In terms of climate change, sea level rise has received a lot of attention. Terrestrial bodies of water, particularly lakes, are also important resources for both humans and wildlife. Until now, the loss of water due to evaporation as the planet warms has been investigated but is poorly understood due to weak models and data.
Using satellite imagery and modeling, new research highlights evaporation volume across natural and artificial lakes in more than 1.42 million lakes.
Lake evaporation around the world
Recent research shows that since 1985, specifically between 1985-2018, about 1500 ± 150 km3 per year have been evaporating from global lakes.
Lake evaporation volume, in some parts of the world, accounts for a significant portion of all evaporation. Regions such as Iraq (Tigris-Euphrates basin), Egypt (Nile basin), Central Asia, and Northeast Canada are all dry and have experienced relatively high evaporation from lakes (over 10%) relative to total evaporation.
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Total rate of increase for lake evaporation is happening at about 1.5% increase per decade, reflecting higher global air temperatures and vapor pressure deficits. A large portion of loss, such as in cold regions in Canada and the Arctic, are from lake ice melting.
Artificial lakes have a higher evaporation rate than natural lakes
One result worth noting is that artificial lakes appear to demonstrate more rapid evaporation than natural lakes. In the United States, Lake Mead is a good example of this.
Artificial lakes account for about 5% of total lake capacity; however, from the 6715 artificial reservoirs studied, they contribute 16% (235 km3 year−1) to global evaporation volume in lakes.
This is probably because so many reservoirs are built in hot and/or dry regions that evaporation becomes more extreme.
An increasing trend in lake evaporation rates
Three key factors are contributing to this increasing trend of evaporation, depending where one is in the world, with overall evaporation rate (affected by temperature), lake surface area, and lake ice duration contributing to total volume evaporation increases witnessed.
Lake area, where larger lakes are located, leads to greater overall loss, particularly in drier regions, while lake ice duration is affected by shorter winters and warmer weather.[1]
HydroLAKES dataset
The calculation of lake volume was conducted using the HydroLAKES dataset, where 1,427,688 water bodies including and over 0.1 km2 surface area are represented.[2]
Three meterological datasets were used for modeling, which include TerraClimate, ERA5, and the Global Land Data Assimilation System (GLDAS). A standard evaporation model is then used with the appropriate climate data from the sources given.
Based on the Penman combination equation, a new evaporation rate is calculated for evaporation model. One reason why this work is novel is because this algorithm for calculating the evaporation rate has now been validated using lakes in the United States.
The algorithm seems to work well for different types of lakes as well (e.g., deep and shallow lakes). The Landsat-based global surface water dataset (GSWD) was used to derive lake surface area. This was combined and compared to the HydroLAKES dataset. With Landsat data, imagery can be classified down to the pixel level for each given year into one of four categories for each pixel: (1) year-round water, (2) seasonal water, (3) not water, and (4) no data.
Existing map data combined with imagery help to reduce the no data category to make the observations more robust. For lake ice duration, critical for determining evaporation, the approach used a method that looks at the relationship between freeze lag and average lake depth; there is a clear relationship between the thaw lag and the average winter temperature as well.
Freeze lag relates to the time between freezing conditions and when a lake freezes, while thaw lag is when temperatures go above freezing and when lakes begin to thaw. How much heat is stored in a lake from the summer and autumn can significantly impact the timing of when a lake freezes.[3]
Overall, in testing the model, an uncertainty of 7.22% in evaporation rate is determined. This is not considered significantly large and enables the model to demonstrate a clear trend, even if some error is to be expected.
The Earth’s water cycle and lakes
What is significant about the results is that lakes are clearly driving a major part of the hydrologic cycle affecting the planet. In fact, far more than previously thought, where oceans and larger bodies of water were accounted for in many other studies with the neglect of many lakes.
Evaporation volume is also important to calculate, and not just evaporation rate, as it tells us about what is lost each year in evaporation. Overall increase in volume evaporation points to the importance of climate change affecting the hydrologic cycle.
Currently, about 71,000 cubic kilometers is evaporated each year and lakes play a major role in this. They contribute about 2.37 percent of water evaporated on land, which may not sound like much but we should keep in mind that they only represent 1.5 percent of global area.
References
[1] For more on global lake evaporation volume and how it was determined, see: Zhao, G.; Li, Y.; Zhou, L.; Gao, H. Evaporative Water Loss of 1.42 Million Global Lakes. Nat Commun 2022, 13, 3686, doi:10.1038/s41467-022-31125-6.
[2] For more on HydroLakes, see: Messager, M.L.; Lehner, B.; Grill, G.; Nedeva, I.; Schmitt, O. Estimating the Volume and Age of Water Stored in Global Lakes Using a Geo-Statistical Approach. Nat Commun 2016, 7, 13603, doi:10.1038/ncomms13603.
[3] For more on the team’s funded work by NASA, see the following press release: https://earthobservatory.nasa.gov/images/150067/lake-evaporation-on-the-rise.
