Context: Cereal-legume intercropping improves water use efficiency (WUE) in dryland agriculture. Yet, how intercrops and soil water interactions drive productivity is poorly understood, particularly under fluctuating rainfall conditions. Objective: The study aims to analyze the spatiotemporal dynamics of soil water, elucidate mechanisms behind water productivity gains, and identify the role of plastic film mulching in maize-soybean intercropping. Methods: A two-year rainfed field experiment (dry 2017 and wet 2018) compared two intercropping systems (maize-soybean intercropping with maize film mulching (FMS) and without mulching (NMS)) with their respective monocultures. Measurements included water equivalent ratio (WER), net effect on WUE (Delta WUE), soil moisture, yield, grain yield and dry matter. Results: The WER values (1.20-1.28) confirmed that both FMS and NMS systems improved water productivity compared to respective sole cropping. This advantage arose from the synergy between interspecific water competition and spatiotemporal niche complementarity, which created a contrasting soil moisture gradient between maize and soybean strips. The gradient facilitated much more grains formation of both crops in intercropping, due to supplying adequate water for maize during silking and grain filling while providing reduced but sufficient water for soybean during flowering. Thereby, it matched intercrops' physiological needs for high yield (maize grain of 14.91 Mg ha(-1) and soybean grain of 1.79 Mg ha(-1)). Moreover, FMS increased intercropping Delta WUE by 77.4%, mainly by boosting maize dry matter and supporting grain yield increase by 9.8% (P < 0.05). It shifted water allocation between intercrops, strengthening maize's competitive edge in dry years and enhancing system-wide water conservation in wet years. It also improved soil water storage for reproductive growth and promoted preferential use of topsoil water, indicating a sustainable strategy for dryland farming. Conclusions: Maize-soybean intercropping boosts water productivity via complementary water-use strategies, and film mulching further enhances system productivity and climate resilience in dryland agriculture. Significance: This work links technology with eco-physiological mechanisms to explain WUE in intercropping, offering practical strategies for sustainable food production under water scarcity and climate variability.