Ali A. Ameli

Integrating geographically isolated wetlands into land management decisions

Wetlands across the globe provide extensive ecosystem services. However, many wetlands - especially those surrounded by uplands, often referred to as geographically isolated wetlands (GIWs) - remain poorly protected. Protection and restoration of wetlands frequently requires information on their hydrologic connectivity to other surface waters, and their cumulative watershed-scale effects. The integration of measurements and models can supply this information. However, the types of measurements and models that should be integrated are dependent on management questions and information compatibility. We summarize the importance of GIWs in watersheds and discuss what wetland connectivity means in both science and management contexts. We then describe the latest tools available to quantify GIW connectivity and explore crucial next steps to enhancing and integrating such tools. These advancements will ensure that appropriate tools are used in GIW decision making and maintaining the important ecosystem services that these wetlands support.

Are all runoff processes the same? Numerical experiments comparing a Darcy‐Richards solver to an overland flow‐based approach for subsurface storm runoff simulation

Hillslope runoff theory is based largely on the differentiation between infiltration excess overland flow, saturation excess overland flow, and subsurface stormflow. Here we explore to what extent a 2-D friction-based overland flow model is useful for predicting hillslope-scale subsurface stormflow, posited here as phenomenologically the same as infiltration excess at depth. We compare our results to a 3-D variably saturated Darcy-Richards subsurface solver for individual rainfall runoff events. We use field data from the well-studied Panola Mountain Experimental hillslope in Georgia USA. Our results show that the two models are largely indistinguishable in terms of their ability to simulate the hillslope hydrograph magnitude and timing for a range of slopes and rainfall depths. Furthermore, we find that the descriptive ability of the overland flow model is comparable to the variably saturated subsurface flow model in terms of its ability to represent the spatial distribution of subsurface stormflow and infiltration across the soil-bedrock interface. More importantly, these results imply that the physics of infiltration excess subsurface stormflow at the soil-bedrock interface is similar to infiltration excess overland flow at the soil surface, in terms of detention storage, loss along the lower boundary, and threshold-like activation at the larger hillslope scale. Given the phenomenological similarity of overland flow and subsurface stormflow and the fact that overland flow model predictions are considerably faster to run (particularly as slope and rainfall depth increase), these findings imply that new forms of hillslope-scale subsurface storm runoff predictions may be possible with the knowledge of bedrock permeability and limited soil information. Finally, this work suggests that the role of soil mantle vis-a-vis subsurface stormflow is mainly as a filter that delays the development of patches of saturation along the bedrock surface. Our model results show that simple realizations of soil based on a few soil depth measurements can possibly be enough to characterize this filtering effect.

Primary weathering rates, water transit times, and concentration-discharge relations: A theoretical analysis for the critical zone

The permeability architecture of the critical zone exerts a major influence on the hydrogeochemistry of the critical zone. Water flow path dynamics drive the spatiotemporal pattern of geochemical evolution and resulting streamflow concentration-discharge (C-Q) relation, but these flow paths are complex and difficult to map quantitatively. Here we couple a new integrated flow and particle tracking transport model with a general reversible Transition State Theory style dissolution rate law to explore theoretically how C-Q relations and concentration in the critical zone respond to decline in saturated hydraulic conductivity (Ks) with soil depth. We do this for a range of flow rates and mineral reaction kinetics. Our results show that for minerals with a high ratio of equilibrium concentration ( Ceq) to intrinsic weathering rate ( Rmax), vertical heterogeneity in Ks enhances the gradient of weathering-derived solute concentration in the critical zone and strengthens the inverse stream C-Q relation. As CeqRmax decreases, the spatial distribution of concentration in the critical zone becomes more uniform for a wide range of flow rates, and stream C-Q relation approaches chemostatic behavior, regardless of the degree of vertical heterogeneity in Ks. These findings suggest that the transport-controlled mechanisms in the hillslope can lead to chemostatic C-Q relations in the stream while the hillslope surface reaction-controlled mechanisms are associated with an inverse stream C-Q relation. In addition, as CeqRmax decreases, the concentration in the critical zone and stream become less dependent on groundwater age (or transit time).

Semianalytical series solutions for three‐dimensional groundwater‐surface water interaction

A semianalytical grid-free series solution method is presented for modeling 3-D steady state free boundary groundwater-surface water exchange in geometrically complex stratified aquifers. Continuous solutions for pressure in the subsurface are determined semianalytically, as is the location of the water table surface. Mass balance is satisfied exactly over the entire domain except along boundaries and interfaces between layers, where errors are shown to be acceptable. The solutions are derived and demonstrated on a number of test cases and the errors are assessed and discussed. This accurate and grid-free scheme can also be a helpful tool for providing insight into lake-aquifer and stream-aquifer interactions. Here it is used to assess the impact of lake sediment geometry and properties on lake-aquifer interactions. Various combinations of lake sediment are considered and the appropriateness of the Dupuit-Forchheimer approximation for simulating lake bottom flux distribution is investigated. In addition, the method is applied to a test problem of surface seepage flows from a complex topographic surface; this test case demonstrated the methods efficacy for simulating physically realistic domains.

Estimating rates of wetland loss using power-law functions

Estimates of rates of wetland loss are important for understanding whether wetland policies meet their objectives. In Alberta, a no-net-area loss interim wetland policy was introduced in 1993. We tested the effectiveness of this interim wetland policy. A historical wetland inventory was established by generating a wetland inventory using digital topographic analysis and calculating a wetland-area vs. wetland-frequency power-law function from these data. Permanent wetland loss (topographic depression no longer exists) was calculated as the deviation from the historical wetland-inventory power-law function (representing the pre-settlement wetland inventory) and was estimated at 32.8% in number and 2.10% in area, with uncertainty estimates well below 1%. Temporary wetland loss (topographic depression remains on the landscape) was calculated as the difference between the historical wetland inventory and a time series of contemporary wetland inventories mapped from aerial photographs. Results indicate that as of 1993, 49.4% of the number of wetlands were temporarily lost (56.6% of wetland area), which increased in 2011 to 56.8% (68.0% of wetland area), with uncertainty estimates well below 1%. From 1993 to 2011, we estimated a rate of loss of 0.63% in wetland area/year. Wetland loss continued despite the introduction of the no-net-area-loss policy in 1993.

Groundwater Subsidy From Headwaters to Their Parent Water Watershed: A Combined Field‐Modeling Approach

Headwater groundwater subsidy, defined here as out of catchment groundwater flow contribution from a headwater catchment to its larger parent watershed (i.e. higher-order stream), can influence the water quality and quantity of regional water resources. But the integrated flow and transport modeling approaches currently being implemented to quantify this subsidy are limited by an absence of critical field observations, such as water table dynamics and groundwater age that are required to test such models. Here, we couple tracer (and hydrometric) observations from the wellstudied 4.5 ha M8 headwater catchment in the Maimai experimental watershed with a new semianalytical free-surface integrated flow and transport model. Our main research goal is to quantify the magnitude, age and flowpaths of the headwaters groundwater subsidies at the Maimai experimental watershed. Additionally, we explore through virtual experiments, the effects of watershed slope, watershed active thickness, and recharge rate on the age, flowpath and magnitude of out of catchment headwaters groundwater subsidies versus within-catchment (or local) groundwater flow contributions. Our results show that more than 50% of groundwater recharged in the Maimai headwaters subsidizes their parent watershed. The relative proportion of headwaters groundwater subsidies is inversely proportional to recharge rate and/or directly proportional to slope angle. Our results also show that the age of the headwaters groundwater subsidies is more than 500 years, almost 9 times older than the age of within-catchment groundwater flow contributions. These findings highlight the need to consider headwaters groundwater subsidies in groundwater management area considerations.