Mimbres Basin, NM
— June 29, 2021
A new method for developing maps of water-tables employed in the Mimbres Basin in southern New Mexico indicates how groundwater pumping and recharge have changed water-table levels through time. Notably, the region from Deming south to Columbus experienced water-level declines of up to 75 feet since 1980, likely a result of pumping for agricultural use.
“There are places where the water levels are really declining and there are places where it’s increased somewhat,” said Bureau Senior Field Geologist and project lead Dr. Geoffrey Rawling, “There’s places where it’s done both over the last 40 years.”
The study, published as an open-file report by the New Mexico Bureau of Geology and Mineral Resources, used a statistical method called spatiotemporal kriging to develop water-table maps for every five years from 1980 to 2020 using individual depth-to-water measurements in wells. The analysis combined measurements collected during a field campaign in the spring of 2020 with forty years of historical measurements collected by the New Mexico Office of the State Engineer and consultants working in the basin.
Agriculture, mining, and ranching form the base of the regional economy in the Mimbres Basin, all industries reliant on groundwater supply. In addition, groundwater serves as the major water supply for municipal and domestic use. This reliance on continued groundwater availability makes careful water management critical for ensuring future supply.
Accurate water-table maps can convey important information, such as groundwater flow direction, locations of recharge and discharge through pumping, connections between surface water and groundwater, and how local geology in the basin influences groundwater movement. However, collecting manual depth-to-water measurements is expensive, labor-intensive, and time consuming. As a result, the number and frequency of measurements in the Mimbres Basin has declined significantly since 1980.
The new method to develop water-table maps employed in this study takes all of the historical measurements since 1980 and new measurements collected at 107 wells, and produces the most accurate maps possible of the water-table through time. This ensures the most value is obtained from the collected data.
“When you think about how much time and effort goes into getting each single one of these measurements, driving around and all the work that goes into it, it makes sense to use the best tools, the best math, whatever you have to get the most information out of it,” said Rawling. “I feel like you’re taking all the data and wringing it out...This method takes all of it and uses all of it.”
Spatial kriging is a statistical method that projects values into the areas between single data points on a map. This produces data value estimates for locations where there are no measurements, in this case, the areas between wells. The resulting maps cover the entire area. Spatial kriging also indicates how uncertain these estimates are.
With spatiotemporal kriging, time is added into the calculations as a third dimension, in addition to elevation and location. Using this method, Rawling was able to project water-table elevations for locations and years where there are no measurements available.
“In a lot of years there was very little data. But there are years around them with lots of data,” said Rawling. “So, when you actually do the math, you see that the precision and accuracy of the predictions in the years with few data points available are almost as good as in the years with lots of data available. That is the advantage of wells with long histories and using this method.”
A number of patterns emerged from the analysis. Water levels have declined up to seventy-five feet since 1980 between Deming and Columbus, a result of groundwater pumping for agricultural use. While the rate of water level declines around Deming have slowed over the last ten years, they have increased around Columbus. But in other regions of the basin, such as east of the Florida Mountains and northeast of Columbus, water levels have increased, likely a result of decreases in pumping for irrigation.
This study focused on identifying changes in water-table level through time, but did not necessarily identify specific causes for those changes. The work done here can serve as a jumping off point for future research into basin hydrology that could identify areas of groundwater recharge and flow paths through the geologically complex basin. In particular, the impact of springs and streamflow in the northern part of the basin near the town of Faywood could link regional precipitation trends to groundwater recharge.
A more clear understanding of the hydrologic system in the basin is crucial for managing pumping in the aquifer to ensure sustainable water resources in the future.
“If you don’t know what you have you can’t manage it,” said Rawling. “Understanding the current state of the aquifer and water levels and how they’re changing, that’s just basic information.”
If applied in other basins in New Mexico, spatiotemporal kriging could help ensure water managers are getting the most information possible from data collected over decades and could add another tool to help regulate groundwater pumping to ensure future sustainability.
This project was funded by the Healy Foundation and the New Mexico Bureau of Geology and Mineral Resources Aquifer Mapping Program.