Kind of a fun study, enabled by the presence of so much public-domain data from CA DWR.

One piece that's missing from this analysis is upstream precipitation. You are sensing precip at Oroville itself, but water in the dam comes from everywhere upstream (3600 square miles according to [1]). As well as (potentially) snowmelt.

People doing forecasting of reservoir levels will be integrating distributed precipitation information, snowpack, and temperature, with a soil runoff and routing model to get predictions.

[1]: http://cdec.water.ca.gov/cgi-progs/profile?s=ORO&type=dam