Stealing the Rain

Image: Storm clouds over Badlands National Park, South Dakota. Photo by Brian Richter

One of the great benefits of my role as the water scarcity editor for the international Water Security journal is that it forces me to read a lot more journal articles than I otherwise would. And sometimes I get a sneak peek at something really cool!

Patrick Keys of Colorado State University and his co-authors recently contributed a paper entitled “Invisible water security: Moisture recycling and water resilience.” In this fascinating paper (OK I’m a water science geek so this stuff excites me) the authors discuss and quantify the global hydrologic processes of water evaporating from land and the ocean surface, flowing as cloud moisture through the atmosphere (sky rivers!), and falling out again as rain or snow. Most of us were introduced to these water processes in grade school when we studied the global water cycle. Some of us might even remember that 60% of the water that falls on land originates from ocean evaporation, the remaining 40% coming from land evaporation.

OK so this is grade-school stuff…….but stick with me. Here’s the kicker of their story:

“Through land-use change, mainly by agricultural expansion, humans are destabilizing and modifying moisture recycling and precipitation patterns across the world.”

“Vegetation plays a particularly important role as it can alter the evaporative flow from land to atmosphere and thus regulates not only regional water availability, but also precipitation elsewhere.”

“Today, in a new era of social-hydro-ecological dynamics, significant anthropogenic impacts to the water cycle are re-wiring human-water dynamics and compromising the capacity of the biosphere to support the human endeavor.”

Wow. We’ve been hearing a lot lately about our influences on the global climate and water availability because of our carbon emissions, but how many of us have contemplated the fact that as we change the vegetation cover of the planet we are also changing the way that moisture moves around and where it falls on the landscape?

Image: Map showing the percentage of precipitation derived from land-based evaporation and therefore subject to being influenced by vegetation changes. Source: Keys et al, 2019.

Keys and his co-authors touch on several case study examples in their paper that illustrate how vegetation changes have affected rainfall or snowpack in downwind areas. Some of those stories really piqued my interest, leading me to dive more deeply into other articles they cite. Here’s one mind-boggling story emerging from our growing understanding of land-atmosphere moisture exchanges.

The Central Valley – Colorado River Connection

• Farmers in the Central Valley of California have been pumping more than 10 million acre-feet of groundwater onto their farms in recent years, causing aquifer levels to plummet. This unsustainable groundwater use supplements what the farmers get from rain/snow and what they divert from the Sacramento and San Joaquin Rivers. (40% of total farm use comes from groundwater)

• Historical conversion of wetlands to farmland had huge impacts on waterfowl, fish, and other wildlife in the Central Valley. The diversion of groundwater and river flows for farms and cities continues to impact wetlands and endangered species.

Image: Pre-1900 land cover (left); land use in 2000 (Source: Faunt 2009)

• The irrigation water poured onto Valley farms results in greatly enhanced evapotranspiration (ET) from crops and soils; ET from the pre-1900 valley landscape is estimated at 12.6 million acre-feet, but today it is more than doubled at 25.6 MAF.

• Much of the evaporated water from the Central Valley drifts away in clouds and falls as rain over the Southwestern US and the Colorado River basin. It increases rainfall in this region substantially, enhancing summertime river flows by 2.1 MAF (+56%) across the Southwest and by 324,000 AF (+28%) in the lower Colorado River basin.

• California recently enacted a Sustainable Groundwater Management Act. As Californians strive to manage groundwater pumping sustainably, considerably less groundwater will be available for farming in the Central Valley. Lesser applications of water to crops in the valley will have the unintended consequence of lowering the volume of water that evaporates and adds to water flows in the Colorado River.

• During recent decades, even with ‘bonus water’ coming from the Central Valley, every drop of the Colorado River is consumed before reaching the Gulf of California. In fact, on average 11% more water has been consumed in the Colorado River basin since 2000 than has been annually available (Bureau of Reclamation data), causing water stored in Lakes Mead and Powell to plummet to 46% of their capacity as of today. As a result, allowable use of the river’s water will be cut back beginning in 2020.

• Climate scientists have recently documented that the Colorado River is already drying because of climate warming and it’s going to get worse: declines in the river’s flow of 20-30% are projected for 2050, posing much-greater risk of water shortages.

To summarize: Farmers and cities in the Colorado River basin are already over-using their available water supply, but things are going to get A LOT WORSE because (1) allowable water use is being throttled back due to plummeting reservoir levels; (2) the climate is warming and shrinking river flows; and (3) less bonus water will be coming from the Central Valley.

Stealing Water?

With that backdrop, one of the major strategies proposed for addressing water shortages in the Colorado River basin really has me scratching my head. Colorado and Wyoming are betting on cloud seeding, a practice in which a chemical such as silver iodide is sprayed into the atmosphere in hope of causing more rain to fall from passing clouds.

Given what we now know about land-atmosphere water exchanges, doesn’t that seem like stealing rain?

Don’t you think it’s only a matter of time before a downwind state such as Nebraska or Kansas files suit against an upwind cloud-seeding state for stealing their water?

A Proven Solution: Pay the Farmers

It should be abundantly clear to water managers and all water users in the Colorado River basin that in the face of imminent water crisis, something big and bold needs to be done immediately.  We need to reverse the downward water storage trends in Lakes Powell and Mead to regain our water security buffer, and the only way to do that is to substantially reduce water consumption in the basin.  The fastest and most reliable means of doing this is to pay farmers to temporarily fallow their farmlands on a voluntary, rotational basis.  For more on this, check out my recent blog on Growing Water.

As always, I’d love to hear your thoughts and ideas for resolving our water crises! (please comment below)