r-hydroeval

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>.