Rainfall-induced　landslides　are　widely　distributed　in　many　countries.　Rainfall　impacts　the　hydraulic　dynamics　of　groundwater　and,　therefore,　slope　stability.　We　derive　an　analytical　solution　of　slope　stability　considering　effective　rainfall　based　on　the　Richards　equation.　We　define　effective　rainfall　as　the　total　volume　of　rainfall　stored　within　a　given　range　of　the　unsaturated　zone　during　rainfall　events.　The　slope　stability　at　the　depth　of　interest　is　provided　as　a　function　of　effective　rainfall.　The　validity　of　analytical　solutions　of　system　states　related　to　effective　rainfall,　for　infinite　slopes　of　a　granite　residual　soil,　is　verified　by　comparing　them　with　the　corresponding　numerical　solutions.　Additionally,　three　approaches　to　global　sensitivity　analysis　are　used　to　compute　the　sensitivity　of　the　slope　stability　to　a　variety　of　factors　of　interest.　These　factors　are　the　reciprocal　of　the　air-entry　value　of　the　soil　α,　the　thickness　of　the　unsaturated　zone　L,　the　cohesion　of　soil　c,　the　internal　friction　angle　ϕ　related　to　the　effective　normal　stress,　the　slope　angle　β,　the　unit　weights　of　soil　particles　γs,　and　the　saturated　hydraulic　conductivity　Ks.　The　results　show　the　following:　(1)　The　analytical　solutions　are　accurate　in　terms　of　the　relative　differences　between　the　analytical　and　the　numerical　solutions,　which　are　within　5.00%　when　considering　the　latter　as　references.　(2)　The　temporal　evolutions　of　the　shear　strength　of　soil　can　be　sequentially　characterized　as　four　periods:　(i)　strength　improvement　due　to　the　increasing　weight　of　soil　caused　by　rainfall　infiltration,　(ii)　strength　reduction　controlled　by　the　increasing　pore　water　pressure,　(iii)　strength　reduction　due　to　the　effect　of　hydrostatic　pressure　in　the　transient　saturation　zone,　and　(iv)　stable　strength　when　all　the　soil　is　saturated.　(3)　The　large　α　corresponds　to　high　effective　rainfall.　(4)　The　factors　ranked　in　descending　order　of　sensitivity　are　as　follows:　α　＞　L　＞　c　＞　β　＞　γs　＞　Ks　＞　ϕ.　©　2025　by　the　authors.