Lessons Learned Measuring and Modeling Evaporation across California
Dennis Baldocchi and Carlos Wang
CA Water Blog
October 27, 2025
Evaporation from the vegetated land is an important component of the water balance. 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 run off. In contrast, agronomists often implement weighing lysimeters 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 size and number.
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. The diverse microclimates and ecosystems of California produce a special challenge on 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. These factors can conspire to down regulate evaporation rates, but by how much?
Over the years, groups of micrometeorologists have developed the eddy covariance method to measure rates of evaporation directly at the field scale and on the noted range of time scales. This method uses a sonic anemometer and an infrared gas analyzer, in combination. The anemometer measures the velocity of each up and downdraft passing a horizontal plane over the vegetation. The gas analyzer measures the change in humidity of those air parcels. When the covariances of vertical wind velocity and humidity fluctuations are averaged over 30 minutes, a flux density of water vapor exchange is produced; in other words, the rate of evaporation.
Evaporation measurements like this have been ongoing by groups on the UC Davis and Berkeley campuses for over 20 years. Here, we report on some of the findings we have produced with a meso-network of long-term measurements sites across California. We have been measuring evaporation over oak savanna and annual grasslands near Ione since 2000. And, we have measured evaporation over a variety of agricultural crops (rice, pasture, alfalfa, corn, sorghum, wheat) in the Sacramento-San Joaquin delta since the early 2010s. Our goal is to produce a biophysical understanding of the processes controlling evaporation in California, so these evaporation rates can be upscaled everywhere and all the time.
Evaporation is a balance between supply and demand for water. If we normalize evaporation rates by the demand, available energy, we can inspect how a set of biophysical factors affect supply. This is quantified by a canopy resistance that is a function of leaf area index, photosynthetic pathway, photosynthetic capacity and soil moisture (Figure 1). The lowest resistances to evaporation are associated with conditions of complete crop cover and high evaporative capacity due to irrigation and fertilization. When the surface resistances are less than about 100 s m-1, evaporation rates reach rates of potential evaporation, which is proportional to 1.26 times available energy. Irrigated crops, like rice and alfalfa, evaporate at rates close to potential evaporation as they are bred to achieve high leaf area indices, are fertilized or are N fixers. The widespread planting of alfalfa across the state is a consequence of fact that highest plant productivity scales with the highest water use.
Higher resistances are associated with crops using the C4 pathway or native vegetation with less than complete cover and suffering from soil moisture deficits. Corn uses less water than other popular C3 crops because it assimilates carbon through the C4 pathway, which does so with higher stomatal resistances. Savannas use the least amount of water by establishing an open canopy and upregulating its stomatal resistance as the soils dry in the absence of summer rain. Plus, the underlying grass dies during the summer period.
The Department of Water Resources is investing in wetland restoration projects in the highly subsided peatlands of the Delta. The rationale revolves around the fact that if the Delta levees fail, the water transfer system for 20 M Californians and its $4T dollar economy will be at jeopardy. We have been measuring evaporation over degraded peatland landscapes and restored wetlands in the Delta since 2007. Our motivation is to provide information on water use by these lands to help protect the water transfer system of the State.
From looking at Figure 1, one may expect wetlands to evaporate at high rates like perennial alfalfa. Hence, there can be a high evaporative cost to restoring wetlands in a State where water is precious and contentious. The answer to this rhetorical question is it depends. The amount of water evaporated over the most productive wetlands is over 1200 mm per year. This value is close to the potential evaporation of central California, given its radiation load and temperature. The surprise was that interannual evaporation among different wetlands varied (Figure 2), and this variation is affected by the amounts of residual dead litter, which affects water temperature, and the fraction of open water and vegetation in the footprint of the flux measurement system. The west pond site, for example, is the oldest wetland, it contains vast amounts of residual litter, which reduced evaporation by 200 mm per year compared to younger wetlands.
One question many of us face with global warming is whether evaporation is increasing with warming and rising carbon dioxide. Based on over 20 years of direct evaporation measurements over an oak savanna, we observe that interannual evaporation amounts are essentially unchanged (Figure 3). From a biometeorological perspective, warming increases the supply of water, but reduces the demand. These conflicting feedbacks offset each other. This may be good news about future water use in the short term.
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 4).
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 advects sensible heat to the downwind field and enhances its evaporation rates beyond the rates of potential evaporation, which we have seen over rice and alfalfa crops and newly developed wetlands in the Delta (Figure 5).
The bottom line of our work is to provide better information to California water managers and decision makers, so they can share water better across the state and use it more effectively during the swings of wet and dry years. We hope this can be achieved by using these data to improve products used by ranchers and managers like the California Irrigation Management System (CIMIS) and OPEN ET.
Dennis Baldocchi is a Distinguished Professor of Biometeorology, Emeritus and Carlos Wang is a Postdoctoral Scientist at the University of California, Berkeley.
References
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.
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.
