New Mexico Geological Society Annual Spring Meeting — Abstracts

The Sacramento Mountain Watershed Study is designed to assess the effects of tree thinning in mountain watersheds as an effective method of increasing groundwater recharge. The project employs a soil water balance to quantify the partitioning of local precipitation before and after tree thinning. We are using the stable isotopes of oxygen and hydrogen to identify tree water sources. The study is being conducted in a 1st order watershed with no perennial outflow stream where vegetation is dominated by Douglas Fir (Pseudotsuga Menziesii). Ridges are capped with San Andres Limestone while lower slopes and the valley bottom are underlain by the Yeso Formation. From March to November 2011, we collected several soil and twig samples from which water was extracted by cryogenic vacuum distillation. Mobile soil water was sampled with passive capillary samplers (PCAPS) placed in soil profiles.

The isotopic composition of bulk soil water appears to be controlled by evaporation of snow melt stored within the soil matrix. The isotopic composition of mobile soil water is a result of mixing of non-evaporated rainfall and evaporated bulk soil water. As the monsoon season progressed and cumulative rainfall increased, the isotopic composition of mobile soil water evolved towards that of local precipitation. The isotopic composition of twig water samples from March and July resembled that of bulk soil water. In August and September, twig water isotope values appeared to have both bulk soil water and mobile soil water contributions.

The conceptual model that we have developed to explain this phenomenon relies on differences in how snow melt and monsoon precipitation enter the subsurface which determines where each water source is stored. Snow melt infiltrates soil and is stored in the matrix while water from short duration, intense rainstorms preferentially flows through macropores and quickly flushes through soil profiles to shallow epikarst features in the underlying bedrock. In the spring and early summer, trees use soil water. During monsoon season when epikarst storage increases, a secondary root system is able to begin exploiting this newly available source. The contribution of this secondary source manifests in tree water as an integrated mixture of bulk soil water and epikarst water.