Water Budgets – a Tool to Quantify the Hydrologic Cycle

“Water availability is an important concern in the 21st century. Ensuring sustainable water supplies requires an understanding of the hydrologic cycle—how water moves through Earth’s atmosphere, land surface, and subsurface. Water budgets are tools that water users and managers use to quantify the hydrologic cycle”.   

A water budget may be used for planning purposes, where the availability of accurate, reliable data is critical when assessing available water and allocating water use in a watershed.  A water budget, like a financial budget, has numerical inputs and outputs. The water budget diagram below shows some of the elements and relationships between elements of a water budget. 

Figure 1: Water Budget Diagram

Data Considerations

In addition to listing a number of related variables that comprise a water budget, the water budget diagram correctly suggests that water budgeting is a data-intensive activity.  The quality and reliability of a water budget is proportional to the quality of the data inputs. 

For planning purposes, “typical” or “average” input and output data over a certain period of time may be selected to represent the “typical” or “average” condition of water flux in a watershed or to forecast extreme conditions, such as forecasting flood or drought conditions (e.g., 100-year return period storm or low streamflow).

The article by Robert Schmaltz “The Spring Creek Water Resource” in the Spring Creek Watershed Atlas cites some of the water resources data and considerations for the Spring Creek Watershed.  Water utilization in the Spring Creek basin is somewhat unusual in that significant water use is largely dependent upon groundwater sources.  In rounded numbers, precipitation accounts for an average input of approximately 333 million gallons per day (MGD), of which approximately half (185 MGD) is returned to the atmosphere as evapotranspiration.  Groundwater withdrawals account for approximately 12 MGD.  

A detailed analysis of the water budget of Spring Creek and nearby, related water resources was performed by the US Geological Survey in 2015.  This detailed analysis reveals some of the factors that would complicate water budget analysis for the Spring Creek basin, including karst aquifer conditions and different surface water and groundwater boundaries.  Some of the precipitation that falls within the topographic (surface) Spring Creek watershed boundary flows as groundwater to the Nittany Creek and Spruce Creek basins.  The USGS analysis utilized data from seven years to build a computer model of the study area, and extreme drought or flood conditions were not specifically addressed.

One of the objectives of water budget analysis is to “quantify the hydrologic cycle”, which may support an assessment of potentially non-sustainable water use, such as extreme (i.e., drought or flood) meteorological conditions during which excessive groundwater and surface water withdrawals can adversely affect the groundwater reservoir or surface water flows and aquatic resources. 

The chart below shows 2016 average daily flow values for the following locations :

• Spring Creek at Milesburg (USGS)

• Buffalo Run Mouth (WRMP)

• Logan Branch Mouth (WRMP)

• Cedar Run Mouth (WRMP)

• Slab Cabin Run Mouth (WRMP)

(USGS = United States Geological Survey, WRMP = Water Resources Monitoring Project)

Figure 2: 2016 Average Daily Flow in the Spring Creek Watershed

Water Resource Management

In 2002, the Pennsylvania legislature passed Act 220 of 2002 as a mandate to update the 1929 version of the State Water Plan.  Act 220 called for the creation of six regional committees representing water resource planning areas, staffed by volunteer experts in water resources and other relevant planning, agricultural, industrial, environmental, and governmental fields and a state-wide Water Resources Committee, which would coordinate with the PA Department of Environmental Protection (PADEP) to update the 1929 Plan.   

Initial elements of Act 220 also included:

• Establish a statewide system to collect and make available data on PA water resources.

• Register certain water withdrawals

• Identify “Critical Water Planning Areas” (CWPAs) – “any significant hydrologic unit where existing or future demands exceed or threaten to exceed the safe yield of available water resources [emphasis added]”.

• Definitions that are relevant to water resource planning and water budgeting, such as “safe yield” (“the amount of water that can be withdrawn from a water resource over a period of time without impairing….. the water resource”, etc.)  

The identification of CWPAs, an early, critical component of Act 220, relies heavily on water budgeting considerations.  An ongoing PADEP initiative is to make water resource data more readily available to water resource management personnel.

Figure 3: PADEP's Regional Water Resource Planning Areas

Permitting Considerations

“The Commonwealth of Pennsylvania has joined with the States of New York and Maryland and the Federal Government (US Army Corps of Engineers) to form the Susquehanna River Basin Compact https://www.srbc.gov/about/about-us/docs/srbc-compact.pdf and the Susquehanna River Basin Commission (SRBC) for the conservation, utilization, control, development, or management of [ground and surface] water resources”. The SRBC’s mission includes:  “reduce damages caused by floods; provide for the reasonable and sustained development and use of surface and ground water for municipal, agricultural, recreational, commercial and industrial purposes [emphasis added}; protect and restore fisheries, wetlands and aquatic habitat; protect water quality and instream uses; and ensure future availability of flows to the Chesapeake Bay.” (Ibid)  SRBC regulations balance sustainable economic development with conserving the Basin’s aquatic ecosystems.

The SRBC reviews and has approval authority for projects that involve:

• Water withdrawals of 100,000 gallons per day (gpd) or more over a 30-day average)

• Consumptive water use (water that is withdrawn and not returned to the Basin) of 20,000 gpd or more over a 30-day average from any water source

• Water diversions from the basin, including diversions out of the Basin (consumptive use) and into the Basin (water quality impacts).

• All water withdrawn for natural gas activities must obtain Commission approval.  (https://www.srbc.gov/regulatory/regulations/)

Pennsylvania regulations address water resources planning through PA Code § 110 and the State Water Plan, which now includes an online Digital Water Atlas (https://storymaps.arcgis.com/stories/d945de2b227b44f5adad48faa36af929).  

Political Considerations

Water resource management relies on science (e.g., water budgeting) but is also subject to political factors. As an unfortunate example of the influence of political influence upon water resource management, the amount of available water in the Colorado River was over-estimated in the 1922 Colorado River Compact by approximately one-third due to use of inadequate data and political considerations by negotiators for Compact States, who ignored an early USGS estimate of 15 million acre-feet per year of annual, available water and arbitrarily selected the annual yield of the Colorado River as 20 million acre-feet.  Current, long-record, science-based data indicate that the annual yield is closer to 12 million acre-feet/year. One acre-foot is a volume consisting of one foot of water over a land area of one acre, or roughly 325,000 gallons. (see “Science Be Dammed: How Ignoring Inconvenient Science Drained the Colorado River”, Kuhn & Fleck, discussed by the authors at https://www.vox.com/2022/9/23/23357093/colorado-river-drought-cuts).

Although some of the Regional Committees identified climate change as an emerging issue in water resources management, this complex topic has not yet been formally addressed in the State Water Plan.  Other priorities of the six Regional Committees are published in the 2009 Principles “Document “, which mentions Climate Change as a factor which may require updated statistical analyses of historic data.

References

1. Barlow, P. M., Cunningham, W. L., Zhai, T., & Gray, M. (2007). Water budgets: Foundations for effective water-resources and environmental management (U.S. Geological Survey Circular 1308). U.S. Geological Survey. https://pubs.usgs.gov/circ/2007/1308/

2. Ferguson, I. M., Huntington, J. L., Troch, P. A., & Zeng, X. (2023). Uncertainties in measuring and estimating water-budget components: Current state of the science. Wiley Interdisciplinary Reviews: Water, 10(3), e1667. https://onlinelibrary.wiley.com/doi/10.1002/wat2.1667

3. Goode, D. J., Senior, L. A., & Subah, A. (2015). Water-budget and recharge-area simulations for Spring Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania, water years 2000–06 (Scientific Investigations Report 2015-5073). U.S. Geological Survey. https://pubs.usgs.gov/sir/2015/5073/

4. Keystone Water Resources Center. (n.d.). Spring Creek Water Resource Monitoring Project (WRMP). Retrieved September 20, 2025, from https://www.keystonewaterresources.org/read-me

5. Pennsylvania General Assembly. (2002). Environmental Resources (27 Pa.C.S.)—Amend Agricultural Advisory Board establishment, powers and review and water resources planning, administration and enforcement, Act of Dec. 16, 2002, P.L. 1776, No. 220 (HB 2302). https://www.palegis.us/statutes/unconsolidated/law-information?sessYr=2002&sessInd=0&actNum=220

6. Commonwealth of Pennsylvania, Department of Environmental Protection. (2009, March). State Water Plan principles: Executive summary (Publication No. 3010-BK-DEP4227). https://files.dep.state.pa.us/water/Division%20of%20Planning%20and%20Conservation/StateWaterPlan/StateWaterPlanPrinciples/3010-BK-DEP4227.pdf

About the author: Mark Ralston is a retired hydrogeologist, PG 000293-G, who has worked for over forty years in water resources management in the US Northeast, Southwest, and elsewhere.  He was President of the Spring Creek Chapter of Trout Unlimited, chair of the Water Resources Monitoring Project (Governor’s Award for Watershed Stewardship from Governor Ridge in 2001) of the Spring Creek Watershed Community, gave presentations at water resource symposia, and other volunteer natural resource management activities.