Introduction
The Mesilla Basin monitoring program was established in 1987, and is conducted by the U.S. Geological Survey (USGS) in cooperation with 8 local, state, and federal agencies. Monitoring program components include the Mesilla Basin observation well network, the Mesilla Valley hydrologic sections, and seepage investigations at selected surface-water sites. The Mesilla Basin monitoring program has helped document recent hydrologic conditions and established a long-term continuous data base to permit future quantitative evaluation of the ground-water flow system and stream-aquifer relations. Collected data was extensively used in interpretive studies conducted by the USGS and other agencies to better define the geohydrologic system (selected references). Hydrologic data collected by the monitoring program will continue to provide valuable information towards future efforts to update, revise, and calibrate basin models of the ground-water flow system. The monitoring program continues to document changing hydrologic conditions and stream-aquifer relations, and identify ground-water trends.

Problem
Historic ground-water data collection efforts within the basin were limited to obtaining data on a nonrecurring, unsystematic basis. Previous studies emphasized the need for a comprehensive monitoring network of the ground-water flow system in the Mesilla Basin. A long-term monitoring program is necessary to document current ground-water conditions and stream-aquifer relations to identify changes with time.

Objectives
The objectives of this monitoring program are to document hydrologic conditions within the Mesilla ground-water basin and establish a long-term continuous data base to permit future quantitative evaluation of the ground-water flow system and stream-aquifer relations. To accomplish these objectives, data collection and maintenance of the Mesilla Basin monitoring program will continue including the Mesilla Basin observation-well network, the Mesilla Valley hydrologic sections, and seepage investigations at selected surface-water sites including associated water-quality analyses.

Approach
The Mesilla Basin monitoring program will continue to document changing hydrologic conditions and identify stream-aquifer relations by conducting the following program elements:

1. Mesilla Basin Observation-Well Network
Conduct annual ground-water-level measurements at sites in the Mesilla Basin observation-well network. Network wells are completed in the Rio Grande flood-plain alluvium/Santa Fe Group aquifer system, with wells located in the Mesilla Valley and West Mesa area (fig. 1). Annual winter water-level measurements and well records are entered into the USGS National Water Information System (NWIS)--Ground-Water Site Inventory (GWSI) data base. Mesilla Basin network wells are web ready with information currently available to the public on NWIS Web at URL http://waterdata.usgs.gov/nwis (table 1.)

2. Mesilla Valley Hydrologic Sections
Maintain water-level record at the Mesilla Valley hydrologic sections. These hydrologic sections consist of a river-stage station and several observation wells aligned perpendicular to the Rio Grande. The location of the Las Cruces hydrologic section (A-A’), Mesquite hydrologic section (B-B’), and Cañutillo well-field hydrologic section (C-C’) are shown on figure 1. Diagrams of the Las Cruces, Mesquite, and Caņutillo well-field hydrologic sections show well completion depths and distances from the Rio Grande. The hydrologic sections currently consist of 43 sites; 3 river-stage stations and 40 observation wells. Continuous water-level records are currently maintained at 25 sites, with monthly water-level measurements at 14 observation wells. Continuous water-level records are stored in the USGS NWIS--Automated Data Processing System (ADAPS) data base and monthly water-level measurements are entered into the GWSI data base. Installation of a real-time ground-water data collection platform (DCP) at observation well group LC-2 was completed on May 18, 2005. Well group LC-2 is located at the Las Cruces hydrologic section approximately 350 feet from the Rio Grande. Real-time ground-water data is available to the public on NWIS Web at URL http://waterdata.usgs.gov/nm/nwis.

3. Seepage Investigations
Seepage investigations were conducted on the Rio Grande from Radium Springs, New Mexico, to El Paso, Texas during water years 1988 through 1998. Seepage investigations of selected irrigation drains including associated water-quality analyses were initiated in the 1999 water year. One irrigation drain will be selected for investigation each water year. Biannual seepage investigations of the selected drain will be conducted at low flow during the nonirrigation season and at high flow during the irrigation season. Discharge measurements will be conducted at selected sites to determine seepage gain/loss from specific channel reaches. Water-quality samples will be collected at approximately four main-stem sites for chemical analyses to determine dissolved solids (salinity), and concentrations of major ions and selected nutrients. Recent drought conditions and decreasing reservoir storage has resulted in a significant reduction in surface-water allocations during the 2003 and 2004 irrigation seasons, with a significant increase in ground-water withdrawals to meet water demand. Annual seepage investigations will shift from selected irrigation drains to the Rio Grande at low-flow during drought years, starting in the 2004 water year (fig. 2). Results of individual seepage investigations and associated water-quality analyses are published in U.S. Geological Survey Water-Data Reports for New Mexico by water year.

Relevance and Benefits
The Mesilla Basin monitoring network extends through New Mexico and Texas to the United States border with Mexico. The monitoring program will contribute to the USGS mission by providing data to help define the Nation’s water resources, and to disseminate impartial hydrologic information to the public. The program will benefit the cooperators and the USGS by providing essential hydrologic data necessary for subsequent evaluation of the ground-water-flow system and stream-aquifer relations. Ground-water data collected by the program are used in many interpretive studies conducted by the USGS and other agencies including the development and refinement of ground-water-flow models. The long-term monitoring program will document ground-water conditions including stream-aquifer relations, and provide essential data to develop and evaluate water-management strategies.