The historic drought punishing Texas has forced about a fifth of the state's water systems to ask or compel customers to follow water restrictions, while leaving 23 systems with either unknown supply levels or within six months of completely running out, a top expert told state senators Tuesday.

Carlos Rubinstein, one of three commissioners of the Texas Commission on Environmental Quality, testified before the Senate Natural Resources Committee that 91 percent of the state is now facing extreme or exceptional drought conditions and 956 public water systems have imposed voluntary or mandatory restrictions on water use. His commission lists 4,721 total community water systems across Texas.

Rubinstein said 55 of those have prohibited all outside watering and at least 23 are in such dire straits that officials can't determine the state of their water stocks, or believe they are within 180 days of drying up.

"We try to talk to them, try to find out where we have alternate water supplies," Rubinstein said of the communities whose water situation was most-alarming. He did not identify the areas, but a map posted on the commission Website showed many regions under mandatory restrictions clustered around Houston and San Antonio.

Through September, Texas has averaged only 8.5 inches of rain, nearly 13 inches less than normal. Rubinstein's commission safeguards water rights and diverts water to ensure all needs are met. But those diversions are based on the priority date of each water right and, during droughts, more senior holders trump newer ones. Rubinstein said authorities have already curtailed water rights in 1,005 cases.

In addition to the most intense single-year drought in history, Texas has had the largest agricultural losses and most destructive wildfires on record this year.

The state tracks water levels at 109 of its 175 major water supply reservoirs and found that by the end of September, they were below 60 percent capacity — the lowest level since 1978. Rubinstein said of the 31.1 million acre feet capacity, just 18.8 million acre feet of water was being stored.

Republican Sen. Troy Fraser, chairman of the Natural Resources Committee, said the drought would be a top challenge during the next legislative session, which opens in January 2013. Rubinstein was part of one of seven panels committee members heard Tuesday and a counterpart House committee has scheduled a drought meeting for Wednesday.

Prospects for the future are even more sobering.

State climatologist John Nielsen-Gammon said Texas has endured droughts five of the past seven years and that La Nina activity in the Pacific, as well as unfavorable conditions in the Atlantic, mean the current drought is likely to continue at least another year — and may rival the 1950 to 1957 drought that was the state's longest prolonged lack of rainfall.

In the meantime, Texas had its hottest summer ever recorded with an 86.8-degree average, which Nielsen-Gammon called a statistical tie with Oklahoma for the hottest in U.S. history. He said sizzling temperatures had increased evaporation in reservoirs, further depleting already decimated water stocks.

Carolyn Brittin of the Texas Water Development Board urged implementing a state contingency plan to bolster future water supplies, but said estimated costs have increased from $31 billion in 2007 to $53 billion today. The plan is now more costly because it calls for more water treatment and desalinization facilities, as well as additional pipes and other infrastructure for transporting water.

Still, the price tag left committee members taken aback. "We're just going to have to get a Washington D.C. perspective on all this," deadpanned Republican Sen. Craig Estes.

Copyright 2011 The Associated Press.