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USGS2025

Reviews and syntheses: Variable inundation across Earth's terrestrial ecosystems

The structure, function, and dynamics of Earth's terrestrial ecosystems are profoundly influenced by how often (frequency) and how long (duration) they are inundated with water. A diverse array of natural and human-engineered systems experience temporally variable inundation whereby they fluctuate between inundated and non-inundated states. Variable inundation spans extreme events to predictable sub-daily cycles. Variably inundated ecosystems (VIEs) include hillslopes, non-perennial streams, wetlands, floodplains, temporary ponds, tidal systems, storm-impacted coastal zones, and human-engineered systems. VIEs are diverse in terms of inundation regimes, water chemistry and flow velocity, soil and sediment properties, vegetation, and many other properties. The spatial and temporal scales of variable inundation are vast, ranging from sub-meter to whole landscapes and from sub-hourly to multi-decadal. The broad range of system types and scales makes it challenging to predict the hydrology, biogeochemistry, ecology, and physical evolution of VIEs. Despite all experiencing the loss and gain of an overlying water column, VIEs are rarely considered together in conceptual, theoretical, modeling, or measurement frameworks and approaches. Studying VIEs together has the potential to generate mechanistic understanding that is transferable across a much broader range of environmental conditions, relative to knowledge generated by studying any one VIE type. We postulate that enhanced transferability will be important for predicting changes in VIE function in response to global change. Here we aim to catalyze cross-VIE science that studies drivers and impacts of variable inundation across Earth's VIEs. To this end, we complement expert mini-reviews of eight major VIE systems with overviews of VIE-relevant methods and challenges associated with scale. We conclude with perspectives on how cross-VIE science can derive transferable understanding via unifying conceptual models in which the impacts of variable inundation are studied across multi-dimensional environmental space.

Biogeosciences↗

USGS2004

Conceptualization and simulation of the Edwards aquifer, San Antonio region, Texas

A new numerical ground-water-flow model (Edwards aquifer model) that incorporates important components of the latest information and plausible conceptualization of the Edwards aquifer was developed. The model includes both the San Antonio and Barton Springs segments of the Edwards aquifer in the San Antonio region, Texas, and was calibrated for steady-state (1939–46) and transient (1947–2000) conditions, excluding Travis County. Transient simulations were conducted using monthly recharge and pumpage (withdrawal) data. The model incorporates conduits simulated as continuously connected (other than being separated in eastern Uvalde and southwestern Medina Counties), one-cell-wide (1,320 feet) zones with very large hydraulic-conductivity values (as much as 300,000 feet per day). The locations of the conduits were based on a number of factors, including major potentiometric-surface troughs in the aquifer, the presence of sinking streams, geochemical information, and geologic structures (for example, faults and grabens). The simulated directions of flow in the Edwards aquifer model are most strongly influenced by the presence of simulated conduits and barrier faults. The simulated flow in the Edwards aquifer is influenced by the locations of the simulated conduits, which tend to facilitate flow. The simulated subregional flow directions generally are toward the nearest conduit and subsequently along the conduits from the recharge zone into the confined zone and toward the major springs. Structures simulated in the Edwards aquifer model influencing groundwater flow that tend to restrict flow are barrier faults. The influence of simulated barrier faults on flow directions is most evident in northern Medina County. A water budget is an accounting of inflow to, outflow from, and storage change in the aquifer. For the Edwards aquifer model steady-state simulation, recharge (from seepage losses from streams and infiltration of rainfall) accounts for 93.5 percent of the sources of water to the Edwards aquifer, and inflow through the northern and northwestern model boundaries contributes 6.5 percent. The largest discharges are springflow (73.7 percent) and ground-water withdrawals by wells (25.7 percent). The principal source of water to the Edwards aquifer for the Edwards aquifer model transient simulation was recharge, constituting about 60 percent of the sources of water (excluding change in storage) to the Edwards aquifer during 1956, a drought period, and about 97 percent of the sources (excluding change in storage) during 1975, a period of above-normal rainfall and recharge. The principal discharges from the Edwards aquifer for the transient simulation were springflow and withdrawals by wells. During 1956, representing drought conditions, the change in storage (net water released from storage) was much greater than recharge, accounting for 75.9 percent of the total flow compared to 14.5 percent for recharge. Conversely, during 1975, representing above-normal rainfall and recharge conditions, recharge constituted 79.9 percent of the total flow, compared to 7.1 percent for the change in storage (net water added to storage). A series of sensitivity tests was made to ascertain how the model results were affected by variations greater than and less than the calibrated values of input data. Simulated hydraulic heads in the Edwards aquifer model were most sensitive to recharge, withdrawals, hydraulic conductivity of the conduit segments, and specific yield and were comparatively insensitive to spring-orifice conductance, northern boundary inflow, and specific storage. Simulated springflow in the Edwards aquifer model was most sensitive to recharge, withdrawals, hydraulic conductivity of the conduit segments, specific yield, and increases in northern boundary inflow and was comparatively insensitive to spring-orifice conductance and specific storage.

Texas↗