Gretchen R Miller

• Ph.D.
• Professor (Associate) at Texas A&M University

In the Gulf Coastal Plains of Texas, a state-of-the-art distributed network of field observatories, known as the Texas Water Observatory (TWO), is developed to better understand the water, energy, and carbon cycles across the critical zone (encompassing aquifers, soils, plants, and atmosphere) at different spatiotemporal scales. Using more than 300...

Hydraulic fracturing (HF) events consume high volumes of water over a short time. When groundwater is the source, the additional pumping by Rig/Frack Supply Wells (RFSW) may impose costs on owners of other sector wells (OSWs) by lowering the hydraulic head. The Carrizo‐Wilcox Aquifer in south Texas is the main source of water for HF of the Eagle Fo...

Understanding the carbon fluxes and dynamics from a broad range of agricultural systems has the potential to improve our ability to increase carbon sequestration while maintaining crop yields. Short-term, single-location studies have limited applicability, but long-term data from a network of many locations can provide a broader understanding acros...

In semiarid agricultural regions, aquifers have watered widespread economic development. Falling water tables, however, drive up energy costs and can make the water toxic for human consumption. The study area is located in central Mexico, where arsenic and fluoride are widely present at toxic concentrations in well water. We simulated the holistic...

This study simulates the dynamics of exchange fluxes between Brazos River Alluvium Aquifer and the Brazos River, TX, USA. Seven conceptual models for the connection between the river and the aquifer were simulated in HYDRUS 2D using small‐scale, high‐resolution transects across the river. These models assumed varying aquifer lithology and river inc...

As we reckon with the effect of COVID‐19 on the research enterprise in hydrologic science, it is important to acknowledge that disruptions will be persistent and that institutional‐level adjustments, while helpful, are not sufficient to mitigate all impacts on hydrologic scientists. Here, we describe the breadth of research contributions in the hyd...

Effective measurement of seasonal variations in the timing and amount of production is critical to managing spatially heterogeneous agroecosystems in a changing climate. Although numerous technologies for such measurements are available, their relationships to one another at a continental extent are unknown. Using data collected from across the Lon...

Small unoccupied aerial systems (sUASs) are increasingly being used for field data collection and remote sensing purposes. Their ease of use, ability to carry sensors, low cost, and precise maneuverability and navigation make them a versatile tool for a field researcher. Procedures and instrumentation for sUASs are largely undefined, especially for...

Small-scale treefall gaps are among the most important forms of forest disturbance in tropical forests. These gaps expose surrounding trees to more light, promoting rapid growth of understory plants. However, the effects of such small-scale disturbances on the distribution of plant water use across tree canopy levels are less known. To address this...

Near-surface remote sensing has been used to document seasonal growth patterns (i.e. phenology) for plant communities in diverse habitats. Phenology from this source may only apply to the area within the images. Meanwhile ecosystem models can accommodate variable weather and landscape differences to plant growth, but accuracy is improved by adding...

This study characterizes the dynamics of exchange fluxes between Brazos River Alluvium Aquifer and the Brazos River, TX, USA. Seven alternative conceptual models for the connection between the river and the aquifer were simulated in HYDRUS 2D using small-scale, high-resolution transects across the river. These models assumed varying aquifer litholo...

This study updates the multi‐layered Community Land Model (CLM‐ml) for hillslopes and compares predictions from against observations collected in tropical montane rainforest, Costa Rica. Modifications are made in order to capture a wider array of vertical leaf area distributions, predict CO2 profiles, account for soil respiration, and adjust wind f...

Interactions between groundwater mounds caused by a geologic layer contrast affect the efficiency of managed aquifer recharge in arid areas. However, research has rarely examined the roles of groundwater mounding size variations on soil water dynamics in a stratified vadose zone in response to a sustained infiltration source. Numerical experiments...

As more ASR systems are employed for management of water resources, the skillful operation of multiwell ASR systems has become very important to improve their performance. In this study, we developed MODFLOW and MT3DMS models to simulate a multiwell ASR system in a synthetic aquifer to assess effects of hydrogeological and operational factors on th...

This study compares the performance of the Community Land Models (CLM4.5 and CLM5) against tower and ground measurements from a tropical montane rainforest in Costa Rica. The study site receives over 4000 mm of mean annual precipitation and has high daily levels of relative humidity. The measurement tower is equipped with eddy-covariance and vertic...

The 2015–2016 El Niño event ranks as one of the most severe on record in terms of the magnitude and extent of sea surface temperature (SST) anomalies generated in the tropical Pacific Ocean. Corresponding global impacts on the climate were expected to rival, or even surpass, those of the 1997–1998 severe El Niño event, which had SST anomalies that...

Abstract. This study compares the performance of the Community Land Models (CLM4.5 and CLM5) against tower and ground measurements from a tropical montane rainforest in Costa Rica. The study site receives over 4,000 mm of mean annual precipitation and has high daily levels of relative humidity. The measurement tower is equipped with eddy-covariance...

Link to full text via NSF-PAR: https://par.nsf.gov/servlets/purl/10179051

The 2015–16 El Niño event ranks as one of the most severe on record in terms of the magnitude and extent of sea surface temperature (SST) anomalies generated in the tropical Pacific Ocean. Corresponding global impacts on the climate were expected to rival, or even surpass, those of the 1997–98 severe El Niño event, which had SST anomalies that were...

Transpiration in humid tropical forests modulates the global water cycle and is a key driver of climate regulation. Yet, our understanding of how tropical trees regulate sap flux in response to climate variability remain elusive. With a progressively warming climate, atmospheric evaporative demand (i.e., vapor pressure deficit, VPD) will be increas...

Several new functions representing groundwater dependent vegetation were incorporated into a coupled subsurface-land surface model, ParFlow.CLM, in order to adequately describe groundwater water uptake, hydraulic redistribution, and plant water stress. The modified model was used to conduct three-dimensional, stand-scale simulations of a Mediterran...

As water grows scarcer in semi-arid and arid regions around the world, new tools are needed to quantify fluxes of water and chemicals between aquifers and rivers. In this study we quantify the volumetric flux of subsurface water to a 24 km reach of the Brazos River, a lowland river that meanders through the Brazos River Alluvium Aquifer (BRAA), wit...

Study region The Gulf Coast and Carrizo-Wilcox aquifer systems in the Gulf Coastal Plains of Texas. Study focus Aquifer storage and recovery is a water storage alternative that is underutilized in Texas, a state with both long periods of drought and high intensity storms. Future water storage plans in Texas almost exclusively rely on surface reser...

Upscaling water use of individual trees to stands using sap flux techniques is a common method for partitioning site water balance, but few such studies have occurred in the tropics. Increasing interests in the role of tropical forests in global cycles have spurred upscaling studies in natural tropical forests, which present challenges from greater...

While it is reasonable to predict that photosynthetic rates are inhibited while leaves are wet, leaf gas exchange measurements during wet conditions are challenging to obtain due to equipment limitations and the complexity of canopy–atmosphere interactions in forested environments. Thus, the objective of this study was to evaluate responses of seve...

Spatial and temporal variation in wet canopy conditions following precipitation events can influence processes such as transpiration and photosynthesis, which can be further enhanced as upper canopy leaves dry more rapidly than the understory following each event. As part of a larger study aimed at improving land-surface modeling of evapotranspirat...

This study presents the results of a survey of new civil engineering students at a large, conservative, land-grant university. Students were asked a set of questions about engineering ethics and professional issues; these were designed to determine their opinions on the relative importance of sustainability in the professional practice of engineeri...

The ISI (Institute for Sustainable Infrastructure) Envision rating system is designed to be a comprehensive sustainability assessment that can be applied to a wide range of infrastructure projects, including water supply. With water supply resiliency, a prominent concern in many arid and semi-arid regions, the implementation of a water sustainabili...

Wet tropical forests are some of the environments with the greatest annual precipitation, but are also considered as the world’s major carbon sink; however, literature postulates that phothsynthesis rates are inhibited while leaves are wet. Yet measurements of photosynthesis during wet conditions are challenging to obtain due to equipment limitatio...

Because of high canopy interception in tropical forests, evaporation from wet canopy surfaces makes up a sizeable portion of the total water vapor flux. The modeling complexities presented by changing canopy wetness, along with a scarcity of land-atmosphere flux exchange data from tropical forests, means evapotranspiration (ET) processes have been...

The Texas Water Observatory (TWO) is a new distributed network of field observatories for better understanding of the hydrologic flow in the critical zone (encompassing groundwater, soil water, surface water, and atmospheric water) at various space and time scales. Core sites in the network will begin in Brazos River corridor and expand from there...

Wet canopy evaporation is a significant component of the water budget in rainforests. Frequent precipitation events followed by drying produce spatial and temporal variation in wet/dry forest canopy conditions that influence processes such as photosynthesis and growth. Upper canopies contribute a disproportionately large fraction of transpiration a...

Modelling groundwater uptake provides a powerful tool for examining the tight linkage of phreatophytic vegetation with spatial and temporal variations in groundwater and soil moisture. Here, a physically based modelling framework was developed to simulate groundwater uptake and hydraulic redistribution (HR), as driven by the potential gradients alo...

Ecosystems which rely on either the surface expression or subsurface presence of groundwater are known as groundwater-dependent ecosystems (GDEs). A comprehensive inventory of GDE locations at an appropriate management scale is a necessary first-step for sustainable management of supporting aquifers; however, this information is unavailable for mos...

The heat pulse method is widely used to measure water flux through plants; it works by using the speed at which a heat pulse is propagated through the system to infer the velocity of water through a porous medium. No systematic, non-destructive calibration procedure exists to determine the site-specific parameters necessary for calculating sap velo...

Ecohydrological connectivity is a system level property that results from the linkages in the networks of water transport through ecosystems, by which feedbacks and other emergent system behaviours may be generated. We created a system dynamics model that represents primary ecohydrological networks to examine how connectivity between ecosystem comp...

The connectivity of ecohydrological and biogeochemical processes across time and space is a critical determinant of ecosystem structure and function. However, characterizing cross-scale connectivity is a challenge due to the lack of theories and modelling approaches that are applicable at multiple scales and due to our rudimentary understanding of...

The ecohydrology of transitional premontane cloud forests is not well understood. This problem is being addressed by a NSF Research Experience for Undergraduates (REU) study at the Texas A&M University Soltis Center for Research & Education in Costa Rica. Exploratory analysis of the water budget within a 20-ha watershed was used to connect three fa...

In the humid tropics, conservationists generally prefer selective logging practices over clearcutting. Large valuable timber is removed while the remaining forest is left relatively undisturbed. However, little is known about the impact of selective logging on site water balance. Because large trees have very deep sapwood and exposed canopies, they...

Antecedent soil conditions create an ecosystem's "memory" of past rainfall events. Such soil moisture memory effects may be observed over a range of timescales, from daily to yearly, and lead to feedbacks between hydrological and ecosystem processes. In this study, we modeled the soil moisture memory effect on savanna ecosystems in California, Ariz...

Savanna ecosystems present a well-known modeling challenge; understory grasses and overstory woody vegetation combine to form an open, heterogeneous canopy that creates strong spatial differences in soil moisture and evapotranspiration rates. In this analysis, we used ParFlow.CLM to create a stand-scale model of the Tonzi Ranch oak savanna, based o...

This National Science Foundation REU site hosted by Texas A&M University allows undergraduate students to conduct original research on various aspects of the ecohydrology of a tropical pre-montane forest at the Texas A&M Soltis Center for Research and Education in Central Costa Rica. Tropical pre-montane forests are biologically diverse ecosystems...

Groundwater can serve as an important resource for woody vegetation in semiarid landscapes, particularly when soil water is functionally depleted and unavailable to plants. This study examines the uptake of groundwater by deciduous blue oak trees (Quercus douglasii) in a California oak savanna. Here we present a suite of direct and indirect methods...

Groundwater can serve as an important resource for woody vegetation in semi-arid landscapes, particularly when soil water is functionally depleted and unavailable to plants. This study examines the uptake of groundwater by deciduous blue oak trees (Quercus douglasii) in a California oak savanna. Here, we present a suite of direct and indirect measu...

In this work, we present the results of a two-year ecohydrologic monitoring program at the Tonzi Ranch oak savanna site. A comprehensive measurement suite, originally designed to assist in coupled canopy-subsurface modeling studies, was assembled at the site. Measurements collected include: groundwater levels, sap flow rates, soil moisture contents...

Since soil moisture is considered to be a primary controller of evapotranspiration in semi-arid systems, the effect of the availability of deep (>5m) water sources is often neglected in ecohydrological modeling. However, plant uptake from groundwater can contribute significantly to a site's water balance. Knowing the level of this contribution is i...

The heat pulse method is widely used to measure water flux in plants and soil; it works by inferring the velocity of water in a porous medium from the speed at which a heat pulse is propagated through the system. No systematic, non-destructive calibration procedure exists to determine the site-specific parameters necessary for calculating sap veloc...

Land application of food-processing waste water occurs throughout California's Central Valley and may be degrading local ground water quality, primarily by increasing salinity and nitrogen levels. Natural attenuation is considered a treatment strategy for the waste, which often contains elevated levels of easily degradable organic carbon. Several k...

California's Central Valley contains over 640 food-processing plants, serving a multi-billion dollar agricultural industry. These processors consume approximately 7.9 x 107 m3 of water per year. Approximately 80% of these processors discharge the resulting wastewater, which is typically high in organic matter, nitrogen, and salts, to land, and many...

Study of water exchange between soil, plants, and the atmosphere in response to seasonal or periodic droughts is critical to modeling the hydrologic cycle and biogeochemical processes in water-controlled ecosystems. The difficulties in such studies arise from insufficient understanding of the complex interactions between the various processes and t...

Measurements of individual tree transpiration, obtained using the sap flow technique, are easier to collect and less expensive than other traditional measurements of ecosystem evapotranspiration, such as eddy-covariance and lysimetery. Up-scaling these point measurements to a stand or a landscape level, however, is a challenge, especially in water-...

Study of water exchange between soil, plant and the atmosphere in response to seasonal or periodic droughts is critical to model the hydrologic cycle and many other processes in water-controlled ecosystems. The difficulties in such studies arise from insufficient understanding of the complex interactions between the various processes and their scal...

Soil moisture data, obtained from four AmeriFlux sites in the US, were examined using an ecohydrological framework. Sites were selected for the analysis to provide a range of plant functional type, climate, soil particle size distribution, and time series of data spanning a minimum of two growing seasons. Soil moisture trends revealed the importanc...

This study details the design of a network of sapflow and soil moisture sensors currently under construction in an oak savanna ecosystem in the Sierra Nevada foothills, the site of an AmeriFlux micrometeorological station. One project goal is to upscale individual tree sapflow measurements into a total canopy flux that can be compared to the latent...

Soil moisture data, obtained from four FLUXNET sites in the US, were examined using an ecohydrological framework. Sites were selected for the analysis to provide a range of plant functional type, climate, and soil grain size distribution. Data at a selected site included at least two years of measurements of volumetric soil water content, air tempe...

A variety of water supply sources are used in the rural Guatemalan Highlands. Formal municipal systems are rare, but villagers frequently form local water committees that construct spring-based supply systems. Recent work has indicated that groundwater may be a viable water supply source. The water quality from these two sources was characterized a...

This paper presents the basis for using groundwater circulation wells (GCWs) for removal actions focused on contaminated domestic water supplies. A properly placed GCW could be used to pretreat water before it enters a domestic well. Commercial software was used to develop a model for GCWs installed at a Nebraska Superfund site to perform a feasibi...

California's oak savanna ecosystems may be subject to increased water stress as climate change fundamentally alters hydrological patterns. Previous studies of these water-limited ecosystems have focused on soil moisture as a regulator of transpiration (Miller et al., 2007), but have not yet addressed the role of groundwater in the system's overall...