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 modeling methods.
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 (explanation 3).
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.
