By defining a closure operator on effective equivalence relations in a regular category C, it is possible to establish a bijec- tive correspondence between these closure operators and the regu- lar epireflective subcategories L of C. When C is an exact Goursat category this correspondence restricts to a bijection between the Birkhoff closure operators on effective equivalence relations and the Birkhoff subcategories of C. In this case it is possible to pro- vide an explicit description of the closure, and to characterise the congruence distributive Goursat categories.
Borceux, F., Gran, M., & Mantovani, S. (2007). On closure operators and reflections in Goursat categories. Universita degli Studi di Trieste. Istituto di Matematica. Rendiconti, 39, 87-104. https://hdl.handle.net/2078.5/73370 (Original work published 2007)