New Mexico State University researchers have gathered data to answer these questions: How much of the water diverted into acequia irrigation systems is lost to seepage and evaporation? What are the benefits of acequia systems to the hydrology of the surrounding areas?
Entering into the seven-year ongoing study of acequias, researchers knew the traditional, earthen irrigation ditches had hydrologic and other benefits, beyond simply delivering water to crops.
“The wisdom of the ancients was the starting point of our study,” said Sam Fernald, NMSU assistant professor of watershed management. “For generations, people have believed that acequias are more important than just being used to water fields. They believed that the seepage affected groundwater quality and recharged the aquifer. But, there were no supporting data.”
The research team has now gathered data that explains how the seepage from the earthen ditches affects groundwater quality; what percentage of the seepage evaporates or returns to the river; and what effect upstream acequia systems have on downstream farming.
The study data show that, on average, only 7 percent of the water diverted from the Rio Grande into a north-central New Mexico acequia irrigation system is lost to evapotranspiration, the sum of evaporation from all sources, including water vapor released by plants. The remaining 93 percent returns to the river: 60 percent as surface water from irrigation tailwater and 33 percent as groundwater.
“We have documented that large water flows are rapidly exchanged between the river, the irrigation system and the fluvial aquifer,” Fernald said of the research conducted in the northern end of the Espanola Valley, between Embudo and Ohkay Owingeh Pueblo, formerly San Juan Pueblo. “A significant amount of water being diverted into the valley returns back to the river after completing important production and ecological tasks.”
The findings show that water entering the groundwater through field irrigation and seepage from the earthen ditches increases the shallow water table during the irrigation season. That water then returns to the river in an average of six to 10 weeks, but can remain underground up to three months.
“The irrigation system collectively takes spring and summer runoff from the river and retransmits the flow later in the year,” Fernald said. “This storage of water underground should help save water on a regional scale by reducing evaporation losses.”
Besides showing that acequias help retain spring and summer runoff used later downstream, the researchers also documented the value of water seepage from the earthen ditches.
When Fernald and Steve Guldan, superintendent at NMSU’s Sustainable Agriculture Science Center at Alcalde, began the study in 2002 at the Alcalde Center, the researchers wanted to determine the amount of water that seeped from the earthen irrigation ditches.
The first phase of the study determined that, although there is seepage from the acequia ditches, the water is also seeping back into the groundwater through the irrigated land.
“There was 16 percent seepage over the five-mile irrigated length of the Alcalde Ditch, while there was between 25 percent and 60 percent percolation below the crops growing in a sandy, loam soil,” Fernald said.
The study also showed that seepage helps dilute contaminants in the groundwater.
“The seepage is diluting the nitrate in the groundwater and creating better-quality water that is recharging the aquifer and going back to the river,” Fernald said.
The researchers compared data on nitrate levels in the groundwater during the winter, when the irrigation systems are not flowing and there is no seepage from them, with nitrate levels when water is in the acequias. The data showed that nitrate levels are lowest closer to the river in winter, but at other times, when water is seeping from the acequias, nitrate levels are lower closer to the ditches, as well as the river.
An additional study developed by NMSU range science research specialist Carlos Ochoa addressed the field-based characterization of water transport through the soil and into the shallow aquifer. He determined how much water percolates below different types of field soils in the agricultural corridor between the Alcalde ditch and the Rio Grande.
The study, complemented with evapotranspiration estimates from crop fields and riparian vegetation along ditches and the river, helped determine how much of the total seepage from ditches and crop fields makes it back to the river as groundwater return flow.
As part of the research, computer modeling developed by NMSU civil engineering graduate student Yeliz Cevik was used to understand how valley-wide surface-water hydrology and groundwater hydrology influence the hydrology of the entire agricultural corridor.
“The main thing the research has shown is that acequias, operating the way they have for generations, seem to have a lot of value for New Mexico,” Fernald said.
“Given the increasing importance of properly managing the state’s water supplies, this research is adding critical information on the hydrology of acequia irrigation systems of north-central New Mexico,” Guldan said.