Soil Water Movement And System Design Parameters Under Subsurface Drip Irrigation

There is an interaction characteristic between system hydraulic properties and soil water movement under Subsurface Drip Irrigation system (SDI). To improve the design theory and management method of SDI, field positioned observations, laboratory experiments, theoretical analysis and mathematical models were conducted for studying the continuous system from pipe network of SDI, soil to crops. Characteristics of soil water movement under SDI were studied. Emitter flow, drip line spacing and depth were discussed, and such main designning parameters of SDI system were proposed. Hydraulic of pipe networks were analyzed. A new method was explored for the monitoring and evaluation of SDI system in the field. The main conclusions obtained as follows:(1) Process of the Soil Water Front (SWF) can be summarized into two independent linear. Process under SDI. There is a saturated zone near the emitter during irrigation. Migration rates of SWF vary with directions. Form downward, level to upward, migration rates of SWF decrease. Different directions of SWF distance trend to a same value during the redistribution process. There is a certain ratio among the migration distance at different directions, and this ratio related to the emitter flow and the soil texture.(2) The potential energy near the outlet of emitter were directly affected by the emitter's flow and soil water conductivity under SDI. The stability pressure near the emitter's outlet increases with the nominal emitter flow rate's increasing, and decreased with the increasing of initial soil moisture for a same soil. The distribution of soil water potential was nonlinear. Using the soil water potential as parameters, equations were given to solute the soil water potential distribution under the infiltration from point and line source(3) For the soil water movement under SDI contains fine soil particles'movement,it couldn't be described directly with Richard equation. Soil water movement under SDI were divided into saturated and unsaturated infiltration diffusion zone, and described by Darcy Law and Richard equations, respectively. A coupled regional model was established. A nonlinear function was used to describe the outside boundary of saturation range, and it was used as the boundary of the coupled ranges of SDI.(4) the emitter flow was a dynamic decreasing process and tended to a stable rate during the process of SDI. The steady flow rate reduced by 5% to 30% compared with the nominal emitter flow. Lesser of the nominal emitter flow, the flow rate of buried emitter closer to its nominal value and longer time to its stable status. So as to smaller initial soil water content. It is suggested that the designed emitter discharge could be discounted on it's nominal discharge. The reduction coefficient is from 0.70 to 95 for the sandy loam.(5) The suitable Drip Line Spacing (DLS)for sandy loam soil is 90 cm. The affected zone of SDI was divided into pressure conduction, soil moisture diffusion zone and crop water uptake zone, based on soil water potential distribution. The formulas were given to calculate the length of corresponding regions, with soil water potential as the variable. DLS equals to 2 times of above 3 parts sum. And formula of drip line spacing, as the function of crops and water relations, was given under SDI, too.(6) The drip line buried depth of SDI were affected by soil texture, crop, and plow pan. The drip line buried depth could be shallower for sandy loam compared with the loam and silt loam, and be deeper with crops. It should be considered that relative position of drip line and plow pan, combined with the main root water uptake areas, agricultural operations and other factors, for selecting of drip line depth.(7) The stability of SDI system was improved with flushing pipes. Field observions show that the drip line working pressure were±30% of nominal pressure. and the difference of emitter flow is less than 20%. Drip line head loss has a positive correlation to the distance from the plot entrance. The drip line working status near the entrance of plot varies with the system working pressure. Field pipes can be analyzied as multi-outlet with outflow at its end, ring pipe networks could be changed into the tree pipe networks. Composed of nonlinear equations to calculate hydraulic of pipe network in SDI.(8) 2 L timing method was proposed to monitor the status of SDI. Reducing drip line flow rate varied with using years. Corresponding to 1, 2, 3 and 4 years, more than 10% of drip line flow decreased rates are 0, 1/3, 2/3, 2/3, respectively. For a same service year, drip line properties changes with their location and their types. Near to inlet of plot, the decreased drip line flow rate is smaller. It is stable for lower flow and thicker drip wall.(9) 25-point sampling method was used to evaluate soil water distribution uniformity under SDI. Results show that soil water consume is as the drip line of center in the space under SDI. Compared with the upper and lower layers, uniformity of the soil moisture vicinity of drip line lower before irrigation; it increase obviously after irrigation.