Spring came early. That's bad for Arizona's water supply

Spring is arriving sooner and warming up faster than ever before, new research shows. And that means more than just early wildflower blooms across Arizona.

A longer, warmer spring can stress water supplies in the West. The longer spring season may also produce ripple effects on agriculture as water demand will likely increase, and growing seasons may shift.

On average spring temperatures have increased by 2 degrees Fahrenheit across the U.S., according to research by the USA National Phenology Center. The season is starting earlier and getting warmer in 97% of the 238 locations surveyed. And more than anywhere in the U.S., the Southwest has experienced the most significant increase in temperatures since 1970.

Since 1970, Tucson has warmed 6.1 degrees Fahrenheit during the spring season, and in Phoenix, temperatures rose 5.5 degrees over the same time. The study suggest that the spring season is prolonged due to an increase in atmospheric levels of carbon that is driving climate change. On average, both Tucson and Phoenix are experiencing more than 30 spring days with above-normal temperatures

Scientist are concerned that an early start to spring could cause snowpack, which plays a key function in the water cycle in the West, to melt faster and earlier than usual.

“In many watersheds it’s the mountain precipitation that runs off and provides water flow and streamflow and that occurs in the spring,” said Sharon Megdal, director of the University of Arizona's Water Resources Research Center. “We get snow in the winter, and it melts in the spring, it’s a really important season.”

One of those watersheds is the Colorado River, which supplies about 36 percent of water in Arizona according to ASU. Megdal says that even with average or above-average precipitation in the basin in recent years, runoff has decreased, likely due to warmer weather specifically in the springtime.

“Nobody is rejoicing yet about the tremendous snow and perception we had this winter in the Colorado River Basin,” she said. “We may have very good snowpack and very good precipitation, but what that means for runoff really does depend on what happens in the spring in terms of temperatures and how quickly they warm up and how much they warm up.”

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How drought affects soil, plants

Mountain snowpack is responsible for about 50% of the West's water supplies. Snow that accumulates during winter months will begin to melt in the spring, ideally creating a slow trickle that will turn into streamflow and ultimately flow into rivers and reservoirs.

But an earlier spring season and warmer temperatures can cause snowpack to melt rapidly and disappear earlier. In the last 40 years, snowpack melt has trended three weeks earlier in many areas of the West, which has stressed the already limited water supply. When this snowpack melts earlier, it creates a lull between its runoff and the summer monsoon season and can extend a period of dry conditions that affects plant and crop health.

“We have a had more rapid increase in temperatures here in the Southwest than in other regions,” said Theresa Crimmins, director of the National Phenology Network, who worked on the study. “You need more moisture to keep the plants happy when it's hotter and that isn’t necessarily happening, so there is greater increase in drought stress on plants.”

Drought stress results when water loss from the plant exceeds the ability of roots to absorb water and when the plant's water content is reduced enough to interfere with normal plant processes. Crimmins says warmer spring temperatures will create a higher demand on organisms for moisture as they will likely have less access to water.

A warmer spring in the Southwest may have pronounced affects because soil health is damaged from extended drought. Drought can affect soil's chemical, physical and biological activities that are essential for plant and soil health. Dried-out soil turns hydrophobic as it loses nutrients and will reject excess water, leaving it to flow across the surface instead of absorbing it.

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How warmer springs affect growing seasons

The seasonal shift also changes the way vegetation grows, whether they're natural or planted crops.

“When it gets warmer earlier it’s going to make for a longer growing season,” said Paul Brierley, executive director of the Yuma Center of Excellence for Desert Agriculture. “Now you have more time when the crops are growing and needing water.

The desert has two growing seasons — spring and fall — and increased temperatures earlier in the year could mean the growing season will shift with temperature patterns.

The challenges of a longer growing season may cause higher irrigation demands and limit the types of crops that are grown. Farmers may also be faced with more unwanted plants and animals — including weeds like poison ivy and pests like mosquitoes — that might reduce crop yields.

“You’re going to need water earlier and you only get so much water,” Brierley said. “A lot of areas in Arizona are limited hydrologically or with water rights, and if the crop needs more water than you have water to put on it, then you have problems.”

And as the growing season could potentially shift due to increased temperatures, less runoff from snowpack may also reduce water availability to farms that rely on surface water.

The Gila River is fed by snow that forms in the high elevations of the Gila National Forest in New Mexico. In parts of the river where water still flows, snow melt and stream flow will typically feed the river until it dries up as summer approaches in June.

“If you think about the Gila River, it typically dries in June and then the monsoons come a few weeks later and that gets it going again,” Brierley said, “So you don’t have a whole long period where there’s no water in the river to divert and water your crops,”

Brierley worries that increased temperatures earlier in the season could leave the river dry for longer stretches which would likely affect how much water gets to crops.

“So, if spring is warmer and earlier it's likely the river will dry up earlier and there will be a longer stretch before the monsoon comes and gets It ready again,” he said. “You could have problems not being able to divert water from the river because of that longer gap of the river being dry.”

Crimmins says measuring the start of spring in Arizona is more difficult than in other parts of the country that have more clear-cut seasons. But the significant increase in temperatures over the last 50 years in the Southwest has shifted what happens during the spring season and will have long term impacts she says.

Jake Frederico covers environment issues for The Arizona Republic and azcentral. Send tips or questions to jake.frederico@arizonarepublic.com.

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This article originally appeared on Arizona Republic: How earlier, warmer spring affects Arizona's water supply