological impacts of El Niņo and La Niņa should
be designed either to improve the resilience of watersheds and its hydrologic
processes in the presence of rainfall pattern disturbances. A well-managed
watershed vegetation normally makes the watershed hydrologically stable.
Therefore sustainable watershed management and rehabilitation of denuded
forests could significantly neutralize the adverse impacts of El Niņo
and La Niņa. Improvement of the ability to capture and store excess rain
during wet season could also help alleviate the problems brought about
by prolonged drought and excessive rainfall. Enhancing the base of information
and technology through research for upgrading the knowledge on El Niņo
and La Niņa and on quantification of its impacts could also help substantially
adapting the El Niņo and La Niņa.
Source:
Rex Victor O. Cruz; Ma. Maureen B. Avanzado; Severo R. Saplaco; Rodel
D. Lasco; Florencia B. Pulhin; Nathaniel A. Cruz. A Collaborative Project
of the Forestry Development Center (FDC), Environmental Forestry Program
(ENFOR); Philippine Atmospheric, Geophysical and Astronomical Services
Administration (PAGASA), and PCARRD. Terminal Report, May 2000.
For more
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