Water Isotopic Composition Traces Source and Dynamics of Water Supply in
a Semi-Arid Agricultural Landscape
Abstract
Climate forecasts for semi-arid landscapes suggest changes in
seasonality and form of precipitation. These shifts are expected to
alter the structure and function of grassland and steppe ecosystems and
present challenges for land management and crop production in regions
like the Northern Great Plains, North America. Precipitation in
lower-elevation, semi-arid areas provides a local supply of soil water
that drives biogeochemical cycling, agricultural production, and
groundwater recharge. However, studies of the fate of precipitation are
far less common in lower-elevation areas compared to studies of the fate
of seasonal snowpack and runoff in alpine areas. This research uses
isotopic composition of water (δ 18O and δ
2H) to explore the sources and fate of soil water in
lower elevation areas of the Judith River watershed, in the headwaters
of the Missouri River in Montana, USA. Extensive non-irrigated crop
production in this area occurs on well-drained soils and depends on
careful water management. Agricultural fertilization and organic matter
mineralization have resulted in excessive nitrate leaching from
cultivated soils into shallow aquifers and streams. Our observations
indicate that colder precipitation contributes isotopically distinct
water to cultivated terrace soils relative to downgradient groundwaters
and streams. Riparian waters also exhibit isotopically distinct
contributions from colder precipitation. Apparent contributions from
colder precipitation in terrace and riparian soil waters suggest that
snowmelt is an important component of water supply to these systems. In
riparian waters, influence of evaporation is also evident, suggesting
sufficient residence times and atmospheric exposure for local processing
to occur. The evolution of isotopic composition from soils to shallow
aquifers to stream corridors indicates source water partitioning as
precipitation moves through a semi-arid agricultural landscape. Mixing
processes apparent in landscape water isotopic compositions reveal
source water dynamics that facilitate plant uptake, solute processing,
and contaminant leaching.