Introduction

SPHY (Spatial Processes in Hydrology) is a conceptual, spatially distributed (raster-based) “leaky-bucket” type water balance model. It integrates dominant hydrological processes, including:

  • Rainfall–runoff
  • Lake/reservoir outflow
  • Cryospheric processes (snow, ice, glaciers)
  • Evapotranspiration
  • Soil hydrological processes

SPHY has evolved over time by incorporating the best components from well-established simulation models, such as SRM, VIC, HydroS, SWAT, PCR-GLOBWB, SWAP, and HimSim. It captures relevant terrestrial hydrological processes across various spatial scales (local, regional, and global) and is adaptable to different land-use changes, extreme weather conditions, and climate scenarios.

An overview of the SPHY model concepts is illustrated in the figure below:

SPHY Model Concept
Conceptual overview of the SPHY model.

Key Features

Mass conservation is the core principle of the SPHY model. The dominant hydrological processes are defined by physical equations and parameters. The model employs a sub-grid variability approach to accurately represent fine-scale processes. Each model cell can be:

  • Glacier-free
  • Partially glacierized
  • Completely covered by glaciers

Non-glacierized cells can include different land-use types. Sub-grid variability is primarily influenced by fractional vegetation coverage, impacting processes such as interception, effective precipitation, and potential evapotranspiration.

The model divides the soil/land column into:

  • Two upper soil stores
  • A third groundwater store

Each store has corresponding drainage components: surface runoff, lateral flow, and base flow. Glacier melt contributes to river discharge via two pathways:

  • Slow component: Percolation into the groundwater reservoir, eventually becoming base flow.
  • Fast component: Direct runoff.

Model Capabilities

SPHY simulates the dynamic behavior of glaciers by integrating key processes such as accumulation, ablation, and ice mass transfer from accumulation to ablation zones. If a glacier loses mass, ice from the ablation zone is redistributed according to volume ice redistribution principles.

Additional modules include:

  • Lake module: Tracks lake levels and storage over time, using an advanced routing scheme to direct water flow from lakes to downstream regions.
  • Erosion module: Calculates soil erosion due to raindrop impact, overland flow, and river flow.

There is no single best hydrological model for all applications; the optimal choice depends on project goals. However, SPHY stands out due to its versatility and wide range of functionalities, including:

  • Spatial scale: SPHY can be applied at various spatial scales, from small farms to large regional and global applications. Users can analyze hydrological variability across different resolutions (e.g., 50m for glaciers, 1000m for general hydrology).
  • Temporal scale: The model supports sub-daily to yearly time steps, allowing flexibility based on data availability and process dynamics.
  • Adaptability: SPHY can be customized for different climatic conditions. Users can deactivate non-relevant processes (e.g., glacier melt in tropical regions) to streamline simulations.
  • Data requirements: SPHY can operate with minimal data or integrate extensive datasets, such as hydrological measurements, cryospheric data, crop coefficients, and lake/reservoir information.
  • User-friendliness: Designed for accessibility, SPHY allows input via static values, time series, or spatial raster data. Outputs include detailed spatial maps and time-series data tailored to user preferences.

FutureWater Applications

FutureWater utilizes SPHY for a variety of hydrological studies and applications:

  • Hydrological change analysis: Assessing past and future hydrological regimes in projects such as IWRM-Bhagirathi and IWRM-Tajikistan.
  • Basin management: Supporting strategic planning for major river basins, such as the Ganga River Basin.
  • Irrigation management: Providing farm-specific irrigation advice, such as in Romania and Angola.
  • Flow forecasting: Used in operational flow prediction in Chile.
  • Land degradation and restoration: Applied in landscape management projects such as Madagascar.
  • Hydropower assessments: Supporting hydropower development in Georgia, Indonesia, Kenya, and other locations.
  • Flood risk assessment: Evaluating extreme weather impacts in projects such as SYSTEM-RISK and IMPREX.

More Information

For further details, visit sphymodel.com.

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