Drought Management and Its Impact on Public Water Systems Report on a Colloquium Sponsored by the Water Science and Technology Board (1986) / Chapter Skim
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2. Causes and Occurrence of Drought
2. Causes and Occurrence of Drought
Pages 24-48
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From page 24... ...
Unfortunately, the brevity of available hydrologic records makes it virtually impossible to distinguish between these two possible causes (Lettenmaier and Burges, 1978~. Nonetheless, the Present increase in meteorological variability should we're used to.t' ~ cause a corresponding increase in the occurrence of hydrologic droughts, at least in the immediate future.
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One source of confusion is the unavoidable diversity in the ways in which various fields of study view drought events. A water resource engineer views drought as a problem in supply and demand.
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From page 26... ...
For instance, since the term low flow denotes an annually occurring minimum flow of short duration, one would expect the annually occurring maximum flow of short duration to be called a high flow; however, this is usually termed a flood event. In addition, because the term high flow may be used to refer to extended periods of above mean discharge, one would expect extended periods of below mean discharge to be called low flows; however, these are usually termed drought events.
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From page 27... ...
—2 7— Di sc ha rge mean o FLOOD ~ I GH FLOW LOW FLOW DROUGHT .
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In the midst of this ambiguity and complexity, one potentially satisfying drought definition has emerged in the past decade as being objective and yet flexible enough to be applicable to a wide variety of drought concepts. In effect the proposed approach merely systematizes the intuitive intentions of the various definitions mentioned above.
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From page 29... ...
YL/tL = Run-length above XO Run-length below XO (drought duration) Drought magnitude 8 9 10 11 Years FIGURE 2-2 Parameters of the runs of a hydrologic series.
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From page 30... ...
The method's flexibility is due to the freedom allowed in the choice of xo; the hydrologist may use mean annual flow, while the agriculturalist may prefer seasonal soil moisture. The combination of these two characteristics in an approach to drought definition and description is particularly attractive because of the wide variety of drought concepts held throughout various scientific disciplines.
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warm, dry air in the middle troposphere. In drought regions, the warm air aloft subsides at the rate of several hundred meters per day.
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—32— tom' ._ .h A, ~ i]
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. Permissions granted World Meteorological Organization and UNESCO Press.
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From page 35... ...
Permissions granted World Meteorological Organization and UNESCO Press.
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One theory holds that the land, rendered hot and dry during drought, further heats the air above it and thereby enhances the regional high pressure zone. A second theory states that drought causes an increase in fine dust particles in the air, which lead to high concentrations of very small cloud droplets whenever cumulus clouds form and thus make it more difficult for precipitation to form (Twomey and Squires, 1959~.
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From page 37... ...
Correlations are then made and teleconnections are used to translate these data into circulation patterns, temperature, and precipitation over continental areas (Namias, 1976~. The hydrological methods of forecasting drought include streamflow-regression-based methods, regression methods, the analysis of cycles in annual streamflow, and other statistical analyses of historical streamflow events.
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Permissions Meteorological Organization and UNESCO Press. granted World
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From page 39... ...
states that these forecasts are physically based using statistical and synoptic methods which employ large scale fields of Northern Hemisphere atmospheric geopotential height and Pacific Ocean sea surface temperatures. The forecasts are made in equally likely tercile classes, light (L)
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The report contains a discussion of results of the project forecasts to date and an analysis using a "skill score." The CDWR translates these forecasts into California Water Supply Outlook report (1985) and a State Water Project Water Delivery Rule Curve and Criteria for 1985 report.
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—42— ran m LL]
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Namias evaluates his meteorological forecasts for California and the western United States using the following skill score equation: Skill = Correct Forecasts - Correct Forecasts Expected by Chance Total Forecasts - Correct Forecasts Expected by Chance For the time period 1975-1976 through 1981-1982, skill scores of approximately 0.67 were experienced. However, during the 1982-1983 water year a skill score of only +0.25 (6 out of 12 correct forecasts)
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From page 45... ...
However, in considering this topic and the broader issue of drought management and its impact on public water systems not only the engineering and technological aspects of the problem should be analyzed, but also the areas of economics, finances, legalities, politics, society, and the environment. For example, suppose an elaborate international network for drought prediction were developed, which included ground sensors, report collection stations, telemetering satellites, simulation models, and information dissemination centers.
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From page 46... ...
1985. State Water Project Water Delivery Rule Curve and Criteria for 1985.
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1976. Seasonal forecasting experiments using North Pacific air-sea interactions, preprints of Sixth Conference on Weather Forecasting and Analysis, Albany, N.Y., American Meteorological Societies.
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From page 48... ...
1972. Relationships between sea surface temperature and wind speed over the Central Arabian Sea and monsoon rainfall over India.
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