California WaterBlog

By Dennis Baldocchi and Carlos Wang

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Rainfall and snow falling across the state have several fates. One is runoff to rivers, reservoirs and the ocean. Another is storage in the snowpack, soil and groundwater. The third is evaporation from vegetation, soil and open water bodies. Historically, the rates and amounts of evaporation from vegetation and soil to the atmosphere have been difficult to assess. For instance, hydrologists have traditionally inferred evaporation at watershed scales as a residual of the water balance, e.g., precipitation minus runoff and storage [Hanak and Lund, 2012]. In contrast, agronomists often use weighing lysimeters (these are freestanding blocks of soil and plants on a scale that changes weight as water evaporates) to study crop water use, like the one at Campbell Tract on the UC Davis campus. While this approach is direct, it constitutes a relatively small sample. Better estimates of evaporation will allow water managers to understand where and how water is used throughout the state, improving water management.

The diverse microclimates and ecosystems of California present a special challenge in assessing and predicting evaporation. Californian vegetation experiences cool, wet winters and hot, rainless summers on an annual basis and a high probability of suffering from drought on a year-to-year basis.

Direct measurements of evaporation from natural and managed landscapes are needed on hourly, daily, seasonal and annual time scales to better manage water in California. For example, high-frequency evaporation measurements produce more accurate sums of evaporation.  The temporal variation in these evaporative fluxes tells us how environmental stresses (high temperatures, low humidity and low soil moisture) or management decisions (planting dates, cropping density, crop choice) may be altering evaporation relative to potential, or reference, conditions. The cited stress and management factors might reduce evaporation rates, but by how much? Knowing how evaporation changes helps water managers have a better idea of how much water may be needed by water users.

Rather than relying on weighing lysimeters, evaporation can be measured directly using the eddy covariance method. This approach measures the velocity of up and downdrafts of wind (eddies) over vegetation and relates the velocity to fluctuations in water vapor in those eddies (Figure 1). These measurements and calculations let researchers generate estimates of evaporation over larger areas under real-world conditions. 

Measurements like this have been ongoing by the biometeorology groups at the UC Davis (K.T. Paw U, Rick Synder and Kosana Suvocarev) and ours on UC Berkeley campus for over 20 years. Here, we report on some of the findings we have produced with a network of long-term measurement sites across California. We have been measuring evaporation over oak savanna and annual grasslands near the town of Ione since 2000. And, we have measured evaporation over a variety of agricultural crops (e.g., rice, pasture, alfalfa, corn, sorghum, wheat) and restored wetlands (e.g., tidal and non-tidal) in the Sacramento-San Joaquin Delta since the early 2010s. Our goal is to produce a biophysical understanding of the processes controlling evaporation of different vegetation types growing in California, so these evaporation rates can be upscaled to improve estimates of evaporation everywhere and all the time. Information like this is needed to provide ground truth for inferred methods that are being used to manage water across the state by farmers, ranchers and hydrologists.

One major lesson is that evaporation rates and annual amounts from vegetated landscapes in California vary a lot, even though the landscapes may experience similar weather conditions. In our research, we see a ranking in the hourly, daily and annual evaporation across different managed and natural landscapes (Table 1). The annual sums of water evaporated by these landscapes depend upon the rates and duration of hourly and daily evaporation (Table 1). Perennial wetlands evaporate1000 to 1200 mm of water per year. Evaporation rates and sums from wetlands are complicated by the amounts of residual dead litter, which affect water temperature, the fraction of open water and vegetation in the footprint of the flux measurement system, and the length of growing season. The tidal wetland at Dutch Slough near Oakley experienced the greatest amounts of evaporation (about 1200 mm per year). This is likely because tidal action brings in warmer water that breaks down residual dead biomass that would otherwise inhibit evaporation. The Twitchell Island 1997 restored wetland, on the other hand, experiences an annual rate of evaporation 50 to 100 mm per year lower than younger wetlands. It is the oldest wetland and contains vast amounts of residual litter and the coldest water. The litter and cold water reduce the length of the growing season and the daily maximum evaporation rate compared to the other wetlands. They also affect how much water evaporates at night.

In comparison, irrigated alfalfa and rice evaporate about 1000 mm per year. Alfalfa, a perennial, achieves a dense canopy of leaves with a high leaf area index and has a long growing season. Plus, it is a nitrogen fixer, which enables it to keep its stomata open wide; stomata are pores on plant leaves through which water transpires to the atmosphere. Plants can open and close the stomata to regulate moisture loss, among other purposes. Alfalfa evaporates more water because its stomata remain open for long periods. Rice fields evaporate freely, too, because they are flooded both during the growing season and during the dormant season, often for waterfowl habitat.

Corn, an annual crop, uses less water than rice and alfalfa, evaporating about 700 to 800 mm per year. Corn depends on a unique pathway for photosynthesis, called the C₄ pathway, which saves water because the stomata are not as far open as vegetation like Alfalfa that use the alternative C₃ photosynthetic pathway. 

Native oak savannas use about 400 mm per year, and near-by grasslands use about 300 mm per year. These landscapes have evolved to use less water for growth than the average rainfall they receive. Native oak savannas achieve lower amounts of evaporation by forming an open leaf canopy, which has fewer leaves than crops, and by having their stomatal pores close gradually during the hot, dry summer. The grasses use even less water by evaporating during the cooler spring and by dying back during the hot dry summer.

Table 1. Ranking of Annual Evaporation Sums measured across Northern California with the eddy covariance system.

Location/Vegetation	Annual Evaporation (mm y-1)	Standard Deviation(mm y-1)	Ameriflux Web Site
Oakley Dutch Slough Tidal Wetland	1235	58	https://ameriflux.lbl.gov/sites/siteinfo/US-Dmg
Sherman Island Restored wetland	1090	248	https://ameriflux.lbl.gov/sites/siteinfo/US-Myb
Bouldin Island alfalfa	1077	95	https://ameriflux.lbl.gov/sites/siteinfo/US-Bi1
Twitchell Island rice	1009	83	https://ameriflux.lbl.gov/sites/siteinfo/US-Twt
Twitchell Island Restored Wetland	995	113	https://ameriflux.lbl.gov/sites/siteinfo/US-Tw4
Twitchell Island 1997 Restored Wetland	949	58	https://ameriflux.lbl.gov/sites/siteinfo/US-Tw1
Bouldin Island corn	706	24	https://ameriflux.lbl.gov/sites/siteinfo/US-Bi2
Sherman Island Pasture	698	117	https://ameriflux.lbl.gov/sites/siteinfo/US-Snd
Ione Oak Savanna	407	64	https://ameriflux.lbl.gov/sites/siteinfo/US-Ton
Ione Annual grassland	329	45	https://ameriflux.lbl.gov/sites/siteinfo/US-Var

Are Trends in Evaporation Emerging?

One question many of us face with global warming is whether evaporation is increasing with warming and rising carbon dioxide. If so, more water may be required for farming. Based on over 20 years of direct evaporation measurements over an oak savanna, we observe that interannual evaporation amounts vary a lot on a year-to-year basis, but there is no long-term trend in evaporation (Figure 2). From a biometeorological perspective, warming increases the supply of water but reduces the demand. In principle, the supply is increased because the saturation vapor pressure of the wet leaves increases with temperature in an exponential manner. But the demand decreases because a warmer surface radiates and convects more heat energy, yielding less energy to drive evaporation. These conflicting feedbacks offset each other. This may be good news about future water use in the short term, as evaporative water use by Californian vegetation may increase as much as feared.

One of our challenges is to distill these measurements into information that ranchers and water managers can use. We have attempted to do this by developing the Breathing Earth System Simulator (BESS) model. It computes evaporation on 1 km resolution using satellite remote sensing data and mechanistic equations that couple carbon, water and energy fluxes. The following map gives an idea of how evaporation may vary across the state (Figure 3). Several factors conspire to affect evaporation rates across the state. First, are areas irrigated or flooded, or are they experiencing soil moisture deficits due to lack of rain and supplemental water? Second, how many layers of leaves are sustained by the landscape, as leaves provide more surface area for evaporation? Third, is the vegetation perennial, with a long growing (and evaporating) season, or an annual crop with a short growing season and long fallow period?

While this model is based on first principles, we suspect it may underestimate rates of evaporation that may occur at smaller farm scales. We have learned that the mosaic of land use in the irrigated regions of California is very complex. Fallow fields next to irrigated fields may differ in surface temperature by up to 20 deg C. This condition moves sensible heat to the downwind field and enhances its evaporation rates, as we have seen over rice and alfalfa crops and newly developed wetlands in the Delta (Figure 4). 

One final lesson we have learned is that we can use meteorological and remote sensing data to estimate evaporation fluxes across time and space [Anderson et al., 2018; Wong et al., 2021].  In addition, weather stations that measure temperature and humidity means and variances and solar radiation allow us to calculate evaporation rates [Wang et al., 2023]. We hope this approach may be applied in the future to the California Irrigation Management Information System (CIMIS) and OPEN ET to improve products used by ranchers and managers, like state-wide estimates of evaporation from crops and wildlands. 

The bottom line of our work is to provide better information to California water managers and decision makers, so they can better share water across the state and use it more efficiently during the swings of wet and dry years. 

About the Authors  

Dennis Baldocchi is a Distinguished Professor of Biometeorology, Emeritus. Carlos Wang is a Postdoctoral Scientist at the University of California, Berkeley.

Further Reading  

Anderson, M., et al. (2018), Field-Scale Assessment of Land and Water Use Change over the California Delta Using Remote Sensing, Remote Sensing, 10(6), 889

Baldocchi, D., D. Dralle, C. Jiang, and Y. Ryu (2019), How Much Water Is Evaporated Across California? A Multiyear Assessment Using a Biophysical Model Forced With Satellite Remote Sensing Data, Water Resour. Res., 55(4), 2722–2741, doi:10.1029/2018wr023884.

Baldocchi, D., S. Ma, and J. Verfaillie (2021), On the inter- and intra-annual variability of ecosystem evapotranspiration and water use efficiency of an oak savanna and annual grassland subjected to booms and busts in rainfall, Global Change Biology, 27(2), 359–375, doi:https://doi.org/10.1111/gcb.15414.

Eichelmann, E., K. S. Hemes, S. H. Knox, P. Y. Oikawa, S. D. Chamberlain, C. Sturtevan, J. Verfaillie, and D. D. Baldocchi (2018), The effect of land cover type and structure on evapotranspiration from agricultural and wetland sites in the Sacramento-San Joaquin River Delta, California, Agricultural and Forest Meteorology, 256, 179–195, doi:10.1016/j.agrformet.2018.03.007.

Hanak, E., and J. R. Lund (2012), Adapting California’s water management to climate change, Clim. Change, 111(1), 17–44, doi:10.1007/s10584-011-0241-3

Wang, T., J. Alfieri, K. Mallick, A. Arias-Ortiz, M. Anderson, J. B. Fisher, M. Girotto, D. Szutu, J. Verfaillie, and D. Baldocchi (2024), How advection affects the surface energy balance and its closure at an irrigated alfalfa field, Agricultural and Forest Meteorology, 357, 110196, doi:https://doi.org/10.1016/j.agrformet.2024.110196.

Wang, T., J. Verfaillie, D. Szutu, and D. Baldocchi (2023), Handily measuring sensible and latent heat exchanges at a bargain: A test of the variance-Bowen ratio approach, Agricultural and Forest Meteorology, 333, 109399, doi:https://doi.org/10.1016/j.agrformet.2023.109399.

Wong, A. J., et al. (2021), Multiscale Assessment of Agricultural Consumptive Water Use in California’s Central Valley, Water Resour. Res., 57(9), e2020WR028876, doi:https://doi.org/10.1029/2020WR028876.