Climate Variability and Planning for Floods and Droughts
by: Upmanu Lall
The estimation of
flood and drought risk has long been part of the training of water resource
engineers and managers. Design for flood control and operation of reservoir
systems for floods and droughts can be done as part of an integrated
risk based river basin development and management plan. Managers also
recognize that changing land use and changing objectives require periodic
re-assessment of the estimate of risk and of the associated management
plan. The recognition that climate may be changing systematically over
long periods of time, either naturally as in the case of periodic episodes
of El Nino and La Nina events, or due to increases in greenhouse gases,
is relatively recent. The thesis of this talk is that while considerably
uncertainty continues to be associated with the projected local impacts
of anthropogenic warming, our understanding of inter-annual climate
variability continues to improve, allowing for new strategies to assess
and cope with the significant changes in regional flood and drought
risk.
Two examples, one
dealing with floods and one with droughts will be discussed. The probability
of regional floods and droughts will be connected to variations in certain
planetary climate precursors. These "teleconnections" form
a basis for understanding how flood and drought risk may vary systematically
over a period of years. Hydrologic records in developing countries are
often short. Where climate varies systematically, the last 10-30 years
may not be representative of what could happen in the next 10 to 30.
Using planetary climate information and a regional perspective, a better
representation of local flood and drought risk and its long term variation
can be developed to help river basin planning and design of projects.
Second, given that many places respond strongly to El Nino/La Nina events
or other measurable ocean conditions that are in evidence a season or
more before the impact, one can consider near term risk estimation and
adaptation strategies. The use of formal and informal seasonal climate
forecasts in this context is discussed.
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