CMIP6-LOCA2 Thornthwaite-Mather Budgets

This display shows a simple Thornthwaite-Mather Water Budgets series of climate simulations from the CMIP6 Climate Model Ensembles and are a collection of the best simulations from each center's model participating in CMIP6.

The output from these models were 'downscaled' from the global to regional scale using the Localized Constructed Analog Method (LOCA) by Pierce et al. (2023).

The results for the downscaling have been averaged over State Climate Divisions chosen by the user.

The user may select a variable from the pulldown menus, and also select a future 30-year period by which to view the monthly trends and compare them to a fixed historical period (1981-2010).

For both the annual and monthly plots, the solid lines represent the ensemble means, while the shading represents the middle 50% range of the collected ensembles.

The Thornthwaite-Mather Budget (Thornthwaite and Mather, 1955) is a simple water-accounting scheme that requires only minimal input data (monthly mean temperature and monthly total precipitation). The budget can be presented as a ledger-style table or, more often, as the line/bar/area graphic shown in the topmost figure above.

Its components are as follows (all units here are in depth of water, in our case mm of water).

Precipitation

As with all budgets, we have an income stream. Here, it is the total monthly precipitation. This data typically comes directly from observed values. In the budget graphic above, it is represented by the solid green line (a reference dashed line is available for the future climate scenarios to show the historical baseline).

Potential Evapotranspiration

This represents the atmospheric demand for your available water. It may exceed the amount actually available in your water budget for that month. Potential Evapotranspiration can be calculated in several ways, but here it is a function of monthly and annual temperatures and latitude. Like precipitation, this is represented by a solid line in our Thornthwaite-Mather budget plots, this time in blue. And again, a historical reference line for the future climate plots is included as the dashed line.

Evapotranspiration

If there is sufficient water from precipitation, the actual amount of evapotranspiration equals Potential Evaporation. But when Precipitation (supply) is outpaced by Potential Evapotranspiration (atmospheric demand), the difference is drawn from the landscape's water storage. The wetter the soil (the more storage), the easier it is to extract water to offset our budgetary imbalance. As the soil dries, resistance to extracting water from the soil increases. In our budget figure, evapotranspiration is shown as the yellow-shaded area. The actual storage amount in the model can be seen in the figure comparing the budgets across the different scenarios, but it is typically not explicitly shown in the classic Thornthwaite-Mather budget plots.

Deficit

Even with evaporation tapping the soil as a supplemental source to close the gap between atmospheric evaporative demand and precipitation, that still won’t be enough to fully close it. This failure to optimally close the water budget is “The Deficit.” We can express it as plant stress (often seen in the summer months), austerity measures in water resources planning, and other adverse impacts. This is represented by the orange-shaded areas in the budget plot.

Recharge

Once atmospheric demand (Potential Evapotranspiration) exceeds inbound water (Precipitation), we can “pay back” the water extracted from the soil reserves. This excess is “Recharge” and is shown as the green-shaded area in the budget chart.

Surplus

Once the recharge is paid off and the soil water storage reaches its maximum capacity, any precipitation within a month that exceeds the Potential Evapotranspiration is presented as excess “Surplus” (the blue-shaded area). In the classical Thornthwaite-Mather budget scheme, this water is flushed from the system, much like streamflow or runoff is removed from a watershed or landscape area unit. Hypothetically, a user could send this water into a supplementary storage (e.g., groundwater or a protected reservoir) or route a fraction of it through a simple routing model.

Snowpack

Yet another reservoir that can persist for months, Snowpack (shown in our budget plot as the light blue shading), can be added to the budget scheme. Here, Precipitation when temperatures are below a given threshold (e.g., near freezing) is not subject to evaporation or loss through Surplus until temperatures rise above that threshold, at which point the “meltwater” can be partitioned latently into Recharge or Surplus.

Pierce, D.W., D.R. Cayan, D.R. Feldman, and M.D. Risser, 2023: Future increases in North American extreme precipitation in CMIP6 downscaled with LOCA, Journal of Hydrometeorology, 24(5), 951-975, doi:10.1175/JHM-D-22-0194.1.

Thornthwaite, C.W., and J.R. Mather, 1955: The water balance. Publications in Climatology, 8(1), Laboratory of Climatology. Drexel Institute of Technology, Centerton, NJ.