With the implementation of the “Grain-for-Green” program, artificial vegetation was introduced on the Loess Plateau, which resulted in high soil water content (SWC) depletion. Currently, lack of soil water recharge is one of the most serious challenges on the Loess Plateau. Soil drying and wetting processes are critical for the sustainability of soil water recycling, but this has not been well studied. There is also a lack of physical definition of the upper bound SWC of dried soil layers (DSL). In this study, soil water dynamics – the change of SWC affected by precipitation and vegetation transpiration – were studied under converted vegetation. In-situ SWC measurements from the 0–5 m or 0–8 m deep profile over consecutive wet years (from 2016 to 2018 with an average precipitation of 660.9 mm) were analyzed to understand soil water depletion and restoration processes. Results showed distinct differences in soil water dynamics in the soil profiles and soil water balances under different vegetation types. SWC under continuous perennial alfalfa (Medicago sativa) had greater fluctuations between 0 and 300 cm than below 300 cm, and a DSL was observed below 300 cm. After converting from alfalfa to soybean (Glycine max), SWC increased greatly during the three wet years. Soil water storage (S) increased at an average rate of 35.8 mm year⁻¹ m⁻¹ within the top 500 cm of the soil profile, average evapotranspiration (ET) was 482.0 mm year⁻¹, and maximum restoration depth of soil water extended to 660 cm. However, SWC gradually decreased over time after replacing food crop with alfalfa. S declined at an average rate of 21.4 mm year⁻¹ m⁻¹ within the top 500 cm of the soil profile, average ET was 680.4 mm year⁻¹ and the maximum depth of soil water depletion extended to 360 cm. These results suggest that SWC in deep layers can be depleted and replenished quickly, and the processes were dominated by vegetation types and precipitation. Taking vegetation types and soil texture into consideration, the calculation of upper bound SWC of DSL was redefined. Given the long-term effects of high water demand from vegetation such as alfalfa on the soil water balance, ET of vegetation should be reduced through conversion to less water-intensive vegetation types or biomass control (i.e. reduced planting density appropriately) in arid areas of the Loess Plateau.

An indoor aquaponic system (i.e., the integration of fish culture with hydroponic plant production in a recirculating setup) was operated for maximizing water reuse and year-round intensive food production (Nile tilapia, Oreochromis niloticus, and leaf lettuce) at different fish feed to plants ratios. The system consisted of a fish culture component, solid removal component, and hydroponic component comprising six long channels with floating styrofoam rafts for holding plants. Fish culture effluents flowed by gravity from the fish culture component to the solid removal component and then to the hydroponic component. Effluents were collected in a sump from which a 1-horsepower in-line pump recirculated the water back to the fish culture tanks at a rate of about 250 L/min. The hydroponic component performed as biofilter and effectively managed the water quality. Fish production was staggered to harvest one of the four fish tanks at regular intervals when fish attained a minimum weight of 250 g. Out of the total eight harvests in 13 mo, net fish production per harvest averaged 33.5 kg/m³ of water with an overall water consumption of 320 L/kg of fish produced along with the production of leaf lettuce at 42 heads/m² of hydroponic surface area. Only 1.4% of the total system water was added daily to compensate the evaporation and transpiration losses. A ratio of 56 g fish feed/m² of hydroponic surface effectively controlled nutrient buildup in the effluents. However, plant density could be decreased from 42 to 25-30 plants/m² to produce a better quality lettuce.