Geographically isolated wetlands (GIWs), those surrounded by uplands, exchange materials, energy, and organisms with other elements in hydrological and habitat networks, contributing to landscape ...functions, such as flow generation, nutrient and sediment retention, and biodiversity support. GIWs constitute most of the wetlands in many North American landscapes, provide a disproportionately large fraction of wetland edges where many functions are enhanced, and form complexes with other water bodies to create spatial and temporal heterogeneity in the timing, flow paths, and magnitude of network connectivity. These attributes signal a critical role for GIWs in sustaining a portfolio of landscape functions, but legal protections remain weak despite preferential loss from many landscapes. GIWs lack persistent surface water connections, but this condition does not imply the absence of hydrological, biogeochemical, and biological exchanges with nearby and downstream waters. Although hydrological and biogeochemical connectivity is often episodic or slow (e.g., via groundwater), hydrologic continuity and limited evaporative solute enrichment suggest both flow generation and solute and sediment retention. Similarly, whereas biological connectivity usually requires overland dispersal, numerous organisms, including many rare or threatened species, use both GIWs and downstream waters at different times or life stages, suggesting that GIWs are critical elements of landscape habitat mosaics. Indeed, weaker hydrologic connectivity with downstream waters and constrained biological connectivity with other landscape elements are precisely what enhances some GIW functions and enables others. Based on analysis of wetland geography and synthesis of wetland functions, we argue that sustaining landscape functions requires conserving the entire continuum of wetland connectivity, including GIWs.
Wetlands provide numerous ecosystem services, from habitat provision to pollutant removal, floodwater storage, and microclimate regulation. Delivery of particular services relies on specific ...ecological functions, and thus to varying degree on wetland ecological condition, commonly quantified as departure from minimally impacted reference sites. Condition assessments are widely adopted as regulatory indicators of ecosystem function, and for some services (e.g., habitat) links between condition and function are often direct. For others, however, links are more tenuous, and using condition alone to enumerate ecosystem value (e.g., for compensatory mitigation) may underestimate important services. Hydrologic function affects many services cited in support of wetland protection both directly (floodwater retention, microclimate regulation) and indirectly (biogeochemical cycling, pollutant removal). We investigated links between condition and hydrologic function to test the hypothesis, embedded in regulatory assessment of wetland value, that condition predicts function. Condition was assessed using rapid and intensive approaches, including Florida's official wetland assessment tool, in 11 isolated forested wetlands in north Florida (USA) spanning a land use intensity gradient. Hydrologic function was assessed using hydrologic regime (mean, variance, and rates of change of water depth), and measurements of groundwater exchange and evapotranspiration (ET). Despite a wide range in condition, no systematic variation in hydrologic regime was observed; indeed reference sites spanned the full range of variation. In contrast, ET was affected by land use, with higher rates in intensive (agriculture and urban) landscapes in response to higher leaf area. ET determines latent heat exchange, which regulates microclimate, a valuable service in urban heat islands. Higher ET also indicates higher productivity and thus carbon cycling. Groundwater exchange regularly reversed flow direction at all sites in response to rainfall. This buffering effect on regional aquifer levels, an underappreciated service of isolated wetlands, was provided regardless of condition. Intensive landscapes may benefit most from the hydrologic services that wetlands provide because that is where certain services (floodwater storage, microclimate regulation) are realized. While the portfolio of wetland services clearly changes with disturbance, our results support a revised approach to wetland valuation that recognizes the services that accrue from sustained or enhanced functions in these "working wetlands."
All wetland ecosystems are controlled by water table and soil saturation dynamics, so any local-scale deviation in soil elevation and thus water table position represents variability in this primary ...control. Wetland microtopography is the structured variability in soil elevation and is typically categorized into a binary classification of local high points (hummocks) and local low points (hollows). Although the influence of microtopography on vegetation composition and biogeochemical processes in wetlands has received attention around the globe, its role in forested wetlands is still less understood. We studied relationships among microtopography and understory vegetation communities, tree biomass, and soil chemistry in 10 black ash (Fraxinus nigra Marshall) wetlands in northern Minnesota, USA. To do so, we combined a 1 cm resolution surface elevation model generated from terrestrial laser scanning (TLS) with colocated water table, vegetation, and soil measurements. We observed that microtopography was an important structural element across sites, where hummocks were loci of greater species richness; greater midstory and canopy basal area; and higher soil concentrations of chloride, phosphorus, and base cations. In contrast, hollows were associated with higher soil nitrate and sulfate concentrations. We also found that the effect of microtopography on vegetation and soils was greater at wetter sites than at drier sites, suggesting that the distance-to-mean water table is a primary determinant of wetland biogeochemistry. These findings highlight clear controls of microtopography on vegetation and soil distributions while also supporting the notion that microtopography arises from feedbacks that concentrate biomass, soil nutrients, and productivity on microsite highs, especially in otherwise wet conditions. We therefore conclude that microtopography is a fundamental organizing structure in black ash wetlands.
Recent U.S. Supreme Court rulings have limited federal protections for geographically isolated wetlands (GIWs) except where a “significant nexus” to a navigable water body is demonstrated. Geographic ...isolation does not imply GIWs are hydrologically disconnected; indeed, wetland‐groundwater interactions may yield important controls on regional hydrology. Differences in specific yield (Sy) between uplands and inundated GIWs drive differences in water level responses to precipitation and evapotranspiration, leading to frequent reversals in hydraulic gradients that cause GIWs to act as both groundwater sinks and sources. These reversals are predicted to buffer surficial aquifer dynamics and thus base flow delivery, a process we refer to as landscape hydrologic capacitance. To test this hypothesis, we connected models of soil moisture, upland water table, and wetland stage to simulate hydrology of a low‐relief landscape with GIWs, and explored the influences of total wetland area, individual wetland size, climate, and soil texture on water table and base flow variation. Increasing total wetland area and decreasing individual wetland size substantially decreased water table and base flow variation (e.g., reducing base flow standard deviation by as much as 50%). GIWs also decreased the frequency of extremely high and low water tables and base flow deliveries. For the same total wetland area, landscapes with fewer (i.e., larger) wetlands exhibited markedly lower hydrologic capacitance than those with more (i.e., smaller) wetlands, highlighting the importance of small GIWs to regional hydrology. Our results suggest that GIWs buffer dynamics of the surficial aquifer and stream base flow, providing an indirect but significant nexus to the regional hydrologic system.
Key Points
GIWs act as hydrologic sinks and sources in response to climate
GIWs effects on regional water table and base flow were modeled
GIWs buffer surficial aquifer variation, impacting stream base flow
Depressional wetlands of the extensive U.S. and Canadian Prairie Pothole Region afford numerous ecosystem processes that maintain healthy watershed functioning. However, these wetlands have been lost ...at a prodigious rate over past decades due to drainage for development, climate effects, and other causes. Options for management entities to protect the existing wetlands, and their functions, may focus on conserving wetlands based on spatial location vis-à-vis a floodplain or on size limitations (e.g., permitting smaller wetlands to be destroyed but not larger wetlands). Yet the effects of such management practices and the concomitant loss of depressional wetlands on watershed-scale hydrological, biogeochemical, and ecological functions are largely unknown. Using a hydrological model, we analyzed how different loss scenarios by wetland size and proximal location to the stream network affected watershed storage (i.e., inundation patterns and residence times), connectivity (i.e., streamflow contributing areas), and export (i.e., streamflow) in a large watershed in the Prairie Pothole Region of North Dakota, USA. Depressional wetlands store consequential amounts of precipitation and snowmelt. The loss of smaller depressional wetlands (< 3.0 ha) substantially decreased landscape-scale inundation heterogeneity, total inundated area, and hydrological residence times. Larger wetlands act as hydrologic “gatekeepers,” preventing surface runoff from reaching the stream network, and their modeled loss had a greater effect on streamflow due to changes in watershed connectivity and storage characteristics of larger wetlands. The wetland management scenario based on stream proximity (i.e., protecting wetlands 30 m and ~450 m from the stream) alone resulted in considerable landscape heterogeneity loss and decreased inundated area and residence times. With more snowmelt and precipitation available for runoff with wetland losses, contributing area increased across all loss scenarios. We additionally found that depressional wetlands attenuated peak flows; the probability of increased downstream flooding from wetland loss was also consistent across all loss scenarios. It is evident from this study that optimizing wetland management for one end goal (e.g., protection of large depressional wetlands for flood attenuation) over another (e.g., protecting of small depressional wetlands for biodiversity) may come at a cost for overall watershed hydrological, biogeochemical, and ecological resilience, functioning, and integrity.
Scientists and policymakers increasingly recognize that headwater regions contain numerous temporary streams that expand and contract in length, but accurately mapping and modeling dynamic stream ...networks remain a challenge. Flow intermittency sensors offer a relatively new approach to characterize wet stream length dynamics at high spatial and temporal resolutions. We installed 51 flow intermittency sensors at an average spacing of 40 m along the stream network of a high-relief, headwater catchment (33 ha) in the Valley and Ridge of southwest Virginia. The sensors recorded the presence or absence of water every 15 min for 10 months. Calculations of the wet network proportion from sensor data aligned with those from field measurements, confirming the efficacy of flow intermittency sensors. The fine temporal scale of the sensor data showed hysteresis in wet stream length: the wet network proportion was up to 50% greater on the rising limb of storm events than on the falling limb for dry antecedent conditions, at times with a delay of several hours between the maximum wet proportion and peak runoff at the catchment outlet. Less stream length hysteresis was evident for larger storms with higher event and antecedent precipitation that resulted in peak runoff > 15 mm/day. To assess spatial controls on stream wetting and drying, we performed a correlation analysis between flow duration at the sensor locations and common topographic metrics used in stream network modeling. Topography did not fully explain spatial variation in flow duration along the stream network. However, entrenched valleys had longer periods of flow on the rising limbs of events than unconfined reaches. In addition, large upslope contributing areas corresponded to higher flow duration on falling limbs. Future applications that explore the magnitude and drivers of stream length variability may provide further insights into solute and runoff generation processes in headwater regions.
The U.S. Mid-Atlantic coastal region is experiencing higher rates of SLR than the global average, especially in Hampton Roads, Virginia, where this acceleration is primarily driven by land ...subsidence. The adaptation plans for coastal flooding are generally developed at the municipal level, ignoring the broader spatial implications of flooding outside the individual administrative boundaries. Flood impact assessments at the watershed scale would provide a more holistic perspective on what is needed to synchronize the adaptation efforts between the neighboring administrative units. This paper evaluates flooding impacts from sea level rise (SLR) and storm surge among watersheds in Hampton Roads to identify those most at risk of coastal flooding over different time horizons. It also explores the implications of flooding on the municipalities, the land uses, and land covers throughout this region within the case study watershed. The 2% Annual Exceedance Probability (AEP) storm surge flood hazard data and NOAA’s intermediate SLR projections were used to develop flooding scenarios for 2030, 2060, and 2090 and delineate land areas at risk of combined flooding. Findings show that five out of 98 watersheds will substantially increase in inundation, with two intersecting multiple municipalities. They also indicate significant inundation of military, commercial, and industrial land uses and wetland land covers. Flooding will also impact residential land use in urban areas along the Elizabeth River and Hampton city, supporting the need for collaborative adaptation planning on hydrologically influenced spatial scales.
Forest management can play an important role in landscape‐scale water balances and thus regional water supply planning, necessitating improved quantification and prediction of forest water yield ...(i.e., rainfall minus evapotranspiration (ET)). We used high frequency soil moisture data to quantify soil ET and interception in 30 pine stands capturing regional variation in aridity, hydrogeology, and forest management. We evaluated typical forest rotation stages (i.e., clear cuts through mature stands), as well as stands restored to historical, lower biomass conditions. Our results supported the expectation that forest management can strongly influence local water yield. A simple model using leaf area index (LAI), hydrogeologic setting, and climate aridity (P:ET) explained nearly 80% of observed water yield variation. LAI emerged as the dominant forest structural control, influencing both soil ET and interception rates, with each unit decrease in LAI increasing water yield by nearly 10 cm. While other forest attributes (e.g., basal area, groundcover, species) were less important for predicting stand‐level water balances, aridity and hydrogeologic setting emerged as highly significant predictors of water yield. We further observed small and short‐lived effects of low‐intensity prescribed fires on soil ET and no discernible effect of pine species, suggesting that maintaining low density pine forests—regardless of species—is a viable management strategy for increasing water yield. Overall, our results illustrate the utility of soil moisture‐based methods for stand‐level water balances and provide useful models for predicting landscape water yield under a range of forest management and hydroclimatic settings now and in the future.
Plain Language Summary
Evapotranspiration is a major part of the water balance in coastal plain forests. In this work we use soil moisture measurements to calculate how much water forests use, and thus how much is left over for aquifer recharge and stream flow generation. We show that this “water yield” is accurately predicted using local climate, hydrogeology, and forest density (using leaf area index). Since water supply planning depends on how much water yield occurs from the landscape, our simple models to predict water yield can be used to understand how forest management decisions impact water supply, and inform how incentives to forest landowners can help meet regional water supply sustainability goals.
Key Points
We quantified soil evapotranspiration (ET) and interception using high frequency time series of soil moisture profiles
Water yield (precipitation minus ET) was well predicted by a model based on aridity, water table depth, and leaf area index
Forest management can meaningfully influence regional water supply planning in the US southeastern coastal plain
Historical land-use conversion and drainage may increase the risk of high intensity, soil-consuming fires in peatlands. Severe fires may degrade ecosystem resilience through changes in hydrology and ...by removing remnant seed and bud banks. Lower ecosystem resilience leaves peatlands highly susceptible to species invasions from aggressive colonizing vegetation post-disturbance. In this work we aimed to address the impacts of smoldering fires on the abundance of the noxious
Phragmites australis
in a large fire scar at the Great Dismal Swamp National Wildlife Refuge (VA and NC, USA). Primarily, we wanted to observe the extent of the relationship between the altered hydrology of post-fire conditions and
P. australis
occurrence. We did so by leveraging high resolution satellite imagery, random forest models, LiDAR data, and water table observations. Our results suggest that
P. australis
is aided by a hydrologic regime generated, in part, from the combined effects of drainage and deep smoldering fires. Our findings emphasize the importance of hydrologic management post-disturbance for limiting
P. australis
invasion. Further, we highlight the potential for feedbacks between deep peat-consuming burns and
P. australis
invasions and spread in degraded peatlands.
PDF
