Brrrrrrrrrrr! It's cold in the rice paddies of Northern California. Maybe too cold.

Sacramento Valley rice producers have more than their share of yield robbing pests, however, a University of California Cooperative Extension farm advisor in Butte County believes his rice farming clients may be losing a lot of money to a manmade pest….cold irrigation water releases from Oroville Dam.

Farm advisor Randall “Cass” Mutters was admittedly surprised to find last year how pervasive the-yield robbing effects of cold-water releases from Oroville Dam can be.

Monitoring two fields and a main canal that delivers water from the Oroville Dam afterbay, Mutters found the water remained constantly cold from the dam to the field — six degrees or more below the 68 degrees considered the threshold where rice yields are impacted based on recent research.

When rice paddy water is too cold, Mutters said, it causes blanking of heads and delays maturity and therefore reduces yields.

“Rice growers have always realized that where the water comes into the field they can expect lower yields because that is where the water is the coldest,” said Mutters. It is expected to warm as it moves into the checks.

Some construct long, narrow warming checks before the water is turned into the growing rice. However, that may not be a totally effective strategy for mitigating cold water effects, particularly early in the season.

Losses extend into field

Mutters discovered that the yield losses due to cold water can extend far into the field-across a half mile check — because the water is too cold coming into the field and never warms ups to above yield-reducing threshold.

Recording thermometers measured water temperatures every hour for the entire growing season. Results from the main canal of a local irrigation district revealed no temperature change in 15 miles of canal movement.

It's too cold and never warms to tolerable levels, rice growers contend, because water releases from Oroville Dam are colder, perhaps colder than agreed upon when the dam was built.

Oroville Dam, completed in 1967 and as the last major reservoir built as part of the State Water Project, it has a gross capacity of 3.5 million acre feet of water.

It is operated for water supply, power generation, flood control, recreation and fisheries and wildlife habitat enhancement.

Many of those are competing interests, especially fish and wildlife and farming when it comes to water temperatures.

Farmers want warm water and fish and wildlife people want colder water.

Colder, deeper

Unlike natural rivers where Mother Nature controls the water temperature, the operator of the Oroville, the State Department of Water Resources (DWR) controls water temperature via release shutters in the dam. The deeper the water-release level, the colder the water.

Farmers knew that and when the dam was proposed, they asked that water be warm enough to farm rice. However, Curtis Creel, chief of the DWR state water project operations branch, said there was no “explicit” temperature agreement, only that the water temperature be “similar” to water the river flow was before the dam was built.

There is an afterbay below the world largest earthen dam where Creel said water is expected to warm before being delivered to ag water districts in the Butte Basin.

Mutters research revealed that the afterbay only slightly warms the water temperature before it goes down the main canals and into fields.

That was a surprise to Creel, who acknowledged that farmers have been complaining about colder water.

“To this point the issue of water temperatures have been antidotal,” said Creel. He was not aware of Mutters' work, but said, “it is good” that scientific information is being collected. “It will help the decision makers in the re-licensing process we are now going through with Oroville. We may have to consider alternatives to how we now operate and see if there is any flexibility in meeting competing requirements.”

Shorter warm-up

Creel acknowledged that the Oroville water releases are colder today. “The retention time in the afterbay is a bit less than it used to be and that means the water does not have the opportunity to warm up,” he said.

Lower retention time in the 57,000 acre-foot capacity afterbay he said is due to higher demands from cities for water; Delta restoration demands as well as farm water demands. “We also lowered the water temperature (released from the dam) at the request of the fish and game department to address a virus problem in the Feather River fish hatchery. More recently, we have been looking at increasing the temperature because of concerns expressed by farmers about the impact of the colder water,” said Creel.

Mutters is also surprised at how significant the impact of cold water has on yield, but when he recorded water temperatures of 58 to 67 degrees all summer long it is not that surprising.

“We are talking about yield reductions that represent substantial economic losses in some fields due to cold water,” said Mutters.

California agriculture is in a fight for its economic life with a dwindling state water supply due to urban growth and fish and wildlife demands. Mutters has discovered that rice producers are also losing out to water quality as well because of that competition with fish.

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