climate_indices

Python library of indices useful for climate monitoring

This project contains Python implementations of various climate index algorithms which provide a geographical and temporal picture of the severity and duration of precipitation and temperature anomalies useful for climate monitoring and research.

The following indices are provided:

• SPI, Standardized Precipitation Index, utilizing both gamma and Pearson Type III distributions
• SPEI, Standardized Precipitation Evapotranspiration Index, utilizing both gamma and Pearson Type III distributions
• PET, Potential Evapotranspiration, utilizing either Thornthwaite or Hargreaves equations
• PNP, Percentage of Normal Precipitation
• PCI, Precipitation Concentration Index

This Python implementation of the above climate index algorithms is being developed with the following goals in mind:

• to provide an open source software package to compute a suite of climate indices commonly used for climate monitoring, with well documented code that is faithful to the relevant literature and which produces scientifically verifiable results
• to provide a central, open location for participation and collaboration for researchers, developers, and users of climate indices
• to facilitate standardization and consensus on best-of-breed climate index algorithms and corresponding compliant implementations in Python
• to provide transparency into the operational code used for climate monitoring activities at NCEI/NOAA, and consequent reproducibility of published datasets computed from this package
• to incorporate modern software engineering principles and scientific programming best practices

This is a developmental/forked version of code that was originally developed by NIDIS/NCEI/NOAA. See drought.gov.

• Documentation
• License
• Disclaimer

Migration Guide for v2.2.0

Breaking Change: Exception-Based Error Handling

Version 2.2.0 introduces a significant architectural improvement in error handling. The library now uses exception-based error handling instead of returning None tuples for error conditions.

What Changed

Before (v2.1.x and earlier):

# Old behavior - functions returned None tuples on failure
result = some_internal_function(data)
if result == (None, None, None, None):
    # Handle error case
    pass

After (v2.2.0+):

# New behavior - functions raise specific exceptions
try:
    result = some_internal_function(data)
except climate_indices.compute.InsufficientDataError as e:
    # Handle insufficient data case
    print(f"Not enough data: {e.non_zero_count} values found, {e.required_count} required")
except climate_indices.compute.PearsonFittingError as e:
    # Handle fitting failure case
    print(f"Fitting failed: {e}")

New Exception Hierarchy

• DistributionFittingError (base class)
  • InsufficientDataError - raised when there are too few non-zero values for statistical fitting
  • PearsonFittingError - raised when L-moments calculation fails for Pearson Type III distribution

Impact on Users

• Direct API users: No changes needed - the public SPI/SPEI functions handle exceptions internally
• Library integrators: If you were checking for None return values from internal functions, update to use try/catch blocks
• Benefits: More informative error messages, better debugging, and automatic fallback from Pearson to Gamma distribution when appropriate

Code Quality Improvements

Version 2.2.0 also addresses floating point comparison issues (python:S1244) throughout the codebase:

Floating Point Comparisons:

# ❌ OLD: Direct equality checks (unreliable)
if values == 0.0:
    handle_zero_case()

# ✅ NEW: Safe comparison using numpy.isclose()
if np.isclose(values, 0.0, atol=1e-8):
    handle_zero_case()

Benefits:

• Eliminates floating point precision issues in statistical parameter validation
• Improves test reliability and numerical robustness
• Follows scientific computing best practices for floating point arithmetic
• See docs/floating_point_best_practices.md for comprehensive guidelines

Citation

You can cite climate_indices in your projects and research papers via the BibTeX entry below.

@misc {climate_indices,
    author = "James Adams",
    title  = "climate_indices, an open source Python library providing reference implementations of commonly used climate indices",
    url    = "https://github.com/monocongo/climate_indices",
    month  = "may",
    year   = "2017--"
}