Vegetation patterns governing the competitive relationship between runoff and evapotranspiration using a novel water balance model at a semi-arid watershed

Vegetation patterns play an important role in precipitation partitioning into hydrological components, especially evapotranspiration and runoff. However, few studies focus on their competitive relationship and the influence of the vegetation on them. In this study, a vegetation threshold was postulated to prevent further decrease of runoff by determining a new hydrological component continuing evapotranspiration (partitioned from total and initial evapotranspiration) through a novel model coupled with the Budyko model (dimensional form) and two-stage partitioning model (nondimensional form) in the semi-arid watershed. The results showed significant correlations between model parameters epsilon (underlying surface index), lambda (ratio of initial evapotranspiration) and vegetation coverage (M) (R-2 = 0.95 and 0.97, p < 0.01) b Based on the modified Budyko model and lambda. The nondimensional model showed high-precise estimations of K-H (Horton index Fraction), K-B (Baseflow Fraction), K-V (evapotranspiration Fraction), K-R (runoff Fraction), and K-C (continuing evapotranspiration Fraction) (R-2 > 0.98, p < 0.01) as a function of a new aridity index phi. K-H, K-B, K-V, K-R, showed symmetrical patterns correlated with phi both at between-subwatershed and between-year scale based on the dimensional model and lambda. However, KC showed asymmetrical different correlation with M-3 and phi (K-C/M-3 similar to phi: in between-subwatershed: R-2 = 0.92, p < 0.01; and between-year scale: R-2 = 0.74, p < 0.01). Based on the solution of continuing evapotranspiration, the vegetation threshold has been solved with M = 0.73 (+0.09/-0.02) to prevent further decreasing runoff. The framework presented can be applied in other semi-arid watersheds worldwide to better protect the sustainability of the hydro-ecosystems.