Dryland crops are planted in plastic-mulched ridges to suppress weed growth and support early plant emergence because of increased soil temperatures in the ridges. However, effects of plastic-mulched ridges on soil water dynamics in humid areas have not yet well inves-tigated. This study aimed to evaluate soil water flow in a field covered with plastic mulch using the HYDRUS (2D/3D) model. Observed soil water contents in an egg-plant (Solanum melongena L.) filed with plastic-mulched ridges in the summer of 2012 were validated with the wa-ter flow model. Boundary conditions and two parameters of the root water extraction model were investigated for several simulation scenarios. A reasonably good agree-ment between the model-predicted and observed soil wa-ter contents in plastic-mulched ridges was found. Simu-lation results revealed the water flows in plastic-mulched ridges during rainfalls. Soil water contents were higher in ridges without mulch than in ridges with mulch. Moreover, soil water contents fluctuate greater in bare ridges than in mulch covered ridges. We concluded plastic mulch at the soil surface could moderate the water content increase in the soil profile during the rainfall in the humid area.
A scheme for determining irrigation depth us-ing a numerical model of crop response to irrigation and quantitative weather forecast was presented. To optimize each irrigation depth, a concept of virtual income, which is proportional to an increment in transpiration amount dur-ing an irrigation interval, is introduced. A field experiment was carried out to evaluate effectiveness of the presented scheme in terms of net income considering the price of water. Potato was grown in summer season of 2015 us-ing a drip irrigation system in Arid Land Research Cen-ter, Tottori, Japan. Two treatments were conducted: auto-mated irrigation and proposed scheme with two replicates for each. Results indicated that predicted water content agreed well with observation although some underestima-tion of water content due to overestimation of transpiration was observed. Proposed scheme could save water by 27 %, while yield was increased by 15 %, resulting in higher net income as compared to automated irrigation. Based on these results together with previous works, we can con-clude that proposed scheme can at least realize similar net income to automated irrigation systems without high ini-tial investment.
To prevent water leakage from direct-sown dry paddy rice fields, a compaction roller 2 m in width with a total load of 9.4 kN was developed. Even when the soil was dry, and thus difficult to compact, the roller success-fully decreased permeability after five times compactions. However, the mechanism of the observed permeability de-crease has not been well investigated. We therefore stud-ied the progressive changes in the pore size distributions based on the water retention curves as a result of the roller compactions. Observations of the soil profile and measure-ments of penetration resistance identified a densely com-pacted layer less than 2 cm above the plough pan. Posi-tion markers inserted in the soil profile revealed that the soil at several centimeters below the surface was highly compacted. The pore size distributions had three stages as the number of roller compactions increased. Firstly, the volume of the larger pores and the total porosity de-creased. Secondly, the pore size distribution shifted from larger pores to smaller pores while keeping the total poros-ity almost constant. Lastly, the pore size distribution did not change anymore regardless of the further roller com-pactions. Although water infiltration decreased steeply in the first stage, the compaction was insufficient to prevent leakage. In the second stage, infiltration reduced suffi-ciently to stop leakage. At this stage, the ratio of pores larger than 0.2 mm decreased, whereas macropores larger than 0.1 mm still remained in the compacted soil layers.