Seasonal water usage by Juniperus ashei : assessment with stable isotopes of hydrogen and oxygen

The recent expansion of Juniperus ashei (Ashe juniper) on the Edwards Plateau of Central Texas has prompted studies on the impact the juniper expansion has had on the hydrologic regime of the region. This study addresses Ashe juniper's impact on the water budget of the Edwards Plateau by examining seasonal variations in the juniper's water source and the juniper's effect on soil water content at the Honey Creek State Natural Area. The study site experiences two distinct periods of seasonal rainfall. The dry season (late spring and summer) is characterized by large infrequent storm events followed by periods of drought. The wet season (fall, winter and early spring) is characterized by large intermittent storms and smaller frequent rainfall events. Stable isotopes of oxygen and hydrogen from precipitation, soil water, Ashe juniper xylem water, and groundwater were compared to identify the Ashe juniper water source. These data, combined with soil water content, identify different routes of soil water loss and soil water conservation as a function of vegetation cover. The USGS Bowen ratio measurements of evapotranspiration (ET), coupled with the isotope data and soil water content data, are used to partition evaporative and transpirative water loss from the terrestrial system. Based on a mass balance calculation, a minimum of 60% of the Ashe juniper's water was obtained from groundwater over the dry season, while over the wet season, on average, only a maximum of 30% of the Ashe juniper's water was drawn from groundwater. The soil water contents, along with groundwater, soil water, and Ashe juniper xylem water δ¹⁸O values, indicate that the presence of Ashe juniper can appreciably reduce groundwater resources both by lateral roots intercepting potential recharge under wet conditions and direct uptake of groundwater by deep roots under dry conditions. Additionally, Ashe juniper usage of soil water at the beginning of the grass growth season (early spring) may decrease herbaceous productivity by drawing water from beneath the grasses. Ashe juniper canopy rainfall interception is another process by which terrestrial system water is lost to the atmosphere. The isotope and soil water content data suggest that juniper canopy interception is an important route of terrestrial water loss over the wet season. Over the dry season, little evidence is seen of juniper canopy interception. Rather, the data suggest the juniper conserves soil water by limiting evaporation beneath its canopy. ET flux measurements are separated into evaporation from the soil, evaporation from the juniper canopy, and juniper transpiration using measured ET, precipitation, soil water content, their respective δ¹⁸O values, and juniper xylem water δ¹⁸O values in a mass balance calculation. This unique approach shows promise for quantifying the individual components of ET water loss from the terrestrial system. More reliable results may be found when water vapor δ¹⁸O values are measured and sampling occurs over shorter time intervalsGeological Science