Water scarcity strongly limits the establishment and productivity of young orchards. Although cover crops are increasingly adopted to improve soil health, their integrated effects on soil–plant–water interactions under drought remain unclear. Here, a two-year field study evaluated Legume, Gramineae, and Legume-Gramineae mixture covers in relation to soil water dynamics, evapotranspiration (ET), and water use efficiency (WUE). Gramineae cover reduced 0–100 cm soil water storage by 5.99% compared with bare soil, whereas the Legume-Gramineae mixture effectively buffered drought-induced water loss. All cover treatments increased total ET, with the mixture showing the highest (10.31%), indicating that enhanced transpiration compensated for reduced soil evaporation. As a result, WUE improved, particularly during winter and spring when water demand was lower. Stepwise analysis identified rainfall as the primary climatic drivers of ET and WUE. Overall, the Legume-Gramineae mixture offers a promising strategy for improving WUE and mitigating drought stress in water-limited orchards.