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Walnut Gulch Experimental Watershed, Arizona (Precipitation)

posted on 2024-02-13, 13:57 authored by David Goodrich

An extensive precipitation database at the 149 km2 Walnut Gulch Experimental Watershed (WGEW) has been developed over the past 53 years with the first records starting in August 1953 and continuing to the present. The WGEW is a tributary of the San Pedro River, surrounds the town of Tombstone in southeastern Arizona, and has a drainage area of approximately 149 km2. Elevation of the watershed ranges from 1220 m to 1950 m above mean sea level (MSL). Average annual precipitation for the period of 1956-2005, as measured with six gauges, is roughly 312 mm, with approximately 60% falling during the summer monsoon. Precipitation consists almost solely of rainfall with relatively rare instances of hail and snowfall. From a historical high of 95 rain gauges, a current network of 88 gauges is operational. This constitutes one of the densest rain gauge networks in the world (0.6 gauges/km2) for watersheds greater than 10 km2. Through 1999, the network consisted of analog recording weighing rain gauges. In 2000, a newly designed digital gauge with telemetry was placed adjacent (1 m) to the analog gauges. Both the analog and digital networks of gauges were in operation from 2000 to 2005 to enable a comparative analysis of the two systems. The analog data were digitized from paper charts and were stored in breakpoint format. The digital data consist of rainfall depths at 1-min intervals during periods of rainfall.

Resources in this dataset:




Data contact name

Armendariz, Gerardo


Agricultural Research Service

Intended use

Observation and measurement of the spatial and temporal variations of precipitation are fundamental to our understanding of watershed and hydrological processes. The WGEW rainfall network database has been used extensively for a great number of different analyses as well as validation of methods and models. For example, temporal trends of precipitation from 1956 to 1996 were investigated by Nichols et al. [2002] who found an increasing trend in total annual precipitation over this time that was due almost entirely to increases in non-summer precipitation as a result of a greater number of rainfall events in these seasons. The high-intensity, summer air mass thunderstorms also produce rainfall totals that vary significantly over short distances.

Use limitations

For the analog network record, different numbers of rain gauges were in operation during different periods of time. The most notable cases were from January 1980 to June 1991 and from October 1998 to October 2004 when the analog operational network was scaled back to nine gauges during the non-monsoon months because of financial considerations. The exact turn-on and turn-off dates for each of the gauges is part of the database and they are returned with any data query via the web database interface. Chart on and off times were set and compared respectively, to the wristwatches of field technicians whose watches were set to a broadcast standard time. If clocks stopped or gained/lost more than 15 min over the course of a 7-day period (weekly gauge visits), the clocks were replaced with spares and the malfunctioning clocks were serviced in Tombstone. In these cases the event start times would be adjusted to account for the fast/slow clock using a linear correction. In the case where a clock stopped, start times from the nearest operating gauge were used. Prior to 1968 there was not a regular schedule of infield rain gauge calibration. Occasional field checks were made by measuring the amount of rainfall accumulated in the collection bucket with a standard volumetric tube measurement. If differences existed between the measurements, a correction factor was developed from the tube measurement and applied.

Temporal Extent Start Date



  • Not specified

Geographic Coverage


ISO Topic Category

  • environment
  • farming

Ag Data Commons Group

  • Long-Term Agroecosystem Research
  • Walnut Gulch Experimental Watershed

National Agricultural Library Thesaurus terms

watersheds; rivers; Arizona; drainage; sea level; summer; monsoon season; rain; hail; snow; rain gauges; telemetry; paper; digital database; temporal variation; hydrology; models; air; storms

OMB Bureau Code

  • 005:18 - Agricultural Research Service

OMB Program Code

  • 005:040 - National Research

ARS National Program Number

  • 211

Primary article PubAg Handle

Pending citation

  • No

Public Access Level

  • Public

Preferred dataset citation

Goodrich, David (2020). Walnut Gulch Experimental Watershed, Arizona (Precipitation). Agricultural Research Service.