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<br />A-3 <br />must be scheduled to preserve the greatest amount of flexibility for the peak storage <br />months of June and July, without unduly risking the over release of water in storage, <br />Often, this requires high releases during the winter to evacuate space in the reservoir <br />and moderate releases during the spring to preserve operational flexibility. This <br />produces the lowest reservoir storage during March and the highest storage during July. <br />Avoiding anticipated spills is one of the 1968 Act provisions that is closely tied to dam <br />safety concerns and also ensures that high flood flows do not damage the canyon <br />ecosystems. With full reservoir conditions there are usually small margins for error in <br />making these release decisions. <br /> <br />If the reservoir storage is in between these two situations, the objectives become more <br />complex. Potential spills as well as over release of water on an annual basis are both <br />risks. Release decisions are a careful balancing act and sometimes planned releases <br />change dramatically with each new runoff forecast, usually occurring monthly and even <br />bi-weeldy during peak inflow periods. Changes in monthly releases on short notice can <br />cause difficulties with power scheduling, recreational use, and research in the Grand <br />Canyon. <br /> <br />Forecast errors introduce a great measure of uncertainty in planning future releases. <br />The scheduling of releases must account for this to avoid the risks cited earlier. <br />Continual updating and reanalyzing of basin conditions help to counter the weather <br />variability which is the greatest cause of forecast errorS. These errors are often as much <br />as several MAF during mid-winter, a significant percentage of the total April - July <br />runoff volume. The 1995 runoff was a good example of how abnormal weather patterns <br />can greatly affect runoff. Since each year is different, both with respect to inflow <br />volumes and forecast errOrs, the monthly release pattern changes continually, adjusting to <br />changing forecasts. However, the basic principles remain the same. <br />