Tree water source variation in semi-arid ecosystems is important to understand because climatic shifts towards lower snowpack and increased drought affect water availability in subalpine forests of the western US. Here, we use daily in situ measurements of stable isotopes (2H) in soil and tree water, soil matric potential and sap flow to study tree water uptake dynamics. We instrumented three soil profiles down to 90 cm, as well as three Aspen and Engelmann Spruce trees near Gothic, Colorado, in the East River watershed. We observed the fate of natural 2H variations in rainfall, soil, and plants from June to October 2022, and in August 2023 we conducted a 2H labeled water irrigation experiment. Our observations showed that transpiration was reduced by all trees, but partially compensated by shifting the dominant water source from 60 to 90 cm within days of a dry period. Intense rainfall quickly shifted the plant water uptake partially to top soil layers. Changes in water uptake depths were similar between aspen and spruce, but rainfall infiltration was low in the spruce stand due to high canopy interception. Therefore spruce transpiration was lower and relied more on snowmelt. However, both species relied on snowmelt to sustain transpiration and groundwater recharge from monsoonal rains was not observed. These findings highlight the important role of snowmelt stored in the deep soil layers for subalpine forest drought response and the dominant fate of monsoonal rainfall to become transpiration rather than recharging groundwater and streams in the Upper Colorado River.

Conventional approaches of training ecohydrologic models usually consider one hydrometric dataset, such as field measured matric potential or a soil moisture time series. In numerical modeling, these metrics can be used to optimize soil physical parameters using equations for water content and hydraulic conductivity. However, training a model on matric potential alone neglects valuable calibration potential available from transport processes, often described by the advection dispersion equation. Our group aims to quantify this potential by using stable isotopes of water as a tracer. In HYDRUS-1D, we couple daily stable isotope ratios and matric potential measurements to optimize soil hydraulic and transport parameters for two forested sites in the Upper Colorado River Basin. Using inputs of δ2H, five parameter optimization schemes were tested across four sampling intervals (daily, weekly, biweekly, and rainfall-based) to assess calibration performance. Results show that when KGEΨ values are highest (indicating strong model performance with regard to matric potential), unincorporated values of KGED are considerably low (indicating poor model performance with regard to stable isotopes). We hypothesize that the maximum average of both metrics, KGEx̄, yields a more robust parameter set, and consequently, a more physically representative model. Comparing KGEx̄ for daily and weekly sampling intervals shows a negligible difference, suggesting that weekly sampling is adequate for model calibration. Rainfall-based sampling (two days after precipitation ≥ 3 mm) shows promise, with a comparable model performance to daily and weekly sampling. Meanwhile, the model underperformed with biweekly sampling. We propose that weekly sampling is sufficient for producing an optimal parameter set, and that daily sampling is likely unnecessary. For good measure, less frequent sampling intervals are not recommended. This study aims to evaluate sampling frequency and model performance to benefit future efforts of ecohydrologic model calibration.

GSA Connects, 2023—Pittsburgh, PAMuch of the western United States experiences significant variability in annual precipitation between wet and dry years. As a result, surface water and groundwater supplies can become depleted during dry years, while in wet years, water infrastructure necessary to replenish these systems is often inefficient and outdated. The unsaturated zone acts as a critical freshwater storage mechanism between wet and dry periods. Agricultural surface treatments, such as cover cropping and reduced tillage, have the potential to modify infiltration rates and route valuable quantities of water, below the root zone of crops, to this subsurface region. At the University Farm in Chico, CA we investigate links between crop and management type on soil structure and preferential flow for locations within an almond orchard, across a conventional wheat and vegetable field, and in an organic vegetable plot. We collected soil cores across 15 sampling locations at half meter depths for subsequent measurements of saturated hydraulic conductivity (Ks) and scans of soil pore structure using x-ray computed tomography (CT). Rates of Ks were compared for soils under different surface treatments, with results showing median rates of 80, 425, and 503 centimeters per day in the conventional wheat and vegetable field (tillage without cover crop), the organic vegetable plot (minimal tillage with cover crop), and the almond orchard (cover crop without tillage), respectively, with the highest variability of Ks measured in the almond orchard. Preliminary analysis of soil core CT scans reveal the pore network structure of samples. These data will be used to spatially characterize expected infiltration rates and estimate storage implications for high magnitude rain events, guiding decision making for future water distribution across the site.

In the coming years, climate models forecast mountainous watersheds to undergo a reduction in snowpack, early season melt, and increases in evapotranspiration. As a result, dry soil conditions will stress vegetation at elevations of 1,850 to 2,900 meters above sea level. In this study we investigate infiltration patterns and root water uptake in response to drying within the East River catchment in Colorado. Our group collected soil cores, measured matric potential and sap flow, and monitored tree xylem and soil for stable isotopes of water (2H, 18O) along two profiles to 90 cm depth—with three Engelmann spruce and three aspen trees instrumented, respectively. Field isotope dynamics were analyzed on a daily basis between mid-July and late October using an in situ cavity ring-down spectrometer. The numerical model HYDRUS-1D was trained and calibrated with pressure head and isotope data, simulating the response to late summer dry spells and monsoonal rainfall for a 128 day period. Lab measured and model derived rates of saturated hydraulic conductivity are consistent for both soil profiles, with a median rate of 1,410 cm d-1. Model simulations reflect the three distinct dry down events from late July to September, each followed by rapid infiltration of rainfall (42, 47, and 68 mm of cumulative precipitation per event). Compared to aspen trees, shallow soil under Engelmann spruce repeatedly dries out beyond the permanent wilting point of -1.5 MPa, likely due to higher rates of canopy interception for spruce. This study highlights the benefits of coupling tracer data and commonly used hydrometric data to better constrain parameters used in numerical modeling. That said, these efforts aim to help predict and better understand quantification of certain plant water responses during ecosystem changes and future climate conditions.

UCSC GEOPATHS is an NSF-supported initiative to improve undergraduate success in the geosciences, driven by a desire to broaden academic engagement. One component of the program is a funded undergraduate summer program that provides authentic, professional experiences – across all employment sectors – to increase commitment in the geoscience pipeline. Many hydrologic basins rely on groundwater to supply domestic, municipal, and agricultural demand, but resources are increasingly stressed by rising demand, changes in land use, and a shifting climate. Consequences of groundwater overdraft include drying surface water systems, land subsidence, and seawater intrusion. Managed aquifer recharge (MAR) can help improve groundwater resources by increasing infiltration of excess surface water. We are part of a research team assessing hydrologic conditions during MAR on an active vineyard in Central California, through diversion of high flows from an adjacent river, a strategy known as “flood-MAR.” Our team collected soil samples from the upper 100 cm below ground surface at 24 locations across the 785-acre field site. We analyzed samples for soil texture at 10-cm spacing using a particle size analyzer based on laser light scattering. Preliminary analysis of fractions of sand, silt, and clay-sized particles indicate some lateral continuity from site to site. The northern part of the field area appears to be finer grained, on average, consistent with regional soil maps, but there is also considerable variability with depth. These data will be used to assess variations in expected infiltration rates by combining soil texture (to estimate infiltration capacity) and potential flood and saturation depths (to bracket vertical head gradients). Studies of this kind are helpful for assessing the efficacy of flood-MAR as a strategy to improve groundwater supplies and quality.