UC Riverside

The need for improvements in the water use efficiency by agricultural ecosystems requires a holistic assessment of the hydraulic functioning of cropped soils, taking into consideration the most relevant interactions and feedbacks that control the soil water budget. We implemented a mechanistic approach to isolate the effects of soil hydraulic properties (K-θ-h) of layered soils on water balance components and land and water productivity, adopting comprehensive scenarios of soil water availability and requirements. The agro-hydrological simulations were performed using the SWAP model integrated with the WOFOST crop growth module. The simulated scenarios included the rainfed crop growth of maize and soybean in three climate zones, evaluating the current climate scenarios as well as two future scenarios, a wetter and a drier one, totaling 108 scenarios simulated for 30 years each. Simulations were performed for six soils, grouped pairwise (3 × 2), where each pair represented the same soil group with two different long-term land uses: natural forest (proxy of a no-tillage system) and conventional agricultural use. The K-θ-h relationships were obtained simultaneously by inverse modeling for the full range of soil water contents commonly found in the domain of crop available water. The agro-hydrological simulations showed that the soil hydraulic properties affect dynamically water balance components and land productivity by relating soil hydraulic functioning to climate patterns and crop water requirements. In general, maize productivity was more sensitive to soil hydraulic properties under future climate scenarios than soybean. While land productivities of maize and soybean increased under the wetter climate scenario, water productivity of both crops was consistently reduced by both future climate scenarios. The K-θ-h of soils under conventional agricultural use over-performed their counterparts under long-term natural forest use, especially regarding land productivity during growing seasons with pronounced dry spells. Depending on the length and timing of drought stress during the growing season, the yield response is determined by soil-specific conditions strictly related to water availability. The long-term average revealed that the sampled loamy sand soils have more favorable hydraulic properties for crop growth; moreover, the reduced unproductive water losses, especially runoff, increased the dynamic water storage of those soils.