One factor to consider is that before hurricane Rita in 2005, there was a much more significant land mass on the eastern edge of the ship channel which reduced the salt water flow into the southern end of the lake. Hurricane Rita and subsequent erosion destroyed that land mass and greatly increased the coupling of salt from the Gulf to the eastern part of Big Lake.
Consequently, prior to 2005, the salinity levels in the lake permitted the weirs to be open most of the time and only a prolonged period with less than average rainfall would raise the salinity in the lake sufficiently to require closing the weirs. (The levees themselves and the open weirs had already reduced saltwater intrusion sufficiently without closing the weirs.)
After 2005, higher salinity levels in the main lake require closing the weirs more often, to the degree that average or higher than average rainfall is usually needed for the weirs to be open 30 days in a month.
In summary, pre-2005 you needed rainfall levels well below average to force closing the weirs.
Post-2005, you need above average rainfall to permit opening the weirs.
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