Pfafstetter Hydrological Codes 
    as cited in Verdin and Verdin (1999) <doi: 10.1016/S0022-1694(99)00011-6> 
    are decoded for upstream or downstream queries.

Compute duration curves of daily flow series, both real and modeled, to be compared through indexes of flow duration curves. The package functions include comparative plots and goodness of fit tests. Flow duration curve indexes are based on: Yilmaz et al., (2008) <DOI:10.1029/2007WR006716>.

Provides a unified, extensible interface to discover
    hydrologic stations and download daily time series (e.g., water
    discharge, water level, water temperature, and several other water
    quality parameter) from national and regional public APIs. Includes a
    provider registry, S3 generics 'stations' and 'timeseries', licensing
    metadata, date-range and 'complete history' modes, rate limiting and
    retries, optional authentication via environment variables, tidy
    outputs, UTF-8 to ASCII transliteration, and WGS84 coordinates.
    Designed for reproducible workflows and straightforward addition of
    new providers.
	Background and use cases are described in Farber et al. (2025)
    <doi:10.5194/essd-17-4613-2025> and Farber et al. (2023)
    <doi:10.57757/IUGG23-2838>.

Computes scalar performance metrics and goodness-of-fit summaries
    for comparing simulated and observed hydrological or regression values.
    Provides error metrics, percentage bias, Nash-Sutcliffe efficiency,
    Kling-Gupta efficiency variants, correlation measures, agreement indices,
    and comparison tables via gof() and gof_compare(). Metric definitions are
    registry-governed with shared validation, provenance-aware wrappers, and
    explicit handling of undefined or degenerate cases. Methods include Nash
    and Sutcliffe (1970) <doi:10.1016/0022-1694(70)90255-6>, Gupta et al.
    (2009) <doi:10.1016/j.jhydrol.2009.08.003>, Kling et al. (2012)
    <doi:10.1016/j.jhydrol.2012.01.011>, and Willmott et al. (2012)
    <doi:10.1002/joc.2419>.