I remember reading something about huge losses due to over-irrigation.
None. The only outlet is the Truckee river which flows east into Nevada. You may be thinking of some other lake.
EVERYTHING from the Tahoe Basin goes east, down the Truckee River to Reno and beyond (ends up in the flats in Pyramid Lake). There is a canal cut through from the northern part of the Truckee to the Carson River basin, which continues to flow east and sinks into the earth, the Carson Sink near the Fallon NAS.
I walked and drove over much of this part of Nevada in the years from 1997 to 2002, as part of my assignment with the Department of Interior. So I saw it up close and in times of both drought and relatively plentiful water.
Big variation between high and low levels.
Lake Tahoe is a tributary watershed drainage element within the Truckee River Basin, and its sole outlet is the Truckee River, which continues on to discharge to Pyramid Lake.