Sankey Diagram
What is it?
Sankey diagrams provide a powerful visual representation of the water balance in SWAT outputs, illustrating the flow magnitudes and interconnections between key hydrologic processes. By extending this approach to transient (multi-year) visualizations, users can better understand the temporal evolution of watershed processes and assess how watershed health and hydrologic balance shift over time.How to use it:
Purpose of the Transient Sankey Visualization
The transient SWAT Water Balance Sankey diagrams are designed to help modelers and watershed managers visualize:• The relative importance of various hydrologic processes (e.g., surface flow, baseflow, evapotranspiration, recharge).
• The connectivity and interrelationships among different water balance components.
• The temporal evolution of watershed health and resilience, represented by changes in the balance between inputs, storage, and outputs.
Creating and Interpreting Transient Sankey Diagrams
Each Sankey diagram represents a specific time slice (e.g., annual, biennial, or quinquennial) of the SWAT simulation. Users can generate transient Sankey diagrams using SwaNET result visualization tool. The diagrams use flow width to indicate process magnitude, allowing direct visual comparison across time steps.Interpretation guidelines:
• A balanced flow distribution indicates a hydrologically stable and resilient system.
• Increasing dominance of surface runoff may suggest reduced infiltration or degraded land cover.
• Declining baseflow or shallow recharge may signal declining groundwater health.
• Shifts in evapotranspiration and soil storage dynamics can reflect changes in vegetation cover or climate variability.
Example: SWAT Water Balance Sankey Diagram
The figure below presents an example of a Sankey diagram generated for Kalamazoo River Watershed, summarizing water balance components for the period 2001–2012. The thickness of each flow path represents the relative magnitude of the process contribution.
Multi-Year or Transient Comparisons
To analyze changes over time, Sankey diagrams can be generated for multiple intervals (e.g., yearly or every five years). Comparing diagrams across periods reveals the dynamics of hydrological connectivity and allows identification of shifts in dominant processes due to land use change, management interventions, or climate variation.Recommended comparison intervals:
• Annual – for high-resolution process variability.
• 2-year – to smooth short-term variability.
• 5-year – for assessing long-term watershed trends.
Assessing Watershed Health Using Sankey Diagrams
Watershed health can be inferred from the relative stability and proportionality of key water balance components:• High recharge and baseflow ratios generally indicate good subsurface water connectivity and infiltration capacity.
• Excessive surface runoff or stream discharge dominance may indicate soil compaction, deforestation, or impervious surfaces.
• Balanced evapotranspiration and soil moisture reflect healthy vegetation dynamics and climate equilibrium.
Tracking these indicators across time-sliced Sankey diagrams provides a visual and quantitative measure of watershed response.