So water rationing is sometimes used, but (A) doesn't explain what factors determine whether the area rations water. This means we can't compare the years with and without rationing to this year, which has unusually heavy rains. (A) doesn't challenge the author's assumption so doesn't weaken.

In other words, some (small) part of the water supply isn't affected by the extra rain this year. That's fine, but (B) doesn't give us any reason to believe the rain won't still prevent water rationing. Even if not all of the area's water comes from reservoirs and streams, abundant water from those sources could still be enough that the area doesn't need rationing.

(C) introduces a new factor that disrupts the author's assumption, thereby weakening the argument. By adding water pumping as a limiting factor on the water supply, (C) tells us that even if the reservoirs and streams are full, that doesn't necessarily mean that water will reach customers. So the rain might not be enough to prevent rationing, if the outdated pumping system just can't transport that water.

We don't know what effect the temperature has on water usage or the water supply to the area, so (D) doesn't affect the argument. If we assume that lower temperatures mean less water use, then (D) could even strengthen—but we don't know enough to make that assumption. What we do know is that (D) doesn't weaken.

The first problem with (E) is that it makes a general statement which might not even apply to the year in question. (E) is just talking about "most" years, but we already know that there's unusually heavy rain at the moment. That alone is enough to eliminate (E), because we can't judge how it affects this specific year.

The second problem with (E) is that it discusses precipitation relative to season, but we care about precipitation relative to the area's needs (if precipitation is the key factor at all). Even though the summer might get less rain, that could still be enough for the area not to ration water. Conversely, if demand is extremely high, even a lot of rain could be insufficient. Because (E) doesn't give us this context, it wouldn't weaken even if it applied to this specific year.