Low-Flow Characteristics of Virginia Streams

Needs

Understanding low flows in Virginia streams is essential to sound management of our water resources and associated riparian and watershed ecosystems. Meaningful characterizations of the magnitude, frequency, and duration of low flows provide valuable insights into the dynamics, variability, and flux associated with water movement through these unique stream systems. Improved estimates of low flows provide data needed for the effective management of riparian systems and ecology associated with forested, agricultural, and urban landscapes. Low-flow estimates provide a basis for analysis of future streamflow response to changes in ecosystem and climate.

Scientific Investigations

These needs are addressed through cooperative study and a series of scientific investigations.

1. Low-flow annual non-exceedance probabilities have been determined for estimating low-flow characteristics of gaged and ungaged streams. The 1-, 4-, 7-, and 30-day average stream-gaging station low-flow characteristics for 290 long-term, continuous-record, stream-gaging stations, adjusted for instances of zero (0) flow using a conditional probability adjustment method, are presented for non-exceedance probabilities of 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, 0.1, 0.05, 0.02, 0.01, and 0.005. They may be found here: https://pubs.usgs.gov/sir/2011/5143/
2. Regional regression equations, and methods to transfer flow estimates to ungaged locations, have been developed for low-flow characteristics of Virginia streams. They may be found here: https://pubs.usgs.gov/sir/2011/5143/
3. Analyses of Virginia stream-flow data collected from 1895 through 2007 have been summarized. They may be found here: https://pubs.usgs.gov/sir/2011/5143/
4. Stream basin characteristics computed using spatial data and a geographic information system have been determined. These characteristics have been used as explanatory variables in regional regression equations to estimate annual non-exceedance flow probabilities at gaged and ungaged sites, and have been summarized for 290 long-term, continuous-record stream-gaging stations, 136 short-term, continuous-record stream-gaging stations, and 613 partial-record stream-gaging stations. They may be found here: https://pubs.usgs.gov/sir/2011/5143/
5. Regional regression equations have been developed for six physiographic areas using basin characteristics to estimate 1-, 4-, 7-, and 30-day average low-flow annual non-exceedance probabilities at gaged and ungaged sites. Weighted low-flow values that combine computed stream-gaging station low-flow characteristics and annual non-exceedance probabilities from regional regression equations have been developed to improve low-flow estimates. They may be found here: https://pubs.usgs.gov/sir/2011/5143/
6. Regression equations prepared using a Maintenance of Variation with Extension (MOVE.1) method have been developed to describe the line of organic correlation (LOC) with appropriate index-sites for low-flow characteristics at 136 short-term, continuous-record stream-gaging stations and 613 partial-record stream-gaging stations. They may be found here: https://pubs.usgs.gov/sir/2011/5143/
7. Monthly streamflow statistics have been computed on the individual daily mean streamflows of selected continuous-record stream-gaging stations. They may be found here: https://pubs.usgs.gov/sir/2011/5143/
8. Curves describing flow-duration have been developed.  They may be found here: https://pubs.usgs.gov/sir/2011/5143/
9. Anticipating drought conditions in Virginia streams is essential to sound management of water resources, riparian areas, and basin ecosystems. Reliable predictions of the likelihood that stream low-flows during drought prone months exceed specific low-flow thresholds can provide advance warning of drought conditions, allowing extended anticipatory lead times for improved drought awareness and effective management response.
   
   Improved knowledge and prediction of low flows in drought prone months can extend lead-time for drought response even as precipitation, water-withdrawals, land use, and climate variables change over time. Drought flow probability estimates can improve the management of riparian systems and the ecology associated with forested, agricultural, and urban landscapes. Drought flow probability estimates provide a basis for analysis of future streamflow response to changes in ecosystem and climate.
   
   Maximum Likelihood Logistic Regressions are being developed for 339 stream gaging sites, to predict stream flow probabilities during drought prone summer months as a function of stream flows from previous winter months. These models have the potential to anticipate summer low-flows, extending drought response lead-times by 5 to 8 months. Check here soon for more information about acquiring and using these models and equations.

The Benefits of Quality Data and Science

Quality low-flow measurements, combined with continued collaborative scientific investigation, provide uniquely useful application oriented information and analyses vital to understanding, anticipating, and better managing Virginia’s stream flows, basins, and ecosystems.