What is Gage Gap?

Gage Gap is a tool for identifying areas of the state that need improved stream monitoring systems. Stream gages provide essential information about one of California's most precious resources — water. Gages that are actively reporting and well-maintained help the state properly allocate water resources for people and nature, adapt to climate change, and anticipate potential flooding. Gaps in the stream gage network make it much more difficult to ensure that both people and nature are getting water when and where they need it. This tool highlights the major gage gaps and ways to fill them.

How do I Use This Tool?

Gage Gap Analysis

The Gage Gap tool was built to help understand the status of stream flow monitoring across California. Making use of existing stream network data, we were able to track the path of a stream to determine what sections of a stream are currently monitored by a gage.

Robust geospatial data on streams was available from the National Hydrography Dataset (NHD Plus V2), including stream network connections, flow, drainage, and stream name. We created a database of gage locations and status by combining six existing datasets of stream gages in California (See the list of data sources below for more detail).

By snapping each gage to the stream it monitors, a network analysis became feasible. To start, any stream segment with an active gage along it was considered to be well gaged. Data from a stream gage can be extrapolated both up and down stream of the gage location, so we applied two network tracing tests, one for upstream, and a second for downstream. Both are described in more detail below.

The gap analysis relies on the drainage area of each stream segment, as provided by NHD. Drainage area is a measure of the land area that drains into a stream segment. The drainage area for any stream segment is always the sum of the contributing segments' drainage areas. For this reason, the drainage area increases as you move downstream.

Upstream Analysis

The goal of upstream analysis was to measure how far upstream a gage could monitor. Using the principle applied by USGS¹ it was determined that a drainage area upstream of a gage was sufficiently monitored until the drainage area of that upstream segment falls below 50% of the drainage area at the gaged location. In other words, as you move upstream the drainage area of the stream segments decreases. Once the drainage area of the ungaged stream is less than half of the gaged stream, the data from the gage is no longer informative to estimate flows in the ungaged stream.

Below is an expanded view showing how segments are analyzed across a stream network. Segments are analyzed in an upstream direction, starting at each segment with an active gage. Connected upstream segments considered "Well Gaged" until the drainage area falls below 50% of the gaged segment's drainage area.

Downstream Analysis

As you move down the stream from a gaged location, it is well gaged until the drainage of the stream segment exceeds 150% of the drainage area of the gaged location.The downstream analysis threshold of 150% was taken from the USGS guidance and applied to the stream segments downstream of gaged locations.

The expanded view below details the downstream analysis over a larger network. Stream segments are analyzed in a downstream direction, starting at each stream segment with an active gage. Connected downstream stream segments are analyzed until the drainage area rises above 150% of the gaged stream segment's drainage area.

Combing the Up- and Downstream Analyses

To be well gaged, a stream segment can be gaged in either the upstream or downstream location or both. To reflect this, the Gage Gap analysis combined the two analyses described in Figures 3 and 5. Lastly, any segments not analyzed in the upstream or downstream script were defined as poorly gaged.

The combined results are show below in Figure 6.

Key decisions:

• Major streams were defined as streams with a drainage area equal to or greater than 5 km². The 5 km² threshold is based on two factors:
  • To remove very small tributaries from the analysis that are less important to provide water supplies to people and nature, and are less likely to cause major flooding damage
  • The 5 km² threshold was validated with a review of existing stream gages. Over 85% of known gages are on streams with a drainage of 5 km² or more.
• Only active gages were included in the analysis.
• Gage adequacy calculations were run in a PostGIS database through two independent scripts, using the Psycopg database adapter for PostgreSQL. The results were then combined in Esri ArcGIS software.

Data Sources

Stream Gages

Inactive, on Poorly Gaged Stream = An inactive gage is present and no other active gages, regardless of flow or reporting frequency, adequately monitors the stream.

Inactive, on Gaged Stream = An inactive gage is present but another active gage can be used to monitor the stream. The active gage might report only stage/height and/or have a significant delay in reporting the data.

Active Gage to Enhance = An active gage is present but only reports stage/height and/or has a significant delay in reporting the data.

Active Gage = An active gage reporting flow in real time is adequate for monitoring the stream segment.

California Data Exchange Center (CDEC)
United States Geological Survey (USGS Gages II, and USGS API)
National Streamflow Information Program (NSIP)
USGS National Water Information System (NWIS)
National Oceanic and Atmospheric Administration (NOAA)
Los Angeles County, Department of Public Works (LADPW)

Limitations:

• Many agencies are involved in stream gages. The gage database only focused on gathering gage data from agencies with a large number of gages. Not all gages are included in the database.
• The process of merging the seven datasets resulted in duplicate gage records. Multiple reviews were done to remove clear duplicates. The gage gap database likely still has some duplicate gage records. Because the existence of a gage in the analysis is binary, duplicate gages do not affect the results.
• Attributes for gages were collected from multiple sources and are incomplete, and some may be out of date.
• Information about agency involvement in gages (owner, operator, maintainer, reporter, funder) is very limited. The values are most frequently from the USGS website, under current conditions for the gage, and is of unknown accuracy or vintage.
• Gages were snapped to streams for analysis, but displayed locations are those reported by the agency and may not fall visually on the stream.

Hydrography

National Hydrography Dataset (NHD Plus V2)

Limitations:

• The NHD Plus database is a 1:100,000 data set. More detailed datasets are available and include smaller hydrological features.
• The Gage Gap application only includes linear hydrological features with a drainage area of 5 square kilometers or more. Artificial, reservoir, lake and pond features were removed.
• Attributes are not complete for all streams in the NHD database. Streams without a drainage area were not analyzed.

Regions

California Water Plan GIS Data (CalWater)

Watersheds

The Nature Conservancy, summary watersheds, 2016

Limitations:

• The edges of some watersheds do not topologically align to the hydrologic regions. Each watershed has been assigned to one, and only one, region.

Project Contacts: Kirk Klausmeyer kklausmeyer@tnc.org, Amanda Recinos amanda@greeninfo.org The GIS files used in Gage Gap are available for download. Contact Kirk Klausmeyer for the full database of gages and streams. Extended methods and metadata are also available.

Creative Commons Icons from the Noun Project: Download (Musmellow), Methods (Gregor Cresnar), and Report (Lakshisha)

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