Mountain-front recharge along the eastern side of the Middle Rio Grande Basin, central New Mexico

Mountain-front recharge, which generally occurs along the
margins of alluvial basins, can be a large part of total recharge to
the aquifer system in such basins. Mountain-front recharge occurs as
the result of infiltration of flow from streams that have headwaters
in the mountainous areas adjacent to alluvial basins and ground-
water flow from the aquifers in the mountainous areas to the
aquifer in the alluvial basin.

This report presents estimates of mountain-front recharge to the basin-fill
aquifer along the eastern side of the Middle Rio Grande Basin in central
New Mexico. The basin is a structural feature that contains a
large thickness of basin-fill deposits, which compose the main aquifer
in the basin. The basin is bounded along the eastern side by
mountains composed of crystalline rocks of Precambrian age and
sedimentary rocks of Paleozoic age. Precipitation is much larger
in the mountains than in the basin; many stream channels
debouch from the mountainous area to the basin.

Chloride-balance and water-yield regression methods were used
to estimate mountain-front recharge. The chloride-balance
method was used to calculate a chloride balance in watersheds in
the mountainous areas along the eastern side of the basin
(subareas). The source of chloride to these watersheds is bulk
precipitation (wet and dry deposition). Chloride leaves these
watersheds as mountain-front recharge. The water-yield
regression method was used to determine the streamflow from the
mountainous watersheds at the mountain front. This streamflow
was assumed to be equal to mountain-front recharge because
most of this streamflow infiltrates and recharges the
basin-fill aquifer.

Total mountain-front recharge along the eastern side of the
Middle Rio Grande Basin was estimated to be about 11,000 acre-
feet per year using the chloride-balance method and about 36,000
and 38,000 acre-feet per year using two water-yield regression
equations. There was a large range in the recharge estimates in a
particular subarea using the different methods. Mountain-front
recharge ranged from 0.7 to 15 percent of total annual
precipitation in the subareas (percent recharge). Some of the
smallest values of percent recharge were in the subareas in
the southern part of the basin, which generally have low
altitudes. The larger percent-recharge values were from subareas
with higher altitudes.

With existing information, determining which of the mountain-
front recharge estimates is most accurate and the reasons for
discrepancies among the different estimates is not possible. The
chloride-balance method underestimates recharge if the
chloride concentration used in the calculations for precipitation is
too small or the chloride concentration in recharge is too
large. Water-yield regression methods overestimate recharge if
the amount of evapotranspiration of water that infiltrates into the
channel bed of arroyos during runoff from summer thunderstorms
is large.