[permaculture] Benefits of traditional acequia irrigation systems

Patrick forgeadams at yahoo.com
Wed Dec 16 03:05:19 EST 2009


http://southwestfarmpress.com/irrigation/acequia-hydrology-1125/

Benefits of traditional acequia irrigation systems
	Nov 25, 2009  8:06 AM, 
	By Jane Moorman, New Mexico State University 
	
	

	





	

















	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?




















SAM FERNALD, right, NMSU
assistant professor of watershed management, and Steve Guldan,
superintendent at NMSU’s Sustainable Agriculture Science Center at
Alcalde, discuss the acequia hydrology study at a monitoring pipe along
the Alcalde acequia. Data gathered from a seven-year study show that
only 7 percent of the river water diverted to the irrigation ditches is
lost to evapotranspiration. Fernald and Guldan presented the data at
the Acequia Hydrology Symposium. (NMSU photo)















	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.




      


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