CORPUS CHRISTI - Evaporation, drought and mandatory environmental pass-through releases into the estuary have combined to trigger Stage 2 water restrictions within the city.

An official announcement on the conservation measure is scheduled for Monday. A grace period for compliance will end July 28, according to Brent Clayton, Corpus Christi�s water resource planner.

Combined, the levels for Choke Canyon Reservoir and Lake Corpus Christi reached 40 percent of capacity this week, which means lawn sprinklers may only be used on a resident�s regular garbage pickup day. Year round rules forbid lawn irrigation within the city between 10 a.m. and 6 p.m. Hand-held hose watering carries no restriction under Stage 2.

Lake Corpus Christi is about five feet low and Choke Canyon is about 25 feet low. During May and June the city was forced by a long-standing agreement with the state to release about 30,000 acre-feet of water from the city�s reservoir system. An acre foot equals the amount of water it takes to fill an acre to a 12-inch depth. A single acre foot equals about 325,851 gallons.

The agreed order for environmental pass-throughs to the bay stems from negotiations with the Texas Commission on Environmental Quality before and since Choke Canyon Reservoir was created. In this legal contract the city and state agreed on target amounts for environmental releases for each month based on historic rainfall amounts.

Each month�s target amount is different. But these targets are not mandated release amounts. The city may ignore a target amount if no rainwater reaches the reservoir system. The city may release less than the target amount if incoming rainfall falls short of the target. And if rainfall exceeds the target amount the city may capture and keep the overage.

In May, a typically wet month, the reservoir system received about 18,000 acre-feet of rain. The May target amount is 23,500 acre-feet. So the city released 18,000 acre-feet to maintain the health of the Corpus Christi Bay estuary system. When it rains below the Wesley Seale Dam on Lake Corpus Christi, the city gets credit for the estimated amount that falls, which may be subtracted from the pass-through target for that month. And each month the city gets 500 acre-feet of credit for wastewater returned to the bay. This also is subtracted from the pass-through target.

And when the combined reservoir level reaches 40 percent of capacity the pass-through target amounts are dramatically reduced. For example, July�s target amount is 4,500 acre-feet because it is historically a dry month. But the new target amount is 1,839 acre-feet because the reservoirs are at 40 percent.

David Lozano, manager of the Wesley Seale Dam, said river water is not fully owned by residents of the city. He said based on natural laws the water is owed to the river itself as a public resource and ultimately to its final intended destination, which in this case is the estuary for Nueces Bay and Corpus Christi Bay. Without freshwater the bays and estuaries would die, biologists say.

Lozano suggests if water were not released to help maintain heathy bays and the economies that rely on them, the alternative would be wasteful. Holding on to the water is not like putting money in a bank, he said. Money gains interest. Water evaporates.

During the first seven days of July, 2,703 acre-feet of water escaped into the atmosphere from the wide and shallow Lake Corpus Christi, which currently holds about 167,157 acre-feet of water. The lake capacity is 257,260 acre-feet.

At Choke Canyon Reservoir, a much deeper body that can retain a greater percentage of its volume, the evaporation rate is 1,918 acre-feet per week. The volume of Choke Canyon currently is about 211,095 acre-feet, Lozano said. It is capable of holding 695,271 acre-feet.

During summer, the reservoirs� combined daily or weekly evaporation rate can exceed the amount of water used by residents of Corpus Christi, surrounding communities, agriculture and industry during the same time period, he said.

�Our objective here is to try to conserve as much water as possible while sticking to the agreed order,� Lozano said. �Believe me, we�re not trying to drain the lake. But holding on to (the water) would give people a false sense of security because of how much we lose from evaporation. That would be a waste.�

David Lozano was either misquoted at the end or needs to provide further explanation.

Am I missing something? A full lake would provide more usable water and the effects of evaporation would be felt less in the future if, next week, it stops raining and doesn't start again for a few years. But by releasing water now, as the lake has come up, they are creating a new normal, which is below the holding capacity of the lake, it appears and evaporation will still occur, only it will be depleting a smaller water supply than if they didn't let water out.

Surely, it can't be as simple as I think it is and I'm missing something obvious.