Water budgets are used to understand the movement of water into and out of a
watershed. Much like a financial budget, inflows, storage, and outflows can
be thought of like income, account balance, and expenses respectively.
Unlike a bank account, we don't have perfect information about the flows of
water into and out of a watershed.
In this data visualization, you can see how three major components of the
water budget vary across the conterminous US. Precipitation is incoming rain
and snow; evapotranspiration is outgoing evaporation from soil and water
bodies and transpiration through plants; and runoff is excess water that
makes its way to lakes and rivers through streams and spring discharges.
Each of these components is estimated with a variety of measurement and
With perfect information, the water budget bar charts shown here would
“balance”, showing that in the long run, inflows to each watershed are equal
to outflows plus changes in storage. The imbalance in the bar charts
highlights that our best estimates of the three major water budget
components don't balance perfectly. Why might the budget not balance? We
know the imbalance can be the result of (1) changes in storage, (2)
unquantified sources and sinks, and (3) uncertainty in data. Although the
imbalance is the difference between in and out, the true and complete (but
unknown) budget could have additional terms on both sides (explanations 1 or
2) and could have larger or smaller values for any of the existing terms
The water budget shown here is missing one of the most difficult terms to
quantify – change in storage. The amount of water stored in a particular
watershed, whether in lakes, aquifers, or any other storage, fluctuates over
time; at times storage shrinks due to larger outflows and/or smaller
inflows, and at other times storage increases due to smaller outflows and/or
larger inflows. Using the bank analogy, storage is the amount of money in a
bank account. For a given period of time, the inflows and outflows may not
be equivalent and the balance may shift. While this can be less important
for long-term balances as is shown here, it becomes more important at
monthly or daily time scales, making it a very important aspect of the water
budget. The National Water Census (NWC) is exploring remotely sensed
products and additional observational datasets such as groundwater well data
to improve our characterization of storage.
The “imbalance” in the water budgets also stems from unquantified
components. These could include human water use or natural processes such as
groundwater flow out of a watershed. The NWC continues to apply cutting-edge
science to expanding our ability to account for the complete water budget. No
measurement is exact. Each of the water budget components shown here has
some unknown uncertainty included in it. The NWC is analyzing observational
and simulated data sets to to understand these uncertainties.
As we go forward, the NWC aims to improve our understanding of the unknowns
in the water budget for every watershed in the country. In addition to
improving our observational datasets and our understanding of their
uncertainties, the NWC is developing a model to combine these datasets to
estimate a complete and closed water budget. Building upon knowledge gained
through comprehensive analysis of observational datasets, this model can be
used to understand how the different water budget components relate to one
another and may change under various land use and water management scenarios.