11:00 AM - 11:15 AM
[U10-02] Foresight of the bucket model and the development of land surface models in 50 years
★Invited Papers
Keywords:hydrology, land surface model, bucket model
In his 1967 paper, which was cited as the reason for the Nobel Prize, Dr. Syukuro Manabe showed the results when the amount of water vapor in the atmosphere is kept constant and when the water vapor pressure in the atmosphere changes with the increase in temperature while the relative humidity is kept constant, in addition to the sensitivity experiment by a vertical one-dimensional radiative-convective equilibrium model to calculate the change in surface temperature under double or half concentration of carbon dioxide in the atmosphere. As such, water is an essential substance, like carbon dioxide and ozone, that determines the basic structure of the Earth's atmosphere, and the general circulation model used in his 1969 paper incorporated the major processes of the water cycle in the three-dimensional atmosphere, ocean, and land surface.
In addition to the radiative absorption, another key function of the water cycle on the atmosphere is how the radiant energy illuminating the land surface is partitioned into sensible heat flux and latent heat flux (evaporation).
The evaporation is determined by the surface roughness, atmospheric stability, wind speed, and surface temperature, etc., and the actual evaporation rate becomes smaller for dry condition than the potential evaporation rate for the sufficiently wet condition. Unfortunately, the field observation of sensible and latent heat fluxes and water movement in soils was insufficient at that time, and theoretical studies were still in their infancy.
Therefore, Dr. Manabe assumed that the depth of surface soil layer is uniformly set at 1m globally, and the field capacity was set at 0.15. This is the land surface model, commonly known as the bucket model, that can hold 15cm of water. The essence of the land surface process was very cleverly simplified and modeled in the bucket model; evaporation occurs at the potential evaporation rate up to the threshold of soil water content, which is set at 0.75 times the field capacity, and below that threshold, the actual evaporation rate decreases proportional to the ratio of soil water content to the threshold. When it was calculated that more than 15 cm of water was stored on the land surface, the water would overflow from the bucket and flow into the river.
In the latter half of the 1980s, a land surface model considering vegetation was developed, based on the awareness that changes in vegetation cover either due to human deforestation or atmosphere-vegetation interactions may affect atmospheric circulation in the African monsoon and other regions through the changes in surface albedo, roughness, and evapotranspiration. Further, as the issue of climate change became mainstream, it became necessary to incorporate the carbon cycle into general circulation models, and land surface models with the photosynthetic process of plants were developed and used in climate simulations in the 1990s.
Following the atmospheric-oceanic interaction studies in the 1980s, which were inspired by the large El Niño of 1982/83, atmospheric-land surface interaction studies became popular in the 1990s, and the climate simulation community criticized bucket model too simple and outdated. However, Dr. Manabe preferred simplified elementary processes, and it was not until the 21st century that such modern land surface models were incorporated into the general circulation model at the GFDL.
Nowadays, land surface models include more human friendly water cycles, such as water storage in reservoirs and irrigation intake from rivers and groundwater, because more focus are put on the impacts and adaptation to climate change. However, Dr. Manabe's bucket model is still effective in understanding the basic role of the hydrological cycle in the climate system, and his achievements and foresight are a milestone in land surface hydrology research, as well.
In addition to the radiative absorption, another key function of the water cycle on the atmosphere is how the radiant energy illuminating the land surface is partitioned into sensible heat flux and latent heat flux (evaporation).
The evaporation is determined by the surface roughness, atmospheric stability, wind speed, and surface temperature, etc., and the actual evaporation rate becomes smaller for dry condition than the potential evaporation rate for the sufficiently wet condition. Unfortunately, the field observation of sensible and latent heat fluxes and water movement in soils was insufficient at that time, and theoretical studies were still in their infancy.
Therefore, Dr. Manabe assumed that the depth of surface soil layer is uniformly set at 1m globally, and the field capacity was set at 0.15. This is the land surface model, commonly known as the bucket model, that can hold 15cm of water. The essence of the land surface process was very cleverly simplified and modeled in the bucket model; evaporation occurs at the potential evaporation rate up to the threshold of soil water content, which is set at 0.75 times the field capacity, and below that threshold, the actual evaporation rate decreases proportional to the ratio of soil water content to the threshold. When it was calculated that more than 15 cm of water was stored on the land surface, the water would overflow from the bucket and flow into the river.
In the latter half of the 1980s, a land surface model considering vegetation was developed, based on the awareness that changes in vegetation cover either due to human deforestation or atmosphere-vegetation interactions may affect atmospheric circulation in the African monsoon and other regions through the changes in surface albedo, roughness, and evapotranspiration. Further, as the issue of climate change became mainstream, it became necessary to incorporate the carbon cycle into general circulation models, and land surface models with the photosynthetic process of plants were developed and used in climate simulations in the 1990s.
Following the atmospheric-oceanic interaction studies in the 1980s, which were inspired by the large El Niño of 1982/83, atmospheric-land surface interaction studies became popular in the 1990s, and the climate simulation community criticized bucket model too simple and outdated. However, Dr. Manabe preferred simplified elementary processes, and it was not until the 21st century that such modern land surface models were incorporated into the general circulation model at the GFDL.
Nowadays, land surface models include more human friendly water cycles, such as water storage in reservoirs and irrigation intake from rivers and groundwater, because more focus are put on the impacts and adaptation to climate change. However, Dr. Manabe's bucket model is still effective in understanding the basic role of the hydrological cycle in the climate system, and his achievements and foresight are a milestone in land surface hydrology research, as well.