Hubbry Logo
Water yearWater yearMain
Open search
Water year
Community hub
Water year
logo
7 pages, 0 posts
0 subscribers
Be the first to start a discussion here.
Be the first to start a discussion here.
Water year
Water year
Water year
View on Wikipedia
from Wikipedia

A water year (also called hydrological year, discharge year or flow year) is a term commonly used in hydrology to describe a time period of 12 months for which precipitation totals are measured. Its beginning differs from the calendar year because part of the precipitation that falls in late autumn and winter accumulates as snow and does not drain until the following spring or summer's snowmelt. The goal is to ensure that as much as possible of the surface runoff during the water year is attributable to the precipitation during the same water year.[1]

Due to meteorological and geographical factors, the definition of the water years varies. The United States Geological Survey (USGS) defines it as the period between October 1 of one year and September 30 of the next,[2][3] as late September to early October is the time for many drainage areas in the US to have the lowest stream flow and consistent ground water levels. The water year is designated by the calendar year in which it ends; for example, the 2025 US water year runs from October 1, 2024 through September 30, 2025.[1]

One way to identify a water-year is to find the successive 12-month period that most consistently, year after year, gives the highest correlation between precipitation and streamflow and negligible changes in storage (i.e., soil water and snow).[4] Usually, the time when the variation of storage from year to year is the smallest is the time with the minimum storage level and minimum flow. However, the practical considerations also affect the water year definitions. For example, in Canada the water year starts in October, apparently to coincide with the US one, although better measurement conditions exist in winter.[5]

To accommodate the regional and climatic variations, some researchers use a per-gauge local water year that starts in the month with the lowest average streamflow.[6]

Classification

[edit]

Water year types (or indices[7]) are used to present the historical hydrological data in a simplified form.[8] These indices help to categorize similar water years for the planning of the rule-based[7] water operations. A typical set includes: very dry year, dry year, normal year, wet year, very wet year.[8] The years are characterized through setting numerical thresholds for the water runoff in the water year. The methods of calculation (and the set of types) naturally vary by the region, therefore many indices exists, for example:[7]

  • Palmer Drought Severity Index (PDSI). Proposed by W. C. Palmer in 1965,[9] PDSI is extensively used in the US since then;[10]
  • Standardized Precipitation Index (SPI) was proposed by McKee et al. in 1993;[11]
  • Reclamation Drought Index;
  • deciles.

Many practically used indices were created ad-hoc. For example, California River Indices[12] are weighted averages of the estimates of spring melt, runoff for the rest of the year, and the result for the previous year, calculated for few river basins separately to classify the water year as a wet, above normal, below normal, dry, and critical ("normal" years in California are extremely rare[13]). These California indices were not created "through a systematic statistical analysis of historic basin conditions and river flows".[14]

All indices by nature reflect the historic values and therefore cannot capture the variations in climate that are known to cause the distribution of water year types to be non-stationary in time.[15]

Uses

[edit]

Examples of how water year is used:

  • Used to compare precipitation from one water year to another.
  • Used to define a period of examination for hydrologic modeling purposes.
  • Used in reports by the United States Geological Survey (USGS) as a term that deals with surface-water supply.[2]
  • The end of the water year is used by the CoCoRaHS project as an opportunity for observers to audit and verify data for their site.

See also

[edit]

References

[edit]

Sources

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Water year

View on Grokipedia
from Grokipedia
A water year is a 12-month period used in to measure and report , defined as the time from October 1 of one through September 30 of the next, and designated by the ending (for example, the water year 2023 spans October 1, 2022, to September 30, 2023). This convention aligns with the natural rhythms of the hydrological cycle, particularly in regions like the where much of the annual falls during the cooler months of fall and winter, contributing to , , and that peaks in the spring and summer of the following year. By starting in early fall, the water year groups related events and their downstream effects together, avoiding the fragmentation of seasonal water patterns that occurs when using the standard January-to-December . The water year is a cornerstone of water data management in the United States, standardized by the U.S. Geological Survey (USGS) for analyzing surface-water supply, streamflow, precipitation totals, and drought conditions. It coincides with the federal fiscal year, facilitating integrated reporting on water availability, flood risks, and resource allocation by agencies such as the USGS, USDA, and state water departments. For instance, annual summaries of river basin hydrology, reservoir levels, and climate impacts are typically framed around water years to provide consistent, seasonally coherent assessments. While primarily a North American practice, similar fiscal or hydrological year concepts exist internationally to track water cycles in varying climates.

Definition and Purpose

Definition

A water year is defined as a 12-month hydrological period that typically spans from October 1 to September 30 in the , designated by the calendar year in which it ends. For instance, Water Year 2025 encompasses the period from October 1, 2024, to September 30, 2025. This structure provides a standardized timeframe for tracking hydrological data, distinct from the year that runs from January 1 to December 31. Also referred to as the hydrological year, discharge year, or flow year, the water year facilitates consistent of water-related phenomena across fiscal and reporting cycles. These alternative designations emphasize its focus on , , and other aquatic metrics rather than civil timekeeping. By centering on this period, the water year aligns hydrological reporting with natural water cycles, capturing seasonal patterns of and runoff more cohesively than calendar-based divisions.

Rationale

The water year serves a critical hydrological purpose by aligning the reporting period with the natural cycles of , accumulation, and melt in regions with seasonal climates, particularly in the . Unlike the , which often bisects these cycles, the water year—spanning from October 1 to September 30—captures the majority of annual and within a single cohesive period. This structure prevents the distortion of data for rivers, reservoirs, and watersheds that would occur if wet-season inputs from late fall and winter were split across two years, ensuring more accurate assessments of and availability. In the , precipitation falling in the latter part of a frequently contributes to in the following year's spring due to delayed runoff from accumulation and gradual melting. For instance, autumn and winter rains or snows build reservoirs of moisture that influence peak flows months later, rendering calendar-year summaries misleading for evaluating annual runoff and storage dynamics. By starting after the typically dry summer low-flow period, the water year uses this seasonal minimum as a natural reset point, facilitating clearer tracking of how inputs translate into outputs over the full hydrological cycle. This synchronization enhances the precision of measurements for water availability, as it encompasses the buildup of beginning in —often the onset of significant mountain —and its subsequent melt-driven contributions to during spring and early summer. In areas like the , where supplies a substantial portion of annual , the water year's design avoids fragmenting these dominant events, thereby supporting reliable analysis of seasonal variability without artificial calendar-induced biases.

Historical Development

Origins

The concept of the water year originated in the early within the , stemming from the pressing needs of in arid western regions for and river basin development. The Reclamation Act of 1902 marked a pivotal influence, authorizing the federal government to fund projects and establishing the Hydrographic Branch of the U.S. Geological Survey (USGS) to systematically collect , , and related hydrological data. This institutional framework laid the groundwork for standardized water assessments, addressing the challenges of uneven seasonal water availability in areas dependent on and river flows. By 1913, the USGS formalized the water year as a 12-month period running from to , coinciding with the launch of the national data publishing system to ensure consistent reporting of resources. This facilitated the analysis of annual hydrological patterns, particularly as federal water projects expanded during the and . A key event was the integration of water year data into records for initiatives like the of 1922, which apportioned the river's waters among basin states based on standardized measurements of annual flows and volumes. The selection of October 1 as the starting date emphasized alignment with natural hydrological cycles in the western U.S., where winter snow accumulation in the and subsequent spring melt dominate streamflow contributions; this timing allows the period to encompass the full cycle of input and runoff response, with systems typically reacting to the prior year's inputs after . Although the formalized U.S. water year emerged in this context, the broader idea of a dedicated hydrological reporting year predated it through informal applications in 19th-century European , exemplified by the publication of Hungary's Hydrological Year Book starting in 1876.

Standardization

Following , the (USGS) played a pivotal role in formalizing the water year for national hydrological and reporting. Building on early 20th-century practices in U.S. water projects, the USGS codified the to period as the standard water year in federal guidelines during the 1950s, aligning annual and cycles for consistent analysis across the country. This standardization was reflected in USGS Water-Supply Papers starting with the water year ending , 1950, where hydrological records were systematically organized by this 12-month interval to facilitate downstream station ordering and data comparability. Concurrently, the International Association of Hydrological Sciences (IAHS), established in , contributed to post-1940s efforts in hydrological through its for global protocols, particularly in the lead-up to international programs. IAHS members influenced the development of uniform methodologies for water resource assessment, emphasizing the need for synchronized annual reporting periods to address transboundary water issues. These efforts complemented USGS initiatives by promoting the adoption of similar fiscal-hydrological years in collaborative research, though the October-September convention remained primarily U.S.-centric in its initial codification. The International Hydrological Decade (IHD), spanning 1965 to 1974, significantly advanced global awareness and standardization of hydrological practices, including the water year concept, by fostering international data exchange and networks. Initiated to enhance understanding of the hydrological cycle, the IHD engaged over 100 countries in coordinated studies, leading to widespread of standardized annual water reporting periods—often aligned with regional patterns—by the 1980s in more than 50 nations, albeit with local adjustments such as variations. This decade's outcomes, including technical reports and training programs, built on USGS and IAHS foundations to promote consistent global benchmarks for water . In the 2000s, the (WMO) updated its hydrological guidelines to integrate variability considerations, particularly through the Global Runoff Data Centre (GRDC), established in 1987 under WMO auspices. These revisions emphasized standardized water year reporting in long-term datasets to better capture trends in runoff and precipitation amid changing , as outlined in WMO's World Climate Programme-Water initiatives for trend detection in hydrological records. The GRDC's protocols, refined in the early 2000s, facilitated international using water year conventions to analyze impacts, ensuring compatibility with global observation systems like the World Hydrological Cycle Observing System (WHYCOS).

Variations and Classifications

Standard Definitions

In the United States, the standard water year is defined as the fixed 12-month period from October 1 to September 30, designated by the calendar year in which it ends. This definition is uniformly applied by federal agencies for hydrological records and water supply assessments, including the (USGS), which uses it to report , , and data across the nation. The (NOAA) also adopts this period for analyzing water year and drought conditions, particularly in regions like where it aligns with the primary . Similarly, the Bureau of Reclamation employs the October 1 to September 30 timeframe for managing reservoir operations and water allocations in major basins, such as the , as stipulated in interstate compacts. In the , the hydrological year typically runs from October 1 to September 30, facilitating the tracking of seasonal recharge from autumn rainfall. However, for performance reporting and , a fixed period from to March 31 is commonly used, aligning with the financial year and encompassing the wetter winter months when river flows and groundwater levels peak. This April-March convention extends to much of , where it supports low-flow analysis and assessments in countries like , capturing the hydrological cycle's emphasis on winter and spring-summer deficits. In the , Australia's standard water year is defined as July 1 to June 30, designated by the ending year to include the summer predominant in many regions. This period is officially used by the for national accounting, market reports, and assessments of availability. The July-June alignment ensures comprehensive capture of monsoon-influenced rainfall patterns, aiding in monitoring and resource planning across diverse climates from arid inland areas to tropical northern zones.

Type Classifications

Water years are commonly classified into categories such as wet, dry, normal, above normal, and below normal based on percentiles of annual or runoff volumes relative to long-term historical . This percentile-based approach allows for standardized assessment of hydrological conditions, where, for instance, flows exceeding the 75th are deemed above normal, while those below the 25th are below normal. The U.S. Geological Survey (USGS) employs a refined system for streamflow rankings, categorizing them as much below normal (less than 10th ), below normal (10th to 24th ), normal (25th to 75th ), above normal (76th to 90th ), and much above normal (greater than 90th ), using data from long-term (e.g., 1930–present). A prominent example is California's water year classification system, implemented by the Department of Water Resources since the 1970s, which uses five types—wet, above normal, below normal, dry, and critical—derived from the Sacramento Valley Index (SVI). The SVI is computed using the : 0.4 × forecasted – unimpaired flow + 0.3 × actual – unimpaired flow + 0.3 × previous water year's SVI, all in million acre-feet (MAF), then compared to fixed thresholds: wet (index ≥ 9.2 MAF), above normal (>7.8 and <9.2 MAF), below normal (>6.5 and ≤7.8 MAF), dry (>5.3 and ≤6.5 MAF), and critical (≤5.3 MAF). These thresholds reflect historical distributions in the Basin. In response to , researchers have developed non-stationary classification frameworks that adjust thresholds dynamically to capture shifting hydrological regimes, as traditional stationary methods may misrepresent evolving conditions. Studies from the 2020s indicate that global warming is altering water year type frequencies, with projections for California's Central Valley showing an increased occurrence of dry and critical years (up to 20–30% more under high-emission scenarios) and fewer wet years due to reduced and earlier . In the broader U.S. Southwest, similar trends suggest more frequent dry years, exacerbating risks as warming amplifies aridity and reduces recharge.

Applications

Water Resource Management

In water resource management, the water year serves as a fundamental unit for annual water rights allocation, particularly in arid regions like the where seasonal variability demands structured distribution. In , for instance, water year types—classified based on unimpaired runoff indices—guide the prioritization of diversions among rights holders, ensuring senior appropriative rights under the post-1914 permit system receive precedence during shortages. This approach, rooted in the Water Commission Act of 1914, enables the State Water Resources Control Board to adjust allocations dynamically, such as setting State Water Project deliveries as low as 5% in critically dry years or up to 100% in wet years, thereby balancing agricultural, urban, and environmental needs. Reservoir operations and flood control also rely heavily on water year delineations to inform real-time decision-making and long-term planning. The U.S. Geological Survey (USGS) provides continuous and level data, which operators use to forecast inflows and schedule dam releases for the ongoing water year, such as projections for Water Year 2025 that incorporate October-to-September hydrologic patterns to mitigate flood risks during wet periods. Additionally, drought declarations by state authorities, like California's, often hinge on cumulative water year deficits, where consecutive dry years—evidenced by below-average and runoff—trigger emergency measures such as enhanced conservation mandates or pumping restrictions. Water year frameworks integrate with planning models to simulate supply-demand dynamics across varying hydrologic conditions. The Water Evaluation and Planning (WEAP) system, for example, models annual water balances by incorporating water year-specific inputs, allowing managers to assess scenarios for non-normal years where unimpaired flows deviate significantly from historical means, such as critical dry types below the 10th percentile. This enables proactive adjustments in infrastructure operations, like optimizing storage in reservoirs during above-normal years to buffer future shortages. Water year type classifications, such as those developed by the California Department of Water Resources, provide the hydrologic thresholds for these simulations.

Climate and Hydrological Analysis

The water year serves as a critical framework for monitoring and analyzing and patterns in studies, enabling consistent tracking of hydrological variability across seasons. In the United States, the (NOAA) provides U.S. Climate Normals with 30-year averages of based on -year periods to identify deviations from typical conditions. Hydrological analyses, however, often align assessments of annual totals and volumes with the October-to-September water year to capture wet-season accumulations more accurately than calendar years. Globally, the (WMO) leverages similar annual hydrological periods—frequently corresponding to water years—to report anomalies, such as the 2023 record, which marked the driest year for rivers in over three decades based on data from major basins like the Amazon and . Trend analysis in flood and streamflow timing benefits significantly from water year delineations, as they allow researchers to standardize seasonal cycles across diverse climates. A 2020 study published in Water Resources Research by the analyzed data from approximately 10,000 global stream gauges, using locally defined water years to reveal shifts in peaks, with earlier occurrences in northern tropical regions and later ones in temperate southern areas, attributed to climate warming. This approach highlights how water years help isolate anthropogenic influences on hydrological timing, providing evidence of altered flow regimes without the confounding effects of calendar-year boundaries. In global water cycle modeling, water years aggregate key variables to quantify changes in , runoff, and storage, informing projections of environmental shifts. The Intergovernmental Panel on Climate Change's Sixth Assessment Report (AR6), Chapter 8, emphasizes the water balance equation aggregated over annual periods like water years: ΔS=P−ET−Q\Delta S = P - ET - Q, where ΔS\Delta S represents change in storage, PP is , ETET is , and QQ is runoff; this formulation enables assessment of intensified dynamics under warming scenarios. Such integrations reveal increasing in some regions and enhanced risks in others, with water year data enhancing the resolution of seasonal storage variations in models like those from the (CMIP6).

References

  1. https://water.usgs.gov/nwc/explain_data.html
  2. https://water.usgs.gov/vizlab/water-availability/glossary
  3. https://www.climatehubs.usda.gov/hubs/northwest/topic/water-year-graphs
  4. https://ecology.wa.gov/blog/october-2022/happy-new-water-year
  5. https://www.epa.gov/hydrowq/definition-and-characteristics-low-flows
  6. https://waterwatch.usgs.gov/2022summary/
  7. https://waterservices.usgs.gov/docs/statistics/statistics-details/
  8. https://water.ca.gov/-/media/DWR-Website/Web-Pages/Programs/Flood-Management/Flood-Data/Climate-summaries/Hydroclimate_Report_2022-ADA-Final.pdf
  9. https://www.weap21.org/webhelp/water_year_definition.htm
  10. https://water.ca.gov/-/media/DWR-Website/Web-Pages/Programs/Flood-Management/Flood-Data/Climate-summaries/Hydroclimate_Report_2015-ADA-Final.pdf
  11. https://www.ausableriver.org/blog/what-water-year
  12. https://watereducationcolorado.org/publications-and-radio/blog/its-a-new-water-year/
  13. https://water.usgs.gov/history.html
  14. https://files.dnr.state.mn.us/publications/waters/water_year_2007-2008.pdf
  15. https://pubs.usgs.gov/pp/0486d/report.pdf
  16. https://unesdoc.unesco.org/ark:/48223/pf0000012343
  17. https://pubs.usgs.gov/wsp/1181/report.pdf
  18. https://wdr.water.usgs.gov/current/documentation.html
  19. https://iahs.info/About-IAHS/History-of-IAHS/IAHS-Timeline/
  20. https://www.unesco.org/en/articles/50th-anniversary-unesco-intergovernmental-hydrological-programme
  21. https://www.sciencedirect.com/topics/earth-and-planetary-sciences/international-hydrological-decade
  22. https://grdc.bafg.de/
  23. https://water.usgs.gov/osw/wcp-water/detecting-trend.pdf
  24. https://www.climate.gov/news-features/featured-images/very-wet-2017-water-year-ends-california
  25. https://www.usbr.gov/lc/region/g1000/pdfiles/ucbsnact.pdf
  26. https://nrfa.ceh.ac.uk/news-and-media/news/happy-new-water-year
  27. https://www.gov.uk/government/publications/water-resources-2023-2024-analysis-of-the-water-industrys-annual-water-resources-performance/water-resources-2023-2024-analysis-of-the-water-industrys-annual-water-resources-performance
  28. https://nora.nerc.ac.uk/id/eprint/529163/1/N529163PP.pdf
  29. https://www.bom.gov.au/water/awid/id-644.shtml
  30. https://www.bom.gov.au/water/about/publications/document/InfoSheet_7.pdf
  31. https://pubs.usgs.gov/fs/2019/3063/fs20193063.pdf
  32. https://waterwatch.usgs.gov/2021summary/
  33. https://www.cbr.washington.edu/sacramento/data/metadata/method_hci.html
  34. https://www.waterboards.ca.gov/waterrights/water_issues/programs/bay_delta/california_waterfix/exhibits/docs/PCFFA&IGFR/PCFFA_72_Null.pdf
  35. https://www.mdpi.com/2225-1154/9/2/26
  36. https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2024EF005465
  37. https://cawaterlibrary.net/document/state-water-project-historical-table-a-allocations-water-years-1996-2024-2/
  38. https://www.waterboards.ca.gov/waterrights/board_info/water_rights_process.html
  39. https://www.ncei.noaa.gov/products/land-based-station/us-climate-normals
  40. https://wmo.int/publication-series/state-of-global-water-resources-2023
  41. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2020WR027233
  42. https://www.ipcc.ch/report/ar6/wg1/chapter/chapter-8/
  43. https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter08.pdf
Add your contribution
Related Hubs
User Avatar
No comments yet.