California’s water balance, what is left after precipitation runs off, drains and evaporates, is complicated by its diverse geography, ecosystems and microclimates, its wet, cool winters and hot dry summers, and its swings between booms and busts in annual rainfall.
Consequently, water in California, is a highly variable and contentious resource that suffers from intense competition among a diverse set of legitimate stakeholders (farms, cities, terrestrial ecosystems and fish), especially during the dry years when it is scarcest.
To use and share this water effectively and efficiently across the Golden State these stakeholders will need to know how much water is lost to evaporation on an annual basis.
One cannot measure evaporation as easily as measuring temperature and rainfall at a weather station or runoff at a gauged stream or river. It takes a complex set of sensors that measures simultaneous fluctuations in wind and humidity ten to twenty times per second.
While this approach, called eddy covariance, produces a direct measurement of evaporation, it only pertains to water loss from fields several hundred meters in scale. At landscape to State scales one must rely on a model to infer how much water is evaporated by natural and managed ecosystems. This approach requires computations of the supply and demand for water.
To evaluate the supply of water we need to know how much leaf area covers the ground, how active the plants are and how moist the soil may be. To evaluate the demand for water we need to know how dry the air is and how much sunlight is absorbed by a collection of plants.
Until, recently, there have been few efforts to evaluate evaporation across the State at high spatial resolution. To remedy this lacking we recently published a high resolution (1 km), California-wide estimate of evaporation in Water Resources Research ; . Our analysis was conducted over the span of 17 years, from 2001 through 2017.
Why is the problem important?
The health of California’s economy, the fifth largest in the world, depends on the transfer of large volumes of water from the wet portions of the State, in the north, to the dry portions of the State, in the south. This is accomplished by pumping four to six million acre-feet of water uphill through a network of State and Federal canals to supply the vast agricultural fields of the San Joaquin Valley and over the Tehachapi Mountains into the Los Angeles basin. Clearly, this movement of vast volumes of water is costly from an energetic and financial standpoint.
If we are to better manage and share water among a diverse set of legitimate stake-holders and sustain the production of fruits, nuts and vegetables by irrigated agriculture, we will need to know how much water is being evaporated by natural and managed ecosystems in the State.
What are we doing differently and new
I was reared on an almond and walnut ranch, near the Delta, where I spent a large amount of time during my formative years irrigating the trees. This work imprinted on me a strong interest in assessing and understanding the amount of water that is evaporated by crops and native ecosystems in California. If farmers and ranchers apply too much water it is not cost effective.
If they apply too little water, their crops may suffer physiological stress and suffer yield losses. So there is value in knowing how much water is needed and being used, to apply the right amount.
As a student, I studied agricultural meteorology at U.C. Davis and University of Nebraska, with an emphasis on measuring and modeling evaporation from crop fields. It has taken the next 40 years of study, measurement and model development to arrive at a stage where we are able to evaluate the amount of water evaporated across California's native and managed ecosystems at high spatial and temporal resolution.
Working with former student, Youngryel Ryu, and other colleagues, we have estimated state evaporation with a numerical model that considers the biophysical processes that control evaporation across a landscape. The model calculations are based on information that is derived satellite remote sensing. Estimates of evaporation that are output by the model have been validated with direct measurements of evaporation we are making across a network of sites we run in California as part of the AmeriFlux program.
What we found:
The average amount of water evaporating across California, is about 393 mm/year (15.4 inches) on a background of annual precipitation equal to 519 mm/year (20.4 inches), leaving a residual of 126 mm/year (4.6 inches) for flows into streams, rivers, reservoir and groundwater. Of course the map shows much geographical variation in annual evaporation across the fields of the Central Valley, the foothills that ring the valley, the forests on the mountains along the coast and on the Sierra Nevada Mountains and the deserts and shrub lands in the south.
To the gardener, this map is analogous to the Sunset Climate Zone map, which based on temperature and informs you about the water demands of native vegetation in your back yard.
We find that crops across the Central Valley use less water than conventional wisdom, based on evaporation from well-watered grass, because many crops evaporate at peak values for a relatively short period during the growing season. Otherwise, a subset of fields are fallow, are at immature or senescent stages of growth, or are suffering from physiological stress during the extremely hot summer days. Forests along the coast and in the mountains, on the other hand, use more water than expected from conventional wisdom because they have a long growing season and absorb more solar energy than crops.
On a yearly basis, we can query how much evaporation varied during certain disturbances or management decisions. By mapping evaporation anomalies for a given year, like 2014, we can quantify the effects of great fires, like the 2013 Rim Fire, near Yosemite. We can also see the effects of management decisions, like fallowing large swaths of agricultural land in the San Joaquin Valley, following the 2013 drought. In this case these areas evaporated about 100 mm (4 inches) less water than on average.
Despite the booms and busts in rain fall over the 2001 through 2017 period, we find that state-wide evaporation is conservative, on a year to year basis, compared to the annual variability in rainfall. Nor do we detect that state-wide evaporation is increasing as the climate has warmed over this period. Feed backs between evaporation, radiation, temperature and humidity dampen the presupposed response between evaporation and temperature. In the short term this is good news. State-wide evaporation by native and managed ecosystems may not increase with a warmer climate as fast as has been inferred with simpler models.
We hope we can provided water managers with new and granular information on evaporative water use to better share water among the various stakeholders, e.g. agricultural, cities, fish, ground water reservoirs and water quality. We hope our mechanistic model will also provide good enough information to manage water use better into the warmer future. Based on these results we may be able to apply less water to agricultural regions, and reserve this savings for groundwater restoration, fish, ecosystems and cities.
But we draw this conclusion cautiously, as farmers need to apply excess water to their fields, periodically, to push accumulated salts out of the root zone.
But, given the steady amount of evaporation from year to year, it may be best to flood the fields during the wettest years, like 2019, and conserve during the dry years. And, we hope this information may be of use to gardeners.