Modeling Approaches to Determination of Appropriate Depth and Spacing of Subsurface Drip Irrigation Tubing in Alfalfa to Ensure Soil Trafficability

Abstract

A major design issue in the implementation of a Subsurface Drip Irrigation (SDI) system for extensively crops such as alfalfa (i.e. crops that cover the entire surface as opposed to row crops), is the determination of the appropriate depth of placement of the drip line tubing. It is important to allow necessary farming operations with heavy equipment at harvesting times while still providing adequate water to meet the crop water requirements. It is also a need to ensure appropriate spacing between the dripline laterals to assure reasonable lateral irrigation uniformity for plant germination. In this study, the program HYDRUS-2D was used to determine the wetting pattern above and laterally from a subsurface drip emitter of an SDI system, for three soils typically found in Southern California and Arizona, a Sandy Clay Loam (SCL), a Clay Loam (CL) and a Loam (L). The design and management conditions from an experimental alfalfa field with an SDI system located at Holtville CA were used and analyzed. The first irrigation design was with a drip line depth of placement of 30 cm and the second design with an installation depth of 50 cm. The two different irrigation management schemes utilized by the farmers and producers in that area were: one with a running time of six hours and a frequency of every three days and the second one with an irrigation running time of twenty-four hours with a frequency of seven days or irrigation every week. After having carried out the analysis and studies of the irrigation designs and management schemes mentioned above, a new model with its corresponding management was proposed to meet the alfalfa water requirements under that particular field and weather conditions while we ensure a sufficiently dry soil surface at harvesting time for each soil case. This irrigation management includes twelve hours or irrigation every three days, for each of the three soils analyzed. It was found that the vertical rise of water above the emitters on the day of the cut, for our recommended SDI management was 26 cm, 29 cm, and 27 cm, with a moisture content at the soil surface of 14.9%, 24%, and 13% for the SCL, CL, and L soils respectively. Then, through the utilization of classical soil mechanics theory, an analysis to calculate the increase in stress on soils at any depth due to a load on the surface from a conventional tractor used during harvest operations was made for the proposed SDI system. The results from the increase in stress were then used together with soil strength properties such as shear strength as a function of soil moisture content to determine the minimum allowable depth of placement of the drip line tubing to ensure that soil failure does not occur. The load increase from a 3,300-kg four-wheel tractor was found to be 0.59 kg/cm2 under a rear tire at 10 cm below the surface and 0.07 kg/cm2 at 70 cm below the surface. To ensure that shearing failure does not occur, a stress analysis using Mohr’s circle indicated that the soil moisture content at 10 cm below the surface should be no greater than 26.8%, 32.7%, and 27% in the SCL, CL, and L soils respectively. The mimimum moisture content of 26.8% occur at 10 cm above the drip line for a SCL soil, which means that the minimum depth placement to avoid failure would be 40 cm below the surface. A similar analysis for the CL and L yielded minimum installation depths of 35 cm and 40 cm respectively. This type of analysis is useful in determining the depth of placement of SDI drip line tubing to ensure adequate trafficability of soil irrigated with subsurface drip irrigation systems. An additional outcome of the modeling study was the determination of the lateral extent of the wetted zone which can be used to determine the appropriate lateral spacing between drip line tubing. Thus, to ensure adequate spatial coverage by a subsurface drip system, the maximum horizontal spacing should be of 80 cm for SCL and L soils and 90 cm in CL soils.