Why does hydrology matter for ecological modelling?#

This page provides a high-level overview of what hydrological modelling is is, what it affects at ecosystem-scale and why we care about it.

Factors affecting hydrology#

The hydrology on an ecosystem is mostly determined by macro- and microclimate, topography, soil and geology, vegetation type and structure, and human activities such as land use change.

Climate#

  • Precipitation: The amount, timing, and type of precipitation (rain, snow, etc.) directly influence water availability and flow patterns.

  • Temperature: Temperature affects evaporation rates and the amount of water that plants and soil can retain.

  • Evapotranspiration: The combined process of evaporation and transpiration affects water loss from the surface to the atmosphere.

Topography#

  • Slope: The steepness of the terrain affects how quickly water runs off the surface and infiltrates the soil.

  • Elevation: Higher elevations tend to receive more precipitation, which impacts water flow and distribution.

  • Landforms: Natural features such as mountains, valleys, and plains influence the direction and speed of water movement.

Soil and Geology#

  • Soil type: Different soil types (sand, clay, loam) have varying capacities to retain and filter water.

  • Permeability: The ability of soil and rock to absorb and transmit water affects groundwater recharge and surface runoff.

  • Rock formations: The composition and structure of underlying rock formations influence groundwater storage and flow.

Vegetation#

  • Plant types: Different species of plants have varying water needs and capacities to absorb and transpire water.

  • Density: Dense vegetation can slow down surface runoff, enhance infiltration, and reduce soil erosion.

  • Root systems: Deep and extensive root systems can increase soil stability and improve water infiltration and retention.

Human activities#

  • Deforestation: Removing trees and vegetation decreases transpiration, increases runoff, and contributes to soil erosion.

  • Agriculture: Irrigation, crop type, and farming practices influence water usage, runoff, and infiltration.

  • Water management practices: Dams, reservoirs, and water diversion projects impact the natural distribution and availability of water.

  • Urbanization: Development and construction alter natural water flow, increase surface runoff, and reduce infiltration.

Key hydrological variables and processes#

Local water balance#

The local water balance is, similar to the microclimate, driven by large scale hydrological patterns and affects the living conditions for organisms at the local scale. The local water balance can be represented by the equation:

\( \Delta S = P − ET − R \) (water_balance)

where \(\Delta S\) represents the net change in water stored in the system, \(P\) stands for precipitation, the total water input, \(ET\) is the evapotranspiration with accounts for water loss to the atmosphere, and runoff \(R\) represents water that flows out of the system.

The water balance include above and below ground processes that together describe the flow of water through the system:

Above ground#

  • Precipitation: This includes all forms of water input from the atmosphere, such as rain, snow, sleet, and hail. The quantity and frequency of precipitation directly affect the amount of water entering the local system.

  • Intercept: Some precipitation is caught and held by plant leaves, branches, and stems before it reaches the ground. This intercepted water can either evaporate back into the atmosphere or eventually drip to the soil.

  • Evapotranspiration: Evaporation describes the process where water is converted from liquid to vapor and released into the atmosphere from surfaces like soil, water bodies, and vegetation. Transpiration refers to the release of water vapor from plants into the atmosphere through small openings in their leaves called stomata. Combined, these processes account for water loss from the surface and vegetation to the atmosphere.

  • Surface runoff: The portion of precipitation that flows over the land surface toward streams, rivers, and other water bodies. Runoff is influenced by factors such as land slope, soil saturation, and land use. High runoff can lead to erosion and nutrient loss.

Below ground#

  • Infiltration: The process where water on the ground surface enters the soil. Infiltration rates depend on soil type, soil moisture, land cover, and land management practices. Enhanced infiltration reduces surface runoff and recharges groundwater.

  • Bypass flow: Some of the water that infiltarted into the soil bypasses the soil matrix and drains directly to the groundwater, for example through soil pipes.

  • Groundwater flow: Water that infiltrates the soil can percolate down to recharge groundwater aquifers. Groundwater flow contributes to maintaining base flow in rivers and streams during dry periods. The rate of groundwater flow is determined by the permeability of subsurface materials and the hydraulic gradient.

  • Storage changes: Water storage can occur in various forms such as soil moisture, surface water bodies (lakes, reservoirs), and groundwater. Changes in storage are influenced by the balance between inputs (precipitation) and outputs (evapotranspiration, runoff, groundwater flow).

  • Root water uptake: A fraction of soil water is extracted by plants. On average, the amount of water extracted from soil is approximately the same as transpiration rates.

Catchment scale water balance#

At catchment scale, horizontal movement and distribution is considered. This includes above and below ground flow of water.

  • Surface runoff and surface water flow: Runoff represents the portion of precipitation that flows over the land surface and into streams, rivers, and lakes. Surface water flow dynamics are influenced by topography, soil characteristics, land cover, and human activities.

  • Groundwater Flow and Storage: Water that infiltrates the soil can move horizontally through aquifers, contributing to groundwater storage. Horizontal groundwater flow interacts with surface water bodies, influencing base flow in rivers and streams.

Methods for hydrology science#

This section gives a broad overview over common methods and models in hydrology research. A review on the progress and future of hydrological modelling is given by Singh (2018).

Data collection#

Effective hydrology modelling relies on accurate data collection, ideally from a combination of sources:

  • Meteorological stations: Meteorological stations provide detailed, real-time data on precipitation, humidity, and sometime soil moisture for locations across an area. Data collection instruments include rain gauges, stream gauges, and soil moisture sensors.

  • Remote sensing: Satellite imagery and aerial surveys provide spatial and temporal data on precipitation patterns, land cover, and surface water dynamics.

  • In-situ measurements: Ground-based sensors complement provide high-resolution data on specific local conditions.

  • Reanalysis: Reanalysis data sets such as ERA5 represent a combination of observations and modelling which can complement measurements by filling gaps in spatial cover and timeseries.

Hydrology modelling#

Hydrology models employ diverse methodologies to simulate the movement and distribution of water within natural systems.

Models#

  • Rainfall-Runoff models simulate the transformation of precipitation into runoff, accounting for factors like soil infiltration, evapotranspiration, and land cover characteristics.

  • Groundwater flow models utilize principles of fluid mechanics to simulate the movement of water through subsurface aquifers, considering geologic properties, recharge rates, and extraction activities.

  • Integrated watershed models combine surface water and groundwater components to provide a comprehensive view of water resources within a watershed, facilitating watershed management and land use planning decisions.

Tools and techniques#

  • Modeling tools include Geographic Information Systems (GIS), statistical software, and specialized hydrological modeling platforms.

  • Techniques such as data assimilation, uncertainty analysis, and scenario testing help improve model accuracy and reliability.

Applications#

  • Hydrological models are used for flood forecasting, water resource management, drought assessment, and climate change impact studies.

  • They support decision-making in agriculture, urban planning, and environmental conservation by predicting water availability and mitigating risks.

Challenges and limitations#

  • Data scarcity and quality: Limited availability of hydrological data in certain regions hinders accurate modeling and forecasting. Data quality issues, such as measurement errors or inconsistencies, can affect model reliability and decision outcomes.

  • Complexity of natural systems: Natural variability and non-linear interactions in hydrological processes pose challenges for model representation and prediction. Incorporating spatial heterogeneity and temporal dynamics requires advanced modeling techniques and computational resources.

  • Uncertainty and assumptions: Uncertainty in model parameters, input data, and future climate projections affects the reliability of hydrological predictions. Assumptions made in model development and simplifications of complex processes can introduce biases and uncertainties.

  • Human and environmental disturbance: Anthropogenic activities such as land use change, urbanization, and water infrastructure development alter hydrological patterns. Environmental changes, including deforestation, climate variability, and extreme weather events, challenge the resilience of hydrological systems.

  • Interdisciplinary integration: Effective integration of hydrological modeling with other disciplines such as ecology, economics, and policy-making requires interdisciplinary collaboration. Addressing societal needs and sustainable water management goals requires holistic approaches that consider socio-economic and environmental factors.