Hydrology

Hydrology Overview

• Definition of Hydrology:

  • Hydrology: The study of the distribution and movement of water along with its physical, chemical, and biological constituents dissolved and suspended in the near-surface environments of the earth.

Water Characteristics

• Most water is not suitable for drinking (non-potable) and is saline.

• Majority of freshwater is found in groundwater.

• The focus of the course will be on near-surface hydrology occurring on shorter timesteps.

Near-Surface Hydrologic Cycle

• Precipitation can follow several pathways once it falls on land:

  • Transpiration: Water loss from plants.

  • Biologically mediated evaporation: Evaporation facilitated by living organisms.

  • Evaporation: General loss of water from surfaces.

  • Interception and stemflow: Water that fails to reach soil and instead gathers on leaves and runs down stems.

  • Interflow: Shallow subsurface flow of water through soil.

  • Overland flow: Surface runoff of water.

  • Streamflow: Movement of water in stream channels.

Predictive Water Movement

• Water moves predictably across landscapes due to:

  • Gravity: Water flows downhill.

  • Pressure: Pressure gradients affect flow direction.

  • Molecular attraction: Cohesion and adhesion impacts distribution.

• As water flows, it undergoes changes in:

  • Chemical composition: Influenced by interactions with the environment.

• Water quantity (discharge, flow) and quality can be inferred from a few easily measurable parameters.

Important Background Terms

• Aquifer: A permeable geologic unit capable of storing and transmitting significant quantities of water.

• Aquitard: Less permeable units that are incapable of storing or transmitting significant amounts of water. Definitions are relative and dependent on conductivity.

Key Definitions

• Water table: The level at which fluid pressure in soil pores equals atmospheric pressure. Marks the transition between saturated and unsaturated zones.

• Capillary fringe: Water can be drawn above the water table in fine-grained media due to tension in capillary pores.

• Hydraulic conductivity: A measure of an aquifer's transmissivity and storativity, reflecting the volume of water that a permeable layer can store or expel.

  • Isotropic: Homogeneous properties—hydrological properties are equal in all directions.

  • Anisotropic: Properties change spatially, resulting in differential pressure.

Aquifer Types

• Confined aquifer: A permeable unit between two aquitards, with the potential for artesian flow if the confining layer is breached.

• Unconfined aquifer: Water table aquifers, where the potentiometric surface is equivalent to the water table.

Water Movement Terminology

• Hydraulic head: The level of water at a specific point in an aquifer, where water moves from areas of high to low head.

• Recharge: Water moving downward in a vertical profile into an aquifer.

• Discharge: Water moving upward in a vertical profile, or out of the aquifer.

• Lateral flow: Water moving horizontally in the direction of groundwater flow.

Darcy's Law

• Formula: Q = -KIA.

  • Q: Discharge or flow.

  • K: Hydraulic conductivity.

  • I: Hydraulic gradient, given as I = rac{dh}{dl} where:

  • dh = change in hydraulic head.

  • dl = length of the flow path.

• This equation serves as the basis for understanding flow through porous media.

Wetland Hydrology Characteristics

• Wetlands contribute to aquifers via recharge and gain water at points of discharge. The hydrology is often variable in space and time.

• Organic layers may act as confining layers, complicating hydrologic flow paths:

  • Gaining or effluent wetland: Receives water.

  • Losing or influent wetland: Loses water.

Hydroperiod

• Definition: The fluctuation of water level in wetlands over time.

• Function: Depicts wetland water level relative to the surface as a function of time.

• Importance: Hydroperiods are a complex interaction of landscape, water source, and climate.

Components of Hydroperiod

• Frequency: Average number of times a wetland is inundated during a specific period.

• Duration: Length of time a wetland remains inundated during flooding events.

• Intensity: The impact of water movement on the substrate during hydrologic extremes.

• Source: The dominant hydrologic source, varying in time, affects water chemistry and flux; sources can include precipitation, surface water, and groundwater.

Ecological Implications

• Intensity: Nonlinear ecological relationships are shaped by stochastic hydrologic events. These events can:

  • Remove organic soils.

  • Deposit wrack, thus impacting substrate.

• Source: Governs water chemistry and potential anthropogenic impacts such as pollutants:

  • Understanding contributing source areas is critical for landscape management and conservation efforts.

Hydroperiod Influences

• Governed by:

  • Landscape characteristics (e.g., topography, conductivity).

  • Climate and weather (seasonality, variability).

  • Local parameters (e.g., soil stratigraphy, size, and volume relationships).

Watershed Concept

• Watershed: An area of land defined by topographic features that captures rain and snow, draining into lakes, streams, or wetlands. Characteristics reflect a combination of watershed hydrology, land cover, and chemical processes.

Importance of Wetlands

• Wetlands are integral hydrologic features in water flow systems, sharing similarities with streams and lakes rather than terrestrial environments. Viewing them as hydrologic entities allows for:

  • Characterization of dominant abiotic constraints.

  • Integration into broader landscape research contexts.

Predictive Modeling of Wetland Conditions

• Investigating whether local wetland conditions can be predicted using landscape criteria involves:

  • Describing and relating patterns of occurrence.

  • Application of models to identify threats to wetland formations and test hypotheses for adaptive land management.

Hydrogeologic Classification

• Hydrogeologic method focuses on wetland type settings based on physiography and climate

  • Examines regional hydrologic landforms and their relation to local groundwater flow systems.

• Hydrogeomorphic Setting (HGM): A means of grouping wetlands based on geomorphic setting, water source, and hydrodynamics.

Data Collection and Analysis

• For a study of 30 NY fens:

  • Landscape data was collected along with water chemistry and vegetation sampling.

  • Data analyzed included dominant cations, anions, pH, conductivity, and ecological community types.

  • Relationships between landscape properties and local conditions were statistically evaluated using:

  • Principal Component Analysis (PCA), Canonical Correspondence Analysis (CCA), stepwise regression, logistic regression, and ANOVA.

Summary of Findings

• Fens are non-randomly distributed and their occurrence is related to specific abiotic properties.

• Hydrogeologic settings (HGS) correlated with fen local conditions, including pore-water ionic composition and vegetation types.

• The scarcity of fens is associated with the rarity of the HGS they occupy.

Modeling and Data Integration

• Employing landscape data to create models predicting fen occurrence involves:

  • Integrating statistical data and GIS to visualize and assess predictions.

  • Refining HGS definitions based on significant landscape predictors and existing conditions.

Concerns and Considerations in Modeling

• The predictive model’s utility and geospatial efficiency must be aligned with current knowledge of fen distribution.

• Statistical metrics indicate how well models fit known distributions vs. predictions made by the model.

Future Directions and Research

• Identifying predictors of fen occurrence and validating through field assessments to address undocumented populations and density predictions.

• Importance of continuous data refinement and validation in conservation efforts, highlighted by examples from Tompkins County regarding specific fen occurrences and their habitat conditions.